Capsaicin-induced Muscle Pain in Humans
Condition: Healthy
MeSH terms: Myalgia
Study type: Interventional

Summary: 
      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:

        1. to describe the development, time course and intensity of capsaicin-induced muscle pain

        2. to evaluate the effectiveness of suprascapular nerve block against capsaicin-induced
           muscle pain

        3. to compare suprascapular nerve block and direct intramuscular local anesthetic
           infiltration for their effectiveness in capsaicin-induced muscle pain
    

Detailed description: 
      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

        1. To determine normative values for endogenous pain modulation in healthy volunteers

        2. To demonstrate that capsaicin-induced pain and hyperalgesia depend on endogenous pain
           modulation

        3. To calculate sensitivity and specificity of suprascapular nerve block for the diagnosis
           of primary muscle pain

        4. To determine whether suprascapular nerve block or intramuscular local anesthetic
           infiltration are more effective against muscle pain

      Methods

        1. intramuscular injection of capsaicin

        2. quantitative sensory tests (pressure pain thresholds)

        3. ultrasound-guided suprascapular nerve block

        4. ultrasound-guided intramuscular local anesthetic infiltration
    

Intervention name(s): Intramuscular capsaicin injection; Suprascapular nerve block; Local anesthetic infiltration
Intervention type(s): Procedure; Procedure; Procedure
Intervention description(s): Intramuscular injection of capsaicin: 50 mcg/0.5 ml; 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.
Intervention MeSH terms: Anesthetics; Anesthetics, Local; Capsaicin
Allocation: Randomized
Intervention model: Crossover Assignment
Masking: Single (Participant)
Primary purpose: Basic Science
Keywords: musculoskeletal pain; regional anesthesia; diagnostic nerve block; intramuscular capsaicin; referred pain; hyperalgesia; experimental pain; central hypersensitivity; human experimental pain