Internuclear ophthalmoplegia

What it is

Internuclear ophthalmoplegia, written INO, is a classic neuro-ophthalmologic finding caused by a lesion in the medial longitudinal fasciculus, or MLF, a brainstem pathway that coordinates eye movements. It results in inability to adduct the eye on the side of the lesion. Because the lesion sits in the pathway that yokes the two eyes together, and not in one nerve, the two eyes move differently on the same attempted gaze.

How it presents

Three findings make it up. Adduction of the eye is impaired on attempted lateral gaze, and this eye is the one ipsilateral to the lesion. The other eye abducts with nystagmus, so the nystagmus is contralateral. Convergence is usually spared, because it uses a different pathway. A worked example fixes the sides: a right INO means impaired adduction of the right eye on left gaze, while the contralateral eye abducts with nystagmus. Nystagmus here is a part of the INO itself, not a separate sign.