Assessment of sympathetic activity

Sympathetic hyperactivity may be present in some peripheral neuropathic pain states as part of complex regional pain syndrome (CRPS; formerly known as cau-salgia or reflex sympathetic dystrophy; see Chapter 27, Complex regional pain syndromes). The clinical aspects of sympathetic hyperactivity include a perception of burning-type pain soon (hours or days) after injury together with the demonstration of swelling, smooth glossy skin, and vasomotor instability. A characteristic localized osteoporosis may be observed in the extremities (Sudeck's atrophy) later on. These features may exist alone or in combination. Sweating may be affected, producing either wet or dry skin. Similarly, the skin may be cooler or warmer, depending on the degree of cutaneous vasoconstriction. In patients suspected of sympathetic dysfunction, tests can be useful to document the degree of sympathetic involvement. These include sweat testing, galvanic skin resistance, plethysmography, skin blood flow measurement (laser Doppler test, thermography), and cutaneous histamine response. Diagnostic sympathetic blocks may also be used to determine the possible involvement of the sympathetic nervous system in a particular pain condition.77 There is at present no single test that can be used to exclude sympathetically maintained pain and there are no known symptoms that predict it.74

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