The syndrome consists of an internuclear ophthalmoplegia combined with a lateral gaze palsy to the same side.
(1) The ipsilateral eye lies fixed in the midline.
(2) The contralateral eye is able to abduct but not adduct.
(3) The contralateral eye shows jerk nystagmus when it is fully abducted.
The condition is caused by a unilateral lesion of the dorsal pontine tegmentum that involves:
(1) the ipsilateral paramedian pontine reticular formation
(2) the internuclear fibers of the ipsilateral medial longitudinal fasciculus crossing from the contralateral sixth cranial nerve nucleus
(3) the ipsilateral abducens nucleus
Clinical signs and symptoms:
(1) diplopia
(2) visual sensation of objects oscillating (oscillopsia)
(3) blurred vision
Conditions that can cause the one-and-a-half syndrome:
(1) primary brain tumor in the brainstem
(2) metastatic tumor to the brainstem
(3) surgery in the posterior fossa
(4) pontine infarct
(5) multiple sclerosis
(6) arteriovenous malformation involving the brainstem
Differential diagnosis (pseudo-one-and-a-half syndrome):
(1) Gullain Barre syndrome
(2) myasthenia gravis
