Q2 LONG ANSWER 10M ★ High-yield

Classify refractive errors. Describe myopia and hypermetropia with their complications and management.

Ophthalmology · 3rd Professional

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

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)

Clinical type	Features
Congenital	Present at birth, 8 to 10 D, often unilateral; associated with congenital cataract, microphthalmos and retinopathy of prematurity
Simple (developmental, physiological, school myopia)	Much the commonest. A physiological variation rather than a disease; appears between 5 and 10 years, progresses slowly, stabilises at 18 to 20 years, and is less than 6 D. Vision is correctable to normal
Pathological (degenerative, progressive, malignant)	A disease of the eye. Onset in early childhood, rapid progression, exceeds 6 D and may reach 25 D or more, and continues to progress into adult life. Strong hereditary basis, and associated with degenerative changes at the posterior pole. Vision is often not fully correctable
Acquired	Post-traumatic, post-keratitic, drug-induced (sulphonamides, topiramate) and that of diabetes with hyperglycaemia — the last being reversible and a recognised presenting feature

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

Feature	Myopia	Hypermetropia
Focus of parallel rays	IN front of the retina	Behind the retina
Far point	Finite, in front of the eye	Virtual, behind the eye
Correcting lens	Concave (minus)	Convex (plus)
Prescribing rule	Weakest lens giving best vision	Strongest lens accepted for best vision
Vision affected first	Distance; near vision good	Near; distance blurs later as accommodation fails
Size of eye and anterior chamber	Large eye, deep anterior chamber, large pupil	Small eye, shallow anterior chamber
Optic disc	Temporal crescent; tigroid fundus; staphyloma	Small and crowded — pseudopapilloedema
Squint	Divergent (exotropia), in high myopia	Convergent (accommodative esotropia)
Glaucoma risk	Open-angle	Angle-closure
Other complications	Retinal detachment, cataract, macular degeneration	Amblyopia, asthenopia, recurrent styes
Presbyopia	Late, or never troublesome	Early

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.

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