grate two totally dissimilar ob-

jects. Therefore, suppression is a

normal physiological phenome-

non.

On the other hand, the total

loss of stereopsis has only been

seen in patients demonstrating a

constant strabismus, amblyopia,

or some other major binocular

anomaly. In fact. stereopsis may

be a strong stimulus for binocu-

lar alignment of the eyes. It is

entirely rare to find a CI who

shows a decrement in stereo

acuity. The authors have only

observed one CI who showed a

complete absence of stereopsis.

Suppressions, however, are com-

mon in CI, and probably serve as

a sensory adaptation to elimi-

nate diplopia and/or overlapping

of field; confusion, and/or symp-

toms by creating functional

monocularity. Therefore, the

more severe the CI and the

longer it has been manifest, the

greater the probability of sup-

pression with a resultant lack of

symptoms.

It should be remembered that

reading is one of the only in-

stances where a person views a

flat fusion stimulus. The loss of

disparity cues in reading may

serve not only as a mechanism

for suppression, but reading ma-

terial itself may be a poorer stim-

ulus for binocular alignment

than stereo stimuli. This, there-

fore, may account for CIs expe-

riencing symptoms while read-

ing, but not while performing

other near tasks.

G.

Refractive Error

There is no correlation between

refractive error and Cl.^9,13,35,36

Passmore and MacLean’8 found

52% of their CI population was

hyperopic, 34% myopic, and 14%

was emmetropic. Smith³⁶ at-

tempted to correlate refractive

error and CI in a population of

473 CIs. He found 38% were low

myopes; 57% were emmetropes

(1 D from Plano), and 5% were

hyeropes (greater than 1 D). In

another study, Hirsch’⁷ found

61% of CIs had ametropia of .75

D or less. These figures are simi-

lar to the findings of refractive

error in the normal population.

H.

Relationship to Learning

Though the exact relationship

of CI to learning has not been

established, it has been impli-

cated as a causative factor.

Eames^17,18,19in studies com-

paring good readers to poor

readers, found that Cl was more

prevalent in the group of poor

readers. Similar findings have

been reported by Park and

Burr40 The authors have ob-

served numerous children who

have demonstrated better atten-

tion, better concentration, less

asthenopia and a better ability

to sit and read after the remedia-

tion of a manifest Cl. However,

one must not assume that CI is

the cause of learning disabilities

nor responsible for severe learn-

ing problems. However, the au-

thors have noted improved class-

room performance of children af-

ter remediation of objective CI

where no subjective symptoms

have been manifest.

Etiology

Duke-Elder’² lists the following

as causes of CI: wide interpupil-

lary distance, delayed develop-

ment or poorly developed accom-

modation or convergence, pres-

byopia, disease or debility which

alters the metabolic state of the

blood supply to the extra ocular

musculature, toxemia, endocrine

disorders, and anxiety neurosis.

Raskind³³ said there are CIs sec-

ondary to systemic disorders

which include: head trauma, en-

cephalitis, drug intoxication,

malnutrition, debility, hepatitis,

and mononucleosis. The implica-

tion that CI is due to weak eye

muscles or other mechanical dif-

ficulties has not been demonstra-

ted. As a matter of fact, Davies⁴’

has stated that the cause of CI is

“not a question of weak eye mus-

cles, but the result of a break-

down of the normal reflex action

between accommodation and

convergence.” Davies also cites

the following systemic causes of

CI: sinusitus and/or dental infec-

tions.

Jampolsky⁴² feels that CI is

most often the result of poor ac-

commodation. The authors agree

with Jampolsky, but caution

that one must not ignore low

positive fusional reserves in the

treatment of CI.

Sasaki feels anemia is a sig

nificant cause of CI. He- has

described five types of anemia

which may result in CI. These in-

clude:

a)

atmospheric anoxia as

found in high flying, mountain

climbing, overcrowded rooms,

and heat.

“The authors have ob-

served numerous chil-

dren who have demon-

strated better attention,

better concentration,

less asthenopia and a

better ability to sit and

read after the remedia-

tion of a manifest con-

vergence insufficiency.”

b)

metabolic anoxia due to

lack of vitamins, minerals, or

amino acids which are necessary

for respiration.

‘c) demand anoxia due to ex-

cessive work or stress.

d)

oculoneurogenic anoxia

e) menstrual or pre-menstrual

anoxia due to sympatheticontia.

Sasaki⁴⁴ has presented case re-

ports of patients who as a result

of heavy tobacco use, developed

anoxia with a resultant CI and

asthenopia. Elimination of to-

bacco use resulted in immediate

recovery from the symptoms.

CI may rarely result from head

trauma incurred in automobile

accidents or gun shot wounds.

According to Chandler⁴⁵ these

patients will respond to orthop-

tic treatment.

As stated earlier, other au-

thors feel that CI is psycho-

genic.^{1,14,20,26,41,46} Only two au-

thors have evaluated the rela-

tionship between psychological

problems and Cl.^20,35 Mellick³⁵

found that 76% of his sample of

63 CIs demonstrated neurotic

tendencies. However, he does not

discuss how he assessed or

measured these neurotic tenden-

cies. Furthermore, high correla-

tions do not imply cause and ef-

fect. Mann,’⁹ in support of the

psychogenic component, states

that there are patients with CI,

676 Journal of the American Optometric Association