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 (oculomotor nerve) and its nucleus is spared bilaterally.

Causes
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 (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 ipsilateral medial 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).

Treatment
There have been cases of improvement in extra-ocular movement with botulinum toxin injection.

See also
 Internuclear ophthalmoplegia

References

Category:Disorders of ocular muscles, binocular movement, accommodation and refraction
Category:Syndromes