Comparison of the Clinical Manifestation & Pathophysiology of Myofascial Pain Syndrome & Fibromyalgia +MP3
Research Review By Dr. Brynne Stainsby©
Audio:
Date Posted:
January 2019
Study Title:
A comparison of the clinical manifestation and pathophysiology of myofascial pain syndrome and fibromyalgia: implications for differential diagnosis and management
Authors:
Bourgaize S, Newton G, Kumbhare D, Srbely J
Author's Affiliations:
Department of Human Health and Nutritional Sciences, University of Guelph, Canada; Department of Medicine, University of Toronto, Canada.
Publication Information:
Journal of the Canadian Chiropractic Association 2018; 62(1): 26-41.
Background Information:
Musculoskeletal (MSK) pain is an extremely prevalent condition, affecting up to 80% of the population (1, 2). Chronic musculoskeletal pain is one of the leading burdens of illness in Canada, with a total financial burden of $5.8 billion (CAD) in 2008 (3). Two of the most common pain conditions are fibromyalgia (FM) and myofascial pain syndrome (MPS), with prevalence of up to 15% of the general population for FM, and between 9-85% for MPS (1, 2, 4-7). Though these two conditions may appear similar on clinical presentation, there are significant differences that affect their diagnosis and treatment, and it is essential for primary care providers to perform accurate and reliable differential diagnosis to ensure optimal management and patient outcomes. FM is a complex, widespread pain condition, which often requires multidisciplinary care including manual, psychological and physical therapies, in addition to pharmacological interventions (8), while MPS is typically well managed with manual and physical therapies (including exercise) (5).
The current body of literature suggests that diagnostic accuracy and reliability between FM and MPS is inadequate, with MPS commonly mistaken for FM as myofascial trigger points (MTrP) may be mistaken for tender points (TP) (1, 9-13). Comorbidities, the fact that MPS may be widespread and the lack of laboratory tests for either condition have also been implicated in the reason for this clinical confusion (1, 12, 13). It has also been hypothesized that limited awareness of the American College of Rheumatology (ACR) 1990 criteria may contribute to incorrect diagnosis (14).
The aim of this narrative review was to compare and contrast the clinical presentation and pathophysiology of FM with MPS.
The current body of literature suggests that diagnostic accuracy and reliability between FM and MPS is inadequate, with MPS commonly mistaken for FM as myofascial trigger points (MTrP) may be mistaken for tender points (TP) (1, 9-13). Comorbidities, the fact that MPS may be widespread and the lack of laboratory tests for either condition have also been implicated in the reason for this clinical confusion (1, 12, 13). It has also been hypothesized that limited awareness of the American College of Rheumatology (ACR) 1990 criteria may contribute to incorrect diagnosis (14).
The aim of this narrative review was to compare and contrast the clinical presentation and pathophysiology of FM with MPS.
Summary:
Pathophysiology:
- Though poorly understood, the current consensus is that MPS is characterized by muscular pain associated with palpable regions of hypersensitivity (i.e. one or multiple MTrPs). It is hypothesized that MTrPs form within the motor endplate of a muscle due to local injury, which results in excessive release of acetylcholine and increased motor endplate activity to allow for the development of a palpable, hyperirritable locus within the muscle (5, 11, 15-18). This persistent contraction leads to biochemical responses, including the release of inflammatory factors, which contribute to muscular pain (11, 15, 17, 18). Concurrently, the release of substance P into the dorsal horn also contributes to neuroplastic changes and can contribute to central sensitization (17, 19). It has also been suggested that neurogenic inflammation subsequent to central sensitization could initiate and facilitate the formation of MTrPs even in the absence of local injury (20).
- The pathophysiology of FM is (unfortunately) also poorly understood. It is believed that maladaptive central processing may be an important mechanism in the development of clinical features (widespread pain of greater than three months duration with symmetrically distributed tender points) (11). It is believed that tender points (TPs) are located within regions of secondary hyperalgesia, as increased levels of synaptic modulators such as substance P have been observed in cerebrospinal fluid samples (21-24). It is important to note that TPs do not typically express inflammatory factors (11).
Epidemiology:
- The prevalence of MPS in chronic pain clinics has been estimated to be as high as 90% (2, 5), and in internal medicine and orthopaedic clinics, MPS has been estimated to make up 30% of pain-related visits (5, 25). In the United States, FM presents in approximately 2% of the general population and 15% of internal medicine hospitalizations (1, 4, 25).
