CO2 Laser stimulation : a modern way for exploring the somesthetic system and its usefulness for the study of the chronic pain state

Pain is one of the most common human experience and the quest for pain relief is one of the oldest preoccupations. In our modern times, pain may still be considered as an enigmatic phenomenon and in many clinical situations pain relief remains a formidable challenge to the physician. Progress in understanding how noxius stimuli are encoded and processed in the nervous system is, like in any other domains of endeavour, tidily linked to the availability and the development of new technical tools (and hance new concepts) which may heldp to push further away the frontier or our ignorance. The present work has been undertaken to study chronic pain state in patients. It was evident that non easy answer was available in our armamentarium to alleviate distressing pain sensations in all situations. Often, the only positive action was bring together various tools (verbal report, scale measurement, reaction time, brain evoked potentials, …) to outline the clinical situation. However what could bring a sophisticated analysis without the basic tool which woud enable us to stimulate selectively the Aδ- and C-fibres, known to conduct nociceptive implusies to the central nervous system. This is what the present work is all about: a CO2 laser stimulator was developed to activate the Aδ- and C-fibres and go further in the experimentation in the hope to unravel some of the mysteries of the functional organisation of the nervous system.
One important aspect of fundamental research in sensory systems is the perfect control over the stimulus. This has always been a limiting factor in experimental investigations on pain, not only in fundamental research but also in clinical evaluation. Indeed, stimulation methodologies should be specific, natural, quantifiable, reproducible and of course avoid to generate lesions. These high level requirements can be achieved, to some extent, by radiative thermal stimulation as it can selectively activate specific categories of peripheral nerve fibres without mechanical contact. Pain induced by radiant heat has been made popular by Hardy et at. (1967) and the Hardy dolorimeter is still is use in some laboratories. However, this conventional heat stimulator is not very powerful and hence the induced increase of cutaneous temperature is too slow (seconds) to study accurately time-locker responses. […]