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Esotropia manifests itself externally by an inward deviation of one cornea, or the inclination of the visual axes toward each other, so that they cross at some point inside of infinity; or more correctly speaking, the visual axis of one eye is directed toward that of the other.3

Etiology.—The factors that individually or in combination produce, esotropia are:

(a) Refractive errors (generally H and Ah conditions).

(b) A congenitally weak fusion-faculty.

(c) Congenital paresis of one or both external recti (to be distinguished from true congenital abducens palsy).

(d) Obstetric injuries to the eyes.

Refractive Errors.—The usual refractive status in most subjects of esotropia is that of compound hypermetropic astigmatism. The error need not necessarily be high. Many esotropic children do not manifest more than 2 diopters of error. In the cases that are solely due to the refractive error the correction of the abnormal refractive status with suitable glasses generally causes the strabismus to rapidly disappear. This variety conforms absolutely to Donder's theory as to the dependence of the deviation on the abnormal convergence stimulus set up by the refractive error, and may properly be called accommodative esotropia. Not all cases of esotropia, however, are explainable on this basis. Ninety-six per cent, of all children are born hypermetropic and the question may justifiably be put, "Why do not all hypermetropic children show esotropia?" Some other factor must, therefore, be operative and the most probable one is a Congenitally Weak Fusion Faculty.

1 Synonyms.—Internal or convergent strabismus; internal squint.

2 The student is urged to read the chapters on "The Evolution of Binocular Vision" and "The Relation between Accommodation and Convergence" before entering upon the study of esotropia.

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