Posterior Communicating Artery (PCOM) Aneurysm


  • Acute CN III palsy with ipsilateral pupil dilation is PCOM aneurysm until proven otherwise
    • Aneurysm compresses the outer fibers (pupillomotor) of CN III → dilation
    • In contrast to CN III palsy due to diabetes mellitus or hypertension in which pupil is spared

Clinical Features

  • Inhibition of ipsilateral medial gaze, upward gaze, downward gaze, ptosis
    • Lateral gaze (abduction) is preserved
  • Ipsilateral dilated pupil

Differential Diagnosis

Binocular Diplopia



  • Emergent BP reduction if hypertensive
  • Neuroimaging
  • Neurosurgical consult



  1. Kusner LL, Puwanant A, Kaminski HJ: Ocular myasthenia: Diagnosis, treatment, and pathogenesis. Neurologist 2006; 12:231-239
  2. Bushra JS: Miller Fisher syndrome: An uncommon acute neuropathy. J Emerg Med 2000; 18:427-430