Patients with Parkinson disease (PD) often have manifestations of autonomic failure. About 40% have neurogenic orthostatic hypotension (NOH), and among PD+NOH patients virtually all have evidence of cardiac sympathetic denervation; however, whether PD+NOH entails extra-cardiac noradrenergic denervation has been less clear.

Microdialysate concentrations of the main neuronal metabolite of norepinephrine (NE), dihydroxyphenylglycol (DHPG), were measured in skeletal muscle, and plasma concentrations of norepinephrine and DHPG were measured in response to i.v. tyramine, yohimbine, and isoproterenol, in patients with PD+NOH, patients with pure autonomic failure (PAF), which is characterized by generalized catecholaminergic denervation, and control subjects.

Microdialysate DHPG concentrations were similarly low in PD+NOH and PAF, compared to control subjects (163±25, 153±27, and 304±27 pg/ml, p<0.01 each vs. control). The two groups also had similarly small plasma DHPG responses to tyramine (71±58 and 82±105 vs. 313±94 pg/ml; p<0.01 each vs. control) and NE responses to yohimbine (223±37 and 61±15 vs. 672±130 pg/ml, p<0.01 each vs. control) and virtually absent NE responses to isoproterenol (20±34 and 14±15 vs. 336±78 pg/ml, p<0.01 each vs. control). PD+NOH patients had normal bradycardia responses to edrophonium and normal epinephrine responses to glucagon

The results support the concept of generalized noradrenergic denervation in PD+NOH, with similar severity to that seen in PAF. In contrast, the parasympathetic cholinergic and adrenomedullary hormonal components of the autonomic nervous system seem intact in PD+NOH.