Effects of Intramuscular Anesthesia on the Expression of Primary and Referred Pain Induced by Intramuscular Injection of Hypertonic Saline

Intramuscular injection of hypertonic saline produces pain in the belly of the injected muscle (primary pain) and, often, pain that projects distally (referred pain). While it is known that referred pain can be induced during complete sensory block of the distal site, there is little evidence as to whether the perception of referred pain depends on ongoing input from the primary stimulus. We assessed whether blocking the noxious input following the induction of pain blocks the primary but not the referred pain. A cannula was inserted into the tibialis anterior muscle in 15 subjects (8 male, 7 female). In a quasi-random crossover design conducted over 2 experimental sessions, each subject received a bolus intramuscular injection of .5 mL of 5% hypertonic saline, followed 90 seconds later by either: A) A second bolus injection or; B) An injection of 2 mL lignocaine through the same cannula. Protocol A was followed 60 seconds later by either a sham injection or an injection of lignocaine, while protocol B was followed 60 seconds later by either a sham injection or an injection of hypertonic saline. Subjects mapped the areas of primary and referred pain, and rated the intensities at these sites every 30 seconds until the cessation of pain. In all subjects, the area and intensity of primary pain rapidly disappeared within 7.5 minutes of intramuscular lignocaine injection (P < .02 relative to the nonanesthesia condition). With the exception of 2 subjects, in whom the referred pain continued in the absence of primary pain, the referred pain declined in parallel with local pain: the mean total pain intensity declined by 74% in both regions. We conclude that the maintenance of referred muscle pain usually depends on ongoing noxious inputs from the site of primary muscle pain.

Perspective

Referred pain is a significant clinical problem, and commonly occurs with pain originating in muscle but not from skin. It is important to know the primary source of the pain so that treatment can be directed to this site rather to the site of referral.