Internuclear ophthalmoplegia (INO) is a specific gaze deficit caused by lesions to the medial longitudinal fasciculus (MLF) of the dorsomedial brain stem. It presents with horizontal eye movement deficits characterized by poor adduction in the affected eye and abduction nystagmus in the contralateral eye. Occasionally INO can be present bilaterally. Smooth binocular vision and the ability to visually track require synchronized ocular movements. The MLF coordinates neuronal signaling between cranial nerves III, IV, and VI to achieve synchronous eye movements. The majority of INO cases are associated with multiple sclerosis, although mass lesions, trauma, and cerebrovascular disease are potential causes as well. The diagnosis of INO is made by neurologic examination, specifically assessing horizontal eye movement. Findings can be subtle and may require specialized neurologic testing.
Internuclear Ophthalmoplegia. Internuclear ophthalmoplegia deficits are present bilaterally and are a common presentation in multiple sclerosis patients. Horizontal gaze is impaired, with poor adduction seen with leftward gaze (A) and rightward gaze (B). (Photo contributor: David Effron, MD.)
Video 02-06: Internuclear Ophthalmoplegia (INO)
(Video Contributor: David Effron, MD)
Management and Disposition
Depending on the underlying cause, symptoms may resolve over months or persist indefinitely. Eye patching may be used to provide symptom relief from diplopia. For patients with multiple sclerosis, treatment of the underlying demyelinating disorder may improve symptoms. All patients with INO require neurologic consultation and MRI. An acute ischemic stroke as a potential cause for new-onset INO should be investigated and may require hospital admission.
INO is the most common ocular movement abnormality in multiple sclerosis.
Ischemic infarction can result in INO and should be investigated with a complete stroke evaluation, especially in older patients.
Bilateral INO is commonly secondary to multiple sclerosis.