Myofascial Trigger Points Elizabeth Demers Lavelle, MDa, William Lavelle, MDb,*, Howard S. Smith, MD, FACPa a Department of Anesthesiology, Albany Medical Center, 43 New Scotland Avenue, Albany, NY 12208, USA b Department of Orthopaedic Surgery, 1367 Washington Avenue, Albany Medical Center, Albany, NY 12206, USA A myofascial trigger point is a hyperirritable point in skeletal muscle that is associated with a hypersensitive palpable nodule [1]. Approximately 23 million Americans have chronic disorders of the musculoskeletal system [2]. Painful conditions of the musculoskeletal system, including myofascial pain syndrome, constitute some of the most important chronic problems that are encountered in a clinical practice. Definitions Myofascial pain syndrome is defined as sensory, motor, and autonomic symptoms that are caused by myofascial trigger points. The sensory distur- bances that are produced are dysesthesias, hyperalgesia, and referred pain. Coryza, lacrimation, salivation, changes in skin temperature, sweating, piloerection, proprioceptive disturbances, and erythema of the overlying skin are autonomic manifestations of myofascial pain. Travell and Simons [1] defined the myofascial trigger point as ������a hyper- irritable spot, usually within a taut band of skeletal muscle or in the muscle fascia which is painful on compression and can give rise to characteristic re- ferred pain, motor dysfunction, and autonomic phenomena������ [1]. When the trigger point is pressed, pain is caused and produces effects at a target, the zone of reference, or referral zone [3,4]. This area of referred pain is the fea- ture that differentiates myofascial pain syndrome from fibromyalgia. This pain is reproduced reliably on palpation of the trigger point, despite the A version of this article originally appeared in the 91:2 issue of Medical Clinics of North America. * Corresponding author. E-mail address: lavellwf@yahoo.com (W. Lavelle). 1932-2275/07/$ - see front matter �� 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.anclin.2007.07.003 anesthesiology.theclinics.com Anesthesiology Clin 25 (2007) 841���851

fact that it is remote from its source of origin. This referred pain rarely co- incides with dermatologic or neuronal distributions, but follows a consistent pattern [5]. Etiology Trigger points may develop after an initial injury to muscle fibers. This injury may include a noticeable traumatic event or repetitive microtrauma to the muscles. The trigger point causes pain and stress in the muscle or mus- cle fiber. As the stress increases, the muscles become fatigued and more susceptible to activation of additional trigger points. When predisposing factors combine with a triggering stress event, activation of a trigger point occurs. This theory is known as the ������injury pool theory������ [1]. Pathophysiology There is no pathologic or laboratory test for identifying trigger points. Therefore, much of the pathophysiologic research on trigger points has been directed toward verifying common theories of their formation. Fig. 1 provides an example of the theory behind the formation of myofascial trigger points. The local twitch response (LTR) has been described as a characteristic re- sponse of myofascial trigger points. LTR is a brisk contraction of the muscle fibers in and around the taut band elicited by snapping palpation or rapid Fig. 1. Myofascial trigger point loci. 842 LAVELLE et al