Use and misuse of conventional electrodiagnosis, quantitative sensory testing, thermography, and nerve blocks in the evaluation of painful neuropathic syndromes.

A number of laboratory tests are critically important in the quest to diagnose presence or absence of organic neuropathic dysfunction and to establish the relevance of such to the subjective pain complaints. However, none of these tests has absolute diagnostic value and their results must be interpreted in the light of the clinical picture. Conventional electrophysiology evaluates function of large caliber afferent and motor fibers leaving the function of small caliber afferent fibers unexplored, and cannot explore the basis for positive sensory phenomena. The quantitative somatosensory thermotest is the best test available to explore function of small caliber afferents. It allows documentation of positive sensory phenomena in the form of thermal hyperalgesia. Because it is a psychophysical test, it lacks localizing value. Thermography sensitively detects and precisely delineates areas of cutaneous thermal change of neural origin. Three types of diagnostic neurologic blocks are used in the clinic: compression-ischemia, local anesthetic and sympathetic blocks. Although they may provide important information about the pathophysiology of pain and hyperalgesias, adequate placebo control is of the essence because chronic neuropathic pain patients may express a high incidence of placebo response.