Table 1.
Inclusion and exclusion Criteria.
Figure 1.
A: Study Algorithm – Experimental day (Top).
All variables were recorded in fixed order throughout the study. MRI was performed on a 3.0 T Philips Achieve Scanner (Philips Medical Systems, Best, The Netherlands) using a 32-channel phased-array send and receive head coil. Initially T1-weighted 3D anatomical images were obtained. Pre-burn BOLD response measurements were performed during painful pin-prick stimulation. Subsequently a first-degree burn-injury was induced on the non-dominant lower leg. After 100 min waiting period the area of secondary hyperalgesia was assessed. The volunteers were then re-positioned in the scanner and BOLD response measurements were repeated twice. There was at least 5 min pause between each BOLD response measurement. B: Stimulation areas (Middle). Areas of noxious pin-prick mechanical stimulation in high- and low-sensitization responders. High-sensitization responders (left) developed large areas of secondary hyperalgesia surrounding the burn-injury site (red square) while low-sensitization responders only developed small areas of secondary hyperalgesia (right). In the first BOLD response measurement, pin-prick stimulation was performed inside the secondary hyperalgesia area one cm from the border of the burn-injury (SH). In the second BOLD response measurement, pin-prick stimulation was performed inside the primary hyperalgesia area (PH). Pin-prick stimulations were delivered within a predetermined area of 1×1 cm. Pain was rated on a verbal numerical rating scale (NRS) after each stimulation. C: Stimulation paradigm (Buttom). To obtain BOLD fMRI measurements we applied noxious mechanical stimulation by weighted-pin stimulators (pin-prick) in a block design. Pain thresholds were individually assessed by pin-prick stimulation and the lightest pin-prick stimulator perceived as painful before induction of the burn-injury was used throughout the study. The stimulation paradigm consisted of rest blocks with no stimulation, alternating with active blocks of pin-prick (pain) stimulation delivered at 1 Hz. One scan session thus consisted of abbrev6 resting blocks (30 s/block) interleaved by 5 stimulation blocks (30 s/block) with a total duration of 5 min 30 sec.
Table 2.
Demographic Data.
Table 3.
Blood pressure (BP), heart rate and end-tidal CO2 during fMRI BOLD scans (Mean + SD).
Table 4.
Baseline blood samples.
Figure 2.
Brain activation during mechanical noxious stimulation before burn-injury.
Group analysis showing brain activation by BOLD fMRI during mechanical noxious stimulation at baseline (pre-burn) in (a) high-sensitization responders and (b) low-sensitization responders. (c) Shows differences in brain activation between high- and low-sensitization responders. Activity is displayed upon standardized inflated brain figures with red areas showing regions-of-interest (ROI) with increased or positive activation and blue areas showing ROI’s with decreased or negative activation. We found significant differences in brain activation with more activation in the right inferior frontal gyrus and less activation in the left superior temporal gyrus, left, and right precentral gyrus in high- versus low-sensitization responders. L = Left side, R = Right side, Lat. = Lateral view, Med. = Medial view.
Table 5.
Pre-burn group activation to noxious mechanical stimulation in high-sensitization responders.
Table 6.
Pre-burn group activation to noxious mechanical stimulation in low-sensitization responders.
Table 7.
Contrast analysis results before burn injury in high- versus low-sensitization responders.
Table 8.
Contrast analysis results for noxious stimulation inside the secondary hyperalgesia in high- vs. low-sensitization responders.
Table 9.
Contrast analysis results for noxious stimulation inside the primary hyperalgesia areas in high- vs. low-sensitization responders.
Figure 3.
Differences in brain activation between high- and low-sensitization responders during post-burn mechanical noxious stimulation.
Group analysis showing differences in brain activation by BOLD fMRI between high- and low-sensitization responders. Activity is displayed upon standardized inflated brain figures with red areas showing regions-of-interest (ROI) with increased activation and blue areas showing ROI’s with decreased activation. The cartoon on the right illustrates the stimulation zones for the pin-prick stimulation. A: Secondary hyperalgesia: We found significant differences in brain activation between high- and low-sensitization responders after stimulation inside the secondary hyperalgesia area with more activation of the right post-central gyrus, left precuneus, left posterior cingulate cortex, right parahippocampal gyrus, right caudate nucleus and less activation of right precuneus in high- compared to low-sensitization responders. B: Primary hyperalgesia: After stimulation inside the primary hyperalgesia areas we found less activation in the left precentral gyrus and anterior lobe of cerebellum (not shown).
Figure 4.
Structural differences between high- and low-sensitization responders.
Volumes of caudate nuclei are shown as a function of secondary hyperalgesia areas in high-sensitization responders (red triangles) and low-sensitization responders (blue diamonds). High-sensitization responders showed smaller volume of the left (p = 0.001) and right (p = 0.01) (corrected p-values) nucleus caudate volume compared to low-sensitization responders.
Figure 5.
Secondary hyperalgesia areas assessed 100 min after a first-degree burn-injury in the screening day and in the experimental day in high- and low-sensitization responders. The table shows the median, inter quartile range (IQR) and range of secondary hyperalgesia areas in high- and low-sensitization responders on the screening day and experimental day respectively. Red lines show the magnitude of secondary hyperalgesia areas in high-sensitization responders, while blue lines show the magnitude of secondary hyperalgesia areas in low-sensitization responders. There were no differences in secondary hyperalgesia areas between the two experimental days in high-sensitization responders (p = 0.71). In low-sensitization responders the secondary hyperalgesia areas were smaller on the experimental day by a median (experimental day – screening day) of −5.4 cm2 (interquartile range; −9.8 to −3.6 cm2) (p < 0.01). This difference could however be subscribed to differences in thermode size between the screening and the experimental days. On the screening day a 12.5 cm2 thermode was used while a smaller MRI compatible thermode of 9 cm2 was used on the experimental day.
Figure 6.
Pain ratings after noxious mechanical stimulation.
Boxplot of pain ratings by numeric rating scale (NRS) in high- (red boxes) and low-sensitization responders (blue boxes). Noxious mechanical stimulation by pin-prick did not result in any differences in pain scores between high- and low-sensitization responders before burn-injury (high-sensitization responders: 3.6 ± 1.9; low-sensitization responders 2.9 ± 1.8; p = 0.29). After burn-injury there were no differences in pain scores after stimulation in the secondary hyperalgesia areas (high-sensitization responders: 5.0 ± 2.1, low-sensitization responders: 4.4 ± 2.1 (p = 0.37)) or after stimulation in the primary hyperalgesia areas (high-sensitization responders: 6.3 ± 2.1, low-sensitization responders 5.1 ± 2.2, p = 0.08).