abducens nerve

The abducens nerve is the sixth cranial nerve, and it carries only somatic efferent fibers to a single muscle, the lateral rectus.¹ The abducens nucleus lies in the pons just ventral to the floor of the fourth ventricle, and the fascicle of the facial nerve wraps around the nucleus.² The nerve leaves the brainstem at the junction of the pons and medulla and ascends through the subarachnoid space along the clivus. The abducens nerve then crosses the petrous apex beneath the petroclinoid ligament, passes through Dorello's canal, and runs through the cavernous sinus lateral to the internal carotid artery. The nerve enters the orbit through the superior orbital fissure, within the annulus of Zinn, to reach the lateral rectus.²

Abducens palsy causes horizontal diplopia that is worse in gaze toward the affected side and at distance, with an esotropia that is largest in that direction of gaze.³ The nerve's long intracranial course, tortuosity, and tight attachment to the skull base make it vulnerable, and palsy has a wide range of causes in children and adults.³ Damage to the abducens nucleus produces an ipsilateral gaze palsy instead, because about 40% of the nucleus's neurons are interneurons that drive the contralateral medial rectus through the medial longitudinal fasciculus.² An abduction deficit without contralateral adduction weakness therefore cannot arise from the nucleus.²