What causes paresis of the 6th cranial nerve (abducens nerve)?

Causes of Abducens (6th Cranial) Nerve Palsy

The most common cause of abducens nerve palsy in adults is vasculopathic disease, particularly associated with diabetes and hypertension, while in children, neoplasms are the predominant cause. 1, 2, 3

Anatomical Considerations

Understanding the anatomical pathway of the abducens nerve helps identify potential causes of palsy:

• The 6th nerve originates in the abducens nucleus of the pons, courses through corticospinal tracts, ascends in the subarachnoid space, and passes underneath the petroclinoid ligament to enter the cavernous sinus 1, 2
• In the cavernous sinus, it lies in proximity to the intracavernous carotid artery, ocular sympathetics, and cranial nerves III, IV, and V1 1, 2
• The nerve then passes through the superior orbital fissure into the orbit 1
• This long, vulnerable course makes the nerve susceptible to various pathologies at different points along its path 1, 2

Major Causes by Category

Vasculopathic Causes (Most Common in Adults)

• Diabetes mellitus - most frequent cause in adults over 50 years 1, 2, 3
• Hypertension - second most common vascular cause 1, 2, 3
• Giant cell arteritis - presents with scalp tenderness, jaw claudication, requires immediate treatment 2, 3

Neoplastic Causes (Most Common in Children)

• Intracranial tumors - particularly common in pediatric patients 3, 4
• Clival chordoma - often causes bilateral 6th nerve involvement 1, 2
• Meningiomas and neurilemmomas at the base of the brain 5
• Brain stem tumors 2
• Tumors at the petrous apex or cavernous sinus 5

Increased Intracranial Pressure

• Can cause bilateral 6th nerve palsies 1, 2
• Associated with papilledema 3
• Can result from pseudotumor cerebri 2, 6

Traumatic Causes

• Head injury with basilar skull fracture 1, 2
• Acute rise in intracranial pressure from intracranial hemorrhage 1, 2

Inflammatory/Infectious Causes

• Meningeal processes (infection or inflammation) 1, 2
• Demyelinating diseases such as multiple sclerosis 1, 2
• Post-viral causes (typically a diagnosis of exclusion) 1, 2

Vascular Anomalies

• Carotid artery aneurysms 2
• Dural sinus thrombosis, including inferior petrosal sinus thrombosis 6, 7

Other Causes

• Guillain-Barré syndrome 6
• Orbital infections, mass lesions, and inflammation 2, 3
• Congenital causes (particularly in children) 4

Age-Related Differences

• In adults over 50: Vasculopathic causes predominate (diabetes, hypertension) 1, 3, 8
• In children: Neoplasms account for approximately 45% of cases 6, 4
• Isolated 6th nerve palsies are relatively uncommon in children (9%) 4

Clinical Course and Prognosis

• Most vasculopathic palsies resolve spontaneously within 6 months, with about one-third resolving within 8 weeks 1, 3
• If no recovery is apparent by 6 months, approximately 40% of patients demonstrate serious underlying pathology 1
• Chronic isolated abducens palsy, even if stable for years, may still be caused by slow-growing tumors at the base of the brain 5

Important Clinical Considerations

• Bilateral 6th nerve involvement should raise suspicion for clival chordoma, increased intracranial pressure, or meningeal processes 1, 2
• Neuroimaging is recommended for all young patients or any patient with other cranial neuropathies, neurologic changes, elevated IOP, or absence of vasculopathic risk factors 1, 3
• In children, neuroimaging should be performed early in the clinical course, even if the palsy is isolated, due to the high risk of neoplasm 4
• Evidence of increased intracranial pressure warrants lumbar puncture (after neuroimaging) 2