Ophthalmology

General Features

• Dimensions — the adult eyeball has an anteroposterior (axial) diameter of 24 mm, a horizontal diameter of 23 mm and a vertical of 23.5 mm; it weighs 7 g and has a volume of 6.5 mL. The axial length is the single most important dimension clinically, since a variation of 1 mm alters the refraction by 3 dioptres

The Cornea

• Dimensions — horizontally 11 to 12 mm and vertically 10 to 11 mm, so it is elliptical from the front and circular from behind; thickness 0.52 mm centrally and 0.67 mm peripherally
• Refractive power +43 to +45 dioptres, which is nearly two-thirds of the total refractive power of the eye (58 to 60 D) — the cornea, not the lens, is the principal refracting surface

• The four factors maintaining corneal transparency — avascularity; the regular lamellar arrangement of uniform collagen fibrils spaced less than a wavelength of light apart; relative dehydration (deturgescence), maintained actively by the endothelial pump and by the epithelial barrier; and non-myelinated nerve fibres

Sclera, Limbus and Uvea

• Sclera — opaque because its collagen fibrils are irregular in diameter and arrangement, the exact converse of the cornea. Thickest posteriorly (1 mm) and thinnest (0.3 mm) just behind the insertions of the recti, which is where it ruptures in blunt trauma and where a staphyloma forms
• Limbus — the 1.5 to 2 mm transition zone between cornea and sclera. It contains the limbal stem cells in the palisades of Vogt, and is the surgical landmark for cataract and glaucoma incisions
• Iris — contains the sphincter pupillae (circular, parasympathetic via the third nerve) and the dilator pupillae (radial, sympathetic). The pupil is normally 3 to 4 mm

Retina, Angle and Blood Supply

• Retina — ten layers. Rods number 120 million, are peripheral, and subserve scotopic (dim light) vision using rhodopsin; cones number 6 to 7 million, are concentrated at the macula, and subserve photopic vision, colour and acuity
• The macula is 5.5 mm across; the fovea is 1.5 mm and the foveola 0.35 mm, containing only cones, with the inner layers displaced aside so that light reaches them directly. The foveal avascular zone has NO retinal vessels and depends entirely on the choroid — which is why a central retinal artery occlusion spares a small central island (the cherry-red spot with a patent cilioretinal artery)
• Optic disc — 1.5 mm across, with no photoreceptors, giving the physiological blind spot 15 degrees temporal to fixation

The Lens, Vitreous and Extraocular Muscles

Applied Aspects

• 1 mm of axial length is 3 dioptres; this single relationship explains axial myopia, axial hypermetropia and why biometry before cataract surgery must be accurate to a fraction of a millimetre
• Inject into the pars plana, 3.5 to 4 mm behind the limbus; anterior to it lies the ciliary body and posterior to it the retina, and the safe window is narrow
• Count the endothelium before intraocular surgery in a compromised cornea; below 500 cells per square millimetre the cornea will decompensate, and the cells do not regenerate

Definitions

• Emmetropia is the state in which parallel rays of light from infinity come to a focus ON the retina with accommodation AT rest.

Myopia

• Myopia (short sight) — parallel rays come to a focus IN front of the retina. The far point is finite and lies at a measurable distance in front of the eye
• Aetiological types — axial, the commonest, from an increased anteroposterior length; curvatural, from increased corneal or lenticular curvature, as in keratoconus; index, from increased refractive index of the lens nucleus in nuclear sclerosis — producing the phenomenon of "second sight", in which an elderly presbyope finds they can read without glasses again; positional, from anterior displacement of the lens; and that due to excessive accommodation (spasm)

• Symptoms — poor distance vision with good near vision; asthenopia; half-shutting the eyes to obtain a stenopaeic (pinhole) effect, which is where the word myopia ("to shut the eye") comes from; and floaters

Myopia Compared with Hypermetropia

Hypermetropia

• Hypermetropia (long sight) — parallel rays come to a focus behind the retina with accommodation at rest. It is the normal refractive state of the infant (+2 to +3 D), reducing with growth — a fact of importance in paediatric practice
• Types — axial, the commonest; curvatural, from a flatter cornea; index, in old age from reduced refractive index of the cortex; positional, from posterior displacement of the lens; and aphakia — the absence of the lens — which produces a hypermetropia of about +10 to +12 D
• The components of hypermetropia, which are examinable and clinically important: total = latent + manifest. latent is that masked by ciliary tone and revealed only by cycloplegia. manifest = facultative + absolute; facultative is that which the patient can overcome by accommodating, and absolute is that which they cannot, and which therefore blurs the vision
• Symptoms — the hallmark is asthenopia: eye strain, frontal headache after sustained near work, watering and a desire to close the eyes — because accommodation is in constant use. Near vision blurs first and distance later, and symptoms worsen with age as accommodation declines
• Signs — a small eye with a small cornea and shallow anterior chamber; and a small, crowded optic disc with blurred margins.

Astigmatism

• Astigmatism is the refractive state in which the refraction differs IN different meridia, so that rays are not brought to a point focus on the retina.
• The word means "without a point".
• Regular astigmatism — the two principal meridia are at right angles to each other and the power changes progressively between them, so it is correctable with a cylindrical lens

Accommodation

The ciliary muscle contracts (parasympathetic, via the third nerve and the ciliary ganglion) → The ciliary ring is drawn forward and inward, so the zonules relax → Released from zonular tension, the lens assumes its own more spherical (convex) shape through the elasticity of its capsule → The anterior surface curves more than the posterior — the anterior radius falls from about 10 mm to 6 mm → The lens thickens, the anterior pole moves forward, and the anterior chamber becomes shallower → The dioptric power increases and the near object is focused → This is helmholtz’S theory, which is the accepted account

• The near reflex is a triad, occurring together and mediated centrally: accommodation, convergence and miosis — the last usefully increasing the depth of focus

Presbyopia

• Presbyopia is the physiological, age-related insufficiency of accommodation, in which the near point recedes beyond the customary working distance.
• It is not a refractive error but a normal consequence of ageing, occurring in every eye.
• Why it occurs at 40 to 45 — comfortable near work at 25 to 33 cm requires about 3 to 4 D of accommodation, and no more than half to two-thirds of the amplitude can be used comfortably for sustained work. The amplitude falls below that threshold at 40 to 45 years, and symptoms begin

Types of Regular Astigmatism

Applied Aspects

• Refract every child under cycloplegia; the latent hypermetropia is otherwise missed, and it is the component that causes accommodative squint and amblyopia
• Suspect keratoconus in rapidly changing or oblique astigmatism, particularly in a young person with allergic eye disease who rubs the eyes, and get topography
• Offer a rigid gas-permeable lens for irregular astigmatism; spectacles cannot correct it, and the improvement with a rigid lens is often dramatic
• Explain that presbyopic glasses do not weaken the eyes; the belief is near-universal and leads people to struggle without them
• Do not exceed +3.00 D for near; a stronger add forces the working distance too close and the patient will not use it
• Suspect diabetes in sudden refractive change in an adult; hyperglycaemia causes a reversible myopic shift and is a recognised presenting feature
• Consider "second sight" in an elderly patient reading without glasses again; it signals index myopia from nuclear sclerosis, and cataract is developing
• With-the-rule astigmatism is normal in the young, from lid pressure, and shifts against-the-rule with age as lid tone falls
• Irregular astigmatism needs a rigid lens, which replaces the irregular surface with a regular one; spectacles cannot correct it
• The circle of least diffusion lies midway between the focal lines, and is where the spherical equivalent places the image

Visual Acuity

• Visual acuity is a measure of the resolving power of the eye — the ability to distinguish two points as separate. The normal eye resolves an angle of one minute of arc
• The snellen chart is constructed so that each letter subtends 5 minutes of arc at the stated distance and each stroke and gap subtends 1 minute. It is read at 6 metres, at which distance rays are effectively parallel
• Recording — V = d / D, where d is the distance at which the patient reads the line and D the distance at which a normal eye should read it. 6/6 is normal; 6/60 means the patient reads at 6 metres what should be read at 60

Objective Refraction — Retinoscopy

• Retinoscopy (skiascopy) is the objective determination of refractive error, requiring NO response from the patient — which makes it indispensable in children, in the uncooperative and in the intellectually disabled
• Principle — light is thrown into the eye and the movement of the reflex in the pupil is observed as the streak is moved. with movement (the reflex moves in the same direction as the streak) indicates that the far point is behind the observer — hypermetropia, emmetropia, or myopia less than the working distance — and is neutralised with plus lenses. against movement indicates myopia greater than the working distance and is neutralised with minus. At neutralisation the pupil fills with a bright reflex that does not move
• The working distance must be subtracted, since retinoscopy is done at a finite distance: at 1 metre subtract 1.00 D; at 2/3 metre subtract 1.50 D. Omitting this step is the classic error, and produces a systematically over-plussed result

