title: Pinprick-evoked brain potentials: a novel tool to assess central sensitization of nociceptive pathways in humans.
abstract: Although hyperalgesia to mechanical stimuli is a frequent sign in patients with inflammation or neuropathic pain, there is to date no objective electrophysiological measure for its evaluation in the clinical routine. Here we describe a technique for recording the electroencephalographic (EEG) responses elicited by mechanical stimulation with a flat-tip probe (diameter 0.25 mm, force 128 mN). Such probes activate Aδ nociceptors and are widely used to assess the presence of secondary hyperalgesia, a psychophysical correlate of sensitization in the nociceptive system. The corresponding pinprick-evoked potentials (PEPs) were recorded in 10 subjects during stimulation of the right and left hand dorsum before and after intradermal injection of capsaicin into the right hand and in 1 patient with a selective lesion of the right spinothalamic tract. PEPs in response to stimulation of normal skin were characterized by a vertex negative-positive (NP) complex, with N/P latencies and amplitudes of 111/245 ms and 3.5/11 μV, respectively. All subjects developed a robust capsaicin-induced increase in the pain elicited by pinprick stimulation of the secondary hyperalgesic area (+91.5%, P < 0.005). Such stimulation also resulted in a significant increase of the N-wave amplitude (+92.9%, P < 0.005), but not of the P wave (+6.6%, P = 0.61). In the patient, PEPs during stimulation of the hypoalgesic side were reduced. These results indicate that PEPs 1) reflect cortical activities triggered by somatosensory input transmitted in Aδ primary sensory afferents and spinothalamic projection neurons, 2) allow quantification of experimentally induced secondary mechanical hyperalgesia, and 3) have the potential to become a diagnostic tool to substantiate mechanical hyperalgesia in patients with presumed central sensitization.
authors.affiliation: Department of Neuroscience, Physiology and Pharmacology, University College London, United Kingdom. g.iannetti@ucl.ac.uk
authors.forename: G D
authors.initials: GD
authors.lastname: Iannetti
authors.forename: U
authors.initials: U
authors.lastname: Baumgärtner
authors.forename: I
authors.initials: I
authors.lastname: Tracey
authors.forename: R D
authors.initials: RD
authors.lastname: Treede
authors.forename: W
authors.initials: W
authors.lastname: Magerl
mesh_terms: D000328:Adult; D002211:Capsaicin / Q000494:pharmacology; D059333:Central Nervous System Sensitization / Q000187:drug effects; D004569:Electroencephalography; D005073:Evoked Potentials, Somatosensory / Q000187:drug effects; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology*; D059225:Nociception / Q000187:drug effects / Q000502:physiology*; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology*; D010147:Pain Measurement / Q000379:methods*; D010812:Physical Stimulation; D018689:Sensory System Agents / Q000494:pharmacology
chemical_list: D018689:Sensory System Agents; D002211:Capsaicin
keywords: central sensitization; electrophysiology; human; neuropathic pain; pain