The pharmacologic treatment of muscle pain.

MYOFASCIAL pain is a significant source of discomfort in individuals with regional pain symptomatology. The prevalence of myofascial pain ranges from around 20% in patients with chronic low back pain (LBP) to 30% in patients with regional pain complaints seen in primary care clinics to upward of 85% in patients presenting to specialized pain management centers. To understand the origin of myofascial pain, it is first necessary to possess a fundamental understanding of two related concepts, muscle tension and trigger points (TPs). Muscle tension is the product of two distinct factors: viscoelastic tone and contractile activity. Viscoelastic tone can be classified into two parts, elastic stiffness and viscoelastic stiffness. Both of these can be quantified only in the absence of electromyographic activity. Elastic stiffness is a function of distance moved, whereas viscoelastic stiffness considers the effect of velocity. Contractile activity is composed of three different subunits: contracture (no electromyographic activity), electrogenic spasm (pathologic), and electrogenic stiffness (normal). Contractures originate endogenously within muscle fibers independent of electromyographic activity. Electrogenic spasm refers to involuntary, pathologic contractions arising from the electrical activity occurring in alpha motor neurons and motor endplates. Electrogenic stiffness refers to muscle tension that derives from electrogenic muscle contraction in individuals who are not relaxed. The latter two terms are associated with measurable electromyographic activity. Trigger points are defined as taut bands of muscle that produce pain in characteristic reference zones. These taut bands of contracted muscle can be classified into two main types, active TPs and latent TPs, the latter of which is more common. Depending on the pain condition, and even within certain subgroups of soft tissue disorders, muscle pain may be associated with TPs, increased muscle tension, or various combinations of these pathologic processes. Common clinical conditions in which muscle pain is caused primarily by spasm include torticollis, trismus, and nocturnal leg cramps. A painful condition that is defined by the presence of active TPs is myofascial pain syndrome (MPS). Tension headache and temporomandibular disorder (TMD) are conditions that may be associated with both increased muscle tone and TPs. Other mechanisms and physiologic processes can contribute to muscle pain in addition to tone and TPs. These include but are not limited to increased metabolism or diminished perfusion leading to local ischemia, peripheral and central sensitization, and autonomic hyperactivity. Not infrequently, psychogenic factors are found to play a role in soft tissue disorders. Although local anesthetic TP injections have been advocated in the treatment of a wide variety of myofascial pain disorders including tension headache, MPS, TMD, and LBP, these injections are beyond the scope of this review article. Fibromyalgia, which shares some characteristics with myofascial pain but which the authors consider a disorder of sensory processing, will also not be considered. MPS is considered to be a distinct disorder with major and minor diagnostic criteria, and the authors will limit the use of this term to the syndrome outlined by Simons. The term myofascial pain is used more broadly and refers to soft tissue pain of unclear etiology.