Local pain and spreading hyperalgesia induced by intramuscular injection of nerve growth factor are not reduced by local anesthesia of the muscle.

Objectives: Injections with local anesthesia for therapeutic and diagnostic purposes are common clinical practice. This double-blind placebo controlled study explores the rational of local anesthetic blocks for the detection of muscle pain as the primary generator in spreading hyperalgesic conditions.

Methods: Experimental muscle pain was induced by injections of nerve growth factor (NGF) in the left and right supraspinatus muscle of 11 healthy volunteers. One day later ropivacaine and saline were administered with ultrasound guidance in the left or right supraspinatus muscle in a randomized double-blind manner. Assessments before NGF, day 1 (before and after ropivacaine/saline), and day 7 included: visual analog scale (VAS) pain scores at rest and during shoulder shrugging, time until exercise interruption owing to pain, cutaneous pain sensitivity, pressure pain thresholds, and pain VAS during tonic pressure stimulation. Cutaneous and pressure pain sensitivity were assessed over the supraspinatus, infraspinatus, and deltoideus muscles, and at the web space between first and second finger.

Results: Increased VAS pain scores and increased pressure pain sensitivity of the supraspinatus and infraspinatus muscles were found in both sides one day after the NGF injection compared with baseline. One day after NGF injections, the time until participants stopped exercising was reduced compared with baseline. The increased muscle pain sensitivity was not normalized by intramuscular ropivacaine. Saline caused increased VAS pain scores compared with ropivacaine.

Discussion: Muscle pain and spreading hyperalgesia induced by NGF is maintained despite anesthesia of the primary nociceptive locus. This indicates that intramuscular injection of local anesthetics may not be a valid diagnostic method for primary muscle pain.