Occurs when medial longitudinal fasciculus (MLF) on one side is damaged.
- Older pts – think Brainstem stroke, tumour, infection, inflammation
- Younger pts – think Multiple sclerosis
- Wernicke’s encephalopathy
- Pernicious anaemia
Loss of ADDuction of the SAME eye on the LATERAL gaze to the other side.
Symptoms – diplopia (side gaze) ; oscillopsia ; brainstem disease symp (vertigo, limb weakness).
- UNILATERAL INO – exotropia (primary position) ; decreased ADDuction on same eye (convergence – adduction improves) ; adducting nystagmus of other eye
- BILATERAL INO – LARGE exotropia ; decreased ADDuction in both eyes ; WEBINO (wall-eyed binocular INO)
Differentials: Partial CN 3 palsy or myasthenia gravis
- MS – resolves itself
- Urgent referral to neurologist
- MRI brain + contrast