Common Errors and Special Situations

Subjective Refraction and Applied Aspects

• Subjective refinement follows the objective result: the duochrome (bichrome) test.
• The jackson cross-cylinder for refining the axis first and then the power of a cylinder; and the astigmatic fan and clock dial
• Binocular balancing before the final prescription, to ensure accommodation is equally relaxed in both eyes
• Always test the pinhole before refracting; if it does not improve the vision, look for disease rather than persisting with lenses
• Subtract the working distance in retinoscopy; forgetting it over-pluses every prescription and is the commonest single error in the technique

Spectacles

• Spectacles remain the commonest, safest and cheapest correction, and uncorrected refractive error is the leading cause of visual impairment worldwide — a problem of access rather than of technology
• Lens materials — crown glass (heavy, but scratch-resistant); CR-39 plastic (light, but scratches); polycarbonate (impact-resistant, and therefore the material of choice for children, sportspeople and anyone with only one seeing eye); and high-index materials for strong prescriptions
• Advantages — safe, easily changed, no infection risk, and they protect the eye. Disadvantages — magnification and minification of the retinal image (2% per dioptre), which limits their use in anisometropia; peripheral aberration and prismatic effect; a restricted field; and cosmetic objection

Contact Lenses

• Optical advantages over spectacles — a wider field with no frame or ring scotoma; minimal image size change, so they are the correction of choice in anisometropia and after unilateral aphakia; no prismatic effect on eccentric gaze; and better cosmesis and utility in sport
• Complications — and these are what the examination asks about: microbial keratitis, the most serious, and dramatically increased by overnight wear; acanthamoeba keratitis, associated with rinsing lenses in tap water or swimming in them, exquisitely painful and difficult to treat; giant papillary conjunctivitis; corneal neovascularisation and hypoxic oedema; solution hypersensitivity; dry eye; and corneal warpage
• Contraindications — poor hygiene or motivation; dry eye; blepharitis and active external infection; a reduced corneal sensation (as after herpes zoster).

Refractive Surgery

• Patient selection is the whole of refractive surgery. Requirements: age at least 18 to 21 with a stable refraction for at least a year; adequate corneal thickness, with a residual stromal bed of at least 250 to 300 micrometres after ablation; normal topography; realistic expectations; and no active ocular or relevant systemic disease
• Contraindications — keratoconus or any suspicion of it on topography, which is the absolute contraindication; a thin cornea; unstable refraction; pregnancy and lactation; uncontrolled autoimmune or connective tissue disease; severe dry eye; glaucoma; cataract; and unrealistic expectations
• The feared complication is post-LASIK ectasia — a progressive corneal bulge resembling keratoconus, occurring months to years afterwards in an eye weakened beyond its biomechanical reserve. It is largely preventable by proper screening, which is why topography is mandatory and why any suspicion of keratoconus disqualifies the patient

Choosing the Correction

Applied Aspects

• Contact lenses, not spectacles, for significant anisometropia and for unilateral aphakia; spectacles create an image size difference the brain cannot fuse
• Prescribe polycarbonate for children and for the one-eyed patient; protecting the remaining eye matters more than any optical consideration
• Warn every contact lens wearer about water and overnight wear; these two behaviours account for most of the serious infections

Anatomy

• Histology — a stratified epithelium of 2 to 5 layers containing goblet cells (most numerous in the fornices, plica and caruncle), over a substantia propria with a superficial adenoid (lymphoid) layer and a deep fibrous layer
• The adenoid layer does not develop until 2 to 3 months of age. This single fact has a direct clinical consequence: a neonate cannot form follicles, so ophthalmia neonatorum is always papillary however it is caused, and the finding of follicles in a newborn should not be expected

Signs and Their Interpretation

Classification of Conjunctivitis

• Infective — bacterial (acute mucopurulent, acute purulent, membranous, angular, chronic); chlamydial (trachoma, adult inclusion, neonatal); viral (adenoviral, herpetic, acute haemorrhagic, molluscum); fungal, rickettsial and parasitic
• Allergic — seasonal and perennial, vernal keratoconjunctivitis, atopic, giant papillary, phlyctenular and contact dermatoconjunctivitis
• Irritative and toxic — chemicals, smoke, drugs and preservatives

Applied Aspects

• Evert the upper lid in every case; the tarsal conjunctiva carries the diagnostic signs, and a conjunctivitis assessed without eversion is assessed blind
• Distinguish conjunctival from ciliary congestion first; the second means cornea, uvea or glaucoma, and is the sign that separates a trivial red eye from a sight-threatening one
• Palpate the preauricular node; its presence points to viral or chlamydial disease and changes both the treatment and the advice about contagion

Acute Mucopurulent Conjunctivitis

• The commonest form of bacterial conjunctivitis, and one of the commonest of all ocular conditions. Organisms: staphylococcus aureus (the commonest overall), Streptococcus pneumoniae, haemophilus influenzae (particularly in children, and often with otitis media), and Moraxella
• Symptoms — discomfort and a foreign-body or "sand in the eye" sensation rather than true pain; redness; discharge; and sticking together of the lids on waking, which is characteristic and is often the presenting complaint
• The coloured haloes — patients may describe rainbow haloes around lights, from the prismatic effect of mucus in the tear film. They disappear when the discharge is cleared by blinking or irrigation — the "FICK'S sign" — which distinguishes them from the persistent haloes of angle-closure glaucoma. This is a genuinely useful bedside discrimination

Acute Purulent (hyperacute) Conjunctivitis

• The organism is neisseria gonorrhoeae (and rarely N. Meningitidis). In adults it is transmitted by autoinoculation from the genital tract, and it is an ophthalmic emergency
• Why it is an emergency — because neisseria can penetrate an intact corneal epithelium. Almost every other organism requires a breach; this one does not, so a corneal ulcer and perforation can develop within 24 to 48 hours in an eye that was normal the day before
• Features — explosive onset over hours; copious thick yellow pus, which wells out as soon as the lids are parted and reaccumulates immediately; gross lid oedema such that the lids may be impossible to open; severe chemosis, sometimes with the conjunctiva prolapsing between the lids; a tender preauricular node; and pain

Membranous, Angular and Chronic Forms

Summary of the Bacterial Conjunctivitides

Applied Aspects

• Treat hyperacute purulent conjunctivitis as an emergency with systemic ceftriaxone and hourly irrigation; the cornea can perforate within two days
• Use Fick’s sign to separate haloes; haloes that clear on blinking or irrigation are mucus, and haloes that persist are corneal oedema and may be angle closure
• Never patch a discharging eye, and never give a steroid without knowing the diagnosis; both convert a self-limiting illness into a corneal disaster
• Look for a cause in chronic conjunctivitis — blepharitis, dacryocystitis, a retained foreign body, an ingrowing lash or uncorrected refractive error — since it is a symptom rather than a diagnosis
• Distinguish a true from a pseudo-membrane by whether it bleeds; a bleeding, adherent membrane means diphtheria until proved otherwise and the child needs antitoxin

Aetiology and Epidemiology

• Serotypes matter — A, B, Ba and C cause trachoma; D to K cause adult inclusion conjunctivitis and genital infection; and L1 to L3 cause lymphogranuloma venereum
• It is the leading infectious cause of blindness in the world, and a disease entirely of poverty — of crowding, water scarcity, poor sanitation and the absence of latrines

Clinical Features

• McCALLAN’S classification is the older scheme — Stage I incipient; Stage II established (IIa follicular, IIb papillary); Stage III cicatrising; and Stage IV healed

The Safe Strategy and Treatment

• The elimination target — a prevalence of TF below 5% in children aged 1 to 9, and of TT below 0.2% in adults, sustained for at least two years with a system in place to manage incident cases
• Individual treatment — azithromycin 1 g orally as a single dose, or a tetracycline; and treat the whole household, since reinfection from family contacts is the rule

Trachoma Compared with Adult Inclusion Conjunctivitis

Applied Aspects

• Evert the upper lid and look at the upper tarsus; trachoma follicles are there.
• Look for Herbert’s pits at the upper limbus; they are pathognomonic and permanent, and identify past trachoma in an adult presenting with trichiasis
• Treat the entire household and the community, not the individual; reinfection is otherwise immediate and treatment achieves nothing durable

