Internuclear ophthalmoplegia involves paralysis of one or more of the extraocular muscles. It is often abbreviated to INO. Its features are a reduction or total loss of ADDuction of one eye and gaze evoked nystagmus of the ABDucting eye. The lesion is in the medial longitudinal fasciculus which is often abbreviated to MLF.

The MLF is the pathway that links the VIN on one side with the IIIN on the other. For example, the lateral rectus (LR) on the RIGHT side of the brainstem links directly with the medial rectus (MR) on the LEFT side of the brainstem through the MLF. This allows the eyes to move (conjugately) to the RIGHT. An MLF lesion means that the link between LR on one side and the MR on the other side of the brainstem is broken and ADDuction is affected. Saccadic, smooth pursuit and vestibulo-ocular reflex systems all affected. It can be partial or total and unilateral or bilateral.

If the affected eye is used to fixate in laevoversion for a right sided INO, extra innervation is sent to the affected MR, this results in excessive innervation of the contralateral LR which results in over action. Simultaneously, the convergence system is being innervated hence the ABDucting nystagmus (caused by the lesion). Bilateral INO in patients less than 50 years is most commonly associated with multiple sclerosis. In patients over 50 years the cause is more likely to be vascular. Unilateral INO in older patients is also most commonly vascular in origin (small vessel occlusion).

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