A 25-Year-Old Man With Painless Diplopia

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


August 09, 2016


INO is one of the ophthalmologic manifestations of neurologic disorders. It is a dysfunction of conjugate horizontal gaze with restriction or loss of adduction in the diseased eye, usually with associated nystagmus in the contralateral abducting eye.[1] Convergence remains preserved in most cases.

Patients experience horizontal diplopia in cases of significant restriction of gaze. Right-sided INO results in diplopia upon attempted left lateral gaze, and left-sided INO results in diplopia upon attempted right lateral gaze. 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.[2]

The lesion has characteristic localization to the medial longitudinal fasciculus (MLF).[3] The MLF consists of interneurons that connect the contralateral medial rectus subnucleus of the third cranial nerve in the midbrain to the ipsilateral sixth cranial nerve nucleus/paramedian pontine reticular formation (PPRF) complex in the dorsomedial pons.[4] 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.

INO is localized to the side of the adduction impairment, which in turn is ipsilateral to the MLF lesion. The cause of the nystagmus in the abducting eye is debated. However, most consider it to be 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.

Older studies proposed that INO can be categorized as anterior INO or posterior INO, on the basis of involvement of adduction or abduction. Impulses from the medial rectus subnuclei were thought to pass through the anterior portion of the medial longitudinal fasciculus, whereas the signals to the lateral rectus nuclei traverse through the posterior part.[6] This may merely be a clinical assumption that has yet to be proved scientifically.


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