Diplopia

1 · First minutes

1. Monocular (optical) vs binocular (alignment): covering either eye kills binocular diplopia.
2. Painful pupil-involving CN III = CTA now (PCOM aneurysm).
3. Age ≥50: ask about jaw claudication and treat GCA on suspicion.

3 · Immediate workup

• Visual acuity, pupils, cover testing; glucose
• Ice-pack or rest test if MG is likely and the patient is stable

4 · Don’t miss

• Posterior communicating artery aneurysm (CN III)
  Painful pupil-involving third-nerve palsy (down-and-out, ptosis, blown pupil) — CTA/MRA now, neurosurgery. A complete pupil-sparing CN III in a vasculopath >50 y is more often microvascular, but pain or incomplete palsies still get imaged.
• Brainstem stroke / INO (internuclear ophthalmoplegia)
  Sudden diplopia with ataxia, dysarthria, or crossed findings — posterior-circulation pathway, MRI-DWI. INO (impaired adduction + contralateral nystagmus) is MLF until proven otherwise.
• Giant cell arteritis
  Age ≥50, new headache, jaw claudication, diplopia or amaurosis — start glucocorticoids immediately to protect the other eye; ESR/CRP can be normal.
• Myasthenic crisis
  Fatigable binocular diplopia and ptosis, worse late in the day; pupils spared. Watch NIF/VC; avoid triggering antibiotics.
• Cavernous sinus thrombosis / orbital apex
  Diplopia + proptosis, chemosis, fever or CN V1 pain — CT/MR venography, antibiotics if septic, anticoagulation if atraumatic.