One and a half syndrome
The one and a half syndrome is a rare weakness in eye movement affecting both eyes, in which one cannot move laterally at all, and the other can move only in outward direction. More formally, it is characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other". Nystagmus is also present when the eye on the opposite side of the lesion is abducted. Convergence is classically spared as cranial nerve III and its nucleus is spared bilaterally.Causes of the one and a half syndrome include pontine hemorrhage, ischemia, tumors, infective mass lesions such as tuberculomas, and demyelinating conditions like multiple sclerosis.Anatomy
The syndrome usually results from single unilateral lesion of the paramedian pontine reticular formation and the ipsilateral medial longitudinal fasciculus. An alternative anatomical cause is a lesion of the abducens nucleus on one side, with interruption of the ipsilateral medial longitudinal fasciculus after it has crossed the midline from its site of origin in the contralateral abducens nucleus.Treatment
There have been cases of improvement in extra-ocular movement with botulinum toxin injection.
