Internuclear Ophthalmoplegia (INO)
Internuclear ophthalmoplegia, or medial longitudinal fasciculus syndrome, consists of ipsilateral weakness of eye adduction with contralateral nystagmus of the abducting eye (ataxic or dissociated nystagmus), but with preserved convergence. This may be obvious with pursuit eye movements, but is better seen when testing reflexive saccades or optokinetic responses when the adducting eye is seen to "lag" behind the abducting eye. INO may be asymptomatic or, rarely, may cause diplopia, oscillopsia, or a skew deviation. INO may be unilateral or bilateral. The eyes are generally aligned in primary gaze, but if there is associated exotropia this may be labeled wall-eyed monocular/bilateral internuclear ophthalmoplegia (WEMINO, WEBINO syndromes).
The most common cause of INO by far is demyelination, particularly in young patients, but other causes include cerebrovascular disease (particularly older patients), Wernicke-Korsakoff syndrome, encephalitis, trauma, and paraneoplasia.
A similar clinical picture may be observed with pathology elsewhere, hence a "false-localizing " sign and referred to as a pseudointernuclear ophthalmoplegia (q.v.), especially in myasthenia gravis.
Zee DS. Internuclear ophthalmoplegia: pathophysiology and diagnosis. In: Büttner U, Brandt Th. Ocular motor disorders of the brain stem. London: Baillière Tindall, 1992: 455-470