Internuclear ophthalmoplegia

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An internuclear ophthalmoplegia (INO) is an ocular motility disorder due to a lesion involving the medial longitudinal fasciculus (MLF).(1-2) The primary function of the MLF is to synchronize adduction and abduction on lateral gaze. Lesions of the MLF disrupt communication between the nucleus of CN VI in the pontine paramedian reticular formation, which initiates lateral gaze, and the nucleus of CN III in the midbrain, hence internuclear (Video). Lateral gaze results in abduction of the ipsilateral eye, but no adduction of its fellow. Typically the abducting eye also has nystagmus (dissociated nystagmus because it is different in the two eyes). By convention, the INO is labeled by the side of the adduction failure. Failure to adduct is an isolated abnormality in the affected eye; normality of the lid and pupil distinguish an INO from a CN III palsy. Some patients have total adduction failure and some may have exotropia in primary gaze.

The most common causes of an INO are MS and brainstem stroke.(3,5,6) INOs due to MS are usually bilateral and seen in young patients; those due to brainstem vascular disease are more often unilateral and seen in older patients. About 25% are due to unusual causes such as trauma or tumor.(4) This patient has INOs bilaterally, worse on the right. On left gaze, adduction of the right eye is severely limited and there is nystagmus in the left eye. MRI showed multiple areas of demyelination, including a lesion in the right MLF.


1. Campbell WW. Clinical signs in neurology : a compendium. Philadelphia: Wolters Kluwer Health, 2016.

2. Campbell WW. DeJong's the neurologic examination, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2013.

3. Hassen GW, Bhardwaj N. Images in clinical medicine. Bilateral internuclear ophthalmoplegia in multiple sclerosis. N Engl J Med. 2013 Jan 17;368(3):e3.

4. Keane, JR. Internuclear ophthalmoplegia: unusual causes in 114 of 410 patients. Arch Neurol. 2005; 62:714-717.

5. McNulty JP, Lonergan R, Bannigan J, O'Laoide R, et al. Visualisation of the medial longitudinal fasciculus using fibre tractography in multiple sclerosis patients with internuclear ophthalmoplegia. Ir J Med Sci. 2016 May;185(2):393-402.

6. Saini M, Bte Idu Jion Y. Periprocedural stroke presenting as isolated unilateral internuclear ophthalmoplegia. QJM. 2015 Feb;108(2):165.