Examqo › Examqo library › Ophthalmology › Optics and pharmacology · IMLE

Refractive errors and refractive surgery

In a refractive error the eye does not focus distant light on the retina: in myopia the focus is in front of the retina and is corrected with a diverging (minus) lens, in hyperopia it is behind and corrected with a converging (plus) lens, in astigmatism the cornea refracts differently in each meridian, and in presbyopia accommodation is lost from about age 40.

לקריאה בעברית

Why it matters in the exam

Basics that are asked: a dioptre is the reciprocal of the focal length in metres — so in myopia of 2 dioptres the far point that is seen sharply lies at 50 cm. In accommodation the ciliary muscle contracts, the zonules relax and the lens becomes rounder. Improvement of acuity through a small hole (pinhole) shows that the problem is refractive and not disease. In children refraction is measured only after cycloplegia (cyclopentolate 1% or atropine), because strong accommodation hides hyperopia. A large difference between the eyes (anisometropia) causes amblyopia and unequal image sizes (aniseikonia).

Myopia in children is rising worldwide, and high myopia (6 dioptres or more) raises the risk of retinal detachment, glaucoma, early cataract and myopic maculopathy. Ways to slow progression: low-concentration atropine (LAMP study: 0.05% was the most effective of 0.01%, 0.025% and 0.05%), outdoor time — at least two hours a day, special defocus spectacles and contact lenses, and orthokeratology. Ordinary under-corrected glasses do not slow progression — they may speed it.

Refractive surgery: LASIK — a corneal flap with laser reshaping underneath, quick recovery; PRK — surface reshaping without a flap, suitable for a thin cornea and contact sports, with a more painful and slower recovery; SMILE — removal of a tissue lenticule through a small incision. Contraindications: keratoconus or a suspicious cornea on topography, a thin cornea, unstable refraction, age under 18, active autoimmune disease and severe dry eye. Complications: dry eye (the commonest), glare and haloes at night, haze after PRK, flap problems and ectasia. In very high myopia a phakic intraocular lens is implanted.

Key points

• Myopia → minus; hyperopia → plus; presbyopia from 40.
• A pinhole improves refractive error.
• Children: cycloplegic refraction.
• Slowing myopia: low-concentration atropine, outdoor time.
• LASIK forbidden in keratoconus; PRK for a thin cornea.

Summary table

A free exam question

In the full version for subscribers

• A simple algorithm
• Memory aid
• Exam traps
• 5 review cards
• Enough for IMLE

The full version is open to subscribers of: Ophthalmology, Medical licensing exam (IMLE), Family medicine, Pediatrics.

Subscriber? Read the full topic Plans and prices 25 free questions

Sources

• AAO Preferred Practice Pattern: Refractive Errors, 2022 — Myopia, hyperopia, astigmatism, presbyopia and refractive surgery
• Low-Concentration Atropine for Myopia Progression (LAMP) Study, 2019 — Results
• AAO Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, 2024-2025 — Optics of the human eye

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.