Chemical mediators of autonomic nervous system activity.

Several modifications have improved the sensitivity and specificity of neurochemical tests to diagnose and distinguish multiple system atrophy and pure autonomic failure. These include [3H]-norepinephrine infusion to estimate plasma norepinephrine clearance; neuroendocrine stimulation tests; and simultaneous measurements of levels of levodopa, catecholamines, and catecholamine metabolites. New clinical imaging approaches have the potential to visualize alterations in neurotransmitter function in the brain and periphery. Patients with multiple system atrophy have decreased striatal radioactivity after injection of positron-emitting ligands at dopamine receptors or uptake sites, and patients with diffuse sympathetic denervation have decreased myocardial radioactivity after injection of radioactive sympathoneural imaging agents.