6th Nerve Palsy
The sixth cranial nerve (the abducens nerve) supplies the lateral rectus, which abducts the eye (moves the eye outwards). When impaired, the affected eye cannot move fully outward, leading to an esotropia and horizontal diplopia.
Aetiology
The abducens nerve originates from the abducens nucleus in the pons and exits the brainstem at the pontomedullary junction. It passes through the subarachnoid space, travels via Dorello’s canal and the cavernous sinus, and enters the orbit through the superior orbital fissure. In the orbit, it innervates the lateral rectus muscle, which abducts the eye by moving it away from the midline.
Localising the Lesion
Associated neurological findings help determine where along the pathway the damage has occurred resulting in the palsy.
Nuclear / Fascicular Lesions (Brainstem)
Damage within the pons may affect nearby structures.
The sixth nerve lies close to:
The facial nerve (VII)
The corticospinal (pyramidal) tracts
Foville Syndrome
Lesion in the pontine tegmentum produces:
Partial sixth nerve palsy
Ipsilateral (same-side) facial weakness
Loss of taste from anterior (front) two-thirds of tongue
Ipsilateral Horner’s syndrome
Reduced facial sensation on the same side
Ipsilateral sensorineural hearing loss
Millard–Gubler Syndrome
Lesion in the ventral pons causes:
Sixth nerve palsy
Contralateral (opposite) hemiplegia
Possible ipsilateral facial paralysis
Peripheral Lesions
After leaving the brainstem, the nerve has a long intracranial course and is vulnerable to injury from:
Head trauma (particularly closed injuries)
Pituitary tumours
Craniopharyngioma
Meningioma
Basilar artery aneurysm
Subarachnoid haemorrhage
Middle ear infection
False Localising Sign
The sixth nerve bends over the petrous temporal bone apex. Raised intracranial pressure (ICP) can stretch it at this point resulting in a palsy. When sixth nerve palsy occurs due to increased ICP, it does not indicate the exact site of pathology. This is known as a false localising sign as it does not tell you the exact location of damage.
Clinical Presentation
Unilateral Sixth Nerve Palsy
Esotropia greater at distance than near
Mild cases may retain binocular single vision (BSV) at near
AHP: Face turn toward the affected side
Larger in distance fixation
May be minimal at near
Field of single vision shifts toward the unaffected side
Some mild cases compensate using a physiological V-pattern with slight chin depression
Muscle Sequelae
Overaction of the contralateral medial rectus
Contracture of the ipsilateral medial rectus
Underaction of the contralateral lateral rectus
Bilateral Sixth Nerve Palsy
Large-angle esotropia
Initially worse for distance
In long-standing cases, near deviation also becomes significant
Head turn toward the fixing eye to maintain central fixation
Chronic cases frequently develop medial rectus contracture, limiting abduction even to midline.
Management
The main aims of management in patients with a 6th nerve palsy is:
Relieve diplopia
Reduce abnormal head posture
Prevent medial rectus tightening
Non-Surgical Options
Prismatic correction
Occlusion
Botulinum toxin injection to medial rectus
Summary:
Unilateral 6th:
ET bigger in distance
AHP: face turn towards affected side (R 6th = face turn R, L 6th = face turn L).
AHP Larger on distance fixation
Muscle sequelae:
L LR underaction → R MR overaction → L MR contracture (overaction) → R LR underaction
R LR underaction → L MR overaction → R MR contracture (overaction) → L LR underaction
Bilateral 6th:
Large ET at near and distance - may initially just start with distance ET but in longstanding cases will develop a large near ET too
AHP: head turn towards fixing eye
