More formally, it is characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other".

Causes of the one and a half syndrome include pontine haemorrhage, ischemia, tumors, infective mass lesions such as tuberculomas, demyelinating conditions like multiple sclerosis, Arteriovenous malformation, Basilar artery aneurysms and Trauma.

[3] 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).

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