Anisometropia

What is anisometropia?

Very few people are born with two eyes of identical optical power, but the brain manages to compensate and it’s usually unnoticeable.

However, when a person has anisometropia, the difference in vision between their two eyes is significant and will interfere with normal binocular vision. In practice, they will see a smaller image in one eye and a larger image in the other eye. The result is that their overall vision is often blurred.

Another potential outcome from anisometropia is amblyopia (lazy eye), which can occur if one eye has blurred vision for some time and becomes permanently weaker.

Sometimes anisometropia can be present at birth, although frequently it won’t become apparent until later in life. It has been estimated that around six percent of all children aged between six and eighteen suffer from this visual condition.

Types of anisometropia

There are three types of anisometropia:

• Simple anisometropia. This is when one eye is affected while the other eye has no refractive error (or spectacle prescription). The affected eye can either be hyperopic (long-sighted) or myopic (short-sighted).
• Compound anisometropia. This is when both eyes are myopic (short-sighted), although there will be a significant difference in their refractive errors (or spectacle prescriptions). This causes one eye to see a more blurred image than the other.
• Mixed anisometropia. This is when both eyes have appreciable refractive errors, with one eye myopic and the other hyperopic.

Symptoms of anisometropia

There are a number of potential symptoms, including:

• Amblyopia (lazy eye). Usually, this is when reduced refractive power in one eye causes a lack of visual stimulation that results in insufficient information being transmitted through the optic nerve to the brain
• Strabismus (crossed eyes). When a patient is unable to align both eyes. This lack of coordination prevents both eyes being able to focus on the same point in space
• Diplopia (often known as double vision). The result includes:
  • Eyestrain
  • Headaches
  • Nausea
  • Light sensitivity
  • Tiredness
  • Dizziness.

Causes

Even people who have normal vision can have up to 5% difference in the refractive power of each eye. However, those with a 5–20% difference will experience uneven vision (anisometropia). Causes include defects in the eye at childbirth as well as uneven size of the two eyes.

Treatment

It is important to treat anisometropia as soon as it is diagnosed. Untreated, the brain can decide to select the eye that presents the clearer image, and then ignore the other eye. This can lead to a dependence on the stronger eye.

The neglected eye will become progressively weaker; therefore it is important to seek treatment before the issue becomes acute.

There are several ways that anisometropia can be treated, depending on the severity. For some, the difference between the two eyes is manageable, especially with a minimal difference. Other patients may require corrective lenses, contact lenses or corrective surgery.

When contact lenses or corrective lenses are used, each lens will need to be a different prescription to be effective.

Generally speaking, glasses are not suitable for those with very large degrees of anisometropia. Due to their magnification effect, glasses can create a considerable difference in the size of the image seen by each eye and can actually prevent good binocular vision.