What causes post-viral diplopia?

Post-Viral Diplopia: Causes and Mechanisms

Post-viral diplopia is typically caused by isolated cranial nerve VI (abducens) palsy, though it represents a diagnosis of exclusion after ruling out more serious etiologies including vascular, neoplastic, demyelinating, and infectious causes. 1

Primary Mechanism

Post-viral sixth nerve palsy is the most commonly recognized post-viral cause of diplopia in adults, though it must be diagnosed only after excluding other serious pathologies. 1 The condition presents with:

• Acute onset horizontal double vision, worse at distance than near 1
• Worsening diplopia on lateral gaze toward the affected side 1
• Possible compensatory head turn to minimize diplopia 1

Differential Diagnosis Framework

When evaluating suspected post-viral diplopia, the anatomical localization determines the differential diagnosis. The lesion may involve: 2

Cranial Nerve Involvement

• Cranial nerve VI (abducens): Most common isolated nerve palsy in post-viral cases 1
• Cranial nerve III (oculomotor): Can occur post-virally but requires urgent evaluation to exclude compressive lesions, particularly if pupil-involving 3
• Cranial nerve IV (trochlear): Rarely post-viral; trauma is the most common cause 1

Other Anatomical Sites

• Brainstem nuclei and connecting tracts (internuclear ophthalmoplegia suggests demyelinating disease like multiple sclerosis in younger patients) 1
• Neuromuscular junction (myasthenia gravis must be excluded) 4
• Extraocular muscles themselves (thyroid eye disease, orbital inflammatory conditions) 1

Critical Exclusions Before Diagnosing Post-Viral Etiology

Vascular Causes (Most Common in Adults)

The majority of acute cranial nerve palsies in adults are vasculopathic, associated with diabetes and hypertension. 1 These typically:

• Resolve within 6 months (one-third within 8 weeks) 1
• May be accompanied by pain 1
• If no recovery by 6 months, approximately 40% have serious underlying pathology requiring further evaluation 1

Neoplastic Causes

Intracranial neoplasms may present with insidious or acute onset diplopia. 1 Key features include:

• Bilateral sixth nerve involvement suggests clival chordoma or increased intracranial pressure 1
• Multiple ipsilateral cranial nerve palsies (III, IV, VI) suggest cavernous sinus or orbital apex lesions 1

Demyelinating Disease

Multiple sclerosis is a primary consideration in younger patients presenting with acute internuclear ophthalmoplegia. 1 Lesions typically involve the pons, often with associated facial palsy. 1

Infectious/Inflammatory Causes

Multiple ipsilateral cranial nerve palsies can occur with: 1

• Infectious meningitis (TB, fungal, Lyme disease)
• Non-infectious causes (sarcoidosis, leptomeningeal tumor spread)

Diagnostic Approach

Initial Imaging

MRI of the brain with and without gadolinium contrast plus MRA or CTA is recommended for acute diplopia to exclude serious pathology. 1, 3 This is particularly urgent for:

• Pupil-involving third nerve palsy (requires urgent evaluation for aneurysm) 3
• Any associated neurological symptoms 1
• Bilateral sixth nerve involvement 1

When Post-Viral Diagnosis is Appropriate

Post-viral sixth nerve palsy should be considered a diagnosis of exclusion only after: 1

• Neuroimaging is normal
• Vascular risk factors are assessed
• Demyelinating disease is excluded (especially in younger patients)
• No evidence of increased intracranial pressure
• No other neurological deficits present

Prognosis and Management

Expected Course

• Most vasculopathic and presumed post-viral palsies resolve within 6 months 1
• If no recovery is apparent by 6 months, further evaluation for underlying pathology is warranted 1

Symptomatic Management

Temporary measures while awaiting recovery include: 3

• Eye patching or occlusion therapy
• Prism glasses
• Botulinum toxin injection

Surgical Consideration

Strabismus surgery may be considered after 6-12 months if no further recovery is expected. 3

Critical Pitfall

Giant cell arteritis must be excluded in elderly patients presenting with diplopia accompanied by scalp tenderness, temporal region pain, or jaw claudication, as this can result in permanent visual loss if not promptly treated. 1