Anisometropia (Unequal Refractive Error)

What anisometropia means

Each eye focuses light onto the retina. A refractive error is a focusing problem: myopia (nearsightedness), hyperopia (farsightedness) or astigmatism. Lens power is measured in diopters (D). In anisometropia, the two eyes' errors differ by about 1 D or more in sphere or astigmatism. Differences of 3 D or more are considered high.

Types:

• Simple: one eye normal, the other nearsighted or farsighted.
• Compound: both eyes nearsighted or both farsighted, by different amounts.
• Antimetropia: one eye nearsighted, the other farsighted.
• Astigmatic: one eye has much more astigmatism.

Causes

• Unequal eye growth during childhood, often with no specific cause.
• Genetics: it can run in families.
• Eye conditions: congenital cataract, drooping eyelid (ptosis) or eyelid tumors pressing on one eye can disturb normal development.
• In adults: a cataract developing in one eye, eye surgery (including cataract or refractive surgery), keratoconus or injury.

Symptoms

Children often have no complaints, because the brain simply relies on the better eye. That is why routine vision screening matters.

Adults and older children may notice:

• blurred vision in one eye
• eye strain, tired eyes or headaches
• trouble with depth perception, such as catching balls or judging steps
• double vision or discomfort with glasses
• dizziness or a "swimming" feeling with new glasses

Aniseikonia

Glasses lenses change image size. When the two lenses are very different, each eye sees an image of a different size (aniseikonia). The brain struggles to fuse them, causing strain, headaches and dizziness. Contact lenses reduce this effect because they sit on the eye.

Anisometropia and amblyopia

During early childhood, the visual system is still developing. If one eye sends a consistently blurrier image, the brain learns to suppress it and that eye's vision does not develop properly. This is anisometropic amblyopia, one of the most common forms of lazy eye. Because the eyes usually look straight, it is often missed until a vision test.

Higher degrees of anisometropia and hyperopic anisometropia carry the highest risk. Amblyopia responds best to treatment before about age 7, though older children can still improve.

Diagnosis

• Visual acuity testing of each eye separately with an eye chart or picture chart.
• Refraction to measure each eye's prescription. In children, eye drops (cycloplegic refraction) relax focusing to reveal the true prescription.
• Eye alignment tests to check for strabismus, which often coexists.
• Examination of the eye's health, including the lens and retina.
• Photoscreening devices used in pediatric offices can detect anisometropia in young children.

Treatment

Children

1. Glasses with the full prescription for each eye. Glasses alone improve amblyopia in many children over several months.
2. Patching the stronger eye for a few hours a day, or atropine eye drops to blur it, if amblyopia persists, forcing the brain to use the weaker eye.
3. Contact lenses for high anisometropia, where glasses cause large image size differences.
4. Regular follow-up to adjust prescriptions and monitor vision.

Newer treatments using binocular video games or digital devices are available for some children.

Adults

• Glasses for small to moderate differences.
• Contact lenses for large differences, giving more equal image sizes.
• Refractive surgery (LASIK, PRK or lens implants) can reduce the difference.
• Cataract surgery for anisometropia caused by a cataract, with the new lens chosen to balance the eyes.

Adults who were not treated as children may have permanent reduced vision in the weaker eye, but correction often improves comfort.

Frequently asked questions

What is anisometropia?

Anisometropia is a condition where the two eyes have significantly different prescriptions, usually a difference of 1 diopter or more. One eye may be more nearsighted, farsighted or astigmatic than the other. It is corrected with glasses or contact lenses.

Can anisometropia cause lazy eye?

Yes. In young children, anisometropia is a common cause of amblyopia (lazy eye). The brain favors the clearer eye and stops developing vision in the blurrier one. Early glasses and, if needed, patching or atropine drops usually restore vision.

What are the symptoms of anisometropia?

Many children have no symptoms. Older children and adults may have blurry vision in one eye, eye strain, headaches, poor depth perception or dizziness with glasses. Vision screening is often the only way it is found in young children.

Can anisometropia be corrected?

Yes. Glasses or contact lenses correct the focusing difference. Contact lenses are often better for large differences. In adults, refractive surgery or cataract surgery can reduce it. In children, early correction also prevents or treats amblyopia.

Why do my new glasses make me dizzy?

If your eyes have different prescriptions, your lenses may make images appear different sizes in each eye, which can cause dizziness and strain. Most people adapt within 1 to 2 weeks. If not, ask your eye doctor about contact lenses or specially designed lenses.