Referral of musculoskeletal pain

Abstract

Pain referral can pose a serious problem for the diagnosis and treatment of muscle pain because it leads to a mislocalization of the pain by the patient. Referral of pain originating in muscles can be elicited experimentally in a relatively high proportion of healthy subjects. Pain and tenderness can be referred to muscle from other muscles, joints, viscera, and as pain originating in the central nervous system. Clinically, muscle pain referred from other muscles has the typical characteristics of deep-tissue pain and can be elicited, e.g., by local pressure also from muscles that appear to be normal. Referral of pain from joint to muscle is frequent; it often occurs in muscles crossing the joint. Finally, pain can be experienced in muscle as an expression of central pain, i.e., pain due to lesions of the central nervous system. A prominent example of such a muscle pain is phantom limb pain. In the second part of the chapter, potential mechanisms of pain referral are discussed as well as the differences between Head zones and referred pain in the strict sense. Basically, pain referral appears to result from nociceptive information taking a wrong path in the spinal cord and reaching (somatotopically) inappropriate dorsal horn neurons. The convergence-projection theory by Ruch is still the central concept for the explanation of referred pain. It states that a given dorsal horn neuron receives synaptic connections from two separate innervation areas (convergent input), and that the neuron induces subjective pain in only one (and always the same) area, even when it is excited from the other area. The theory explains the referred pain in the skin from painful viscera. Typical examples of muscle pain referred from viscera include the chest-wall pain of cardiac infarction and the flank pain of renal calculi. A more recent version of the theory states that normally only one of the convergent connections is sufficiently effective to fire the neuron; the other elicits just subthreshold potentials in the neuron. However, the ineffective connections can become effective if there is a long-lasting lesion in the region of the ineffective connection (somatotopically inappropriate connections are opened). Thus, the nociceptive information takes a wrong course in the spinal cord and the pain is mislocalized.