Refractive Errors and Their Management
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Direct answer
A normal eye focuses parallel rays of distant light exactly on the retina (emmetropia); refractive error is the mismatch between refractive power and axial length. In myopia the image falls in front of the retina and a concave (minus) lens corrects it; in hypermetropia it falls behind and a convex (plus) lens is needed; astigmatism differs in different meridians and requires a cylindrical lens; presbyopia is the age-related failure of accommodation managed with a near addition. Beyond spectacles, rigid and soft contact lenses, corneal laser surgery (LASIK, PRK, SMILE) and, for extreme myopia, implantable collamer lenses reshape or supplement the optical system.
What you must remember
- Myopia: axial myopia is the commonest type; far point lies at a finite distance (1/dioptres in metres, so −2.5 D focuses at 40 cm); corrected by the weakest concave lens giving 6/6.
- Simple versus pathological myopia: simple school myopia stabilises in the twenties; pathological (degenerative) myopia shows progressive axial length with posterior staphyloma, lacquer cracks, Fuchs spot, lattice degeneration and a lifelong retinal detachment risk.
- Hypermetropia: total hypermetropia = latent (masked by accommodation) + manifest (facultative + absolute); young patients compensate, so always refract children under cycloplegia.
- Astigmatism: regular (with-the-rule, against-the-rule, oblique) corrected by cylinders; irregular astigmatism — classically keratoconus — is corrected by rigid gas-permeable contact lenses, never by spectacles.
- Presbyopia additions by age: about +1.0 D at 40 years, +1.5 D at 45, +2.0 D at 50, +2.5 D at 55, +3.0 D at 60 — the table examiners ask for directly.
- Surgical options: LASIK (flap plus excimer ablation) and PRK (surface ablation) for roughly −1 to −8 or −10 D with adequate corneal thickness and stable refraction; SMILE avoids a flap; phakic implantable collamer lens for myopia beyond laser range; clear lens exchange in presbyopic high myopes.
- Anisometropia above about 2.5–3 D makes spectacle correction intolerable through aniseikonia — contact lenses or the surgical route win.
- Every child with subnormal vision or squint needs cycloplegic refraction (atropine 1 per cent in young children, cyclopentolate otherwise) before any surgical talk.
Numbers worth prescribing by
Put a 45-year-old tailor in the chair. He has never worn glasses, reads at 40 cm but now pushes the newspaper to arm's length. Distance vision is 6/6 each eye, so his amplitude of accommodation has simply fallen — from about 14 D at age ten to a couple of dioptres now — and his near point has receded beyond a comfortable working distance. Prescribe the age-appropriate +1.5 D addition over his distance correction, check it at his actual working distance, and warn him the addition will rise about half a dioptre every five years. Contrast him with a 22-year-old student at −3.5 D each eye: her far point is roughly 28 cm, her near work unaided is fine but the blackboard is blurred; full correction in the weakest minus lenses, yearly review, and a dilated peripheral retina check because even moderate myopia thins the peripheral retina. If she asks about LASIK at −3.5 D with stable refraction for a year, normal topography and adequate pachymetry, she is an ideal candidate; her cousin at −14 D with a thin cornea is not — the implantable collamer lens is his answer.
How the exam frames it
Expect one-liners built on arithmetic and definitions: the far point of a −2.5 D myope (40 cm), the power of the correcting lens for a hypermetrope whose near point is 50 cm, or which addition suits a 55-year-old (answer +2.5 D). Stems describing "best vision with a pinhole in a young patient" point to refractive error, and "progressive myopia with night-driving halos and streaks" points to keratoconus. Viva examiners probe why contact lenses beat spectacles in anisometropia and in irregular astigmatism — retinal image size and surface tear-lens correction respectively.
Frequently asked questions
What is the far point of a −2.5 D myopic eye?
Forty centimetres; the far point in metres equals one divided by the myopia in dioptres, which is also how a myope sees clearly without glasses by bringing objects to that distance.
What lens corrects hypermetropia and why?
A convex (plus) lens, because it adds converging power so that parallel rays focus on the retina rather than behind it; part of the error may be hidden by accommodation, hence cycloplegic refraction.
What near addition is typical at 50 years of age?
About +2.0 D over the distance correction, rising to +2.5 D at 55 and +3.0 D at 60, reflecting the falling amplitude of accommodation.
How is irregular astigmatism corrected and why not with spectacles?
A rigid gas-permeable contact lens, because its tear lens replaces the irregular corneal surface with a regular optical front; spectacles cannot compensate meridian-to-meridian irregularity.
Which surgical option suits a −14 D myope with thin corneas?
An implantable collamer (phakic intraocular) lens, since excimer laser ablation of that magnitude would leave an unstable, ectasia-prone cornea.
What fundus changes mark pathological myopia?
Posterior staphyloma, lacquer cracks, Fuchs spot at the macula, peripheral lattice degeneration and areas of chorioretinal atrophy — with regular dilated examination for treatable detachment-risk lesions.