internuclear ophthalmoplegia

A supranuclear disorder caused by a lesion in the medial longitudinal fasciculus (MLF) of the brainstem resulting in impaired adduction of the ipsilateral eye and nystagmus of the contralateral abducting eye during horizontal gaze. It may be unilateral or bilateral and is often associated with other brainstem or neurological signs depending on the location of the MLF lesion. INO helps localize the site of dysfunction to the MLF pathway and differentiates from other causes of horizontal gaze palsy like abducens nerve palsy.