An isolated paralysis of the medial rectus muscle without involvement of other muscles supplied by cranial nerve III is very rare. With this type of paralysis the greatest defect of ocular motility occurs when the affected eye moves into adduction. Since the action of the antagonistic lateral rectus muscle is unopposed, an exotropia usually is present in primary position (Fig. 20–15). The patient’s face turns toward the uninvolved side. The differential diagnosis of an isolated medial rectus paralysis includes internuclear ophthalmoplegia (see p. 441).
FIGURE 20–15. Right medial rectus paralysis. Exotropia in primary position and overaction of left lateral rectus muscle.
A rare, bizarre phenomenon referred to as synergistic divergence, which consists of a congenital adduction deficit with simultaneous abduction of each eye on attempted gaze into the field of action of the paralyzed medial rectus muscles, has been described in patients with congenital medial rectus paralysis.14; 36; 45, p.246; 280The etiology is unknown although innervational anomalies similar to those found in Duane’s syndrome have been implicated.52, 96Extirpation of the ipsilateral lateral rectus muscle has been suggested to eliminate simultaneous abduction.
Other substitution phenomena in congenital andacquired supranuclear disorders of eye movements have been described.39