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standards in longitudinal studies. However, it must be appreciated that QST is a psychophysical test and therefore is dependent upon patient motivation, alertness, and concentration. Patients can willingly perform poorly, and even when not doing so there are large intra- and interindividual variations. Further, abnormal findings are not specific for peripheral nerve dysfunction; central nervous system disorders will also affect sensory thresholds.

Autonomic function testing

The evaluation of autonomic functions in patients suspected of having neuropathic pain can be important because of anatomic similarities between pain and autonomic fibers outside the central nervous system (CNS), and because disorders productive of neuropathic pain frequently have signs and symptoms of autonomic dysfunction (dry eyes or mouth, skin temper- ature and color changes, sweating abnormalities, orthostatic hypotension, heart rate responses to deep breathing, edema, and so forth). The majority of autonomic tests study skin temperature, and sudomotor, baroreceptor, vasomotor, and cardiovagal functions; they have been extensively reviewed [9,10]. A semiquantitative composite autonomic symptoms score (CASS), composed of the results of sudomotor, cardiovagal, and adrenergic testing, has been devised [11]. Less frequently, pupillary, gastrointestinal, and sexual function tests are helpful.

The value of autonomic testing in patients with a general neuropathic pain disorder, painful small-fiber neuropathy with burning feet, has been illustrated in several studies [12,13] in which many patients had normal or only mildly abnormal electrophysiologic (NCVs/EMG) findings. Auto- nomic abnormalities were seen in more than 90% of patients, the most use- ful tests being the quantitative sudomotor axon reflex test (QSART), thermoregulatory sweat test, heart rate responses to deep breathing, Val- salva ratio, and surface skin temperature [12,13]. However, in a recent study of patients with diabetic polyneuropathy, discordance was noted between efferent C-fiber responses in sudomotor tests (QSART and sweat imprint), and primary afferent (nociceptor) C-fiber axon-reflex flare responses. These findings indicate that these two C-fiber subclasses can be differentially affected in diabetic small fiber polyneuropathy. There may be involvement of one subclass and not the other or there may be different patterns of re- generation and reinnervation [14]. Autonomic functions can also be abnor- mal in peripheral neuropathies not associated with pain.

The relationship between autonomic dysfunction and pain is more com- plicated in CRPS in which focal sudomotor and vasomotor abnormalities are thought to be essential for the diagnosis [15] and sympathetic blockade has been a mainstay of diagnosis and therapy for decades. As would be ex- pected, the vast majority of patients with CRPS were found to have auto- nomic abnormalities, particularly involving sweating and skin temperature [16]. However, there are patients with identical focal pain, but no clinical

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