An exhaustive review of the various tests of autonomic function in PD is beyond the scope of this chapter. Briefly, the tests that have been commonly employed in the study of PD autonomic failure include heart rate variation (with deep breathing and valsalva), tilt table testing for orthostatism, sudomotor axon reflex testing, and thermoregulatory sweat testing. Abnormalities may be seen in one or more of these depending on the presenting symptoms. Several studies have suggested that as a group, autonomic failure is more severe in MSA than in PD, but these studies often use the severity of autonomic failure to help assign the diagnosis; the more severe the autonomic symptoms, the more likely the patient will be classified as having MSA. Riley and Chelimsky (13) have noted the circular nature of this reasoning and performed a comparative study of autonomic tests in patients with PD and MSA, in whom autonomic failure was not used for diagnosis. They found a high frequency of abnormal test results in patients with dysautonomia with both PD and MSA, and the severity of autonomic failure did not differ between the groups. They suggested that thermoregulatory sweat testing might be the most sensitive test for PD-related autonomic failure, since all patients with dysautonomia with either diagnosis who received this test had abnormal results. There is some controversy on whether quantitative sudomotor axon reflex testing (QSART) is helpful for detecting sympathetic dysfunction in PD. In one study (14), patients with abnormal blood pressure responses to valsalva and decreased cardiac fluorodopa PET scanning (indicating sympathetic dysfunction) had normal QSART test results. They interpreted this finding as indicating that the sympathetic dysfunction in PD involves the loss of noradrenergic but not cholinergic function. A number of studies (15-18) have confirmed that heart rate variability is decreased in patients with PD.
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