Refractive Errors

Refractive errors are failures of the eye to focus images sharply on the retina, causing blurred vision. In the emmetropic (normally refracted) eye, the cornea and lens focus light sharply on the retina; during accommodation, the ciliary muscles adjust lens shape to maintain focus.

Types

Type	Mechanism	Effect	Correction
Myopia (nearsightedness)	Focus point falls in front of the retina — cornea too steeply curved and/or eye axial length too long	Distant objects blurred, near objects clear. Often progresses in children until growth stops	Concave (minus) lens
Hyperopia (farsightedness)	Focus point falls behind the retina — cornea too flat and/or axial length too short	Adults: both near and distant blurred. Children/young adults with mild hyperopia may see clearly via accommodation	Convex (plus) lens
Astigmatism	Non-spherical corneal/lens curvature — light of different orientations focuses at different points	Distorted vision at all distances	Cylindrical lens (no power along one axis, concave/convex along the other)
Presbyopia	Age-related loss of the lens’s ability to change shape for near focus, noticeable from the early-mid 40s	Near vision blurred	Convex (plus) lens — separate reading glasses, bifocals, or variable-focus lenses
Anisometropia	Significant difference (usually >3 dioptres) between the two eyes’ refractive error	Corrective lenses produce a difference in image size (aniseikonia) between the eyes, risking fusion difficulty or suppression of one image	Case-by-case; contact lenses can reduce aniseikonia vs spectacles

Clinical features

• Blurred vision — distant, near, or both
• Headaches (from excessive ciliary muscle tone, or prolonged squinting/frowning)
• Ocular surface desiccation from excessive staring — irritation, itching, visual fatigue, foreign body sensation, redness
• In children: frowning/squinting when reading, excessive blinking or eye rubbing

Diagnosis

Visual acuity testing, refraction, and a comprehensive eye exam by an optometrist or ophthalmologist.

Management

• Eyeglasses — convex (hyperopia/presbyopia), concave (myopia), cylindrical (astigmatism)
• Contact lenses
• Refractive surgery

---

Source: MSD Manual — refractive error classification.