Anisometropia

Comorbidity

Hyperopic children with anisometropia of one diopter or more are at risk for refractive accommodative esotropia, even with low hyperopia. Excessive accommodation in farsighted children triggers convergence, leading to an inward eye turn. Anisometropia also raises the risk of amblyopia. Both strabismus and anisometropia can cause amblyopia, and each can result from amblyopia.

Diagnosis

Most people with anisometropia show no clear symptoms, so the condition is usually detected during a routine eye exam. For preverbal children, photoscreening uses specialized cameras to evaluate light reflexes; if risk factors appear, an ophthalmologist performs a complete exam with pupil dilation and refractive measurement of each eye.

Treatment

For large degrees of anisometropia, standard spectacles may cause aniseikonia—a difference in image magnification between the eyes—that hinders binocular vision and can produce headaches or eyestrain. Earlier treatment makes adjustment easier. Iseikonic lenses can be made to equalize image sizes; the formula involves center thickness, refractive index, front base curve, vertex distance, and back vertex power. For a difference up to 3 diopters, such lenses can compensate, though at 3 diopters the lenses would be visibly different, with one lens at least 3 mm thicker and a base curve increased by 7.5 spheres.

For example, correcting −1.00 m⁻¹ in one eye and −4.00 m⁻¹ in the other with identical parameters (t=0.001 m, n=1.6, P=5 m⁻¹, h=0.015 m) yields retinal image sizes 1.17% smaller in the first eye and 5.36% smaller in the second. Adjusting the second lens to t=0.005 m and P=14.5 m⁻¹ brings its magnification closer to the first. Anisometropia also increases the risk that accommodative esodeviation will not be fully eliminated with hypermetropic correction.

Epidemiology

Determining the prevalence of anisometropia is difficult because refractive measurements vary and different criteria define the condition. Anisometropia is more frequent and severe in persons with high ametropia, especially myopes. It follows a U-shaped age distribution: common in infants, rare in young children, more frequent in teenagers and young adults, and increasingly prevalent after presbyopia sets in. One study estimated 6% of those aged 6 to 18 have anisometropia.

The underlying causes remain poorly understood. Anisometropic persons with strabismus are mostly farsighted, and almost all have or have had esotropia. Anisometropia may influence the outcome of strabismus surgery, and poor binocular function may be a risk factor for anisometropia to develop or increase.

Did You Know?

• Anisometropia is likely the result of both genetic and environmental influences.
• In older adults, developing asymmetric cataracts may cause worsened anisometropia.
• Astigmatism may additionally be present and may differ among the eyes.