Page:Handbook of Ophthalmology (3rd edition).djvu/160

154 gradually, during recovery from the paralysis, passing into concomitant strabismus. Diplopia is generally present in all cases of strabismus convergens which develop later than childhood.

The most frequent form of strabismus convergens is the stationary monolateral squint. One and the same eye is always used for fixation, while the other remains turned inward.

We have before said that in general the angle between the visual axis of the squinting eye and its normal direction remains constant as well during associated movements as during the secondary squint, which affects the eye usually employed for fixation when the other is so employed. This rule must not be regarded as absolute; for aside from certain fluctuations which it exhibits in the same individual within short periods of time, it generally increases during fixation of near objects and when the squinting eye is turned outward. The mobility of the eyes does not by any means remain unchanged in strabismus convergens; as a rule, the movement outward is less and that inward greater than normal. This limitation of movement may be divided symmetrically between the two eyes, but oftener the lateral movement of the squinting eye is noticeably less than that of the eye used for fixation; this causes an increase of the squinting angle when looking toward the side of the squinting eye.

Often, too, the associated deviation occurring upon change of fixation is observed to be stronger than the primary squint. Particularly if there be hypermetropia on both sides, but with difference in refraction, the squinting angle is always greater when the eye with the higher degree of hypermetropia is used for fixation than when the other is used. The stronger tension of accommodation which the more hypermetropic eye requires, causes immediately an increase of convergence. This symptom is so characteristic that one may diagnose from it, both the existence of different refraction in the two eyes and which eye is the more hypermetropic, even when the hypermetropia is latent in both eyes.

Alternating strabismus convergens is comparatively rare. Even when vision is so good in both eyes that either may be used for fixation, the eyes are but seldom used alternately. Slight differences in the acuity of vision generally determine the habitual use of one particular eye.