Q5 LONG ANSWER 10M ★ High-yield

Describe the optical correction of refractive errors by spectacles, contact lenses and refractive surgery.

Ophthalmology · 3rd Professional

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

Type	Features
Rigid gas-permeable (RGP)	The lens of choice for irregular astigmatism, keratoconus and corneal scarring. Excellent optics and good oxygen transmission; but uncomfortable initially and requiring adaptation
Soft (hydrogel and silicone hydrogel)	Much the commonest; comfortable at once; available as daily, fortnightly and monthly disposables. Toric for astigmatism, and multifocal for presbyopia. Silicone hydrogel has far higher oxygen transmissibility
Therapeutic (bandage) lenses	Used for pain relief in bullous keratopathy and recurrent erosion, to promote healing of persistent epithelial defects, and to seal small perforations
Cosmetic and prosthetic	To alter iris colour, and to mask a disfigured blind eye
Orthokeratology	Rigid lenses worn overnight to reshape the cornea; used for myopia control in children

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).

Procedure	Principle and place
PRK (photorefractive keratectomy)	The epithelium is removed and the excimer laser applied to the surface. No flap, so no flap complications and it suits thin corneas; but painful for some days and with a risk of haze
LASIK (laser in situ keratomileusis)	A hinged corneal flap is raised (by microkeratome or femtosecond laser), the stromal bed ablated, and the flap replaced. Rapid, painless recovery, which is why it became the standard; but flap complications, dry eye and the risk of ectasia
Smile (small incision lenticule extraction)	A lenticule is cut within the stroma by femtosecond laser and removed through a small incision, with NO flap; less dry eye and greater biomechanical stability
Phakic intraocular lens (ICL)	A lens implanted in front of, or behind, the iris with the natural lens left in place. For very high myopia beyond the range of laser ablation, and for thin corneas. Risks cataract and pupil block glaucoma
Refractive lens exchange	Clear lens extraction with an intraocular lens; used in older patients and in high hypermetropia
Incisional (radial keratotomy)	Historical; abandoned because of progressive hyperopic shift and diurnal fluctuation

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

Situation	Preferred correction and reason
Ordinary myopia or hypermetropia	Spectacles — safe, cheap and adequate
Significant anisometropia or unilateral aphakia	Contact lens — it sits at the corneal plane and produces almost no image size difference.
Irregular astigmatism and keratoconus	Rigid gas-permeable lens — the tear film behind it forms a new regular refracting surface
Child, sportsperson, or a patient with one seeing eye	Polycarbonate spectacle lenses — protection of the remaining eye outweighs every optical consideration
Adult with stable moderate refractive error wanting freedom from spectacles	LASIK, PRK or smile, after topography and pachymetry
Very high myopia or a thin cornea	Phakic intraocular lens, since ablation would leave too little stroma
Older patient with early lens changes	Refractive lens exchange or cataract surgery with an intraocular lens
Any suspicion of keratoconus on topography	NO laser refractive surgery at all — the absolute contraindication

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

All questions in Ocular Anatomy, Physiology & Refraction