In the sympathetic or galvanic skin-resistance test, a set of electrodes placed on the skin measures the resistance to the passage of a weak current through the skin; in all likelihood, the change in electrical potential is the result of an ionic current within the sweat glands, not simply an increase in sweating that lowers skin resistance. This method can be used to outline an area of reduced sweating because of a peripheral nerve lesion, as the response depends on sympathetic activation of sweat glands (Gutrecht). However, the galvanic skin response is subject to habituation with repeated stimuli and will show no response if there is a sensory neuropathy. The silicone imprint method is a quantitative postganglionic test that measures sweat droplets induced by iontophoresis of acetylcholine, pilocarpine, or methacholine. Although in theory quantitative, the imprint methods are prone to artefacts (see Stewart et al).
A more quantitative and reproducible examination of postganglionic sudomotor function, termed QSART (quantitative sudomotor axon reflex test), has been developed and studied extensively by Low. It is essentially a test of distal sympathetic axonal integrity utilizing the local axon reflex. A 10 percent solution of acetylcholine is iontophoresed onto the skin using 2 mA for 5 min. Sweat output is recorded in the adjacent skin by sophisticated circular cells that detect the sweat water. The forearm, proximal leg, distal leg, and foot have been chosen as standardized recording sites. By this test, Low has been able to define patterns of absent or delayed sweating that signify postganglionic sympathetic failure in small-fiber neuropathies and excessive sweating or reduced latency in response, as is seen in reflex sympathetic dystrophy. This is the preferred method of studying sweating and the function of distal sympathetic fibers, but its technical complexity makes it available only in specially equipped laboratories.
The thermoregulatory sweat testing (TST) measures the integrity of the central and peripheral sympathetic sudomotor pathways. The test is conducted by raising the core body temperature by increasing the ambient temperature and the sweating pattern is visualized with an indicator dye. This may show striking results but is time consuming, needs special patient preparation, and large clinical space. The advantage of TST is that in combination with tests measuring postganglionic functions, it can differentiate between postganglionic (all tests abnormal) versus preganglionic (only TST abnormal) failure.
Tearing can be estimated roughly by inserting one end of a 5-mm-wide and 25-mm-long strip of thin filter paper into the lower conjunctival sac while the other end hangs over the edge of the lower lid (the Schirmer test). The tears wet the strip of filter paper, producing a moisture front. After 5 min, the moistened area extends for a length of approximately 15 mm in normal persons. An extent of less than 10 mm is suggestive of hypolacrima. This test is used mainly to detect the dry eyes (keratoconjunctivitis sicca) of the Sjögren syndrome, but it may also be helpful in fully studying various autonomic neuropathies.
Tests of Bladder, Gastrointestinal, and Penile Erectile Function
Bladder function is best assessed by the cystometrogram, which measures intravesicular pressure as a function of the volume of saline solution permitted to flow by gravity into the bladder. The rise of pressure as 500 mL of fluid is allowed to flow gradually into the bladder, the emptying contractions of the detrusor, and the volume at which the patient reports a sensation of bladder fullness can be recorded by a manometer. (A detailed account of cystometric techniques can be found in the monograph of Krane and Siroky.) A simple way of determining bladder atony (prostatic obstruction and overdistention having been excluded) is to measure the residual urine (by catheterization of the bladder) immediately after voluntary voiding or to estimate its volume by ultrasound imaging.