Autonomic profile of subjects prone to fainting.
Syncope is the most common form of fainting that may occur at least once during a life-time in up to one-third of the general population. In 50% of patients the cause remains unknown. In an attempt to identify subtle disturbances of the autonomic nervous system, we examined 70 subjects, aged from 14 to 39 years, who suffered from recurrent neurally mediated syncope. We performed a battery of non-invasive tests assessing cardiovagal, sympathetic cholinergic and sympathetic adrenergic function. We compared the results with a group of 30 healthy, non-fainting subjects matched for age and sex. Basal records were similar in both groups. Patients had preserved cardiovagal function. The multivariate cluster analysis allowed us to find a homogeneous group of cases (46%) that simultaneously presented: greater fall in systolic and diastolic blood pressure after standing, increased 15:30 ratio, exaggerated absolute heart rate rise in response to standing and subclinical reduced sudomotor function in the foot. The results suggest the existence of a subclinical autonomic profile, with subtle sympathetic postganglionic impairment, evident in lower limbs. These findings may contribute to proving the existence of different types of neurally mediated syncope, all different in their onset and mechanism but with a common final manifestation: syncopal loss of consciousness.