Capsaicin-induced Muscle Pain in Humans

Capsaicin-induced Muscle Pain in Humans: From Investigating Central Pain Processing to Developing Clinical Applications

There is currently no specific diagnostic test for primary muscular pain. The present study investigates whether selective blockade of the suprascapular nerve can effectively abolish experimental pain arising from the supraspinatus muscle. Experimental muscle pain is induced by intramuscular injection of capsaicin, an alkaloid from red chili peppers. The study consists of three parts: to describe the development, time course and intensity of capsaicin-induced muscle pain to evaluate the effectiveness of suprascapular nerve block against capsaicin-induced muscle pain to compare suprascapular nerve block and direct intramuscular local anesthetic infiltration for their effectiveness in capsaicin-induced muscle pain

Background Musculoskeletal pain is one of the most common reasons for consulting a physician. Many musculoskeletal pain disorders are associated with a widespread decrease in pain thresholds, indicating a state of hyperexcitability of the central nervous system. This central hypersensitivity can be assessed by measuring pain thresholds to different experimental stimuli, using so-called quantitative sensory tests (QST). Several experimental pain models are being used in healthy volunteers in order to study the central mechanisms of pain processing. Injection of capsaicin is one of them, which is associated with spreading of local pain, development of referred pain and hyperalgesia to thermal, mechanical or electrical stimuli. Intramuscular injection of capsaicin is therefore potentially interesting for investigating primary muscle pain, central hypersensitivity and endogenous pain modulation. The present research project comprises three substudies which use intramuscular capsaicin to detect changes in central pain processing and to evaluate clinical tools for the diagnosis of primary muscle pain. Part 1: The investigators hypothesize that the intensity, duration and expansion area of capsaicin-induced muscle pain depends on a subject's endogenous pain modulation. The investigators expect that subjects with strong endogenous pain modulation develop less pain and hyperalgesia after capsaicin injection than subjects with poor endogenous pain modulation. A large sample of healthy volunteers will therefore be screened using QST and then recruited for capsaicin injection if they show either very strong or very weak endogenous pain modulation. The data of the whole sample can be used to determine normative values of endogenous pain modulation. Part 2 investigates the ability of suprascapular nerve block to abolish primary muscle pain. Healthy volunteers will receive capsaicin injection into the supraspinatus muscle or the trapezius muscle as a control condition in a randomized, blinded fashion. The investigators expect that the nerve block is more effective in the former compared to the latter muscle. Part 3 compares the analgesic efficacy of suprascapular nerve block and direct intramuscular local anesthetic infiltration of the supraspinatus muscle after capsaicin injection. This will determine which procedure is more effective to treat primary muscle pain. Objective To determine normative values for endogenous pain modulation in healthy volunteers To demonstrate that capsaicin-induced pain and hyperalgesia depend on endogenous pain modulation To calculate sensitivity and specificity of suprascapular nerve block for the diagnosis of primary muscle pain To determine whether suprascapular nerve block or intramuscular local anesthetic infiltration are more effective against muscle pain Methods intramuscular injection of capsaicin quantitative sensory tests (pressure pain thresholds) ultrasound-guided suprascapular nerve block ultrasound-guided intramuscular local anesthetic infiltration

musculoskeletal pain, regional anesthesia, diagnostic nerve block, intramuscular capsaicin, referred pain, hyperalgesia, experimental pain, central hypersensitivity, human experimental pain

Pain arising from supraspinatus muscle vs. pain arising from trapezius muscle. Nerve block is only expected to be effective in the former.

Suprascapular nerve block vs. intramuscular local anesthetic against pain arising from the supraspinatus muscle.

Injection of capsaicin 50 mcg/0.5 ml into the supraspinatus or trapezius muscle in randomized order, followed by suprascapular nerve block (ultrasound-guided) using Lidocaine 1%; nerve block is expected to be effective only in pain arising from the supraspinatus muscle. Trapezius pain serves as control condition. Evaluates the diagnostic validity of suprascapular nerve block for muscle pain.

Injection of capsaicin 50 mcg/0.5 ml into the supraspinatus muscle in two different sessions; suprascapular nerve block in one session and intramuscular local anesthetic infiltration in one session (in randomized order) using Lidocaine 1%; the aim is to investigate which procedure provides faster and more efficient pain relief. Compares effectivity of suprascapular nerve block vs. intramuscular local anesthetic infiltration.

Inclusion Criteria: Age 18-80 years Written informed consent Exclusion Criteria Any acute pain at the moment of testing Intake of analgesics 24hours before testing Intake of antidepressants, anticonvulsants or benzodiazepines Allergy to local anesthetic or skin disinfectant Pregnancy or breast-feeding

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