Classification and Mechanism

• Allergic conjunctivitis is the commonest of all conjunctival disorders, and comprises a spectrum from trivial seasonal itching to sight-threatening corneal disease
• Seasonal and perennial allergic conjunctivitis (sac and PAC) — a pure type I (IgE-mediated) hypersensitivity to pollen, dust mite or animal dander. Bilateral itching, watering, chemosis and lid oedema, with a papillary reaction; often with rhinitis. It is uncomfortable but not sight-threatening
• Vernal keratoconjunctivitis (VKC, spring catarrh) — the important one in India. A combined type I and type IV reaction. Affects boys between 5 and 20 years, in hot, dry, dusty climates, with a strong atopic family history; it is bilateral, seasonal (worse in summer) and self-limiting at puberty in most cases

Vernal Keratoconjunctivitis in Detail

• Symptoms — intense itching, which is the cardinal symptom and out of proportion to the signs; ropy, thick, stringy mucoid discharge which the child pulls out in strands; photophobia; and a burning, gritty sensation
• The single most important piece of advice is TO stop rubbing. Persistent vigorous eye rubbing in these children causes keratoconus, which is far more damaging in the long run than the conjunctivitis itself and is entirely preventable

Treatment

• General measures — avoid the allergen where identifiable; cold compresses, which are simple and genuinely effective; dark glasses for photophobia; frequent lubricants to dilute allergen; and moving to a cooler climate, which is classically advised and rarely practicable
• Mast cell stabilisers — sodium cromoglycate, nedocromil; slow in onset, so they are started early in the season and continued prophylactically
• Dual-action agents — olopatadine, ketotifen, azelastine — antihistamine and mast cell stabilising, with rapid onset and a long duration; these are now first-line

Applied Aspects

• Tell the child to stop rubbing, and tell the parents why; keratoconus from rubbing causes more permanent visual loss than the vernal disease does
• Measure the intraocular pressure at every visit in a child on topical steroids, and ask specifically whether drops are being bought without prescription
• Use dual-action drops and ciclosporin as the mainstay, and reserve steroids for short supervised pulses in severe exacerbations
• Look for tuberculosis and malnutrition in phlyctenular disease; in India this association remains real and the eye sign may be the presenting feature
• Suspect drug allergy when an eye deteriorates on treatment; neomycin, gentamicin, atropine and preservatives are the usual culprits and the answer is to stop, not to add
• Reassure that vernal disease resolves at puberty in most children; it is a long haul but it ends.
• Distinguish vernal from atopic disease — upper tarsus in a boy that resolves, against lower tarsus in an adult that scars — because the prognosis and the surveillance differ entirely
• Vernal disease affects boys in hot dry climates and resolves at puberty; atopic disease affects adults and progresses

Adenoviral Conjunctivitis

• Adenovirus causes the great majority of viral conjunctivitis, and the two named syndromes are examinable and clinically distinct
• Pharyngoconjunctival fever (PCF) — serotypes 3, 4 and 7. The triad of fever, pharyngitis and follicular conjunctivitis, with a tender preauricular node. Affects children, spreads in swimming pools and within families, and is self-limiting in 1 to 2 weeks. Corneal involvement is mild
• Epidemic keratoconjunctivitis (EKC) — serotypes 8, 19 and 37; the most severe form. Abrupt onset, beginning in one eye and involving the other within days, with watering, redness, gross follicles, lid oedema, a tender preauricular node, and often a pseudomembrane and subconjunctival haemorrhages

Other Viral Conjunctivitis

Adult Inclusion Conjunctivitis

• Caused by Chlamydia trachomatis serotypes D to K, transmitted from the genital tract by autoinoculation or in a swimming pool
• Features — a chronic, unilateral or bilateral follicular conjunctivitis lasting weeks to months, characteristically with large follicles in the lower fornix (the converse of trachoma), mucopurulent discharge, a preauricular node, and often superior micropannus and subepithelial infiltrates
• It is regularly mistaken for adenoviral conjunctivitis and treated symptomatically for months. The clue is the duration — a follicular conjunctivitis that does not resolve in two to three weeks is chlamydial until proved otherwise

Distinguishing the Common Conjunctivitides

Applied Aspects

• Wash your hands and disinfect the tonometer; you are the vector in epidemic keratoconjunctivitis, and an outbreak in a clinic is an infection-control failure rather than bad luck
• Do not prescribe an antibiotic for viral conjunctivitis; it is useless, and it teaches the patient to expect one next time
• Withhold steroids unless there is a membrane or visually significant infiltrates; they prolong viral shedding and the infiltrates rebound when stopped
• Never give a steroid to a red eye with lid vesicles; herpes simplex is the diagnosis and a steroid converts a dendritic ulcer into a geographical one
• Inspect the lid margin in chronic follicular conjunctivitis; a molluscum nodule will keep the eye inflamed indefinitely and removing it is curative

Definition and Predisposing Factors

• A corneal ulcer (infective keratitis) is a discontinuity of the corneal epithelium with necrosis and inflammatory infiltration of the underlying stroma.
• It is the leading cause of monocular blindness in India, and is overwhelmingly a disease of the rural working population.
• The essential precondition is a breach in the epithelium, since the intact epithelium is an effective barrier. Hence the importance of the small number of organisms that can penetrate an intact epithelium: neisseria gonorrhoeae and N. Meningitidis, corynebacterium diphtheriae, haemophilus (Koch-Weeks) and listeria

Clinical Features and the Organisms

• Symptoms — pain, which is severe and is what distinguishes keratitis from conjunctivitis; photophobia, lacrimation and blepharospasm — the classical triad of corneal irritation; blurred vision; and discharge
• Signs — ciliary (circumcorneal) congestion; an epithelial defect staining with fluorescein; a stromal infiltrate beneath and around it, which is the essential sign of infection as opposed to a simple abrasion; surrounding corneal oedema; an anterior chamber reaction with cells and flare; and often a hypopyon
• The four stages (sattler) — infiltration, active ulceration, regression, and cicatrisation

Investigation and Treatment

• Corneal scraping is the key investigation, and it must be taken before any antibiotic is started → Under topical anaesthesia at the slit lamp, using a KIMURA spatula or a sterile blade, scrape the advancing edge and the base of the ulcer — not the surface slough, which is sterile debris → smears — gram (bacteria), GIEMSA (Acanthamoeba cysts, chlamydia), 10% KOH wet mount and calcofluor white (fungal hyphae, which the KOH mount demonstrates within minutes and which is cheap and available anywhere) → culture — blood agar and chocolate agar (bacteria); sabouraud dextrose agar at 25 to 27°C (fungi); thioglycolate broth (anaerobes); and non-nutrient agar with an E.
• Coli overlay for acanthamoeba → Material is plated directly onto the media in C-shaped streaks at the bedside, since transport reduces the yield → confocal microscopy where available demonstrates fungal hyphae and Acanthamoeba cysts in vivo, without waiting for culture
• Treatment of bacterial keratitis — intensive topical antibiotic, hourly day and night for the first 48 hours. Either fortified preparations — cefazolin 5% with tobramycin 1.4% or gentamicin, which cover Gram-positive and Gram-negative organisms respectively — or monotherapy with a fourth-generation fluoroquinolone (moxifloxacin or gatifloxacin 0.5%).

