33 moisture. Color change of the indicator is photographed and processed into a sweat density map generated on standard anatomical drawings. The outcome measure of TST is the percentage of anhidrosis at the anterior body. In healthy subjects, sweat distribution is equal over the whole anterior body. Abnormal sweat distributions can result in distal, segmental, regional, mixed, focal or global anhidrosis.76,109 Quantitative Sudomotor Axon Reflex Testing (QSART) QSART assesses the indirect axon reflex mediated sweat response over time. A sweat capsule is used to apply acetylcholine and to measure the humidity caused by increased sweat production.76,127 Latency, duration and magnitude of the sweat response are determined with real-time measurements. Healthy individuals start to produce sweat in approximately 1-2 minutes after the exposure to acetylcholine. Sweat production peaks approximately after 5 minutes and decreases after 10 minutes. Mean sweat output is 2-3 µl/cm2 for males and 0.25-1.2 µl/cm2 for females.127 Sweat output can increase, decrease or persist (“hung up” response). Persistent sweating often relates with hyperalgesia.109 Advantage of QSART is its temporal resolution. Limitations of QSART are disability of measuring preganglionic lesions, it requires special equipment and is rather expensive.76 Silicone impressions With silicone impressions, direct sweat response is measured. Sweat glands are stimulated by iontophoresis, for example with the same module used for QSART. After 10 minutes, the capsule is removed, the skin is dried and a silicone mold is applied. The silicone mold is liquid and cures after approximately 5-10 minutes. Sweat droplets leave imprints in the silicone mold. Number of droplets, droplet area and droplet volume are used as measure for small nerve fiber function.76,128 Healthy individuals have 311±38 sweat droplets/cm2 on hands and 281± 38 droplets/cm2 on feet. Abnormal impressions are predominantly seen in patients with anhidrosis or hyperhidrosis.76 Anhidrosis might be caused due to SFN, since sympathetic cholinergic nerve function is innervated by small nerve fibers. However, many other disorders are associated with anhidrosis. Moreover, this method lacks temporal resolution and is therefore disabled to determine latency and duration of the response. Quantitative Direct and Indirect Reflex Testing (QDIRT / Acetylcholine sweat-spot test) QDIRT is developed in order to combine the advantages of QSART and the silicone imprint technique. Advances in photography are sufficient to enable quantification of dyed sweat droplets like the silicone imprint technique. Iontophoresis of acetylcholine is applied in combination with an indicator dye. Sudomotor function is determined with temporal resolution in the same way as QSART, while spatial resolution (droplet size and number) is determined similar to the silicone imprint technique.129 Autonomic function has limitations as it is influenced by body temperature, humidity, hydration status, nicotine and room temperature.127,129 An important additional limitation is near absent response in female subjects. Female subjects show low sweat volumes, in combination with rapid evaporation in cool dry air.129 Sympathetic Skin Response (SSR) SSR measures changes in skin potentials. The sources of skin potentials are sweat glands and epidermis. Multiple stimuli have been used to elicit SSR, but most of those show no precise onset or indefinable strength of the stimuli. The best definable stimulus is an electrical square wave pulse.109 The stimulus disturbs the autonomic nervous system, generating a change in skin potential. When the autonomic nervous system is affected by SFN, SSR latency will be delayed. The skin potential is measured with standard EMG electrodes. Presence or absence of the response as well as amplitude and latency are reported from the SSR. Absence of SSR might be caused by habituation or an inefficient stimulus.76 Latency is hard to determine for multiple stimuli, and amplitude varies. Advantages of SSR are easy performance, it requires little additional training, no special equipment next to standard EMG, it is believed to be useful, clinically meaningful, reliable and has extensive published data.109 2 35 2
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