Fig 1.
A total of 170 individuals replied to the advertisement. Ninety-four subjects were assessed for general eligibility on an enrichment selection session (Session 0). Twelve subjects were excluded before the assessments, three tested positive for tetrahydrocannabinol (THC) in their urine, one used prescription drugs and seven had scars or tattoos in the assessment areas. Eighty-three subjects received a cutaneous heat injury (CHI) on Session 0. Subsequently, three subjects were excluded, two subjects did not develop secondary hyperalgesia areas (SHAs), and one subject had a vasovagal syncope following the CHI. Eighty subjects’ SHAs were analyzed and ranked according to magnitude, assessed by planimetric measurements. Through an enrichment process, 40 subjects with SHAs in the two middle quartiles (Q2/Q3) were discontinued, while the 20 subjects belonging to the lower quartile (Q1) and the 20 subjects belonging to the upper quartile (Q4) were randomized and continued to the experimental sessions. Two subjects from Q4 were, however, excluded before the allocation procedure in Session 1 for reasons unrelated to the trial, one subject was unavailable, and another was diagnosed with acute lymphocytic leukemia (ALL). All other subjects completed per-protocol the trial sessions, and the final analysis, thus, included 38 subjects.
Table 1.
Inclusion and exclusion criteria.
Fig 2.
Cutaneous heat injuries were induced on an enrichment session (Session 0), a selection process uncovering ‘high-sensitizers’ (large secondary hyperalgesia areas: upper quartile [Q4]) and ‘low-sensitizers’ (small secondary hyperalgesia areas: lower quartile [Q1]). Sessions 1 and 3 included repeat cutaneous heat injuries in ‘high- and low-sensitizers’. Target-controlled infusion sessions were Sessions 2 and 4, with randomized allocation between placebo and naloxone. The time interval between Sessions 1 and 2, and, Sessions 3 and 4, was 7 days. The time interval between Sessions 0 and 3 was > 8 weeks.
Fig 3.
Session 1 and 3 (cf. Fig 1) included baseline assessments (green rectangle, 0 min), induction of a cutaneous heat injury (CHI, red rectangle, 20 min) and post-injury assessments (blue rectangles: 1 h 27 min and 2 h 27 min). Session 2 and 4 (cf. Fig 1) included a pre-drug assessment (magenta rectangle; post-injury 165 h), drug-infusions (naloxone or placebo; 167 h 35 min, 168 h 0 min, and 168 h 25 min), and assessments during target-controlled infusion (TCI; grey rectangles; 167 h 35 min, 168 h 0 min, and 168 h 25 min). The estimated TCI plasma concentrations are superimposed in dashed red line. Numbers (1 to 3) during the infusion period, indicate the three TCI-steps. Assessments included secondary hyperalgesia areas and online reaction time indicated by green stars and pin-prick pain thresholds indicated by red stars.
Fig 4.
Test-algorithm Session 2 and 4 for subjects with superimposed naloxone plasma-concentration curves. Median plasma-concentration (red) with 10% and 90% percentiles ranges (dashed black lines) during a three-step target-controlled infusion (TCI). Naloxone was administered at step 1 (ST1: 15 min to 25 min) with 0.25 mg/kg, step 2 (ST2: 39 min to 49 min) 0.75 mg/kg, and step 3 (ST3: 65 min to 75 min) 2.25 mg/kg. Magenta columns represent ratings with Clinical Opiate Withdrawal Scale (COWS) and blue columns indicate sensory testing (BL and ST1 to ST3: secondary hyperalgesia areas; BL and ST3: pin-prick pain thresholds). BL: baseline assessments [22].
Table 2.
Target-controlled infusion (TCI) of naloxone.
Table 3.
Demographics and anthropometrics.
Fig 5.
Sensitizer categorization following cutaneous heat injuries.
Secondary hyperalgesia areas as measured at baseline on Sessions 0, 1, and 3. Red lines illustrate ‘high-sensitizers’ and blue lines illustrate ‘low-sensitizers’. The blue rectangle shows the Q1 interval and the red rectangle shows the Q4 interval as defined on Session 0.
Fig 6.
Magnitude of secondary hyperalgesia areas following naloxone or placebo.
Comparison of individual maximum secondary hyperalgesia area (SHAMAX) with subtraction of baseline areas after administration of naloxone or placebo. When all subjects were analyzed without partitioning into ‘high- and low-sensitizers’, there were no significant difference between SHAs following naloxone infusion (median [95% CI]: 0 [0–0.3] cm2), compared to the placebo infusion (median [95% CI]: 0 [0–0] cm2; Wilcoxon signed-rank test: P = 0.215). Furthermore, no significant difference for SHAMAX was found between ‘high-sensitizers’ (median [95% CI]: 0 [0–14.4] cm2) and ‘low-sensitizers’ (median [95% CI]: 0 [0–0] cm2; Mann-Whitney U test: P = 0.757). Red lines represent ‘high-sensitizers’ and blue lines represent ‘low-sensitizers’.
Table 4.
Mixed-effects model (secondary hyperalgesia area data).
Table 5.
Post hoc mixed-effects model (secondary hyperalgesia area data).
Table 6.
Estimates of fixed effects for the mixed-effects model (secondary hyperalgesia area data).