Hypopyon and the Assessment of Progress

• Hypopyon is an accumulation of inflammatory cells in the anterior chamber, forming a white or yellow layer with a horizontal fluid level in the lower angle
• The essential point is that a hypopyon in a corneal ulcer is sterile — a toxin-driven outpouring from the iris and ciliary body in response to the corneal infection, with an intact Descemet membrane keeping the organisms out. It settles as the ulcer settles and should not be drained
• The exception — a hypopyon in fungal keratitis may contain organisms, since fungi penetrate Descemet; and a hypopyon after intraocular surgery or a penetrating injury means endophthalmitis and is an emergency

Complications and Applied Aspects

• Complications — descemetocele, a clear bulge of Descemet membrane through the ulcer floor, which signals imminent perforation; perforation, often precipitated by coughing or straining, with iris prolapse and a flat anterior chamber; anterior staphyloma; secondary glaucoma; endophthalmitis and panophthalmitis; and, in every healed ulcer, a corneal scar whose density and position determine the visual outcome
• Management of perforation — cyanoacrylate tissue adhesive with a bandage contact lens for a small perforation; a patch graft; or therapeutic (tectonic) keratoplasty for a large one, done to preserve the globe rather than for vision
• Scrape before you treat; once antibiotics are started the yield falls sharply, and a patient who fails to respond then cannot be investigated

Fungal Keratitis

• Fungal keratitis (keratomycosis) is of exceptional importance in India, accounting for a third to a half of all corneal ulcers in some regions — among the highest proportions in the world — because of the combination of agricultural work, a hot humid climate and uncontrolled steroid use
• Organisms — filamentous fungi predominate: aspergillus (the commonest in most Indian series) and fusarium, together with Curvularia and Alternaria; and yeasts — candida — which affect an already diseased or compromised cornea rather than a healthy one
• Risk factors — trauma with vegetable matter, which is the classical history: a paddy stalk, sugarcane leaf, straw, twig or mud in the eye of an agricultural worker at harvest; topical steroid use, which permits proliferation; contact lens wear; chronic surface disease; and immunosuppression

Acanthamoeba Keratitis

• acanthamoeba is a free-living protozoan present in tap water, swimming pools, hot tubs, soil and dust, existing as a motile trophozoite and a highly resistant double-walled cyst which survives chlorination and most disinfectants
• Risk factors — contact lens wear with rinsing in tap water, swimming or showering in lenses, and poor hygiene, which together account for most cases in developed countries; and, in India, trauma with soil or contaminated water in non-lens wearers
• The hallmark IS pain out OF all proportion TO the signs, which is severe and unremitting and is the feature that should prompt the diagnosis

Distinguishing the Infective Keratitides

Applied Aspects

• Suspect fungus after vegetative trauma, and do a KOH mount before starting anything; the history alone should raise it in every Indian agricultural worker
• Suspect Acanthamoeba when the pain exceeds the signs, and especially in a contact lens wearer who has been swimming or using tap water
• Reconsider the diagnosis of herpes in any "dendritic" ulcer that fails to respond; pseudodendrites of Acanthamoeba are the classic mimic and weeks are lost
• Never use a steroid in fungal or amoebic keratitis; it is the single commonest reason these eyes are lost
• Continue antifungals for weeks after apparent improvement; relapse on premature withdrawal is the rule

Herpes Simplex Keratitis — Pathogenesis

• Primary infection occurs in childhood, subclinically; where it is apparent it presents as a blepharoconjunctivitis with vesicles on the lid margin, a follicular conjunctivitis and a tender preauricular node, and it is self-limiting
• The virus then becomes latent in the trigeminal ganglion, and recurrent disease follows reactivation and axonal transport back to the cornea. Recurrence, not primary infection, is what damages the eye
• Triggers for reactivation — fever (hence "fever blister"), ultraviolet light, trauma and ocular surgery, menstruation, stress, and immunosuppression including topical steroids

Herpes Zoster Ophthalmicus

• Reactivation of varicella-zoster virus latent in the trigeminal ganglion, affecting the ophthalmic division. It occurs in the elderly and in the immunosuppressed, and zoster in a young adult is an indication to test for HIV
• Features — a prodrome of pain, fever and malaise; then a vesicular rash in the dermatome which respects the midline and progresses through papules, vesicles, pustules and crusts
• Hutchinson’S sign — vesicles on the side or tip of the nose, indicating involvement of the nasociliary nerve, which also supplies the globe. It predicts ocular involvement and should be looked for in every case

The Place of Steroids in Herpetic Disease

Applied Aspects

• Test corneal sensation before instilling anaesthetic; it is the sign that points to herpetic disease and it is destroyed by the first drop
• Never give a steroid to a dendritic ulcer; the geographical ulcer that follows is iatrogenic and entirely avoidable
• Give steroid for disciform keratitis, but under antiviral cover; the stromal disease is immune and will scar if left untreated

Types and Indications

• Keratoplasty (corneal grafting) is the replacement of diseased corneal tissue by healthy donor cornea.
• It is the oldest and the most successful of all solid tissue transplants, because the cornea is avascular and has NO lymphatics, giving it a degree of immune privilege.

The Donor Cornea and Eye Banking

• Corneas are retrieved within 6 hours of death (longer if the body is refrigerated), by whole-globe enucleation or by in-situ corneoscleral excision, which is preferred as it leaves the globe intact for the family
• Storage — McCAREY-kaufman medium at 4°C for up to 4 days; optisol-GS for up to 14 days, which is the standard; and organ culture at 34°C for up to 4 weeks
• Assessment of the donor tissue — slit-lamp examination and specular microscopy for the endothelial cell count, which should exceed 2,000 to 2,400 cells per square millimetre; the endothelium is what determines graft survival

Graft Rejection and Complications

• Graft rejection is an immune response to donor antigen, and is the commonest cause of late graft failure. It is reversible if treated promptly, which is why the patient must know the warning symptoms
• Warning symptoms — "RSVP": redness, sensitivity to light, vision reduced, pain. Every graft recipient should be told to attend immediately if any of these occur
• Signs by layer — epithelial rejection line, an elevated irregular line advancing across the graft; subepithelial infiltrates (krachmer spots), resembling those of adenoviral keratitis; stromal haze and oedema; and endothelial rejection — the khodadoust line, a line of keratic precipitates advancing across the endothelium, which is the most serious form since endothelial loss is irreversible

Factors Determining Graft Success

Applied Aspects

• Teach every graft patient "RSVP"; rejection is reversible if it is treated within days and irreversible if it is not, and the patient is the one who detects it
• Choose a lamellar procedure where the disease is lamellar; DALK for keratoconus and DMEK for Fuchs give better vision and far less rejection than penetrating keratoplasty
• Reduce vascularisation and inflammation before grafting; operating on a vascularised, inflamed cornea is the commonest reason a graft fails

Degenerations Compared with Dystrophies

Corneal Degenerations

Corneal Dystrophies

• The TGFBI gene on chromosome 5q31 is mutated in granular, lattice, Reis-Bucklers and Thiel-Behnke dystrophies — a useful unifying fact; macular dystrophy is caused by mutations in CHST6
• The staining triad is examinable — granular = hyaline = MASSON trichrome (red); lattice = amyloid = CONGO red; macular = mucopolysaccharide = ALCIAN blue. Remembered as "Marilyn Monroe Always Gets Her Man in LA County" — Macular-Mucopolysaccharide-Alcian, Granular-Hyaline-Masson, Lattice-Amyloid-Congo

The Stromal Dystrophies Compared

Applied Aspects

• Request a lipid profile for arcus under 40; it is one of the few incidental ocular signs with genuine systemic consequence
• Look for band keratopathy in a child with uveitis; it is the classical sign of juvenile idiopathic arthritis-associated disease and may be the presenting feature
• Measure serum calcium in band keratopathy without obvious ocular cause; hyperparathyroidism and sarcoidosis present this way

Anatomy of the Uveal Tract

• The uvea (uveal tract) is the middle, vascular, pigmented coat of the eye, comprising the iris, the ciliary body and the choroid.
• The name is from the Latin uva, a grape, which the dissected coat resembles.

• Blood supply — and the division explains the clinical pattern of disease: the anterior uvea (iris and ciliary body) is supplied by the major arterial circle OF the iris, formed by the two long posterior ciliary arteries and the seven anterior ciliary arteries; while the posterior uvea (choroid) is supplied by the short posterior ciliary arteries. The two territories are largely separate.

Classification of Uveitis

• The sun (Standardization of Uveitis Nomenclature) classification is anatomical, based on the primary site of inflammation:
• Anterior uveitis — the primary site is the anterior chamber: iritis, iridocyclitis and anterior cyclitis. The commonest form by far
• Intermediate uveitis — the primary site is the vitreous: pars planitis, posterior cyclitis, hyalitis

Anterior Uveitis

• Symptoms — pain, dull and aching, worse at night; photophobia, which is marked and is caused by spasm of the inflamed iris on constriction; lacrimation; redness; and blurred vision. Note what is absent: there is NO discharge and NO itching, which distinguishes it from conjunctivitis at once
• Signs — ciliary (circumcorneal) congestion, a violaceous ring that does not blanch with phenylephrine; keratic precipitates on the endothelium; cells and flare in the anterior chamber; miosis and a sluggish pupil, from irritation of the sphincter; iris nodules; posterior synechiae; hypopyon in severe cases; and an intraocular pressure that may be low (from ciliary shutdown) or high (from trabeculitis or clogging)
• Posterior synechiae are adhesions between the iris and the anterior lens capsule, and are the reason cycloplegia is urgent. If they become complete through 360 degrees the result is seclusio pupillae, which prevents aqueous passing from the posterior to the anterior chamber; pressure then builds behind the iris, bowing it forward as iris bombe, which closes the angle and causes secondary angle-closure glaucoma. If a fibrous membrane covers the pupil the result is occlusio pupillae

