(1) Isolated medial rectus paralysis without involvement of other muscles supplied by the third nerve is very rare.
(2) In the foreground of diagnostic features is a variable exotropia that decreases or may be completely absent when the affected eye is in abduction and increases exponentially as the paralyzed eye attempts adduction (secondary deviation).
(3) A head turn toward the nonparetic side may allow the patient to attain single binocular vision.
(4) Isolated medial rectus paralysis may be associated with ptosis and upper lid retraction on attempted adduction (aberrant regeneration) in patients with partial third nerve paralysis (see 2.36). A ptosis with aberrant regeneration, causing the lid to lift when the paralyzed eye attempts to adduct, is treated by resecting the contralateral medial and recessing the contralateral rectus muscles.42
(5) Medial rectus paralysis must be distinguished from internuclear ophthalmoplegia (INO), caused by lesions in the medial longitudinal fasciculus. In this condition unilateral or bilateral limitation of adduction is associated with nystagmus of the abducting eye. Convergence may or may not be normal.
(6) A clinical picture similar to INO or medial rectus paralysis may be simulated by myasthenia gravis. A Tensilon test may be indicated (see 2.56).
(7) Postoperative medial rectus paralysis after surgery on the medial rectus muscle may be caused by a "slipped" or "lost" muscle.24, p.426
(8) Surgery may be done on the affected eye or may be divided between the paralyzed and the sound eye and consists of resection of the paralyzed medial rectus and recession of its yoke muscle, the lateral rectus of the sound eye, or recession of the ipsilateral lateral rectus muscle. In the case of complete paralysis with exotropia in the primary position and with the head passively straightened, or when a "lost" muscle cannot be located, a full tendon transfer of the vertical recti to the insertion of the medial rectus muscle may be indicated.
(9) See 2.55.
(10) Restriction of adduction may be caused by an excessively resected lateral rectus muscle.