Capsaicin-induced Muscle Pain in Humans

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

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