Treatment

• Topical steroid — prednisolone acetate 1%, given intensively at first (hourly, or even every 15 minutes for the first few hours in severe cases) and then tapered slowly over weeks. Abrupt withdrawal causes rebound, and inadequate initial treatment causes synechiae — so the rule is hit it hard and withdraw slowly
• Cycloplegic and mydriatic — homatropine 2% or atropine 1%, for the reasons above. Where synechiae are already forming, a subconjunctival "mydricaine" injection of atropine, adrenaline and procaine may break them
• Treat the cause — and above all, exclude and treat infection before immunosuppressing: tuberculosis, syphilis, herpes and toxoplasmosis

Intermediate Uveitis

• Intermediate uveitis is inflammation in which the primary site is the vitreous, with the peripheral retina and pars plana secondarily involved.
• pars planitis is the idiopathic subset showing snowbanking, and accounts for most cases.
• The characteristic clinical situation is a quiet eye with floaters — the patient complains of floaters and blurred vision, and the eye is white, painless and without photophobia. It therefore presents late and is easily dismissed, particularly in a child

Posterior Uveitis

• Symptoms — floaters and blurred vision, with NO pain, NO redness and NO photophobia unless the anterior segment is involved — because the choroid has almost no sensory innervation. A central lesion causes profound visual loss with a white quiet eye, and metamorphopsia where the macula is involved
• Signs — vitritis overlying the lesion, which gives the classical "headlight IN A fog" appearance in ocular toxoplasmosis; an active lesion, which is yellowish-white, fluffy and ill-defined with indistinct margins; and a healed lesion, which is a sharply demarcated, atrophic, punched-out white scar with pigmented margins — the distinction between the two is the whole of the assessment of activity
• Retinal vasculitis — perivascular sheathing and cuffing, characteristically of the veins (periphlebitis); with haemorrhage, occlusion, capillary non-perfusion and neovascularisation

Investigation

• Investigation is directed by the clinical pattern, not by a routine panel. A first episode of mild, unilateral, non-granulomatous acute anterior uveitis in an otherwise well young adult needs NO investigation at all — a point worth emphasising.
• Investigate when the uveitis is bilateral, granulomatous, recurrent, chronic, posterior, or associated with systemic symptoms
• Baseline tests — full blood count, ESR and CRP; syphilis serology (treponemal), which should be done in almost every case; mantoux or interferon-gamma release assay and a chest radiograph, which in India are near-routine; serum ACE and calcium for sarcoidosis; HLA-B27 in recurrent acute anterior uveitis; ANA in a child with arthritis; and HIV serology where the pattern suggests it

Localising the Uveitis BY its Symptoms

Applied Aspects

• Indent and examine the far periphery in any young patient with floaters and a quiet eye; snowbanking is invisible without indentation and pars planitis is otherwise missed
• Ask about neurological symptoms in intermediate uveitis; it may precede multiple sclerosis by years, and the finding changes the whole management
• Dilate and examine the fundus in every unexplained visual loss; posterior uveitis is painless and the eye looks normal from the front

Complications of Uveitis

• The complications, rather than the inflammation itself, are what blind the patient — and most are preventable by adequate early treatment

Cystoid Macular Oedema — the Principal Cause of Visual Loss

• Cystoid macular oedema (CMO) is the accumulation of fluid in cystic spaces of the outer plexiform (HENLE) layer of the macula, following breakdown of the blood-retinal barrier. It is the commonest cause of visual loss in uveitis of every anatomical class

The Ladder of Treatment

• Step 0 — exclude and treat infection first.
• Immunosuppressing a patient with tuberculous, syphilitic, herpetic or toxoplasmic uveitis is catastrophic, and this step comes before every other → step 1 — topical: prednisolone acetate 1% intensively with a cycloplegic.
• Sufficient for most anterior uveitis; ineffective for posterior disease, because drops do not reach the back of the eye → step 2 — periocular: posterior sub-Tenon or orbital floor triamcinolone, for unilateral posterior or intermediate disease and for macular oedema → step 3 — intravitreal: triamcinolone, or a sustained-release dexamethasone or fluocinolone implant.

Monitoring and Applied Aspects

• Monitor at every visit — visual acuity; anterior chamber cells (not flare) to titrate treatment; intraocular pressure, which both the disease and the treatment raise; the lens; and the macula by OCT
• Systemic monitoring on immunosuppression — full blood count, liver and renal function; blood pressure and glucose on steroid; bone protection with calcium, vitamin D and a bisphosphonate where appropriate; and screening for tuberculosis and hepatitis before biological agents
• Exclude infection before immunosuppressing; this is the single most important rule in uveitis management, and in India it means thinking about tuberculosis in every case

Choroidal Melanoma

• Uveal melanoma is the commonest primary intraocular malignancy of adults (as retinoblastoma is of children); 90% arise in the choroid, and the remainder in the ciliary body and iris
• Risk factors — fair skin and light iris; oculodermal melanocytosis (naevus of Ota); the BAP1 tumour predisposition syndrome; dysplastic naevus syndrome; and increasing age. It is rare in dark-skinned populations, and therefore much less common in India than in Europe
• Presentation — frequently asymptomatic and found incidentally; otherwise blurred vision, field loss, floaters or photopsia; and rarely pain from secondary glaucoma or necrosis

Other Uveal Tumours

Choroidal Naevus Compared with Melanoma

Applied Aspects

• Apply the TFSOM criteria to every pigmented choroidal lesion; thickness, fluid, symptoms, orange pigment and proximity to the disc distinguish a naevus to watch from a melanoma to treat
• Look for drusen over the lesion; they indicate a long-standing benign naevus and are reassuring
• Get an A-scan; low internal reflectivity supports melanoma and high reflectivity supports haemangioma or metastasis, and it costs nothing

Definitions and Classification

• Endophthalmitis is inflammation of the intraocular cavities and their contents — the aqueous and vitreous — infective.
• panophthalmitis is inflammation of all the coats of the eye together with the orbital tissues, and is the more advanced and destructive condition.

Clinical Features and Investigation

• Symptoms — rapidly worsening pain and profound loss OF vision in an eye that was improving after surgery. Any patient whose vision or comfort deteriorates after cataract surgery has endophthalmitis until proved otherwise.
• Signs — lid oedema and chemosis; marked ciliary and conjunctival congestion; corneal oedema; hypopyon; fibrin in the anterior chamber; vitritis with loss of the red reflex, which is the cardinal sign; a relative afferent pupillary defect; and, where the retina can be seen, retinitis and periphlebitis
• Panophthalmitis adds — proptosis, restricted or absent ocular movements with pain, marked lid swelling, fever and systemic upset, and eventually orbital cellulitis. The involvement of the orbit and the systemic illness are what distinguish it, and it carries a risk of cavernous sinus thrombosis and meningitis

Treatment

• The endophthalmitis vitrectomy study (EVS) governs practice, and its conclusion is decided by the presenting vision:
• If vision is perception OF light only — immediate pars plana vitrectomy with intravitreal antibiotics.
• If vision is hand movements or better — vitreous tap and intravitreal antibiotics, with vitrectomy reserved for those who fail to improve

Prophylaxis and Applied Aspects

• Preoperative povidone-iodine 5% into the conjunctival sac is the single best-evidenced prophylactic measure in all of ophthalmic surgery, and takes three minutes
• Intracameral cefuroxime at the end of cataract surgery was shown by the ESCRS study to reduce endophthalmitis several-fold, and is now widely standard
• Treat blepharitis, dacryocystitis and any lid or lacrimal infection before operating; the organisms come from the patient’s own flora, and an unrecognised dacryocystitis is an absolute contraindication
• Assess and act within hours, not days; the outcome depends on the interval from onset to intravitreal antibiotic more than on anything else

Anatomy of the Lens

• The crystalline lens is a transparent, biconvex, avascular body suspended behind the iris by the zonules OF ZINN.
• It has NO blood supply and NO nerve supply, and is nourished entirely by the aqueous humour.

