Intermittent exotropia

Definition

Exodeviation may be controlled by a fusional mechanism and is classified as exophoria (X), intermittent exotropia (X(T)), or constant exotropia (XT) ( Fig. 80.1 ). X(T) is a disease that becomes manifest when fusional convergence of exophoria is intermittently lost. It differs from exophoria as manifest strabismus naturally occurs with decreased alertness or fatigue. Monocular eye closure may occur with exotropia. Near stereoacuity is mostly preserved, but stereopsis may deteriorate with progression.

Etiology

The causes of X(T) are not fully understood. The following have been proposed:

• 1.

  An imbalance between active convergence and divergence, although it is not clear that divergence is active. Bielschowsky believed that exodeviation is governed by static (anatomic) and dynamic (innervational) factors. During the dynamic interplay between convergence and divergence, any imbalance favoring divergence may result in X(T).

• 2.

  Abnormal orbital anatomy. Exotropia often occurs in disorders associated with hypertelorism such as Apert and Crouzon syndrome. One study found that patients with X(T) have longer interorbital distances compared to patients with esotropia or no strabismus, but there is no evidence that the orbital axis of patients with X(T) is wider than normal.

• 3.

  Abnormalities of extraocular muscle proprioception, although the existence of active proprioceptors in extraocular muscles is disputed. Corsi et al. reported alterations in the ultrastructure of the myotendinous junctions in patients with congenital esotropia. Another study suggested intermittent exotropia might be caused by convergence problems of the medial rectus muscle arising from Schwann cell degeneration. Axonal profiles of the medial rectus muscles were shown to differ in patients with X(T) and acquired sensory exotropia.

Epidemiology

X(T) is the most common form of divergent strabismus. Divergent strabismus is much more common than convergent strabismus in Asia. In Singaporean children aged between 6 and 72 months, the prevalence of strabismus was 0.8%, with an exotropia : esotropia ratio of 7 : 1; 63% of cases of exotropia were intermittent. By contrast, X(T) is rare in South African black and mixed-race children. A female predominance has been found in some studies. According to a study of 1514 non-Hispanic White children and 1522 Asian children aged 6 months to 5 years, the prevalence of strabismus was 3.55% for the Asian group; of these, 2.1% were exotropia and 1.38% were esotropia. In contrast, the prevalence of strabismus for the non-Hispanic White group was 3.24%; of these, 0.73% were exotropia and 2.31% were esotropia.

Clinical Features

X(T) typically has its onset in the second to third year of life when an intermittent outward drift of one eye or monocular eye closure in bright sunlight is noted ( Fig. 80.2 ). However, Costenbader analyzed 472 patients with X(T) and found that 62% of them developed the condition before age 1 year. Hiles and Biglan coined the term “infantile exotropia” to describe children who had the onset of exotropia during the first year of life. However, others use the term X(T) when referring to children with an exotropia presenting during the first year of life. X(T) usually is more pronounced with fatigue, fever, or an upper respiratory infection. Older children or adults may complain of eye fatigue or headaches. It is very rare for young children to complain of these symptoms, presumably because of their well-developed suppression mechanism. Monocular eye closure in X(T) has been attributed to photophobia, although it is difficult to explain why more light should enter exotropic than orthotropic eyes. Monocular eye closure may be due to diplopia or abnormal visual perception, or a reduction in the threshold for bright light, induced by divergence of the eyes. While it is true that monocular eye closure is common with X(T), it also occurs in patients with a constant exotropia as well as people without strabismus. It may improve following exotropia surgery.

Burian reported that the distribution of refractive error in patients with exotropia is similar to the general population and the cause of exotropia is largely unrelated to refractive error. However, recent studies have reported that myopia and exotropia are significantly linked.

Comparison of the angle of distance and near deviations and a determination of the type of exotropia is important when planning exotropia surgery but some clinicians dispute whether these findings are helpful in predicting the surgical outcome.

X(T) is traditionally classified into four types ( Table 80.1 ):

1. True divergence excess : the deviation is more than 10 prism diopters (PD) larger when measured at distance fixation than at near.

2. Simulated (pseudo) divergence excess : initially greater at distance fixation, but the misalignment at near fixation increases to within 10 diopters of the angle at distance following disruption of near binocular vision by monocular occlusion and/or +3.00 D lenses.

3. Basic : the size of the misalignment is within 10 PD, when the misalignment is measured at distance and near fixation.

4. Convergence insufficiency : the deviation is more than 10 PD larger when measured at near fixation than in the distance.

Duane first used, the terms “divergence excess” and “convergence insufficiency” to describe a difference between the distant and near angle of deviation. However, he did not provide a mechanism to account for the difference between the distant and near angles of deviation. It is reasonable to assume that differences in distant and near angles of deviation are affected by relative convergence excess in basic exodeviation (i.e. tonic fusional, accommodative or proximal convergence). With pseudodivergence excess type X(T) the distant angle of deviation is initially greater than the near angle of deviation, but after covering one eye for an extended period of time (30 minutes to 2 hours) they become similar. This is because fusional convergence to a manifest near deviation is blocked after one eye is covered for a long period of time. A high ratio of accommodative convergence to accommodation (AC/A) ratio can be diagnosed if +3.00 D lenses increase the near angle of deviation. Kushner reported that 60% of patients with true divergence excess have a high AC/A ratio, while the rest have normal or increased proximal convergence. However, the X(T) type is not fixed and can change. According to one study, after a part-time occlusion therapy of the dominant eye for 3 months, the basic and convergence-insufficiency types converted to pseudo-divergence excess and basic types in approximately one-half of the subjects. It has been hypothesized that patching may improve fusional ability or fusional convergence at near.

Psychosocial Effect

Exotropia has psychosocial effects in older children and adults. Burian reported that symptoms of exotropia include blurred vision, asthenopia, visual fatigue, and rarely diplopia in older children and adults. Patients may feel more exhausted, but reduced stereopsis generally does not adversely affect vision-related quality of life. One report described significant improvements in appearance-related symptoms regardless of age, especially in patients with constant exotropia.

Clinical Evaluation