Brain, Vol. 122, No. 2, 239-246,
February 1999
© 1999 Oxford University Press
Paradoxical heat sensation in healthy subjects: peripherally conducted by A
or C fibres?
1 Technion Faculty of Medicine, Israel Institute of Technology, 2 Department of Neurology, Rambam Medical Centre and Technion Faculty of Medicine, Haifa, Israel
Correspondence to:
Dr David Yarnitsky, Neurology, Rambam Medical Centre, Haifa, Israel
Paradoxical heat sensation upon cooling of the skin has been reported in central as well as in peripheral neurological conditions. In our study, we examined this phenomenon in 35 naive healthy test subjects, of whom 23 experienced paradoxical heat sensation under test conditions. We measured the peripheral conduction velocities of cold sensation, warm sensation and of paradoxical heat sensation by using a quantitative sensory testing model of indirect peripheral conduction velocity measurement. This was based on comparison of measurements at a proximal and a distal site using two measurement methods, one inclusive and the other exclusive of reaction time. We found that the conduction velocity of paradoxical heat sensation (0.70 m/s) was similar to that of warm sensation (0.68 m/s), and that the conduction velocity of cold sensation (7.74–8.01 m/s) was considerably faster. Thus, we conclude that paradoxical heat sensation in healthy subjects is conducted peripherally via slow unmyelinated C fibres and not via the faster A
fibres. Consequently, we propose that paradoxical heat sensation is encoded via the heat sensing pathway, in accordance with the labelled-line code theory. The mechanisms proposed suggest a malfunctioning cold-sensing pathway disinhibiting the heat-sensing pathway, at peripheral, central or both levels, thus facilitating a paradoxical heat sensation.
paradoxical heat sensation; quantitative sensory testing; conduction velocity; reaction time
ANOVA = analysis of variance; HPC = heat, pinch, cold; PHS = paradoxical heat sensation; QST = quantitative sensory testing
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