Bitesize Orthoptics - Sixth Nerve Palsy: Causes, Localisation, and Red Flags
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The sixth cranial nerve (abducens nerve) controls the lateral rectus muscle, responsible for abducting the eye. When affected, patients often present with horizontal diplopia that worsens on distance fixation and towards the affected gaze. While congenital sixth nerve palsy is rare, acquired palsy is much more common, and careful assessment is needed to identify the underlying cause.
Localising Sixth Nerve Lesions
The sixth nerve’s long intracranial course makes it particularly vulnerable to injury. Localisation depends on whether other neurological signs are present:
Fascicular Damage (within the brainstem)
Lesions affecting the nerve fascicle in the pons may result from demyelination, vascular disease, or tumours. Because of the close anatomical relationship between the sixth nerve, seventh nerve, and pyramidal tracts, characteristic syndromes can occur:
Foville’s Syndrome – damage to the pontine tegmentum causes partial sixth nerve palsy, ipsilateral (same side) facial weakness, loss of taste in the anterior (front) two-thirds of the tongue, ipsilateral Horner’s syndrome, facial sensory loss, and peripheral deafness.
Millard-Gubler Syndrome – damage to the ventral pons produces sixth nerve palsy, contralateral (opposite side) hemiplegia, and sometimes ipsilateral facial paralysis.
Peripheral Damage (outside the brainstem)
Peripheral sixth nerve palsy can result from a range of causes, including:
• Closed head injury
• Tumours (pituitary, craniopharyngioma, meningioma)
• Aneurysms involving the basilar artery
• Subarachnoid haemorrhage (ruptured aneurysms)
• Severe middle ear infections
False Localising Sign
The sixth nerve is highly vulnerable to raised intracranial pressure (ICP), particularly where it passes over the apex of the petrous temporal bone. In such cases, a sixth nerve palsy may occur not because of a lesion at the nerve itself, but as a secondary effect of ICP. This is termed a false localising sign, as it does not provide accurate information about the site of pathology.
Summary
Acquired sixth nerve palsy is far more common than congenital cases and requires careful investigation. The presence of associated neurological features can help localise the lesion within the brainstem, while peripheral and systemic causes must also be considered. As an Orthoptist you must always be cautious of a sixth nerve palsy as a possible false localising sign of raised intracranial pressure, warranting urgent further investigation.
