| One and a half syndrome | |
|---|---|
| Other names | On-and-a-half syndrome |
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| Diagram of normal eye movement compared to left one-and-a-half syndrome (i.e. left lateral gaze palsy, with leftInternuclear ophthalmoplegia (inability to adduct)) | |
| Causes |
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| Differential diagnosis | |
The one and a halfsyndrome is a rareweakness in eye movement affecting both eyes, in which one cannot move horizontally at all, and the other can move only in outward direction. More formally, it is characterized by "a conjugate horizontalgaze palsy in one direction and aninternuclear ophthalmoplegia in the other".[1][2]Nystagmus is also present when the eye on the opposite side of the lesion is abducted. Convergence is classically spared as cranial nerve III (oculomotor nerve) and its nucleus is spared bilaterally.
Causes of the one and a half syndrome includepontine haemorrhage,ischemia,tumors, infective mass lesions such astuberculomas, demyelinating conditions likemultiple sclerosis,Arteriovenous malformation, Basilar artery aneurysms and Trauma.[3]
The syndrome usually results from single unilateral lesion of theparamedian pontine reticular formation and the ipsilateralmedial longitudinal fasciculus. An alternative anatomical cause is a lesion of the abducens nucleus (VI) on one side (resulting in a failure of abduction of the ipsilateral eye and adduction of the contralateral eye = conjugate gaze palsy towards affected side), with interruption of the ipsilateralmedial longitudinal fasciculus after it has crossed the midline from its site of origin in the contralateral abducens (VI) nucleus (resulting in a failure of adduction of the ipsilateral eye).[citation needed]
There have been cases of improvement in extra-ocular movement withbotulinum toxin injection.[4][5] Cases related tomultiple sclerosis sometimes subside with adequate treatment.[3]