grate two totally dissimilar objects. Therefore, suppression is a normal physiological phenomenon.

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 binocular 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 common in CI, and probably serve as a sensory adaptation to eliminate diplopia and/or overlapping of field; confusion, and/or symptoms by creating functional monocularity. Therefore, the more severe the CI and the longer it has been manifest, the greater the probability of suppression with a resultant lack of symptoms.

It should be remembered that reading is one of the only instances 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 material itself may be a poorer stimulus for binocular alignment than stereo stimuli. This, therefore, may account for CIs experiencing symptoms while reading, but not while performing other near tasks.

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 similar to the findings of refractive error in the normal population.

Though the exact relationship

of CI to learning has not been established, it has been implicated as a causative factor .Eames^17,18,19 in studies comparing 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 observed numerous children who have demonstrated better attention, better concentration, less asthenopia and a better ability to sit and read after the remediation of a manifest CI. However, one must not assume that CI is the cause of learning disabilities nor responsible for severe learning problems. However, the authors have noted improved classroom performance of children after remediation of objective CI where no subjective symptoms have been manifest.

 

Etiology

Duke-Elder’² lists the following as causes of CI: wide interpupillary distance, delayed development or poorly developed accommodation or convergence, presbyopia, 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 secondary to systemic disorders which include: head trauma, encephalitis, drug intoxication, malnutrition, debility, hepatitis, and mononucleosis. The implication that CI is due to weak eye muscles or other mechanical difficulties has not been demonstrated. As a matter of fact, Davies^4’ has stated that the cause of CI is “not a question of weak eye muscles, but the result of a breakdown of the normal reflex action between accommodation and convergence.” Davies also cites the following systemic causes of CI: sinusitus and/or dental infections.

Jampoisky⁴² feels that CI is most often the result of poor accommodation. 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 significant cause of CI. He- has described five types of anemia which may result in CI. These include:

found in high flying, mountain climbing, overcrowded rooms, and heat.

“The authors have observed numerous children who have demonstrated better attention, better concentration, less asthenopia and a better ability to sit and read after the remediation of a manifest convergence insufficiency.”

lack of vitamins, minerals, or amino acids which are necessary for respiration.

‘c) demand anoxia due to excessive work or stress.

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 tobacco 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 orthoptic treatment.

As stated earlier, other authors feel that CI is psychogenic.^{1,14,20,26,41,46} Only two authors have evaluated the relationship 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 tendencies. Furthermore, high correlations do not imply cause and effect. Mann,’⁹ in support of the psychogenic component, states that there are patients with CI,

676 Journal of the American Optometric Association