Brief Description, Types, Sign and Symptoms, Causes And Treatment Of Heterophoria

This is a condition in which the tendency of the eyes to deviate is kept latent by fusion. When fusion is removed the visual axis of one eye deviates away.

It is also called latent strabismus. There are four main types of heterophoria and they include:

Esophoria

This is a tendency of the eyes to converge. It is more common in younger age groups. It may come in form of the following:

Convergence Excess; This occurs when there is high esophoria at near than at far and is usually found in uncorrected hyperopia and recently corrected myopia. It comes with the following symptoms; Frontal headache, ocular fatigue, blurred vision and initial of accommodation at prolonged near work. There is high AC/A ratio and reduced negative fusional reserve (NFR). 
It is corrected with plus lens (or bifocal) or visual training but when the two fails, base-out prism is prescribed for near work. The optometrist should instruct myopic paitients to remove their prescription during near work.

Divergence Insufficiency; This occurs when there is high esophoria at far than at near. The patients complains of headache and asthenopic symptoms usually at prolonged far work. Full correction is prescribed for hyperopes. It is corrected with base-out prisms for constant wear.

Basic Esophoria; This occurs when there is significant esophoria at both far and near (esophoria does not vary significantly in degree for both distance). It is corrected with base-out prisms.

Exophoria

It is a tendency of the eyes to diverge and is more often seen in elderly. It may come in form of the following;

Convergence Insufficiency; This occurs when there is high exophoria at near than at far with the patient complaining of double vision (diplopia), fatigue, drowsiness and headache after prolonged near work. There is low AC/A ratio, reduced positive fusional reserve and poor near point of convergence.

The optometrist should undercorrect hyperopes and fully correct myopia or give visual training and when these fails to relieve symptoms, he should prescribe base-in prisms for near work.

Divergence Excess; This occurs when there is high exophoria at far than at near with the patient complaining of occasional double vision while performing distance tasks such as driving or watching television. It is corrected with minus lens for distance wear and if this fails, base-in prism is prescribed.

Basic Exophoria; This occurs when exophoria does not vary significantly in degree for any distance.

It is corrected with base-in prisms.

Hyperphoria

Hyperphoria is the tendency of the eyes to deviate upwards while hypophoria is the tendency of the eyes to deviate download. In practice it is customary to use the term right or left hyperphoria depending on the eye which remains up as compared to the other.

Cyclophoria

Cyclophoria is the tendency of the eyes to rotate around the antero-posterior axis. It could be incyclophoria (inward or nasally) or excyclophoria (outward or temporally).

Note: Convergence Insufficiency, Convergence Excess, Divergence Insufficiency and Divergence Excess are collectively referred to as binocular vision syndrome because they occur as a result of accommodative and convergence anomalies.

Treatment Options

Treatment is indicated only in symptomatic cases (decompensation heterophoria) and they include the following:

1. Correction of refractive error when detected is most important.
2. Orthoptic treatment is indicated when symptoms persist even with glasses or when phoria occurs without refractive error. The aim is to improve convergence insufficiency and fusional reserves. The exercise is performed with synoptophore.
3. In troublesome cases, prism is prescribed with its apex towards the direction of phoria. The prism should correct only half or at most two-third of the heterophoria. For more information on how to prescribe prisms, click here. 
4. Surgical treatment is performed to strengthen the weak muscle or weaken the strong muscle when other measures fail to relieve symptoms.