Anisometropia: a difference in refraction between the two eyes, usually taken as 1D or more of spherical equivalent (or of cylinder)
Aniseikonia: a difference in the size or shape of the images perceived by the two eyes. It is caused by the optics of the correction as well as by the eyes themselves
Causes of anisometropia

Developmental: unequal axial growth (the commonest)
Acquired: unilateral aphakia or pseudophakic refractive surprise, unilateral cataract (index myopia), keratoconus, refractive surgery on one eye, trauma, scleral buckling (which lengthens the eye), corneal grafts

## Effects

Hypermetropic anisometropia is the most amblyogenic: the child accommodates only for the less hypermetropic eye, so the other is never in focus
In myopic anisometropia, the more myopic eye is still used for near, so amblyopia is less likely below about −3.00D of difference

Aniseikonia, from different spectacle magnification in each eye
Prismatic imbalance: the two lenses induce different prism when looking away from the optical centres
Reduced stereopsis and binocular function

## Aniseikonia

Rule of thumb: about 1% of aniseikonia per dioptre of anisometropia corrected with spectacles
Differences up to about 3% rarely cause symptoms, up to about 5% are usually tolerated, and more than that often cause asthenopia, diplopia or suppression
Symptoms: headache, eye strain, distorted spatial perception (tilted or sloping surfaces), and difficulty fusing
Measurement: the space eikonometer, the New Aniseikonia Test, or comparing the two images on a split target
Worked example: unilateral aphakia corrected with a +12.00D spectacle lens gives about 25–33% magnification compared with the phakic eye: intolerable. A contact lens (about 7–10%) or an IOL (about 1–3%) is needed (see Magnification)

## Axial or refractive: Knapp's law

Knapp's law: in axial ametropia, a correcting lens at the anterior focal point of the eye (about 15.7 mm in front of the cornea) gives a retinal image the same size as an emmetrope's (see Magnification)

Anisometropia	Equal retinal images with	Why
Axial (difference in length)	Spectacles, in theory	Spectacles sit near the anterior focal point (Knapp's law)
Refractive (difference in power, e.g. aphakia, keratoconus, refractive surgery)	Contact lenses or IOLs	The correction sits at or close to the eye's own optics, so it hardly changes image size

In practice, many axial anisometropes still do better with contact lenses. The stretched retina of the longer eye has more widely spaced photoreceptors, which offsets the larger image, and contact lenses also remove the prismatic imbalance

## Vertical prismatic imbalance

When both eyes look down to read, each looks below its optical centre. Unequal powers induce unequal vertical prism (Prentice's rule, P = c × F)
Worked example: R +1.00D, L +4.00D, reading 10 mm below the optical centres

Only the vertical powers matter: for a cylinder, use the power in the 90° meridian
Solutions

Slab-off (bicentric grinding): grinding the lower part of the more minus (least plus) lens creates a second optical centre and adds base up prism at reading level. In the example, 3Δ slab-off on the right lens
Reverse slab: moulded base down prism on the more plus lens
Separate reading spectacles with the optical centres lowered to the reading position
Dissimilar bifocal segments, whose different prism offsets the imbalance
Contact lenses, which move with the eye, so there is no imbalance
Teaching the patient to drop the head rather than the eyes

## Managing anisometropia

Children: correct the full anisometropia early, with occlusion or atropine penalisation for amblyopia. Children adapt to aniseikonia well
Adults: try the full correction. If it is not tolerated, partially reduce the difference while keeping good vision in the dominant eye, or use contact lenses
Iseikonic (size) lenses: change spectacle magnification by altering the shape factor and power factor (see Magnification)

To magnify: steepen the front curve, thicken the lens, or increase the vertex distance. To minify: the reverse
Worked example: a lens with a +10D front surface, 4 mm thick (n = 1.5) has a shape factor of 1 ÷ (1 − (0.004 ÷ 1.5) × 10) = 1.027. With a +12D front surface and 6 mm thickness it becomes 1.050, about 2% more magnification

Refractive surgery or IOL choice: in cataract surgery, aim for a target close to the fellow eye, or plan prompt second-eye surgery. A difference of more than about 2–3D between the eyes is rarely tolerated in spectacles
Monovision is deliberate anisometropia (about 1.00–1.50D) for presbyopia. It reduces stereopsis, so it should be trialled with contact lenses first

After surgery for retinal detachment with a scleral buckle, the eye may become 1–3D more myopic: a new cause of anisometropia
A cataract in one eye can cause anisometropia through index myopia, which resolves after surgery