Binocular Vision & Eye Muscle Surgery Qtrly°

Major Review: Intermittent Exotropia; Basic and Divergence Excess Type

Summer of 1993 Volume 8 (No.3): 185-216

I: NOMENCLATURE and
INCIDENCE

Intermittent exotropia (X(T)) is a unique strabismus with a specific set of sensory motor findings (1).

Duane originally, in 1897, used the term divergence excess to describe almost all exodeviations especially those with a deviation larger at distance than near (2).

According to Duane, these exodeviations were a result of active divergence and their sensory motor findings were uniquely different from those found with convergence insufficiency type exodeviations. Conversely, Knapp (3), Windsor (4), and Jampolsky (5) have all felt that since the distance exodeviation was not a result of active divergence, a more appropriate term should have been simply intermittent exotropia.

Various other descriptive terms have been used to identify this entity. Each has been an attempt to describe the condition by its major characteristic. They include Pugh’s “occasional exotropia” (6), Worth’s “neuropathic exotropia” (7), Bielschowsky’s “periodic exotropia” (8), Sugar’s “hyperkinesis of divergence” (9), and Burian’s “exotropia of inattention” (10). The current descriptive term is divergence excess intermittent exotropia, abbreviated DEX(T).

Patients with DEX(T) typically have intermittent deviations with infrequent reports of diplopia. Knapp (3) & Jampoisky (5) have used patient complaints of diplopin and/or hemiretinal suppression to differentiate exophoria from exotropia. However, this has only served to confuse the issue since some deviating DEX(T)s have diplopia at times, while others suppress or avoid diplopia with anomalous retinal correspondence (ARC).

Dunnington (11) suggested DEX(T) should be defined by its major characteristics. He reported seven chief characteristics which included marked exophoria for distance, excessive prism divergence, normal prism convergence, normal near balance, normal near point of convergence, normal rotations, and diplopia, if present, comitant to the left and right. Sugar (9) defined DE as “Exophoria or exotropia greater at distance than at near, good vision in each eye, suppression when manifest, fusion when latent and increased prism divergence for distance particularly”.

Costenbader (12) presented the most complete and clinically accurate description of the entity: His description included exo

phoria-exotropia at distance, normal near point of convergence, adequate prism convergence, intermittency, equal vision, good stereopsis, and ARC when exodeviated.

One can define any strabismic condition by its major motor characteristics, realizing that motor anomalies have commonly associated sensory motor findings. Thus, DEX(T) may be defined by both its spatial (distance/near relationship) and temporal (constant, intermittent or phoria) motor characteristics. Thus, the proper nomenclature might be intermittent exotropia of the divergence excess type (1).

Duane (2) also described another type of XT where the objective angle at distance and at near are equaL He stated that this exodeviation was a DEX(T) with an acquired secondary convergence insufficiency. Burian (10) labelled this third type of XT' a basic XT (distance and near exodeviation are approximately equal).

Divergence excess and basic intermittent exotropes have similar sensory/motor findings and are probably variations of the same conditions; thus, they are subsets of X(T). These patients have an intermittent deviation which occurs from 1% to 99% of the time, have stereopsis when aligned, and suppression and/or ARC when tropic. Their sensory motor characteristics are different from those of constant exotropias, exophorias, or convergence insufficiency type X(T)s (13). (These latter types will not be discussed in this review.)

Exotropia appears less frequently than esotropia (El). The approximate ratio of XT to ET is 1:3 (14). Friedman et al (15) screened 38,000 children aged 1 to 2.5 years. They found 498 had a strabismus, of which approximately 25% were exotropic. Von Noorden (16) has suggested that XT is more prevalent in the Middle East, the Orient, and Africa. This observation supports the findings of Eustace et al (17) and observations of Romano (18) that XT appears more commonly in areas with greater sunlight.

Jenkins (19) reported that in a survey performed by the International Orthoptic Association 30% of all strabismics are exotropic. Japan and Nepal have the highest incidence of XT as compared to ET, i.e., 54% and 76%, respectively. This higher incidence of XT among Orientals/ Asians as compared to Caucasians was noted previously by Ing & Pang (20). The prevalence of XT in Arabian countries, the United States, and United Kingdom are all similar, i.e. 30%.

Exotropia usually appears within the first few years of life and may progress. Von Noorden (21) followed 51 patients with XT who did not have surgery. He reported 75% showed signs of progression, 9% did not change, and 16% improved with time. Jampolsky (22,23) described DEX(T) as a progressive disease in which an exophoria, due to suppression, develops into an X(T) and finally a constant XT. Presbyopia results in a decrease in accommodation and an increase in both the frequency and size of the near deviation. These fmdings support Burian & Franceschetti’s observation (24) that it is rare to find DEX(T) later in life. On the other hand, Hiles et al (25) followed 48 X(T)s for 6-22 years (average age 11.7) who had an initial deviation greater than 18A and who elected not to nave surgery. Thirly-nine of 48 showed no increase in their exodeviation with time, 12 actually showed a decrease with time, and only 8 showed an increase which was less than 15g. Fifty-two percent actually had smaller exodeviations at the end of the study. The near point of convergence did not change with time. Hiles’ study (25) “contradicted the general popular impression that all exophorias in childhood progress to constant exotropia in childhood”. Friendly (26) has reported that Hiles et al used occlusion on some of the X(Y)s who they followed over time. Their findings might have been influenced by this.

Women clearly are affected more frequently by X(T). Cass (27) reported that 70% of DEX(T) are women, while Krzystkowa & Pajakowa found 67% to be female (28), and Gregersen 61% (29). Contrary to Donders (30), who reported a higher incidence of XT with myopia, most studies report a normal distribution of refractive errors with XT (31,32).

modation (AC/A) ratio:

Divergence excess exotropia has by definition a larger distance deviation as compared to the near deviation. Duane (2) originally specified (arbitrarily) that the difference between the distance and near findings had to be at least 15 pd However, most authors have adopted Burian’s arbitrary difference of 10 pd between distance and near (10).

By this last definition, if one assumes a

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