QDIRT: quantitative direct and indirect test of sudomotor function

Abstract

Objective: To develop a novel assessment of sudomotor function.

Background: Postganglionic sudomotor function is currently evaluated using the quantitative sudomotor axon reflex test (QSART) or silicone impressions. We hypothesize that high-resolution digital photography has advanced sufficiently to allow quantitative direct and indirect reflex testing of sudomotor function (QDIRT) with spatial and temporal resolution comparable to these techniques.

Methods: Sweating in 10 humans was stimulated on both forearms by iontophoresis of 10% acetylcholine. Silicone impressions were made and topical indicator dyes were digitally photographed every 15 seconds for 7 minutes after iontophoresis. Sweat droplets were quantified by size, location, and percent surface area. Each test was repeated eight times in each subject on alternating arms over 2 months. Another 10 subjects had silicone impressions, QDIRT, and QSART performed on the dorsum of the right foot.

Results: The percent area of sweat photographically imaged correlated with silicone impressions at 5 minutes on the forearm (r = 0.92, p < 0.01) and dorsal foot (r = 0.85, p < 0.01). The number of sweat droplets assessed with QDIRT correlated with the silicone impression, although the droplet number was lower (162 +/- 28 vs 341 +/- 56, p < 0.01, r = 0.83, p < 0.01). The sweat response and sweat onset latency assessed by QDIRT correlated with QSART measured at the dorsum of the foot (r = 0.63, p < 0.05; r = 0.52, p < 0.05).

Conclusions: The quantitative direct and indirect reflex test of sudomotor function (QDIRT) measured both the direct and the indirect sudomotor response with spatial resolution similar to that of silicone impressions, and with temporal resolution similar to that of the quantitative sudomotor axon reflex test (QSART). QDIRT provides a novel tool for the evaluation of postganglionic sudomotor function.

Figures

Figure 1

Correlation scatter plots of QDIRT vs. silicone impression techniques using both povidone-iodine and alizarin red at 1 minute and 5 minutes in male and female subjects. The total sweat production in a 4.5 cm2 region of the forearm comparing silicone impressions to QDIRT. QDIRT results are shown at 1 minute (A&B), and 5 minutes (C&D), for both povidone-iodine (A&C) and alizarin red (B&D). All silicone impressions are only measured at 5 minutes following standard protocols. Distribution of male and female subjects can be seen in each figure; male subjects exhibited greater sweat production at all time points using all testing methods (P<0.01).

Figure 2

QDIRT images and analysis: example in healthy control and small fiber neuropathy subject. QDIRT images of a healthy control (A) and (SFN) small fiber neuropathy subject (B) tested on the lateral thigh. The large circle indicates the total area of sweat analyzed (4.5 cm2), the inner circle is the direct sweat response (1.13 cm2). Indirect sweat response is derived by subtracting the inner from the outer circle. Total sweat response over time is shown for each subject (C) and is separated into direct and indirect response (D).

Figure 3

Change in sweat area over time in the healthy control and small fiber neuropathy subject shown in Figure 2. The change in sweat area over time (Δ sweat area/Δ time) between consecutive 15 second images identifying the total (A), direct (B) and indirect (C) sweat response for both the healthy control and small fiber neuropathy (SFN) subject. The maximal rate of sweat production is reduced in the SFN subject in the total, direct and indirect regions. There is also a delay in latency to peak sweat production in the indirect and total regions (the delay in the indirect latency appears to be the major contributor to the total response).