What are the signs and symptoms of internuclear ophthalmoplegia (INO in vertebrobasilar stroke?

Updated: Aug 09, 2021
  • Author: Vladimir Kaye, MD; Chief Editor: Stephen Kishner, MD, MHA  more...
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Clinically, internuclear ophthalmoplegia (INO) is a horizontal gaze palsy; it results from a brainstem lesion affecting the MLF between the nuclei of CN VI and III, most commonly in the pons. (See the image below.)

Lesion of the medial longitudinal fasciculus (MLF) Lesion of the medial longitudinal fasciculus (MLF) resulting in internuclear ophthalmoplegia (INO). (Courtesy of BC Decker Inc.)

When a patient with a lesion in the right MLF attempts to look to his/her left (ie, away from the involved side), he/she shows no adduction of the right eye and full abduction of the left eye with the end-point abduction nystagmus.

By the same logic, in the case of bilateral INO, there is no adduction to either side with nystagmus of the abducting eye in both directions. Convergence is preserved, because the nuclei of CN III and peripheral innervation of the medial recti muscles are intact.

Because horizontal gaze requires coordinated activity of the ipsilateral CN III and contralateral CN VI (relative to the lesion), disruption of the communication pathway (ie, the MLF) between the nuclei of CN III (in the midbrain) and CN VI (in the pons) results in the inability of the eye ipsilateral to the lesion to adduct and the contralateral eye to exhibit abduction nystagmus when looking away from the involved side.

In elderly patients, INO is caused most often by occlusion of the basilar artery or its paramedian branches. In younger adults, it may occur due to multiple sclerosis (MS), commonly with bilateral involvement.

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