Quantitative Sudomotor Axon Reflex and Related Tests

Abstract

Abstract Small-fiber nerves may be selectively involved in various disorders. The function of the postganglionic sympathetic sudomotor C fibers can be assessed using the quantitative sudomotor axon reflex test (QSART), noninvasively and reproducibly, to define the severity and distribution of sudomotor failure. The neural pathway evaluated by QSART consists of an axon reflex mediated by the postganglionic sympathetic sudomotor axon. The axon terminal is activated by iontophoresis of acetylcholine, which leads to an impulse that travels antidromically to the branch point, then orthodromically to release acetylcholine from the nerve terminal to stimulate muscarinic receptors on eccrine sweat glands and evoke a sweat response. Normal responses indicate integrity of the postganglionic sympathetic sudomotor axon, while abnormal QSART patterns can indicate sudomotor damage, such as length-dependent reduction responses, indicating distal small-fiber neuropathy. Concomitant use of QSART and a thermoregulatory sweat test may help determine the site of the lesion.