Can cranial nerve (CN) 6 palsy cause diplopia?

Cranial Nerve 6 Palsy and Diplopia

Yes, cranial nerve 6 (abducens nerve) palsy definitively causes diplopia—specifically horizontal double vision that is worse at distance than at near and worse when looking laterally toward the side of the affected nerve. 1

Clinical Presentation of CN6 Palsy-Related Diplopia

The diplopia from abducens palsy has characteristic features:

• Horizontal double vision that patients typically notice in primary gaze position, though with partial paralysis it may only appear on lateral gaze 1
• Distance diplopia is worse than near diplopia, distinguishing it from other causes of binocular diplopia 1
• Worsening when looking toward the affected side due to the lateral rectus muscle weakness 1
• May be accompanied by compensatory head turn to avoid the diplopic field and maintain single vision 1

The diplopia results from incomitant horizontal misalignment that can be disabling at onset, though symptoms may be less bothersome if the palsy develops gradually, allowing horizontal fusional amplitudes to develop 1

When Diplopia May Be Less Apparent

Certain clinical scenarios reduce the subjective complaint of diplopia:

• Partial palsies that develop gradually and chronically permit adaptation 1
• Exhaustion or systemic illness may unmask diplopia that was previously compensated 1
• Extreme horizontal gaze positions may be the only field where diplopia is noticed in mild cases 1

Anatomical Basis for Diplopia

The sixth nerve innervates the lateral rectus muscle, which abducts the eye 2, 3. When this nerve is damaged:

• Esotropia develops (inward deviation of the affected eye) that is greater at distance 4
• Ipsilateral abduction deficiency prevents the eye from moving laterally 4
• Binocular misalignment creates two separate images that the brain cannot fuse 1

The nerve's long course from the pons through the subarachnoid space, under the petroclinoid ligament, into the cavernous sinus, and finally to the orbit makes it vulnerable to injury at multiple locations 3

Common Pitfalls in Diagnosis

Do not assume all horizontal diplopia is CN6 palsy. Other conditions can mimic this presentation:

• Myopic strabismus fixus presents with restrictive esotropia but is distinguished by severe myopia (typically ≥-8.00 diopters), axial length >27mm, and characteristic orbital imaging showing SR-LR dislocation angle of 121° ± 7° 1
• Chronic complete CN6 palsy can develop restrictive changes that complicate the clinical picture 1
• Thyroid eye disease may cause restrictive esotropia 1
• Divergence insufficiency causes distance esotropia but typically has full ductions on examination 1

Clinical Management Implications

The presence of diplopia from CN6 palsy warrants specific evaluation:

• Neuroimaging is indicated in young patients, those with other cranial neuropathies, neurologic changes, elevated IOP, signs of increased intracranial pressure, or absence of vasculopathic risk factors 1, 3
• In older patients with vascular risk factors (diabetes, hypertension), observation for 6 months is reasonable as most vasculopathic palsies resolve, with about one-third resolving within 8 weeks 1, 3
• If no recovery by 6 months, approximately 40% demonstrate serious underlying pathology requiring further evaluation 1, 3
• Giant cell arteritis must be excluded in elderly patients with scalp tenderness or jaw claudication, as this can cause permanent visual loss if untreated 1, 3

Treatment goals focus on eliminating diplopia in primary position and creating a reasonable field of single binocular vision, though patients will likely continue to have diplopia in extreme lateral gaze even after successful treatment 1