Cranial nerve VI, also known as the abducens nerve, innervates the ipsilateral lateral rectus (LR), which functions to abduct the ipsilateral eye. The sixth cranial nerve has a long subarachnoid course. The sixth nerve nucleus is located in the pons, just ventral to the floor of the fourth ventricle and just lateral to the medial longitudinal fasciculus (MLF). About 40% of its neurons project into the ipsilateral MLF only to cross over to the contralateral side and ascend to innervate that contralateral medial rectus subnucleus to participate in contralateral eye adduction.
The abducens nerve emerges from the brainstem at the pontomedullary junction to enter the subarachnoid space, coursing upward between the pons and clivus to enter the Dorello canal. At the petrous apex, it angulates to enter the cavernous sinus and travels in close proximity to the internal carotid artery. The abduces nerve then proceeds through the superior orbital fissure and innervates the lateral rectus muscle.
Any pathology which leads to downward pressure on the brainstem (e.g. brain tumour, extradural haematoma) can lead to the nerve becoming stretched along the clivus of the skull. Wernicke-Korsakoff syndrome (caused by thiamine deficiency and generally seen in alcoholics) is a rare cause of sixth nerve palsy.
Other causes of abducens nerve damage include diabetic neuropathy and thrombophlebitis of the cavernous sinus – in these cases, it is rare for the abducens nerve to be affected in isolation.
Congenital sixth nerve palsy (Duane syndrome) is a well-recognized entity in pediatric ophthalmology, but not common.
Overview of causes
- Elevated intracranial pressure can result in downward displacement of the brainstem, causing stretching of the sixth nerve secondary to its anatomic location within the Dorello canal. This is believed to be the reason that about 30% of patients with pseudotumor cerebri have an isolated abducens nerve palsy and explains how lesions remote from the sixth cranial nerve can cause abducens paresis (false localizing sign).
- Subarachnoid space lesions can be causes of abducens nerve palsy (eg, hemorrhage, infection, inflammation, space-occupying tumor, cavernous sinus mass). Inflammatory (eg, postviral, demyelinating, sarcoid, giant cell arteritis)
- Metabolic (eg, vitamin B, Wernicke-Korsakoff syndrome)
- Neoplasm (children) - Pontine glioma
- Infectious (eg, Lyme disease, syphilis)
- Congenital absence of the sixth nerve (eg, Duane syndrome)
- Trauma, particularly if it results in a torsional head motion
- Post–lumbar tap
Not all abduction deficits are cranial nerve VI palsies. Mimickers are orbital lesions, medial wall fractures, thyroid-associated orbitopathy, myasthenia gravis, and spasm of the near reflex.