Abducens (VI) Nerve Palsy

Abducens (VI) Nerve Palsy

An abducens (VI) nerve palsy causes a selective weakness of the lateral rectus muscle. This muscle is responsible for abducting the eye (moving it outward, away from the nose). Dysfunction of this nerve results in an inability to fully abduct the affected eye.

Clinical Presentation

The primary symptom is horizontal diplopia (double vision), where the two images appear side-by-side. This diplopia is most pronounced when the patient attempts to look towards the side of the affected eye (e.g., looking to the right in a right abducens palsy). Patients may adopt a compensatory head turn toward the affected side to maintain binocular vision and reduce double vision.

Common Causes

The abducens nerve has a long intracranial course, making it vulnerable to various pathologies:

• Microvascular Ischemia: Often related to diabetes or hypertension, this is a common cause of isolated palsies in older adults.
• Raised Intracranial Pressure: The nerve can be stretched as it passes over the petrous temporal bone, leading to a "false-localizing sign" that indicates high pressure rather than a specific local lesion.
• Trauma: Head injuries, particularly those causing basal skull fractures, can damage the nerve.
• Tumors or Aneurysms: Masses can compress the nerve anywhere along its path.
• Nuclear Pontine Lesions: Congenital conditions like Duane retraction syndrome and Möbius syndrome affect the nerve nucleus in the brainstem.
• Inflammation or Infection: Conditions like meningitis can affect the cranial nerves.

Differential Diagnosis

It is important to distinguish a true nerve palsy from other causes of abduction failure:

• Myasthenia Gravis: This neuromuscular junction disorder can mimic an isolated muscle weakness. To avoid overlooking this treatable condition, it is sometimes preferable to initially label the sign as a "lateral rectus palsy" rather than an abducens nerve palsy until a definitive cause is found.
• Pseudo-abducens Palsy: This refers to slow or restricted abduction caused by excessive convergence associated with a midbrain lesion, also known as a "midbrain pseudo-sixth."
• Thyroid Eye Disease: Restrictive myopathy can limit eye movements, including abduction.

 

Cross References

Diplopia; "False-localizing signs"