Session-Order Sensitivity of TS and OA in a Capsaicin Crossover Model (TS-OA-Cap)

Session-order Sensitivity of Temporal Summation and Offset Analgesia in a Randomised Crossover Capsaicin Model of Secondary Hyperalgesia

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.

Study Overview

• Status: Completed
• Conditions: Hyperalgesia; Central Nervous System Sensitization; Pain Perception; Pain Threshold
• Intervention / Treatment: Drug: Topical 0.075% capsaicin cream (Zostrix); Other: Topical vehicle lotion (Lubriderm)

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).

• Study Type: Interventional
• Enrollment (Actual): 16
• Phase: Not Applicable

Contacts and Locations

Study Locations

• Canada
  • Ontario
    • Guelph, Ontario, Canada, N1G2W1
      • Human Health Sciences, University of Guelph

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

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

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: Triple

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Arm 1: Sequence A-to-B (Capsaicin first); Participants received topical 0.075% capsaicin (Zostrix) cream (5 mL) in Period 1, followed by topical inert (Lubriderm) vehicle (5 mL) in Period 2. Periods were separated by a minimum 1-week washout. n = 6.	Drug: Topical 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.; Other: Topical 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.
Active Comparator: Arm 2: Sequence B-to-A (Vehicle first); Participants received topical inert (Lubriderm) vehicle (5 mL) in Period 1, followed by topical 0.075% capsaicin (Zostrix) cream (5 mL) in Period 2. Periods were separated by a minimum 1-week washout. n = 10.	Drug: Topical 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.; Other: Topical 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.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Mechanical temporal summation (Sumscore)	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.	Baseline and 10, 20, 30, and 40 min post-intervention (within each of two sessions, separated by a minimum 1-week washout)
Offset analgesia magnitude (OffA)	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.	Baseline and 10, 20, 30, and 40 min post-intervention (within each of two sessions, separated by a minimum 1-week washout)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Within-session temporal stability (ICC(2,1)) of Sumscore and OffA	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.	10, 20, 30, and 40 minutes post-intervention
Differential design-sensitivity (Measure × Intervention × Period interaction)	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.	10, 20, 30, and 40 minutes post-intervention, across two crossover periods separated by a minimum 1-week washout.

Collaborators and Investigators

• Sponsor: University of Guelph

Publications and helpful links

General Publications

• Grill JD, Coghill RC. Transient analgesia evoked by noxious stimulus offset. J Neurophysiol. 2002 Apr;87(4):2205-8. doi: 10.1152/jn.00730.2001.
• Dwan K, Li T, Altman DG, Elbourne D. CONSORT 2010 statement: extension to randomised crossover trials. BMJ. 2019 Jul 31;366:l4378. doi: 10.1136/bmj.l4378.
• Clouse JA, et al. The reliability of a temporal summation Sumscore. (2021)

Study record dates

Study Major Dates

• Study Start (Actual): May 1, 2024
• Primary Completion (Actual): April 30, 2025
• Study Completion (Actual): April 30, 2025

Study Registration Dates

• First Submitted: May 22, 2026
• First Submitted That Met QC Criteria: June 3, 2026
• First Posted (Actual): June 4, 2026

Study Record Updates

• Last Update Posted (Actual): June 4, 2026
• Last Update Submitted That Met QC Criteria: June 3, 2026
• Last Verified: June 1, 2026

More Information

Terms related to this study

• Keywords: secondary hyperalgesia; offset analgesia; central sensitisation; temporal summation; crossover trial; topical capsaicin; quantitative sensory testing (QST); session-order effect
• Additional Relevant MeSH Terms: Neurologic Manifestations; Nervous System Diseases; Sensation Disorders; Somatosensory Disorders; Pathological Conditions, Signs and Symptoms; Signs and Symptoms; Hyperalgesia; Organic Chemicals; Heterocyclic Compounds; Fatty Acids; Lipids; Alkenes; Hydrocarbons, Acyclic; Hydrocarbons; Hydrocarbons, Cyclic; Alkaloids; Hydrocarbons, Aromatic; Amides; Catechols; Phenols; Benzene Derivatives; Fatty Acids, Unsaturated; Solanaceous Alkaloids; Polyunsaturated Alkamides; Fatty Acids, Monounsaturated; Capsaicin
• Other Study ID Numbers: GUELPH-REB-19-06-003; REB #19-06-003 (Other Identifier: University of Guelph)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: De-identified individual participant data (IPD) and statistical analysis code will be available from the corresponding author on reasonable request, subject to a data-use agreement and institutional ethics/data-sharing requirements. Materials required to reproduce the intervention and assessment protocol are described in the published Methods and Supplementary Materials.
• IPD Sharing Time Frame: On request following publication of the primary manuscript.
• IPD Sharing Access Criteria: Reasonable request to the corresponding author for non-commercial research use, subject to a data-use agreement consistent with University of Guelph data-sharing policy.
• IPD Sharing Supporting Information Type: STUDY_PROTOCOL; SAP; ICF; ANALYTIC_CODE

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No