To investigate whether activation of afferent and central baroreceptor pathways could differentiate between
pure autonomic failure (PAF) and
multiple system atrophy with autonomic failure (MSA), we determined the effect of upright tilt on circulating levels of
vasopressin in patients with PAF and patients with MSA. We also studied 14 normal subjects, nine of whom developed acute
hypotension due to
vasovagal syncope. In patients with PAF and in normal subjects with
vasovagal syncope, upright tilt induced marked
hypotension and a pronounced increase in the plasma concentration of
vasopressin (1.1 +/- 0.3 to 38.0 +/- 8.0 pmol/l in PAF and 1.0 +/- 0.2 to 27.4 +/- 7.2 pmol/l in
vasovagal syncope, p less than 0.005 for both). In patients with MSA, upright tilt also elicited profound
hypotension but circulating levels of
vasopressin increased little (0.5 +/- 0.1 to 1.5 +/- 0.3 pmol/l, p less than 0.05). During upright tilt, the plasma concentration of
norepinephrine significantly increased in normal subjects but did not increase in patients with autonomic failure. Our results indicate that afferent and central baroreceptor pathways involved in
vasopressin release are normal in patients with PAF but are impaired in patients with MSA. Thus, measurement of baroreceptor-mediated
vasopressin release appears to provide a clear marker to differentiate between patients with PAF and patients with MSA.