Physiology and Metabolism

• Energy metabolism — the lens is almost entirely dependent on anaerobic glycolysis, which supplies about 80% of its ATP despite the low yield, because the tissue is avascular and hypoxic. The hexose monophosphate (pentose phosphate) shunt supplies 15% and generates NADPH for glutathione reduction; the Krebs cycle only 3%, being confined to the epithelium; and the sorbitol (polyol) pathway 5% — which is the pathway that matters in diabetes
• The pump-leak mechanism — a Na-K ATPase in the anterior epithelium actively transports sodium out and potassium in, while ions leak passively in the opposite direction posteriorly. The lens therefore has a high potassium and low sodium content, and maintains its state of relative dehydration. Failure of the pump allows the lens to hydrate, and hydration is the first step of cataract formation
• Antioxidant defence — the lens contains the highest concentration of glutathione of any tissue, together with ascorbate, superoxide dismutase and catalase. oxidative damage is the central mechanism of age-related cataract, and the fall in glutathione with age is what permits it

Classification of Cataract

The Lens Compared with the Cornea as a Refracting Medium

Epidemiology and Applied Aspects

• Cataract IS the leading cause OF blindness IN the world, and remains so in India, where it accounts for the majority of avoidable blindness. It is entirely curable by a single operation, which is why cataract surgery is among the most cost-effective interventions in all of medicine and why the Indian national programme is built around it
• Risk factors for age-related cataract — age above all; ultraviolet-B exposure, which is high in India; diabetes; smoking; corticosteroid use; malnutrition and low antioxidant intake; severe dehydrating illness such as cholera and dysentery.
• The lens has no nerve supply, which is why cataract is entirely painless — and a painful eye with a cataract has something else wrong with it

Types of Age-related Cataract

The Three Types of Age-related Cataract Compared

Stages of Cortical Cataract

• 1.
• Incipient (early) — lamellar separation, water clefts and vacuoles; wedge-shaped cuneiform opacities at the periphery, or a saucer-shaped cupuliform opacity posteriorly.
• Vision is minimally affected unless the opacity is posterior and axial → 2.

Symptoms, Signs and Assessment

• Symptoms — painless, progressive, painless loss of vision, which is the cardinal feature; glare and difficulty with oncoming headlights; coloured haloes, which have to be told apart from those of glaucoma; monocular polyopia or diplopia, which persists on covering the other eye and is diagnostic of a lenticular cause; frequent changes of spectacles; "second sight" in nuclear cataract; loss of contrast and colour desaturation; and black spots that do not move with eye movement, unlike vitreous floaters
• Signs — reduced acuity not improving with a pinhole beyond a point; a dull or absent red reflex on distant direct ophthalmoscopy, with the opacity seen as a black shadow against the red glow; the opacity itself on slit-lamp examination after dilation; and leucocoria in a mature cataract
• Preoperative assessment of the posterior segment is essential, because the outcome depends on the retina and optic nerve rather than on the lens: perception of light with accurate projection of rays in all four quadrants; pupillary reactions with a careful search for a relative afferent pupillary defect, which if present indicates optic nerve or extensive retinal disease and a poor prognosis; and, where the fundus cannot be seen, a B-scan ultrasound to exclude retinal detachment, vitreous haemorrhage and a mass

Complications of Untreated Cataract

• These complications are the reason not to let a cataract go to hypermaturity — the older teaching of "waiting until it is ripe" belongs to the era of intracapsular surgery, and is now actively harmful.
• Iris shadow present means immature and absent means mature, and the test takes seconds with an oblique slit beam
• Second sight signals a developing nuclear cataract, and an elderly patient reading without glasses again should be examined

Indications and Preparation

• Indications for cataract surgery — visual, which is the usual one: the vision no longer meets the patient’s needs, and there is no fixed acuity threshold, since a driver or a surgeon needs more than a bedbound patient; medical, where the cataract itself threatens the eye — phacomorphic or phacolytic glaucoma, phacoantigenic uveitis, lens subluxation; diagnostic and therapeutic, where the lens prevents examination or treatment of the retina, as in diabetic retinopathy needing laser; and cosmetic in a blind eye with a white pupil
• Anaesthesia — topical (proparacaine or lidocaine gel, with intracameral lidocaine), now used for most phacoemulsification; sub-TENON, safe and effective; peribulbar; retrobulbar, now largely abandoned because of its complications — globe perforation, retrobulbar haemorrhage, optic nerve injury and brainstem anaesthesia from inadvertent injection into the optic nerve sheath, which causes apnoea and requires resuscitation; and general anaesthesia for children, the uncooperative and the deaf

Surgical Techniques

• The steps of modern cataract surgery → incision — a self-sealing clear corneal or sclerocorneal tunnel, with a side-port for the second instrument → viscoelastic (ophthalmic viscosurgical device) to maintain the chamber and protect the endothelium → capsulorhexis — a continuous curvilinear capsulorhexis (CCC) of 5 to 5.5 mm, whose smooth continuous edge resists tearing and is what allows the whole of modern surgery.
• It should be slightly smaller than the optic, so the capsule overlaps the lens edge through 360 degrees and suppresses after-cataract → hydrodissection — fluid injected beneath the capsule to separate the cortex from it; and hydrodelineation to separate nucleus from epinucleus → nucleus removal — by phacoemulsification (divide-and-conquer, stop-and-chop, or phaco-chop) or by prolapse and delivery in SICS → cortex aspiration by irrigation-aspiration, with careful capsular polishing to remove residual epithelial cells → intraocular lens implantation in the capsular bag → Removal of viscoelastic — which must be complete.

Biometry and Intraocular Lenses

• Biometry determines the power of the lens to be implanted, and errors in it are the commonest cause of an unhappy patient after technically perfect surgery. It requires two measurements:
• Axial length — by ultrasound, either applanation (which indents the cornea and therefore under-estimates the length, leading to a hypermetropic surprise) or immersion (more accurate); or by optical biometry (IOLMaster, Lenstar) using partial coherence interferometry, which is non-contact, more accurate and now the standard, though it fails in a dense cataract where ultrasound still succeeds. An error of 1 mm in axial length produces 2.5 to 3 dioptres of refractive error
• Keratometry — the corneal curvature; an error of 1 dioptre in K gives 1 dioptre of error

Choosing the Technique

Applied Aspects

• Get the biometry right; a perfect operation with a wrong lens power is a failure from the patient’s point of view, and refractive surprise is the commonest source of dissatisfaction
• Use immersion or optical biometry rather than applanation; applanation indents the cornea, shortens the measured axial length and produces a hypermetropic result
• Do not wait for a cataract to "ripen"; a hypermature lens is harder to remove, has weaker zonules and may present with phacolytic or phacomorphic glaucoma

Intraoperative Complications

Early Postoperative Complications

Late Postoperative Complications

• Posterior capsular opacification — the commonest late complication, occurring in 20 to 40% within five years and in nearly all children. Treated with Nd:YAG laser capsulotomy
• Cystoid macular oedema (IRVINE-GASS syndrome) — peaking at 6 to 10 weeks; commoner after complicated surgery, vitreous loss, in diabetics and in uveitic eyes. Presents as vision that improves and then deteriorates again a few weeks later. Treated with topical non-steroidal drops and steroid, acetazolamide, and periocular or intravitreal steroid
• Pseudophakic bullous keratopathy — from endothelial cell loss, presenting months to years later with vision worse on waking; treated by DMEK or DSAEK

Tass Compared with Acute Endophthalmitis

Applied Aspects

• Distinguish TASS from endophthalmitis; TASS is earlier, painless, limbus-to-limbus and without vitritis, and responds to steroid.
• Warn every patient of the symptoms of endophthalmitis before discharge — increasing pain and falling vision — and give them a route back; the outcome depends on how quickly they return
• Do not chase a dropped nucleus into the vitreous; it causes retinal damage. Finish safely and refer for vitrectomy

Aetiology and Types

• Congenital and developmental cataract is a leading cause of treatable childhood blindness, and the outcome depends almost entirely on how early it is detected and operated
• Aetiology, in thirds — about a third are hereditary, autosomal dominant, and bilateral; about a third are associated with a systemic or metabolic disorder; and about a third are idiopathic — a proportion which is higher among unilateral cases, most of which are sporadic and isolated

Assessment

• Screening — the red reflex test should be performed on every newborn and at every routine child check. The bruckner test (comparing the two red reflexes simultaneously with a direct ophthalmoscope from a metre away) detects asymmetry from a cataract, a refractive error, a squint or a tumour, and takes seconds
• Assessment of visual significance — a cataract is significant if it is central and more than 3 mm, or dense enough to obscure the fundus view; and the surest sign in an infant is a poor fixation response, nystagmus or a squint
• Examination — both eyes dilated; a search for microphthalmos, glaucoma, PFV, coloboma and retinal disease; measurement of axial length and keratometry; and B-scan where the fundus cannot be seen

