Correct! The eye movement abnormalities here may be subtle, but if you look carefully, you will see that the adducting eye is lagging behind the
abducting eye, and that the abducting eye has a jerk nystagmus in extremes of horizontal gaze. This is a common presentation of internuclear ophthalmoplegia (INO), a lesion of the
medial longitudinal fasciculus (MLF).
Because the MLF connects the sixth nerve nucleus to the third nerve nucleus, a lesion here will impair adduction during horizontal gaze. In the severest lesions, the eye on the
affected side will not adduct at all and the eye on the side of the INO will be outwardly deviated (exotropia). Your challenge, of course, is to detect these milder forms. In young
patients, the likely cause is MS; in older adults, it is stroke. Many other types of lesions can cause INO. Tip: when INO is bilateral (it is here, although subtle), you can bet
on MS as the most likely cause! Trap: a similar eye movement abnormality can occur in myasthenia gravis. In that case, the term “pseudo INO” is applied.
