Contact Heat Evoked Potentials (CHEPs) in Patients with Mild-Moderate Alzheimer's Disease and Matched Control--A Pilot Study.

OBJECTIVE: Clinical studies have found that patients with Alzheimer's disease report pain of less intensity and with a lower affective response, which has been thought to be due to altered pain processing. The authors wished to examine the cerebral processing of non-painful and painful stimuli using somatosensory evoked potentials and contact heat evoked potentials in patients with Alzheimer's disease and in healthy elderly controls. DESIGN: Case-control study SETTING AND SUBJECTS: Twenty outpatients with mild-moderate Alzheimer's disease and in 17 age- and gender-matched healthy controls were included METHOD: Contact heat evoked potentials and somatosensory evoked potentials were recorded in all subjects. Furthermore, warmth detection threshold and heat pain threshold were assessed. Patients and controls also rated quality and intensity of the stimuli. RESULTS: The authors found no difference on contact heat evoked potential amplitude (P = 0.59) or latency of N2 or P2 wave (P = 0.62 and P = 0.75, respectively) between patients and controls. In addition, there was no difference in regard to pain intensity scores or pain quality. The patients and controls had similar warmth detection threshold and heat pain threshold. Somatosensory evoked potentials, amplitude, and latency were within normal range and similar for the two groups. CONCLUSIONS: The findings suggest that the processing of non-painful and painful stimuli is preserved in patients with mild to moderate Alzheimer's disease.

Assessment of small fibers using evoked potentials.

Background and purpose Conventional neurophysiological techniques do not assess the function of nociceptive pathways and are inadequate to detect abnormalities in patients with small-fiber damage. This overview aims to give an update on the methods and techniques used to assess small fiber (Aδ- and C-fibers) function using evoked potentials in research and clinical settings. Methods Noxious radiant or contact heat allows the recording of heat-evoked brain potentials commonly referred to as laser evoked potentials (LEPs) and contact heat-evoked potentials (CHEPs). Both methods reliably assess the loss of Aδ-fiber function by means of reduced amplitude and increased latency of late responses, whereas other methods have been developed to record ultra-late C-fiber-related potentials. Methodological considerations with the use of LEPs and CHEPs include fixed versus variable stimulation site, application pressure, and attentional factors. While the amplitude of LEPs and CHEPs often correlates with the reported intensity of the stimulation, these factors may also be dissociated. It is suggested that the magnitude of the response may be related to the saliency of the noxious stimulus (the ability of the stimulus to stand out from the background) rather than the pain perception. Results LEPs and CHEPs are increasingly used as objective laboratory tests to assess the pathways mediating thermal pain, but new methods have recently been developed to evaluate other small-fiber pathways. Pain-related electrically evoked potentials with a low-intensity electrical simulation have been proposed as an alternative method to selectively activate Aδ-nociceptors. A new technique using a flat tip mechanical stimulator has been shown to elicit brain potentials following activation of Type I A mechano-heat (AMH) fibers. These pinprick-evoked potentials (PEP) have a morphology resembling those of heat-evoked potentials following activation of Type II AMH fibers, but with a shorter latency. Cool-evoked potentials can be used for recording the non-nociceptive pathways for cooling. At present, the use of cool-evoked potentials is still in the experimental state. Contact thermodes designed to generate steep heat ramps may be programmed differently to generate cool ramps from a baseline of 35◦C down to 32◦C or 30◦C. Small-fiber evoked potentials are valuable tools for assessment of small-fiber function in sensory neuropathy, central nervous system lesion, and for the diagnosis of neuropathic pain. Recent studies suggest that both CHEPs and pinprick-evoked potentials may also be convenient tools to assess sensitization of the nociceptive system. Conclusions In future studies, small-fiber evoked potentials may also be used in studies that aim to understand pain mechanisms including different neuropathic pain phenotypes, such as cold- or touch-evoked allodynia, and to identify predictors of response to pharmacological pain treatment. Implications Future studies are needed for some of the newly developed methods.