Management and the Critical Period

• Timing IS everything, and it is determined by deprivation amblyopia. The critical period for visual development begins at 6 weeks of age, when fixation matures, and the outcome falls steeply with delay beyond it
• Unilateral dense cataract — operate by about 4 to 6 weeks. The prognosis is worse than for bilateral disease, because the good eye competes and suppression is profound; and aggressive occlusion therapy afterwards is essential and is what actually determines the result
• Bilateral dense cataract — operate by about 8 to 10 weeks, the second eye within a week or two of the first, or under the same anaesthetic in some protocols

Timing and the Critical Period

Applied Aspects

• Check the red reflex in every newborn; it is the only screening test for congenital cataract and for retinoblastoma, it takes seconds, and it is frequently omitted
• Treat leucocoria as retinoblastoma until proved otherwise, and arrange urgent dilated examination and B-scan
• Operate a dense unilateral cataract by 6 weeks; every week of delay costs vision, and referral must be immediate rather than routine

Definition

• Note what the modern definition does not contain: a pressure figure. Glaucoma is defined by the optic neuropathy, not by the intraocular pressure. This is not a semantic point — it is the reason that normal-tension glaucoma exists (a typical glaucomatous neuropathy with a pressure that never exceeds 21 mmHg) and that ocular hypertension exists (a pressure above 21 with a healthy nerve and normal fields.
• Pressure remains central to management despite this.

Intraocular Pressure

• The normal intraocular pressure is 10 to 21 mmHg, with a mean of 16 and a standard deviation of 2.5. The upper limit of 21 is therefore simply two standard deviations above the mean — a statistical convention rather than a physiological threshold.
• Diurnal variation — the pressure fluctuates by 3 to 5 mmHg through the day in a normal eye, and is characteristically highest in the early morning. A variation exceeding 8 mmHg is abnormal and is itself a risk factor. The practical consequence is that a single normal afternoon reading does not exclude glaucoma, and where suspicion persists a diurnal variation test (phasing) is performed with readings every two to three hours
• Goldmann’S equation summarises the determinants: IOP = (F ÷ C) + Pv, where F is the rate of aqueous formation, C the facility of outflow and Pv the episcleral venous pressure. Every antiglaucoma treatment alters one of these three terms: drugs either reduce F or increase C, and the rare glaucomas of raised episcleral venous pressure — carotid-cavernous fistula, Sturge-Weber, thyroid eye disease — act through the third

Tonometry

• Goldmann applanation tonometry is the gold standard. It rests on the IMBERT-FICK principle, which states that for an ideal thin sphere the pressure inside equals the force applied divided by the area flattened. The cornea is not an ideal thin sphere, and the tonometer resolves this by flattening a specific area — a circle of 3.06 mm diameter — at which the surface tension of the tear film (which pulls the prism on) exactly cancels the resistance of corneal rigidity (which pushes it away). At that diameter the force in grams multiplied by ten gives the pressure in millimetres of mercury
• Technique — fluorescein and anaesthetic are instilled, the prism is advanced under cobalt blue light, and the dial is turned until the inner margins of the two semicircles just touch, pulsating gently with the ocular pulse
• Sources of error in applanation tonometry — and each is commonplace: excess fluorescein gives thick semicircles and an over-reading, while too little gives an under-reading; astigmatism over 3 dioptres requires the prism to be rotated, since the flattened area becomes elliptical; corneal oedema gives a false low reading and corneal scarring a false high one; and squeezing the lids, holding the breath or a tight collar all raise the reading substantially

Classification of Glaucoma

• Congenital and developmental — primary congenital glaucoma (buphthalmos), from isolated maldevelopment of the angle; and glaucoma associated with other ocular or systemic anomalies — Axenfeld-Rieger anomaly, Peters anomaly, aniridia, Sturge-Weber syndrome and neurofibromatosis
• Primary adult — open angle: primary open-angle glaucoma (POAG), in which the pressure exceeds 21; and normal-tension glaucoma, in which it does not
• Primary adult — angle closure: a spectrum from primary angle-closure suspect, through primary angle closure, to primary angle-closure glaucoma; and by presentation into acute, subacute (intermittent), chronic and absolute
• Secondary open-angle — pseudoexfoliative; pigmentary; steroid-induced; lens-induced (phacolytic, lens particle); uveitic (trabeculitis); traumatic (hyphaema, angle recession, ghost cell); and glaucoma of raised episcleral venous pressure

Definition and Epidemiology

• It is the commonest form of glaucoma in most populations, and it is characteristically asymptomatic until it is advanced — which has earned it the name "the silent thief of sight". Central acuity is preserved until very late, the peripheral loss is gradual and binocular overlap conceals it, so patients present having already lost a great deal of field
• The consequence for practice is that POAG is a disease that must be sought rather than waited for — by examining the disc of every adult whose fundus is being viewed for any reason, and by screening those at risk

Pathogenesis

• The site of obstruction is the juxtacanalicular (cribriform) meshwork, immediately adjacent to Schlemm canal, where the great majority of the outflow resistance normally resides. The changes include thickening of the trabecular beams, accumulation of extracellular material, loss of trabecular endothelial cells and fusion of the beams — so the resistance rises while the angle remains anatomically open and normal-looking
• Damage at the optic nerve head is explained by two theories.
• The mechanical theory — raised pressure deforms and bows back the lamina cribrosa, the sieve-like plate through which the axons leave the eye; the pores become misaligned and directly compress the axon bundles, blocking axoplasmic flow and depriving the ganglion cell body of neurotrophic factors transported from the brain

Clinical Features and Diagnosis

• Symptoms — none, in the great majority, until the disease is advanced. Late symptoms are of missing objects to one side, difficulty in dim light, bumping into things and difficulty driving; and central acuity is normal until the very end, which is why acuity is a useless screening test for glaucoma
• The diagnosis rests on five examinations, and all five are required:
• 1. Tonometry — the pressure, with pachymetry to interpret it and diurnal phasing where there is doubt
• 2. Gonioscopy — to confirm that the angle is open, which is what makes it open-angle glaucoma, and to exclude secondary causes such as recession, neovascularisation or pigment
• 3. Optic disc assessment — ideally stereoscopically with a slit lamp and a 78 or 90 dioptre lens, looking for cupping, rim notching, violation of the ISNT rule, asymmetry between the eyes, disc haemorrhage and retinal nerve fibre layer defects
• 4. Perimetry — standard automated perimetry.
• 5. Imaging — optical coherence tomography of the retinal nerve fibre layer and ganglion cell complex, which detects structural loss before the field defect appears and provides objective, quantitative monitoring
• Normal-tension glaucoma deserves separate mention: typical glaucomatous damage with a pressure never above 21. It is associated with vasospasm, migraine, Raynaud phenomenon, nocturnal hypotension and sleep apnoea; disc haemorrhages are commoner; and the field defects tend to be deeper, steeper and closer TO fixation. Before accepting the diagnosis, exclude a missed pressure peak on diurnal phasing, previous steroid use, a past episode of raised pressure such as uveitis or trauma, and — critically — a compressive lesion of the optic nerve or chiasm. Neuroimaging is indicated where the patient is young, the acuity or colour vision is disproportionately reduced, the field defect respects the vertical midline, or there is pallor of the rim exceeding the cupping
• Central acuity is normal until the end, so visual acuity is useless as a screening test for glaucoma
• Binocular overlap conceals the field loss, so patients do not notice a defect until it crosses fixation or both eyes are affected
• Ask about a first-degree relative; it is the single most useful question in the history and multiplies the risk several-fold
• Tell the patient to bring their siblings; case-finding through families is the most efficient screening there is

Anatomical Predisposition and Mechanism

• PACG is proportionately more important in ASIA than in the West, and accounts for a large share of glaucoma blindness in India, China and South-East Asia — because it blinds faster and more often than open-angle disease
• The predisposed eye is a small eye, and every anatomical feature follows from that: a short axial length; hypermetropia; a shallow anterior chamber; a small corneal diameter; a thick and anteriorly positioned lens; and a steep, narrow angle recess

Acute Angle-closure Attack

• Symptoms — severe ocular and periorbital pain; headache; nausea and vomiting, which may be so prominent that the patient is admitted to a medical or surgical ward with a suspected abdominal emergency, migraine or intracranial event — a genuinely common and serious misdiagnosis; coloured haloes around lights from corneal oedema; and rapid, profound loss of vision
• Signs — the eye is dramatically abnormal and the diagnosis is made across the room:
• Lid oedema and marked ciliary (circumcorneal) congestion

