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Describe anisometropia and aniseikonia.

Ophthalmology · 3rd Professional

Anisometropia is a difference IN refractive error between the two eyes.
aniseikonia is a difference IN the size or shape of the two retinal images.
The first is the usual cause of the second.
The relationship — each dioptre of spectacle anisometropia produces about 2% of aniseikonia; and the brain can fuse a difference of up to about 5%, that is 2.5 D. Beyond that, fusion fails

Consequence	Explanation
Amblyopia	The commonest and most important. The more ametropic eye receives a persistently blurred image and is suppressed — anisometropic amblyopia. It is often unsuspected.
Loss of binocular single vision	Where the difference exceeds what can be fused
Asthenopia	Headache, eye strain and difficulty with near work, from the effort of fusing dissimilar images
Squint	The suppressed eye may deviate
Anisophoria (differential prismatic effect)	On looking away from the optical centres, the two lenses induce different prism, causing vertical diplopia in downgaze — the classic complaint of a spectacle-corrected anisometrope reading or descending stairs

Spectacles — adequate for differences of up to about 2.5 to 3 D in adults, and well tolerated by children even at higher differences, because the developing brain adapts. In a child the full correction should be given regardless of the difference.
Contact lenses are the correction of choice in significant anisometropia, because they sit at the corneal plane and produce almost NO image size difference; they also abolish the differential prismatic effect
Intraocular lens — the definitive answer in aphakia.
Refractive surgery in suitable adults
Iseikonic lenses — spectacles designed with altered base curve and thickness to equalise image size; theoretically neat, rarely used, and expensive

Test each eye separately in every child; anisometropic amblyopia produces no squint, no symptom and no sign, and is missed unless the good eye is covered
Give a child the full correction whatever the difference between the eyes; preventing amblyopia matters more than the theoretical aniseikonia, and children adapt
Offer contact lenses to the adult anisometrope; they eliminate both the image size difference and the prismatic effect at a stroke
Implant an intraocular lens in unilateral cataract; leaving an adult aphakic in one eye creates an anisometropia that cannot be managed with spectacles
Ask about vertical diplopia in downgaze in a spectacle-corrected anisometrope; anisophoria explains it, and a slab-off prism or contact lenses solve it

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