Spatial summation of mechanically evoked muscle pain and painful aftersensations in normal subjects and fibromyalgia patients

Abstract

Impulse frequency and number of recruited central neurons are relevant for pain encoding and temporal as well as spatial summation of pain (SSP). Whereas SSP of heat-induced pain is well characterized, mechanical SSP (MSSP) has been less studied. MSSP may be relevant for chronic pain conditions like fibromyalgia (FM) and play an important role in the pathogenesis of this chronic pain syndrome. Our study was designed to determine MSSP in 12 normal controls (NC) and 11 FM subjects. MSSP testing consisted of 5 s suprathreshold pressure-pain stimulations of forearm muscles by up to three identical probes (separated by 4 or 8 cm). The stimulated areas ranged between 0.79 and 2.37 cm2. The subjects rated the pain intensity of mechanical stimuli as well as pain aftersensations. Although MSSP increased monotonically in NC and FM subjects, pressure pain and pressure pain aftersensations were greater in FM subjects and highly associated with clinical pain intensity (r2=.44-.64), suggesting that spatial and temporal summation factors may contribute to overall clinical pain. However, despite higher experimental pain ratings, the magnitude of MSSP was not statistically different between NC and FM subjects. Furthermore, muscle stimuli elicited more MSSP when separated by 8 cm than 4 cm and this finding was not different between NC and FM subjects. Thus, mechanisms of MSSP were similar for both FM and NC subjects. The important role of MSSP for pain encoding suggests that decreasing pain in some muscle areas by local anesthetics or other means may improve overall clinical pain of FM patients.

Figure 1

Location of pressure stimuli applied to the volar surface of the forearms. The surface area of each probe was identical (0.79 cm²). Three different probe combinations (A, B, C) were utilized for two-probe experiments. Two probe tips were always separated by 4 cm (A, B), except for combination C (8 cm). The locations of individual probes during MSSP experiments with one or three probes are shown in Panel D.

Figure 2

Time course of mechanical stimuli used for MSSP experiments in NC and FM subjects. Duration of all pressure stimuli was 5 sec. At the beginning of the stimulus pressures slowly increased at a rate of 1 kg/sec from zero to 2.5 kg, remained at this level for 2.5 sec, and rapidly returned to zero at the end of the stimulus

Figure 3

Apparatus used for MSSP experiments in NC and FM subjects. Three cylinders with identical pistons were mounted on a metal frame that allowed adjustments in vertical and horizontal directions. All cylinders were powered by a pressurized air in parallel fashion, thus assuring equal pressures at each piston during the experiments. When controlled air pressure was applied, pistons extended from the cylinders for 5 sec during each trial and actively retracted after each stimulus. All piston tips were circular and flat (surface area: 0.79 cm²) with smooth edges. The distance between piston tips was always kept at 4 cm except during some of the two probe experiment C (8 cm). One probe was equipped with an electronic pressure transducer connected to a digital read-out. Because identical air pressures were applied to the cylinders during each stimulus trial, pressure readings at one probe were representative for all other probes. Each subject’s forearm was stabilized in a plastic mold underneath the probes during the MSSP experiments.

Figure 4

Average mechanical pain ratings (SD) of NC and FM subjects during mechanical stimulation (2.5 kg) of the forearms using single probes at three different locations (4 cm apart). There was no significant difference between ratings of single probes regardless of location (p > .05) in NC or FM subjects. There was, however, a significant interaction effect of diagnostic group (p < .001), indicating that FM subjects’ mechanical pain ratings were higher than those of NC.

Figure 5

Average (SD) mechanical pain ratings of NC and FM subjects during mechanical stimulation of the forearms with two probes. Probe positions were varied in three different ways (see Figure 1 A-C) and pressure at each probe was maintained at 2.5 kg during the 5 sec stimuli. mechanical pain ratings of FM subjects were higher than NC regardless of stimulus combination (p < .001). MSSP of NC subjects was greater with two probes separated by 8 cm than by 4 cm (p < .01) and this finding was not statistically different in FM subjects (p > .05).

Figure 6

Average mechanical pain ratings (SD) of NC and FM subjects during 5 sec mechanical stimuli using one, two, or three probes. There was significant MSSP noted in both groups of subjects (p < .001). Although mechanical pain ratings of FM subjects were higher than NC (p < .002), the increase (slope) of mechanical pain ratings was not different between diagnostic groups (p > .05).

Figure 7

Correlations of mechanical pain ratings with clinical FM pain. mechanical pain ratings were obtained after 5 sec of mechanical stimulation of forearm muscles with one probe (●), two probes (■), or three probes (▲). Pearson product-moment correlations of clinical FM pain with mechanical pain ratings were significant for one (r² = .47; p < .05), two probes (r² = .47; p < .05), and three probes experiments (r² = .44; p < .05).

Figure 8

Average ratings (SD) of AS by NC and FM subjects during the MSSP experiments. Pain ratings of AS were obtained 15 sec (Panel A) and 30 sec (Panel B) after the end of 5 sec pressure stimuli using one, two, or three probes on the forearms. mechanical pain-AS were significantly more painful 15 sec and 30 sec after mechanical stimuli in FM subjects compared to NC. Although MSSP-AS seemed to be dependent on number of probes used, this relationship was not consistently seen for three probe combinations. AS15: 15 sec aftersensations; AS30: 30 sec aftersensations; ns: non-significant

Figure 9

Correlations of mechanical pain-AS with clinical FM pain. Ratings of pain AS were obtained 15 sec (Panel A) and 30 sec (Panel B) after the end of 5 sec forearm muscle stimulation with one, two, or three probes. Clinical FM pain ratings (VAS) highly correlated with ratings of 15 sec and 30 sec AS (p < .001). AS = aftersensations

Figure 9

Correlations of mechanical pain-AS with clinical FM pain. Ratings of pain AS were obtained 15 sec (Panel A) and 30 sec (Panel B) after the end of 5 sec forearm muscle stimulation with one, two, or three probes. Clinical FM pain ratings (VAS) highly correlated with ratings of 15 sec and 30 sec AS (p < .001). AS = aftersensations