A 25-Year-Old Man With Diplopia and Loss of Body Control

Sumaira Nabi, MBBS, FCPS; Shahzad Ahmed, MBBS; Fateen Rashid, MBBS; Mazhar Badshah, MBBS, FCPS

Disclosures

September 03, 2020

Discussion

Internuclear ophthalmoplegia (INO) is an ocular movement disorder that presents with horizontal diplopia. INO occurs secondary to damage to the interneurons that connect two nuclei of cranial nerves VI and III, resulting in dysfunction of conjugate horizontal gaze with restriction or loss of adduction in one eye, usually with associated nystagmus in the contralateral abducting eye.[1] Convergence remains preserved in most cases.

The presentation localizes to a myelinated interneuron pathway, the medial longitudinal fasciculus (MLF).[2] The MLF consists of interneurons that connect the contralateral sixth cranial nerve nucleus/paramedian pontine reticular formation (PPRF) complex in the dorsomedial pons to the ipsilateral medial rectus subnucleus of the third cranial nerve in the midbrain.[3] INO results from damage to the white-matter fibers of the MLF within the dorsomedial pons or the tegmentum of midbrain. The MLF exists as a pair of tracts in close proximity to each other in the midline of the above-mentioned brainstem structures; this often leads to bilateral pathology.

Patients experience horizontal diplopia and dysconjugate gaze upon examination. Some patients may remain asymptomatic, whereas others may experience blurring of vision. Rare manifestations include vertigo, visual confusion, oscillopsia, fatigue while reading, loss of depth perception, falls, and accidents while driving.[4]

INO is localized to the side of the adduction impairment, ipsilateral to the MLF lesion. The contralateral abducting eye frequently demonstrates horizontal nystagmus, likely an adaptive phenomenon in response to the primary adduction weakness in the affected eye.[5] This is best supported by the fact that the medial rectus of one eye exists as a yoke pair with the lateral rectus of the opposite eye in order to maintain smooth conjugate eye movement.

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