If in a case of orthophoria a single muscle becomes paretic, deviation in the primary position is always present, being latent or manifest according to the intensity of the paresis and the function of the paretic muscle. A latent or only temporarily manifest deviation results if, for instance, the internal rectus is weakened, since paretic divergence of not too high a degree can be overcome by a convergence impulse. But even the slightest paresis of one of the elevators or depressors cannot be kept latent, because the vertical range of fusion is very small. That is true only of cases of orthophoria. In a case of heterophoria the paretic deviation is modified by the anomalous position of rest—increased or diminished, or even completely abolished or compounded of various components, as will be shown presently in discussing the palsies of individual muscles.
In any case of paresis the position of rest
BIELSCHOWSKY A. LECTURES ON MOTOR ANOMALIES OF THE EYES: II. PARALYSIS OF INDIVIDUAL EYE MUSCLES. Arch Ophthalmol. 1935;13(1):33–59. doi:10.1001/archopht.1935.00840010043006
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