Intermittent exotropia is a large exophoria that intermittently breaks down to an exotropia. Occluding one eye breaks fusion and will manifest the exotropia (Figure 5.1). When fusing, the eyes are straight and stereo acuity is excellent, usually 40 seconds of arc. When tropic, there is large hemi-retinal suppression of the deviated eye. It is common for patients to show a preference for one eye, however, resist the temptation to label the deviation as a right or left exotropia. You can easily change the deviated eye by covering the dominant eye. Patients with late onset exotropia during late childhood or adulthood may experience diplopia when tropic. The exotropia is typically manifest when the patient is fatigued, daydreaming or ill. Approximately 80% of intermittent exotropia patients will show progressive loss of fusion control and an increase in the exotropia over several months to years. Adult patients can have extremely large deviations (Figure 5.2). The patient in Figure 5.2 has alternating intermittent exotropia. Despite the large angle exotropia she was able to fuse intermittently.
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