Concomitant exotropia is divided into primary and secondary types. Primary types include exophoria, intermittent exotropia, and constant exotropia. Secondary types include sensory exotropia, caused by poor vision in one eye, and consecutive exotropia, which results from overcorrection after surgery for esotropia.

bubble_chart Etiology

Anatomical, mechanical, and innervational factors play a role in all concomitant strabismus. In concomitant exotropia, innervational factors likely play a significant role. Theoretically, primary concomitant exotropia arises from an imbalance in the tension between convergence and divergence. Electromyographic studies have demonstrated that divergence is an active physiological process, not merely the result of passive inhibition of convergence. Duane was the first to propose that exotropia is caused by an imbalance in innervation, which disrupts the relationship between convergence and divergence.

bubble_chart Clinical Manifestations

1. Age of Onset: Most patients have an early onset of the disease, with 35–70% occurring before the age of 2. Exotropia initially presents as exophoria, which further progresses to intermittent exotropia and then constant exotropia. This progression may be related to the weakening of convergence tension with age.

2. Gender: Approximately two-thirds of exotropia patients are female, for reasons that remain unclear but may be related to genetic factors.

3. Refractive Errors: In addition to the interaction between convergence and divergence mechanisms, refractive errors can weaken neural control and affect eye alignment. It was previously believed that exotropia was more common in myopia. However, recent statistical data show that it is also frequently observed in emmetropia and hyperopia, with only 8–15% of cases associated with myopia. Jampolsky emphasized a significant correlation between anisometropia and exotropia. Unequal retinal image clarity can disrupt fusion and easily lead to suppression, thereby contributing to the development of exotropia.

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1. Divergence excess type: The angle of deviation at distance is greater than that at near, with a difference of at least 15△, and the AC/A ratio is high. This type progresses relatively quickly but is generally stable. Surgical treatment may involve bilateral lateral rectus recession.

2. Basic exodeviation type: The angle of deviation at distance is equal to that at near, with a difference not exceeding 10^△, and the AC/A ratio is normal. This type tends to progress. Surgical treatment may consist of lateral rectus recession combined with medial rectus resection.

3. Convergence insufficiency type: The angle of deviation at near is greater than that at distance, and the AC/A ratio is low. Muscle strength during adduction is normal. This type is characterized by rapid progression, intermittency, and a quick transition to constant deviation with loss of fusion function. Close monitoring and timely surgery are necessary, and postoperative chances of achieving binocular single vision remain high.

4. Simulated divergence excess type: Initial examination shows a greater angle of deviation at distance than at near, but special testing reveals that the near deviation equals or exceeds the distance deviation, indicating it is not a true divergence excess type. Surgery involving only the lateral rectus is ineffective; medial rectus resection should also be performed.

This classification method based on the mechanism of disease has clinical advantages. For example, in true divergence excess type, due to excessive neural drive to the lateral rectus, bilateral lateral rectus recession is appropriate. For convergence insufficiency type, bilateral medial rectus resection followed by convergence training can enhance central convergence. Burian observed that divergence excess type is generally stable and can be monitored, whereas convergence insufficiency type progresses rapidly and requires prompt treatment. Basic exodeviation type has a tendency to progress, so treatment should not be delayed.