Management of the Acute Attack

• 1.
• Systemic acetazolamide — 500 mg intravenously, followed by 250 mg orally six-hourly.
• It reduces aqueous production immediately and is the first drug given → 2.
• Intravenous mannitol 20% (1 to 2 g/kg over 30 to 45 minutes) or oral glycerol, to draw fluid osmotically from the vitreous and deepen the chamber.
• Avoid glycerol in diabetics (it is metabolised to glucose) and mannitol in cardiac or renal failure → 3.
• Topical agents — a beta-blocker, an alpha-2 agonist and a topical carbonic anhydrase inhibitor, given together → 4.
• Topical steroid — to reduce the intense inflammation, which itself contributes to synechiae and to a persistently raised pressure → 5.
• Pilocarpine 2% — but only once the pressure has fallen below 40 to 50 mmHg.
• Above that the iris sphincter is ischaemic and will not respond, so pilocarpine is useless; and by shifting the lens forward it may make matters worse.
• Giving pilocarpine first is the classic error → 6.
• Corneal indentation with a gonioprism or a squint hook, which mechanically forces aqueous into the angle and may break the attack → 7.
• Laser peripheral iridotomy as soon as the cornea is clear enough — this is the definitive treatment, creating an alternative route for aqueous and abolishing the pupil block → 8.
• Prophylactic iridotomy to the fellow eye, which is mandatory: the risk of an attack in the untreated fellow eye is of the order of 50% within 5 years

Secondary Open-angle Glaucomas

• Pseudoexfoliation syndrome — a systemic fibrillopathy in which a greyish-white fibrillar material is deposited throughout the anterior segment and in systemic tissues including the heart, lung, liver and skin. It is the commonest identifiable cause of secondary open-angle glaucoma worldwide
• Its ocular signs — a "target" pattern on the anterior lens capsule: a central disc, a clear intermediate zone (swept clear by the moving iris) and a peripheral granular band, seen best after dilation; deposits on the pupillary margin; peripupillary iris transillumination defects; heavy, irregular trabecular pigmentation with a sampaolesi line (pigment anterior to Schwalbe line); and, critically, poor dilation and zonular weakness
• Its surgical importance — pseudoexfoliation predicts a difficult cataract operation: a small pupil, weak zonules with a risk of capsule rupture and dropped nucleus, and late in-the-bag lens dislocation years afterwards. Recognising it before surgery changes the plan

Secondary Angle-closure Glaucomas

• Neovascular glaucoma — considered separately, and much the most important
• Phacomorphic glaucoma — an intumescent cataract producing pupil block and acute closure. It presents exactly like a primary acute attack, and is distinguished by the obvious cataract and, decisively, by the normal, open angle in the fellow eye. Treated by pressure control and then urgent cataract extraction, which is curative
• Uveitic angle closure — from seclusio pupillae with iris bombe, or from extensive peripheral anterior synechiae

Applied Aspects

• Dilate to look for pseudoexfoliation; the target pattern on the lens capsule is invisible through an undilated pupil and it changes the surgical plan
• Do gonioscopy in every glaucoma patient; angle recession, neovascularisation, synechiae, heavy pigment and blood in Schlemm canal are all invisible without it
• Ask about exercise-related blurring in a young myopic man; pigment-dispersion spikes are characteristic and the diagnosis is otherwise missed
• Ask about steroids by every route, including skin creams and inhalers; patients do not regard these as medicines and do not volunteer them
• Monitor the pressure lifelong after any blunt ocular injury; angle recession glaucoma appears decades later and is entirely silent
• Check the fellow eye angle in any acute attack; a normal open fellow angle means the cause is secondary, and phacomorphic
• Suspect ice syndrome in unilateral glaucoma with iris changes in a young woman; the corneal endothelial appearance confirms it
• Think of topiramate in a young patient with bilateral acute myopia and shallow chambers; stopping the drug reverses it and an iridotomy does not help
• Pseudoexfoliation is a systemic fibrillopathy, deposited in heart, lung and skin as well as the eye, and is associated with vascular disease
• The Sampaolesi line is pigment anterior to Schwalbe, and is a useful gonioscopic marker of pseudoexfoliation
• Pseudoexfoliative glaucoma progresses faster with higher pressures and wider diurnal swings than primary open-angle disease
• Laser trabeculoplasty works especially well in pseudoexfoliation, because of the heavy trabecular pigmentation
• The Krukenberg spindle is vertical, formed by aqueous convection carrying pigment down the endothelium
• Pigmentary glaucoma may burn out with age as the lens thickens and the iris flattens.
• A third of the population are steroid responders, and the rise appears after two to six weeks of treatment
• Children and myopes respond more strongly to steroid, and a family history of glaucoma raises the risk further
• Steroid glaucoma is reversible if caught early, which is why the pressure must be checked in anyone on prolonged treatment
• Distinguish the mechanism in uveitic glaucoma; trabeculitis, steroid response, synechial closure and pupil block need different treatments
• Angle recession is a wide ciliary body band on gonioscopy, and is compared with the fellow eye to be certain
• Ghost cells are khaki-coloured degenerate red cells from an old vitreous haemorrhage, obstructing the meshwork directly
• Blood in Schlemm canal means raised episcleral venous pressure, and should prompt a search for a fistula or an orbital cause

Principles and the Target Pressure

• The aim of treatment is to preserve visual function for the patient’s lifetime, at a cost in inconvenience and side effects that they can accept. It is not to normalise a number
• The target pressure is the level judged unlikely to permit further damage in that particular eye. It is individual, not universal, and is set from: the severity of existing damage; the pressure at which the damage occurred; the rate of progression; the patient’s age and life expectancy; and other risk factors
• A common approach is to aim for a 20 to 30% reduction from baseline in early disease, and 30 to 50% in advanced disease, revising it if progression continues

Laser Treatment

• Laser peripheral iridotomy (LPI) — the treatment of angle closure. A hole is made in the peripheral iris, superiorly under the lid to avoid dysphotopsia, creating an alternative route for aqueous and abolishing pupil block. Performed with Nd:YAG, sometimes preceded by argon in a thick brown iris. Indications: acute attack, and prophylaxis in the fellow eye and in the occludable angle. Complications: pressure spike, iritis, haemorrhage, and linear dysphotopsia where the iridotomy is not covered by the lid
• Laser iridoplasty — contraction burns to the peripheral iris to pull it away from the angle; used in plateau iris and where an attack will not break
• Laser trabeculoplasty — for open-angle disease. selective (SLT) uses a frequency-doubled Nd:YAG laser targeting pigmented trabecular cells without thermal damage, and is repeatable; argon (ALT) is the older thermal technique. Lowers the pressure by 20 to 30%, works particularly well in pseudoexfoliative and pigmentary glaucoma, and the effect wanes over some years. Its great advantage is that it removes the problem of compliance

Filtration Surgery

• Trabeculectomy is the standard glaucoma operation, and remains the reference against which all others are judged. It creates a guarded fistula: a channel from the anterior chamber to the subconjunctival space, partly covered by a scleral flap that limits the flow and prevents catastrophic hypotony
• Indications — progression despite maximal tolerated medical treatment and laser; a pressure far above target; poor compliance or intolerance of drugs; and advanced disease at presentation, where surgery may reasonably be offered first
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Applied Aspects

• Set a target pressure and write it in the notes; treatment without a stated target cannot be judged as succeeding or failing
• Revise the target if the disease progresses; the target is a hypothesis about that eye and is often wrong the first time
• Ask about compliance non-judgementally; most patients miss doses, few admit it unprompted, and the question is more useful than another drug
• Consider SLT early rather than late; it removes the compliance problem entirely and the LiGHT trial supports it as first-line
• Use an antimetabolite in trabeculectomy; without it the bleb scars and the operation fails, particularly in young, dark-skinned and previously operated eyes
• Review a trabeculectomy frequently in the first weeks; the bleb is won or lost in that period and suture release is time-critical
• Teach every trabeculectomy patient the symptoms of blebitis; bleb-related endophthalmitis occurs years later and is fulminant
• Consider lens extraction in angle closure; the EAGLE study showed it outperforms iridotomy where the pressure is high, and it treats the mechanism directly
• Do not offer MIGS for advanced glaucoma; it is safer but weaker, and an eye with a split fixation field needs a trabeculectomy
• Simplify the regimen wherever possible; fixed combinations, once-daily dosing and preservative-free preparations all improve both compliance and the ocular surface
• Check the drop technique; many patients instil nothing at all, and watching them once is more informative than any history
• Teach punctal occlusion after instilling drops; it reduces systemic absorption and improves ocular penetration at no cost