Characteristics of sensitization associated with chronic pain conditions

Abstract

Objectives: To describe and understand varieties and characteristics of sensitization contributing to hyperalgesia in participants with chronic pain conditions.

Methods: Thermal stimulation was delivered to the face, forearm, and calf of pain-free participants and individuals with irritable bowel syndrome, temporomandibular pain disorder (TMD), and fibromyalgia syndrome (FM). Three-second contacts by a preheated thermode occurred at 30-second intervals in ascending and then in descending series (0.7°C steps).

Results: Thermal pain ratings during ascending series were greater at each site in individuals diagnosed with chronic pain. Intense pain at the time of testing further enhanced the ratings at all sites, but mild or moderate clinical pain did not have this effect. Thermal pain in all participants was greater during descending series compared with the ascending series of arm and leg stimulation. The hypersensitivity during the descending series was comparable in pain-free, FM and TMD participants but was increased in duration for arm or leg stimulation of FM participants.

Discussion: The widespread sensitization for irritable bowel syndrome and TMD participants does not rely on mechanisms of spatial and temporal summation often invoked to explain widespread hyperalgesia associated with chronic pain. Increased sensitivity during descending series of stimulation of an arm or leg but not the face indicates a propensity for sensitization of nociceptive input to the spinal cord. Abnormally prolonged sensitization for FM participants reveals a unique influence of widespread chronic pain referred to deep somatic tissues.

Figure 1

Ratings of pain intensity elicited by ascending series of 3 sec heat stimuli at interstimulus intervals of 30 sec. Each series ended when the eVAS rating equaled or exceeded 40. Stimulation was delivered to the face (left column), arm (middle column) and leg (right column). Pain sensitivity for the ascending progression of stimulus intensities was consistently greater (rating were higher) for subjects with chronic pain (closed squares: TMD, upper row; IBS, middle row; FMS, bottom row) than for pain-free control subjects (open circles). Errors bars depict between subjects’ variability as standard errors of the mean (S.E.M).

Figure 2

The average eVAS ratings of patients with chronic pain (TMD, IBS, FMS) minus the average eVAS ratings of pain-free control subjects for ascending series of stimulation at all sites (with S.E.M. error bars). The differences in ratings peaked for temperatures (48.6°C and 49.3°C) rated below eVAS 20 by control subjects (see Figure 1).

Figure 3

eVAS ratings of clinical pain immediately prior to psychophysical test sessions are plotted against temperatures sufficient to evoke eVAS ratings of 15 with thermal stimulation of the face, arm or leg. Lower temperatures indicate greater sensitivity to thermal stimulation. Error bars represent S.E.M.

Figure 4

Average eVAS ratings of the last 8 stimuli in ascending series and the first 8 stimuli in descending series, normalized to the last stimulus in ascending series. The range of temperatures spans 4.9°C, progressing in 0.7°C steps of ascending (up) and then descending (down) intensities delivered to the face (left column), forearm (middle column) and calf of control subjects (top row) and TMD (second row), IBS (third row) and FMS (bottom row) patients. Ratings were consistently and significantly higher for descending series of stimulation delivered to the arm and leg of each group of subjects, but ratings did not differ significantly for ascending and descending series of facial stimulation. Errors bars depict between-subjects’ variability (S.E.M.)

Figure 5

Average differences between ratings of stimuli in descending and ascending series of stimulation (down – up) for control subjects vs. each group of pain patients (TMD: top row; IBS: middle row; FMS: bottom row). For TMD and FMS subjects, there was no significant difference in sensitization following an ascending series of stimulation to the forearm (left column) or leg (right column). In contrast, sensitization was significantly greater for FMS subjects, compared to controls, at both sites. Errors bars depict between-subjects’ variability (S.E.M.)