BACKGROUND: Juvenile myoclonic epilepsy (JME) is characterized by myoclonic seizures on awakening and giant somatosensory evoked potentials. Those characteristics suggest that JME is related with dysfunction of the brainstem reticular formation. Cardiovascular reflex is also closely related with the brainstem reticular formation, and head-up tilt-table test (HUT) is frequently employed to evaluate the cardiovascular reflex. We performed HUT to assess the cardiovascular reflex in patients with JME and syncope/presyncope. METHODS: We included 52 patients with alleged JME. HUT was performed in 20 of them with a history of presyncope or syncope. The protocol of HUT included 70 degree table tilting test and isoproterenol infusion test (1~5 microgram/min). We determined the positive responses according to the change of blood pressure and heart rate. The positive responses were classified as vasodepressive, cardioinhibitory, or mixed types. We retrospectively analyzed the clinical findings and responses of HUT. RESULTS: Syncope or presyncope occurred during the seizure-prone situation in 3 (15%), and during the syncope-prone situation in 17 (85%) patients. Fifteen (75%) patients had positive responses in HUT. The positive responses were during the baseline test in 1 (5%), during the low dose ((< or = 3 microgram/min) of isoproterenol infusion in 10 (50%), and during the high dose (> or = 4 microgram/min) of isoproterenol infusion in the remaining 4 (20%) patients. All the positive responses were vasodepressive type. CONCLUSIONS: In patients with JME, vasomotor reflex may be impaired. Our results suggest dysfunction of the brainstem reticular formation in JME.