INTRODUCTION AND EPIDEMIOLOGY
Muscles, representing approximately 50% of the body by weight, have long been recognized by clinicians as a source of common pain problems. Various terms have been applied to muscles and soft tissue pain, generally reflecting the prevailing concepts of muscle pain mechanisms or clinical observations of painful muscles. The changing concepts about the nature of muscle pain make it difficult to collect data on the incidence and impact of muscle-related pain. A brief review of the history of these concepts is presented.
The terms muscular rheumatism1 and nonarticular rheumatism2 were used to suggest that pain and stiffness in the region of a joint were caused by soft tissue rather than articular dysfunction. Inflammation as the cause of muscle pain led to the terms fibrositis and myofibrositis,3 which were abandoned with the awareness that inflammation could generally not be demonstrated in painful muscles.
The palpable changes in the muscles, revealing increased resilience, ropiness, and nodularity, led to the terms Myogelosen (muscle gelling),4 Muskelhärten (hardened muscle),5 and Muskelschwiele (muscle callus). The term myofascial pain, suggesting pain originating in muscle and connective tissue, first used by Reynolds in 1952, is now confusingly used interchangeably with myofascial pain syndrome, suggested by Travell and Simons,6 referring to muscle pain originating in myofascial trigger points (TrPs).
PAIN DISTRIBUTION AND PUTATIVE ETIOLOGY AFFECT NOMENCLATURE
Littlejohn7 suggests that the term regionalized musculoskeletal disorders be used for muscle pain when the causes appear to be inflammation, sprain, strain, or degeneration of muscles and tendons. When no obvious etiology is present to account for muscle pain, tenderness, stiffness, and often associated nonanatomic dysesthesias and autonomic disturbances, four overlapping and confusing diagnoses are often used: (1) regional pain syndrome, (2) complex regional pain syndrome, and when muscle pain is widespread, (3) chronic widespread pain (CWP), or (4) fibromyalgia syndrome (FMS). Central nervous system (CNS) dysregulation is generally thought to underlie these diagnoses rather than peripheral muscle pain generators.
MUSCLE PAIN AND NEUROPLASTICITY
When muscle nociceptors (discussed later in the chapter) are sensitized, they stimulate and may sensitize dorsal horn neurons, which may result in opening previously ineffective synaptic pathways, leading to stimulation of neurons at adjacent and distant spinal levels. This mechanism may produce referred muscle pain. Sensitized CNS neurons and primary dysfunction in descending inhibitory pathways may lower the threshold for nociceptive stimulation and result in enhanced muscle pain. Pain from other tissue (nerve, joint, viscera) may refer to muscle and may result in the production of independently self-sustaining muscle pain and a confounded clinical presentation.
MUSCLE PAIN AS A CAUSE OF OR COEXISTING WITH OTHER SUSPECTED PAIN DIAGNOSES
Although diagnoses such as nonspecific low back, ...