Abstract

A brainstem lesion of any type that involves the medial longitudinal fasciculus (MLF) can cause internuclear ophthalmoplegia (INO). This primarily affects conjugate horizontal gaze and classically manifests as impaired adduction ipsilateral to the lesion and abduction nystagmus contralateral to the lesion. Here, we describe the anatomy of the MLF and review the clinical features of INO. We also describe conjugate horizontal gaze palsy and some of the ‘INO-plus’ syndromes.