How to conduct diagnostic tests in patients suspected of neuropathic pain, How to record laser and cool evoked potentials

Neuropathic pain is a clinical description which refers to a pain caused by an impairment of the somatosensory nervous system. Neuropathic pain is commonly associated with damage of the nociceptive somatosensory system. Several tools have been developed to investigate the nociceptive pathways. In electrophysiology, laser-evoked potentials (LEPs) represent the best tool to assess the function of thermo-nociceptive pathways. The technique consists of briefly heating small patches of the skin using short-lasting pulses of laser at a temperature which activates thermosensitive receptors, i.e. A- and C-fibers. Concomitantly the cerebral activity elicited by these stimuli is recorded using surface electroencephalography (EEG). In the time domain, LEPs characterized by their amplitudes and latencies can provide information relating to the integrity of the nociceptive system, i.e. globally from thermonociceptors to the cortical regions to which the information is conveyed. In this hands-on presentation, we will show how to obtain LEPs. After presenting the technical aspect of the technique and characterizing the type of responses recorded, possible interpretations and some of the pitfalls that can be encountered will be discussed. As example, data obtained in patients suffering from small fiber neuropathy will be presented. In addition, cool evoked potentials (CEP), an alternative to LEP recently developed thanks to a new contact thermal stimulator, will be briefly presented