SN7 SHORT NOTE 5M ★ High-yield

Classify errors of refraction and describe the mechanism of accommodation.

Physiology · 1st Professional — Pre-Clinical

Introduction

Errors of refraction occur when parallel rays of light are not brought to a focus on the retina with accommodation at rest.

The eye has a total refractive power of about 60 dioptres, of which the cornea contributes about 43 D (the largest single element) and the lens about 17 D at rest
The cornea contributes most because the difference in refractive index between air and cornea is greatest — which is why vision is blurred under water, where that difference disappears

Types of Refractive Error

Error	Defect	Image falls	Correction
Myopia (short sight)	Eyeball too long (axial) or refractive power too great	IN front of the retina	Concave (minus) lens
Hypermetropia (long sight)	Eyeball too short or refractive power too weak	Behind the retina	Convex (plus) lens
Astigmatism	Unequal curvature in different meridia, usually corneal	Rays in different meridia focus at different points — no single point focus	Cylindrical lens
Presbyopia	Age-related loss of lens elasticity, so accommodation fails; not a true refractive error	Near objects cannot be focused	Convex lens for near work; begins around 40 years
Aphakia	Absence of the lens	Far behind the retina	Strong convex lens or intraocular lens implant

CLINICAL PEARL

Presbyopia is not a refractive error and happens to everyone. The eye is optically normal; what fails is the lens’s ability to change shape, which declines steadily from childhood and crosses the threshold for comfortable reading at around 40. That is why it appears at much the same age worldwide, and why it needs a separate correction for near vision even in someone with perfect distance sight.

Accommodation

A near object is viewed → Impulses via the optic nerve to the edinger–westphal nucleus → Parasympathetic outflow in the oculomotor (III) nerve to the ciliary ganglion → ciliary muscle contracts → the suspensory ligament slackens → The elastic lens becomes more convex — power increases by up to about 14 D in a child → Accompanied by convergence of the eyes and pupillary constriction — the near reflex triad

Amplitude of accommodation falls with age — about 14 D at 10 years, 4 D at 40, and under 1 D by 60
Cycloplegic drugs paralyse the ciliary muscle and abolish accommodation, which is why they are used for accurate refraction in children, in whom active accommodation would otherwise mask hypermetropia

Applied Aspects

Uncorrected refractive error is the commonest cause of visual impairment worldwide, and one of the most easily and cheaply remediable — a pair of spectacles
School vision screening detects it at the age when it most affects learning, and is among the most cost-effective public health measures available
Myopia is rising sharply in urban Asian populations, and time spent outdoors in childhood appears protective; near work and indoor living are implicated
High myopia carries added risks — retinal detachment, myopic degeneration, glaucoma and early cataract; it is not merely an optical inconvenience
An adult who suddenly needs less reading correction may be developing nuclear cataract, which increases lens power and causes "second sight" — a useful and frequently missed clue
Sudden change in refraction can indicate diabetes, as osmotic swelling of the lens alters its power; blood glucose should be checked before prescribing new spectacles
Refraction in children must be done under cycloplegia, since their powerful accommodation conceals hypermetropia and produces a falsely low reading
Uncorrected error in one eye during childhood causes amblyopia, which becomes irreversible after the critical period — another reason early screening matters
Refractive surgery reshapes the cornea and is effective in suitable candidates, but does not remove the retinal risks of high myopia, which persist for life

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