Description of the condition
Myofascial pain (MP) is a form of muscle pain arising from myofascial trigger points (MTrPs). MTrPs are hyperirritable points within muscle taut bands that are painful to palpation, reproduce the patient's symptoms, and cause referred pain (Borg-Stein 2002). Patients may experience regional pain, and may also suffer from an inability to work, mood changes, and reduced quality of life (Borg-Stein 2002). In some cases, the discomfort usually resolves in a few weeks without any medical intervention. When pain persists or worsens, it is referred to as a myofascial pain syndrome (Bennett 2007; Borg-Stein 2002).
MP is a common disorder mainly caused by acute muscle injury, overuse or repetitive strain (Bennett 2007). The prevalence varies from 30% to 93% in clinics of different specialities and pain management centres (Cummings 2007; Fishbain 1986; Fricton 1985; Han 1997; Skootsky 1989).
The precise pathophysiological basis of MP is complex and still unclear. Currently-available evidence suggest that abnormal neurophysiology and a perturbed biochemical milieu are relevant to MTrPs (Bennett 2007). An important finding in the pathophysiology of MP is a pathologic increase in release of acetylcholine by the nerve terminal of an abnormal motor endplate (Mense 2003; Simons 2002). Increased release of acetylcholine can result in sustained depolarisation of the postjunctional membrane of the muscle fibre and produce sustained sarcomere shortening and contracture. The abnormality may greatly increase local energy consumption and reduce local circulation that produces local ischemia and hypoxia. Mechanical, chemical, or other noxious stimuli or injury may mediate the abnormal release of acetylcholine (Liley 1956). Compared to normal muscle, the active trigger points have an acidic milieu and elevated levels of several biologically relevant molecules such as tumour necrosis factor-alpha, interleukin-1b, calcitonin-gene-related polypeptide, substance P, bradykinin, serotonin and norepinephrine (Shah 2005). These active factors can also stimulate the local autonomic nervous system fibres to release more acetylcholine, completing a "positive feedback loop" (Mense 2001; Simons 1999).
Description of the intervention
Many pharmacologic and nonpharmacologic treatments are used in the management of MP. Drugs such as analgesics (e.g. tramadol), non-steroidal anti-inflammatory drugs, tricyclic antidepressants (e.g. amitriptyline), alpha-2 adrenergic agonists (such as tizanidine), and anticonvulsant drugs. Recently, botulinum toxin has also been used for the treatment of MP (Bennett 2007; Borg-Stein 2002; Borg-Stein 2006; Fleckenstein 2010; Leite 2009; Soares 2012). However, the effectiveness of these drugs and treatments can often be unsatisfactory and the adverse effect of synthetic drugs is common. As a consequence, non-pharmacologic therapies, including acupuncture, dry needling, local injection, low-power laser, muscle-stretching technique and massage, are often used as alternative complementary therapies (Borg-Stein 2002; Borg-Stein 2006; Renan-Ordine 2011; Sun 2010; Tough 2009).
Acupuncture originated in China, and has become popular both in eastern and western countries. Acupuncture is a traditional remedy involving the insertion of acupuncture needles into specific sites (acupuncture points) located in the body for treating a variety of symptoms and conditions. There are several types of needling therapies evolved for traditional Chinese acupuncture, such as western medical acupuncture, dry needling, electro-acupuncture, laser acupuncture and acupoint injection. Western medical acupuncture can not adhere to concepts of Traditional Chinese Medicine but according to current knowledge of anatomy, physiology and pathology (Filshie 1999; White 2009).
How the intervention might work
The mechanism of acupuncture analgesia remains not completely understood. Nevertheless, the neuroendocrine mechanisms have been gradually revealed since the 1970s (Sims 1997; Staud 2007; Zhao 2008). Acupuncture stimulations in skin and muscle deliver signals to the spinal cord and lead to the activation of the spinal cord, midbrain and hypothalamus-pituitary gland, which cause the release of neuroendocrine factors such as endorphin, enkephalin, serotonin, and dopamine in the plasma and brain tissue (Peng 1978; Plotnikoff 1985; Tee 2007). It has been determined that these neuroendocrine factors mediate acupuncture analgesia and have an effect on psychological adjustment (Bennett 2007; Cabýigky 2006; Han 1997; Kiser 1983; Mayer 1977; Mendelson 1977; Skootsky 1989). Furthermore, a remarkably close association has been found between acupuncture points and trigger points, with 71% of trigger points sharing location and pain distribution patterns with acupuncture points (Hong 2000; Melzack 1977). Acupuncture may break the "positive feedback loop" of MP by modifying the release of neurological and chemical factors. Acupuncture, therefore may have potential benefits for the treatment of MP.
Why it is important to do this review
No "gold standard" of management has been suggested for MP until now. A large number of clinical trials have now been done to test the effectiveness of acupuncture for MP. However, the clinical trials usually generate contradictory results; and the benefit of acupuncture remains controversial. Therefore, a systematic review is needed to evaluate the evidence of acupuncture for MP.