Fourth nerve palsy as an initial sign of internal carotid posterior communicating artery aneurysm

A 54-year-old female complained of vertical diplopia on downward gaze with a slight headache lasting two days. When she gazed in a nasal lower direction, right hypertropia became obvious and the Bielschowsky head-tilt test was positive. A trochlear nerve palsy of right eye was diagnosed. Several examinations were performed, including blood chemical and serological examinations, plain CT examination, and cerebrospinal fluid examination. Three days after appearance of the trochlear nerve palsy, oculomotor nerve palsy of right eye occurred with severe ocular and head pain, and a four-vessel study disclosed a large aneurysm at the IC-PC portion and two more aneurysms at the distal portion of the middle cerebral artery. This is a very rare case of trochlear nerve palsy as the initial sign of IC-PC aneurysm and the pathogenesis of trochlear nerve palsy was discussed.