Syncope in the adult Chiari anomaly.

A patient with an adult Chiari malformation had syncopal episodes induced by modest exertion. Low-amplitude slowing of the electroencephalogram occurred at the time of syncope, with no change in pulse rate, blood pressure, or arterial gases in attacks reproduced by exercise, compression of jugular veins or standardized Valsalva maneuver. The attacks were attributed to transient increases in intracranial pressure that caused dysfunction of the ascending reticular activating system.