bubble_chart Overview

Exophoria is a tendency for the eye position to deviate outward. It is usually controlled by corrective fusion reflexes, preventing deviation and maintaining binocular single vision. Generally, symptoms may appear if exophoria exceeds 5^△, but this also depends on the patient's fusion function. For example, some individuals may have 10^△ exophoria at near vision but remain asymptomatic due to good fusional convergence. Conversely, others may experience asthenopia with only 4^△ exophoria at near vision, which falls within the normal range, if their fusional convergence is poor.

bubble_chart Etiology

1. Insufficient convergence excitation of the central nervous system.

2. Strong lateral rectus muscle or anterior insertion point, weak medial rectus muscle or posterior insertion point, abnormal anatomy of the check ligament and intermuscular membrane.

3. Myopic refractive error, leading to decreased convergence function due to lack of accommodation at both near and far distances.

bubble_chart Clinical Manifestations

Working at close range soon causes forehead pain, eye soreness, inability to read for long periods, blurred vision, overlapping or double vision, and other symptoms. One must close their eyes and rest briefly before continuing to read, but the symptoms soon reappear. In severe cases, it becomes impossible to continue studying or working. Clinically, sometimes a series of tests are conducted for refractive errors, glaucoma, or neurasthenia, and corrective glasses are prescribed, but the symptoms still persist.

bubble_chart Treatment Measures

1. Correction of refractive errors: Adolescents should undergo optometry after cycloplegia. If myopia is present, it should be fully corrected to enhance accommodation, which has a better effect on eliminating exophoria. If myopic astigmatism, hyperopic astigmatism, or mixed astigmatism is present, correction should be performed to improve visual acuity and increase fusional convergence function, which also aids in treating exophoria.

2. Base-out prisms and synoptophore training: Primarily aimed at increasing fusional convergence, though the effect is limited for severe cases. Pencil near-point training can also be used, where the pencil tip is moved from far to near until diplopia occurs, then retracted to the original position for repeated training. Consistent practice of these methods can help treat exophoria.

3. Prescription of base-in prisms: Wearing base-in prisms does not enhance muscle strength but significantly alleviates visual fatigue. Parks (1975) particularly praised this method. The prism prescription should be the minimum power required to eliminate symptoms, correcting 1/4 to 2/3 of the exophoria degree. The prisms can be placed in front of one or both eyes. While base-in prisms can relieve visual fatigue, they may also worsen fusional convergence and lead to symptom recurrence.

4. Surgery: The primary approach is to strengthen the medial rectus muscles, performing medial rectus resection on one or both eyes. Each 1 mm shortening of the medial rectus roughly corrects 3–5^△, with a fluctuation range of about 2^△. Surgical intervention must be carefully considered, with a large exophoria degree at distance as the main indication. If the distance eye position is normal or shows slight exophoria, but the near exophoria degree is large, the following two outcomes may occur post-surgery: ① Symptoms may be alleviated or disappear initially, but exophoria and symptoms could recur. ② Near symptoms may disappear, allowing sustained reading, but distance diplopia may develop. Hermann (1981) performed bilateral medial rectus resections on 14 patients with convergence insufficiency, resulting in distance diplopia post-surgery, and recommended using Fresnel prisms (base-out) to resolve the diplopia.