QUANTITATIVE SOMATOSENSORY THERMOTEST: A KEY METHOD FOR FUNCTIONAL EVALUATION OF SMALL CALIBRE AFFERENT CHANNELS

Abstract

The quantitative somatosensory thermotest (QST) assesses the function of afferent channels concerned with sensory submodalities served by small calibre fibres. Measured ramps of ascending or descending temperature are applied to the skin through a Peltier contact thermode, and detection thresholds are recorded as the subject signals the onset of a particular sensation. The present study describes underlying principles, methodological aspects and normal reference values for the QST. In patients, measurement of thresholds for cold sensation, warm sensation, cold-induced pain and heat-induced pain, applied to 465 individuals, yielded 13 abnormal patterns segregated into three main groups: (i) thermal (cold or warm) hypoaesthesia; (ii) thermal hyperalgesia (abnormally reduced threshold for cold and/or heat induced pain); (iii) thermal hypoaesthesia combined with thermal hyperalgesia. Critical analysis of these results yielded a number of observations of general relevance: (i) thermal specific (warm or cold) hypoaesthesia and thermal (heat or cold) hyperalgesia may occur in the absence of hypoaesthesia for tactile submodalities served by large calibre afferents; (ii) cold hypoaesthesia and warm hypoaesthesia may dissociate from each other; (iii) thermal pain hyperalgesias may occur in the absence of hypoaesthesias for specific cold or warm sensations; (iv) cold hyperalgesia and heat hyperalgesia may dissociate from each other. Thus, a negative routine sensory examination and unimpaired sensory nerve action potentials do not exclude possible somatosensory dysfunction. Furthermore, while most methods of sensory testing only document normality or deficit, the QST permits additional documentation of hyperalgesia, a positive sensory phenomenon that implies unusual pathophysiologies such as sensitization of receptors, central hyperexcitability, disinhibition or, possibly, ectopic nerve impulse discharge. This psychophysical test does not specify the level within afferent channels, between skin and brainmind, where the abnormality resides. It is recommended that the QST for all four thermal specific and thermal pain functions be incorporated in routine neurological assessment.