Isolated inferior rectus muscle palsy resulting from a nuclear third nerve lesion as the initial manifestation of multiple sclerosis

Isolated inferior rectus muscle (IRM) palsy due to a nuclear or fascicular third nerve lesion is uncommon (1-4), and third cranial nerve palsy as the presenting sign of multiple sclerosis (MS) is rare (5-9). We report an unusual patient with an isolated IRM palsy caused by a demyelinating lesion in the midbrain seen on magnetic resonance imaging (MRI).