Reproducibility of axon reflex-related vasodilation assessed by dynamic thermal imaging in healthy subjects

Introduction: Small nerve fiber dysfunction is an early feature of diabetic neuropathy. There is a strong clinical need for a non-invasive method to assess small nerve fiber function. Small nerve fibers mediate axon reflex-related vasodilation and play an important role in thermoregulation. Assessing the reflex vasodilation after local heating might elucidate some aspects of small fiber functioning. In this study, we determined the reproducibility of the reflex vasodilation after short local heating in healthy subjects, assessed with thermal imaging and laser Doppler imaging.
Methods: Healthy subjects underwent six heating rounds in one session (protocol I, N = 10) or spread over two visits (protocol II, N = 20). Reflex vasodilation was elicited by heating the skin to 42 °C with an infrared lamp. Skin temperature and skin blood flow were recorded during heating and recovery with a thermal imaging camera and a laser Doppler imager. Skin temperature curves were fitted with a mathematical model to describe the heating and recovery phase with time constant tau (tauHeat and tauCool1).
Results: The reproducibility of tau within a session was moderate to excellent (intra-class correlation coefficient 0.42–0.86) and good (0.71–0.72) between different sessions. Within one session the differences in tauHeat were small (bias ± SD − 1.3 ± 18.9 s); the bias between two visits was − 1.2 ± 12.2 s. For tauCool1 the differences were also small, 1.4 ± 6.6 s within a session and between visits − 1.4 ± 11.6 s.
Conclusions: The heat induced axon reflex-related vasodilation, assessed with thermal imaging and laser Doppler imaging, was reproducible both within a session and between different sessions. Tau describes the temporal profile in one parameter and represents the effects of all changes including blood flow and as such, is an indicator of the vasodilator function. TauHeat and tauCool1 can accurately describe the dynamics of the axon reflex-related vasodilator response in the heating and recovery phase respectively.