Central Nervous System Sensitization, Hyperalgesia, Pain Perception, Pain Threshold
Conditions
Keywords
temporal summation, offset analgesia, topical capsaicin, secondary hyperalgesia, quantitative sensory testing (QST), crossover trial, session-order effect, central sensitisation
Brief summary
Temporal summation (TS) and offset analgesia (OA) are widely used psychophysical endpoints in pain research that index different components of central nociceptive processing. While crossover designs are commonly used in experimental pain studies to reduce between-participant variability, the design-stability of these endpoints under repeated testing during experimental sensitisation is not well characterised. This study compared the design-stability of mechanical TS (Sumscore) and offset analgesia magnitude (OffA) in a two-period, vehicle-controlled crossover trial of capsaicin-evoked secondary hyperalgesia in healthy adults. The primary aim was methodological: to determine whether session-order effects differ between TS and OffA when these are used as outcome measures in two-period crossover designs of capsaicin-induced central sensitisation. Topical capsaicin was used as a reversible experimental intervention to create a controlled, transient state of secondary hyperalgesia rather than as a therapeutic intervention. The study informs endpoint selection in future quantitative sensory testing (QST) crossover trials.
Detailed description
This was a single-site, two-period, vehicle-controlled, allocation- and formulation-blinded, randomised crossover study conducted at the Department of Human Health Science, University of Guelph, Canada. Sixteen healthy adult volunteers (10 female, 6 male; mean age 21.9 years) were recruited between May 2024 and April 2025. Participants were randomised in a 1:1 intended allocation to one of two sequences: capsaicin-first then vehicle (Sequence A-to-B) or vehicle-first then capsaicin (Sequence B-to-A); actual allocation was 6:10. The allocation sequence was computer-generated by a research assistant not involved in outcome collection. Sessions were separated by a minimum 1-week washout. Topical 0.075% capsaicin cream (Zostrix; Hi-Tech Pharmacal) was applied as a 5 mL dose to a 5 x 10 cm target area on the lateral elbow and to bilateral C5-C6 cervicothoracic dermatomes. The comparator was an equivalent 5 mL volume of inert vehicle lotion (Lubriderm; Johnson & Johnson) applied identically; creams were matched for colour and texture and prepared by a research assistant in unlabelled containers. Mechanical temporal summation (Sumscore; 256 mN pinprick, 16 stimuli at 1 Hz) and thermal offset analgesia (OA; 32-46°C stepped-stimulus protocol with three thermal hold durations of 5, 10, and 15 s) were assessed at baseline and at 10, 20, 30, and 40 min post-intervention. Continuous pain ratings during thermal stimulation were captured via CoVAS. Primary outcomes (Sumscore and the duration-averaged baseline-normalised OffA) were analysed using REML linear mixed-effects models with fixed effects for Intervention, Time, Intervention x Time, Period, and Sequence, with random intercepts for Participant; sex was included as a pre-specified covariate. Within-session temporal stability was assessed using ICC(2,1). A combined Measure x Intervention x Period model tested for differential design-sensitivity between TS and OffA. The study was approved by the University of Guelph Research Ethics Board (REB #19-06-003).
Interventions
DRUGTopical 0.075% capsaicin cream (Zostrix)
5 mL of 0.075% capsaicin cream (Zostrix; Hi-Tech Pharmacal, Amityville, NY, USA) applied to a 5 x 10 cm target region on the lateral elbow and to bilateral C5-C6 cervicothoracic dermatomes (total 15 mL). The cream was massaged into the skin by a gloved investigator until no residue was visible. Used as an experimental probe to evoke transient, reversible secondary hyperalgesia; not under therapeutic evaluation.
OTHERTopical vehicle lotion (Lubriderm)
5 mL of inert vehicle lotion (Lubriderm; Johnson \& Johnson, Montgomery, NJ, USA) applied to the same dermatomes as the capsaicin condition (total 15 mL), using the identical preparation and massage technique. Matched to the capsaicin cream for colour and texture.
