Diplopia

Usually acute onset and distressing. Binocular diplopia (resolves when either eye is covered alone) reflects an extra-ocular muscle/nerve problem; monocular diplopia (persists despite covering the unaffected eye, and resolves only when the affected eye itself is covered) reflects a problem within that eye — always test both monocularly and binocularly to distinguish them.

New-onset diplopia in any adult >50 years should prompt consideration and work-up of giant cell arteritis (GCA) — even without classic features (scalp tenderness, jaw claudication); check ESR/CRP/platelets (each only ~65-71% sensitive alone) and consider temporal artery ultrasound/biopsy. Ocular GCA is treated with IV methylprednisolone for 3-5 days then a prolonged oral taper over 1-2 years, with tocilizumab increasingly used as a steroid-sparing adjunct; refer any ocular GCA manifestation to ophthalmology promptly — corticosteroids should not be delayed pending biopsy.

Causes

Probability diagnosis: ocular nerve palsies (CN III/IV/VI); CVA/TIA; ophthalmoplegic migraine; alcohol/benzodiazepines. Monocular: cataract, refractive error, corneal disease.

Serious disorders:

• Vascular — CVA/TIA, diabetic microvascular disease.
• Infective — intraocular abscess.
• Neoplastic — slow-growing tumour, or raised ICP.
• Autoimmune — Guillain-Barré syndrome (a very late sign), multiple sclerosis, myasthenia gravis, orbital myositis, giant cell arteritis.
• Traumatic — orbital wall fracture.
• Endocrine — thyroid eye disease.

Examination

Test the direction of gaze provoking diplopia, both monocularly and binocularly (to exclude a monocular cause):

• Lateral gaze → 6th nerve palsy.
• Downward gaze, with the eye elevated upward → 4th nerve palsy.
• Eye “down and out” → 3rd nerve palsy.

Investigations

Screen for diabetes; ESR for GCA; TFTs for hyperthyroidism; lipids (6th nerve palsies are more commonly microvascular); CT brain if MRI unavailable, MRI preferred (identifies microvascular disease, TIA/CVA, tumours).

Management

Discuss with ophthalmology or neurology. Microvascular 6th nerve palsies typically resolve over 3–6 months. Surgery or botulinum toxin for a permanent deficit.