One and a half syndrome

One and a half syndrome
Other names	On-and-a-half syndrome
Diagram of normal eye movement compared to left one-and-a-half syndrome (i.e. left lateral gaze palsy, with left Internuclear ophthalmoplegia (inability to adduct))
Causes	Demyelinating disease, e.g. Multiple sclerosis Infarction Infections, e.g. Brainstem encephalitis and Neurocysticercosis Tumours Arteriovenous malformation Pontine haemorrhage Basilar artery aneurysms Trauma
Differential diagnosis	Neuromyelitis optica spectrum disorder Myasthenia Gravis

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".^[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.