Diplopia

Types & Causes

• Monocular Diplopia (persists when one eye covered):

  • Refractive errors (corrected with lenses)

  • Cataract, keratoconus

  • Retinal pathology (e.g., macular degeneration)

• Binocular Diplopia (resolves with either eye covered):

  • Neurological: CN III, IV, VI palsies (urgent)

  • Thyroid eye disease: Restricted movement (esp. inferior rectus)

  • Myasthenia gravis: Fatiguable weakness

  • Intracranial pathology: Tumours, aneurysms, ↑ ICP

  • Microvascular ischaemia: Diabetes, hypertension

  • Trauma: Orbital fractures

Red Flags (Urgent Referral)

• Binocular diplopia + pain, anisocoria, or ptosis

• Suspected GCA: Headache, jaw claudication, scalp tenderness, ↑ ESR/CRP

• Aneurysm or intracranial tumour suspicion

Cranial Nerve Palsies

• CN III:

  • Features: Eye "down and out," ptosis, ± mydriasis (pupil-involving)

  • Pupil-involving: Compressive lesion (e.g., aneurysm) → urgent imaging

  • Pupil-sparing: Likely microvascular → monitor for resolution in 3 months

• CN IV:

  • Features: Eye "up and out," worsens on downgaze, head tilt compensates

  • Causes: Trauma, idiopathic, microvascular, compressive

• CN VI:

  • Features: Eye fails to abduct

  • Causes: ↑ ICP, microvascular, trauma

  • Urgent referral: If headache or papilloedema present

Common Causes

• Microvascular ischaemia (older adults with diabetes, hypertension)

• Post-viral mononeuropathy (e.g., herpes zoster)

• Intracranial pathology (tumours, aneurysms, MS)

• Idiopathic

Notes

• Myasthenia gravis: Fatiguable diplopia, test with ice pack or edrophonium

• GCA: Start high-dose steroids, urgent biopsy to prevent vision loss