The one-and-a-half syndrome consists of an ipsilateral horizontal gaze palsy and an ipsilateral internuclear ophthalmoplegia, such that the only preserved horizontal eye movement is abduction in one eye; vertical movements and convergence are spared. This results from a brainstem lesion which involves both the abducens (VI) nerve nucleus or paramedian pontine reticular formation, causing ipsilateral horizontal gaze palsy, and the adjacent medial longitudinal fasciculus, causing internuclear ophthalmoplegia. In young patients this is most often due to demyelination, in the elderly to brainstem ischemia; brainstem arteriovenous malformation or tumor may also be responsible.
Myasthenia gravis may cause a pseudo-one-and-a-half syndrome. A vertical one-and-a-half syndrome has also been described, characterized by vertical upgaze palsy, and monocular paresis of
downgaze, either ipsilateral or contralateral to the lesion.
Pierrot-Deseilligny C, Chain F, Serdaru M et al. The "one-and-a-half " syndrome. Electro-oculographic analyses of five patients with deductions about the physiological mechanisms of lateral gaze. Brain 1981; 104: 665-699 Wall M, Wray SH. The one-and-a-half syndrome. A unilateral disorder of the pontine tegmentum: a study of 20 cases and a review of the literature. Neurology 1983; 33: 971-980