A myofascial trigger point is defined as a hyperirritable point in skeletal muscle, which is associated with a tender, palpable lump in a narrow fascia. When compressed, a MTrP can lead to characteristic referred pain, hypersensitivity, motor dysfunction, and autonomic phenomena. MTrPs have been described as active or latent. Active MTrPs are associated with complaints of spontaneous pain, while latent MTrPs are clinically inactive and painful only when palpated or stung. Another feature of MTrPs is the local contraction response (LTR), which is a sudden contraction of muscle fibers within a tight fascia caused by a click on palpation or insertion of a needle into the MRP. The minimum criterion for identifying an active MTrP is an exquisite point on palpation of a lump in a taut band which, when properly palpated, causes the patient to recognize the pain in progress. A latent MTrP, due to its lack of relationship to spontaneous pain, is defined as an exquisite point on palpation of a lump in a narrow band.

The "X" indicates only potential MTrP positions and should be considered as a general guide. Accurate palpation, using the recommended criteria, is key to identifying the MTrP in a single muscle, and the examiner must realize that each muscle can have multiple MTrPs. MTrPs often don't rely on their own derivation models. MTrPs will generally refer distally, meaning that often the muscle responsible for the pain will be proximal to the pain pattern. MTrPs were described in the 16th century by the French physician Guillaume de Baillou (1538-1616), who used the term muscular rheumatism to describe what is now recognized as myofascial pain. Many other doctors have described trigger points; however, Travell and Simons are considered authoritative sources. Travell (1901-1997) initially trained in cardiology and later became interested in pain related to palpation of narrow bands in skeletal muscles. As a side note, she Travell became Presidents Kennedy and Johnson's personal physician and was the first female physician in the White House. Later in her career, she collaborated with Dr. David Simons (1922-2010), a physiatrist, and they were co-authors of widely distributed trigger point manuals. Many other important textbooks on myofascial pain and MTrP have been published.

The prevalence of myofascial pain has been reported in various populations, but the prevalence in the general population is unknown. Researchers reported that between 84% and 93% of patients in pain management centers had myofascial pain. 30% of patients presenting with pain in a primary care general medical clinic had myofascial pain, making myofascial pain the largest diagnostic pain group. Additionally, patients with upper body pain were more likely to have myofascial pain than pain localized elsewhere. In elderly people with low back pain, MTrP was identified in 96% of symptomatic subjects compared with 10% of controls. MTrP was identified in 93.9% of migraine patients compared with 29% of control subjects. Myofascial pain has been described by various clinical specialties in selected patient groups. Regarding the shoulder, it was reported that patients with a medically diagnosed rotator cuff tendinopathy (n = 58) lasting greater than 6 weeks and less than 18 months had MTrP in the supraspinatus (88%), infraspinatus (62 %), minor round muscles (20.7%) and subscapularis (5.2%). Shoulder impingement patients had more MTrPs active in the supraspinatus (67%), infraspinatus (42%), and subscapularis (42%) than normal control subjects. Patients demonstrated generalized hypersensitivity to pressure and the presence of active MTrP which, on examination, could reproduce recognized pain. A study of patients with chronic non-traumatic unilateral shoulder pain (n = 72), conducted in a Dutch physiotherapy clinic, identified active MTrP in all subjects with the following prevalence: infraspinatus (78%); upper trapezius (58%); medium trapezius (43%); anterior, middle and posterior delts (47%, 50%, 44%, respectively); and teres minor (47%). Brukner and Khan considered MTrPs to be among the most common causes of shoulder pain from a sports medicine perspective and recommended evaluating MTrPs in a clinical setting.

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