Sponsors
University of Guelph
Study design
Allocation
RANDOMIZED
Intervention model
CROSSOVER
Primary purpose
BASIC_SCIENCE
Masking
TRIPLE (Subject, Investigator, Outcomes Assessor)
Masking description
By design, participants, the investigator who applied the creams and collected the outcome data, and the data analyst were blinded to allocation. The randomisation sequence was computer-generated by a research assistant not involved in testing and concealed in sequentially sealed, opaque envelopes; capsaicin and vehicle creams were matched for colour and texture and dispensed in unlabelled containers. Because topical 0.075% capsaicin can produce perceptible burning and visible erythema, participant and investigator blinding may have been incompletely maintained. Participant allocation guesses were collected and investigator awareness of allocation was not formally assessed. The data analyst remained blinded through data lock and application of pre-specified exclusion criteria, and was unblinded only after the analysis dataset was finalised.
Intervention model description
Two-period crossover. Participants were randomised in a 1:1 intended allocation (actual 6:10) to one of two sequences (capsaicin-first or vehicle-first). Sessions were separated by a minimum 1-week washout.
Eligibility
Sex/Gender
ALL
Age
18 Years to No maximum
Healthy volunteers
Yes
Inclusion criteria
* Healthy adult volunteers (≥18 years of age) * Able to provide written informed consent in English
Exclusion criteria
* Current or chronic pain * Neurological disease * Skin conditions or sensitivity over the test sites to capsaicin * Contraindications to thermal stimulation of the skin * Musculoskeletal conditions that could alter somatosensory processing * Use of analgesic medications within 24 hours prior to each session * Use of caffeine within 24 hours prior to each session * Use of alcohol within 24 hours prior to each session
Design outcomes
Primary
| Measure | Time frame | Description |
|---|---|---|
| Mechanical temporal summation (Sumscore) | Baseline and 10, 20, 30, and 40 min post-intervention (within each of two sessions, separated by a minimum 1-week washout) | Sumscore is a summed pinprick temporal-summation response computed from a train of 16 repeated 256 mN pinprick stimuli delivered perpendicularly to the skin at 1 Hz, paced by a metronome. Pain ratings (NPRS 0-10) immediately after each stimulus were summed (Clouse et al., 2021). The primary Sumscore endpoint was pre-specified as the average of the lateral and inferior testing sites at the lateral elbow, expressed as a baseline-normalised ratio. |
| Offset analgesia magnitude (OffA) | Baseline and 10, 20, 30, and 40 min post-intervention (within each of two sessions, separated by a minimum 1-week washout) | OffA was measured during a three-phase stepped-stimulus thermal protocol (32-45-46-45-32°C; T1/T2/T3 phases) using a 32 x 32 mm Peltier contact thermode (TSA-II NeuroSensory Analyzer, Medoc AMS). Continuous pain ratings were collected via CoVAS (0-100) at 10 Hz. OA magnitude was quantified as peak minus nadir CoVAS during the 46°C hold and the subsequent 45°C phase. The primary OffA endpoint was pre-specified as the duration-averaged value across three thermal hold durations (5, 10, and 15 s at 46°C), expressed as a baseline-normalised ratio. |
Secondary
| Measure | Time frame | Description |
|---|---|---|
| Within-session temporal stability (ICC(2,1)) of Sumscore and OffA | 10, 20, 30, and 40 minutes post-intervention | Intraclass correlation coefficient (ICC(2,1); Shrout and Fleiss, 1979) computed across the four post-intervention time points (10, 20, 30, 40 min) within each intervention condition, separately for Sumscore and OffA. Reported with bias-corrected bootstrap 95% confidence intervals. |
| Differential design-sensitivity (Measure × Intervention × Period interaction) | 10, 20, 30, and 40 minutes post-intervention, across two crossover periods separated by a minimum 1-week washout. | A combined linear mixed-effects model using within-measure z-scored outcomes tested whether the period-adjusted intervention estimate differed between TS and OffA. Three-way Measure x Intervention x Period interaction tested via Wald F-tests with residual degrees of freedom. |
Countries
Canada
Outcome results
None listed