Binocular Vision &

Eye Muscle Surgery Qtrly°

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

Cooper, MS, OD and N. Medow, MD

Summer of 1993

Volume 8 (No.3): 185-216

I: NOMENCLATURE and

INCIDENCE

Nomenclature

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 exodevia-

tions were a result of active divergence

and their sensory motor findings were

uniquely different from those found with

convergence insufficiency type exodevia-

tions. 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 “hyper-

kinesis of divergence” (9), and Burian’s

“exotropia of inattention” (10). The current

descriptive term is divergence excess inter-

mittent exotropia, abbreviated DEX(T).

Patients with DEX(T) typically have

intermittent deviations with infrequent re-

ports of diplopia. Knapp (3) & Jampoisky

(5) have used patient complaints of diplo-

pin and/or hemiretinal suppression to dif-

ferentiate exophoria from exotropia. How-

ever, 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 cor-

respondence (ARC).

Dunnington (11) suggested DEX(T)

should be defined by its major character-

istics. He reported seven chief character-

istics which included marked exophoria for

distance, excessive prism divergence, nor-

mal prism convergence, normal near bal-

ance, 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 mani-

fest, 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 con-

vergence, 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 as-

sociated sensory motor findings. Thus,

DEX(T) may be defined by both its spat-

ial (distance/near relationship) and temp-

oral (constant, intermittent or phoria)

motor characteristics. Thus, the proper

nomenclature might be intermittent exo-

tropia of the divergence excess type (1).

Duane (2) also described another type

of XT where the objective angle at dis-

tance and at near are equaL He stated

that this exodeviation was a DEX(T) with

an acquired secondary convergence insuf-

ficiency. 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 find-

ings 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 exo-

tropias, exophorias, or convergence in-

sufficiency type X(T)s (13). (These latter

types will not be discussed in this review.)

Incidence

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 exotrop-

ic. 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 high-

est 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%.

Progression

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 re-

ported 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 accom-

modation and an increase in both the fre-

quency and size of the near deviation.

These fmdings support Burian & Frances-

chetti’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. Thir-

ly-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. Fif-

ty-two percent actually had smaller exo-

deviations 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 con-

stant exotropia in childhood”. Friendly (26)

has reported that Hiles et al used occlu-

sion on some of the X(Y)s who they fol-

lowed 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).

II.

SENSORY-MOTOR FINDINGS

Motor Findings

Accommodative convergence / accom-

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 arbi-

trary difference of 10 pd between distance

and near (10).

By this last definition, if one assumes a

188