- The gender distribution of FM and MPS is quite similar, and recent evidence challenges the belief that women are more commonly affected by FM (26). It has been suggested that the earlier differences reported in gender may be primarily related to behavioural differences, where women demonstrate health seeking behaviours more frequently than men (26) and males with FM have lower health awareness (1, 26, 27) and are socialized to suppress pain (27). It has been noted that females with FM display greater pain sensitivity, greater impact on daily life, more frequent work absenteeism and lower quality of life (1, 26, 27). In comparison, MPS is balanced between genders, though women tend to report higher pain scores, reduced pain thresholds and more frequent work absenteeism (2, 28). Overall, the collective findings suggest there are likely gender differences in the development and maintenance of chronic MSK pain due to social and behavioural factors, despite the lack of gender effect on prevalence.
- Current research suggests chronic MSK pain is heavily influenced by age, with the highest prevalence seen in adults over the age of 60 (26, 29). In MPS specifically, the highest occurrence has been found to be between 59-74 years of age (29), while FM is commonly considered a disorder affecting those between 20-50 years of age (27). Further research is needed to understand the prevalence of MPS and FM in children and adolescents.
- The current evidence regarding the role of ethnicity on chronic MSK pain is limited and equivocal. Ethnic background was found to play a role in the prevalence of FM in Europe, but not in America (29).
Clinical Presentation:
- MPS typically manifests as MTrPs and may be associated with weakness or altered muscle function without atrophy and loss of range of motion (1, 5, 11, 30). A local, rapid, transient twitch in the MTrP (but not the entire muscle) can often be observed subsequent to physical stimulus or intramuscular needle injection (1, 17). It is possible that MPS patients may experience symptoms of autonomic dysregulation (diaphoresis, flushing, temperature changes, etc.) but this is not common (5).
- In comparison, FM is defined by chronic widespread pain with symmetrical TPs (defined as discrete areas of soft tissue that are painful to less than 4 kg of palpatory pressure) (1, 11, 26, 30-32). Although tender, TPs are otherwise indistinguishable from the surrounding muscle. FM often presents with additional systemic findings such as sleep disorders, irritable bowel syndrome, nervous bladder, fatigue, cognitive dysfunction, anxiety, depression, headaches and Raynaud’s phenomenon (11, 32, 33).
Diagnosis:
Fibromyalgia (FM):
The diagnosis of FM is based on the American College of Rheumatology’s (ACR) 1990 criteria and the updated 2010/2011 criteria (31, 32). Originally, diagnosis required a history of widespread pain (bilateral pain above and below the waist, with or without axial skeleton pain) lasting at least three months with the presence of 11 out of 18 standardized bilateral tender points (31). Over the next twenty years, it became apparent that these criteria were inadequate, as they did not address the complete clinical presentation of patients with FM and the identification of 11/18 tender points was deemed to be arbitrary (32). The 2010 criteria were intended to provide an alternative diagnostic approach and included a Symptom Severity Score (SS) and a Widespread Pain Index (WPI) (32). The SS aimed to address pain and secondary symptoms, and the WPI includes a questionnaire and body pain diagram for patients to document their pattern of pain (6). A combined SS and WPI score (known as the polysymptomatic distress scale [PSD]) ≥ 13 is the threshold for a diagnosis of FM (32). These criteria have been reported to have a sensitivity of 96.6% and specificity of 91.8% (32). The 2016 revision to these criteria emphasizes the chronic widespread pain aspect of FM, requiring it for diagnosis (34, 35).
Myofascial Pain Syndrome:
The diagnosis of MPS was originally based in the “Trigger Point Manual” by Travell and Simons, which included the presence of a MTrP, local twitch response, referred pain and weakness, without atrophy, autonomic symptoms or restricted range of motion (15, 16). However, these criteria rely heavily on clinical judgement (36, 37). More recent reviews have examined the role of the physical exam in detecting MTrPs and have found it to be inadequate (36). The current research regarding identification of MTrPs is highly variable and significantly limited, and the current consensus is that physical examination should not be used in isolation when diagnosing patients with MPS (36-39).
Given the abovementioned challenges in diagnosing FM or MPS based on history or physical examination, the need for objective diagnostic criteria is clear. Emerging research on the role of biomarkers to indicate normal or pathological processes suggests a promising role in the differential diagnosis of patients with chronic MSK pain (11, 17). Diagnostic ultrasound and MRI studies have demonstrated changes in local muscle tissue thought to represent MTrPs (25, 40). Needle electromyography has been used to identify abnormal motor neuron activity associated with MTrPs (41). While these tools have excellent application in research, it must be acknowledged that their clinical utility is limited at this time.
Fibromyalgia (FM):
The diagnosis of FM is based on the American College of Rheumatology’s (ACR) 1990 criteria and the updated 2010/2011 criteria (31, 32). Originally, diagnosis required a history of widespread pain (bilateral pain above and below the waist, with or without axial skeleton pain) lasting at least three months with the presence of 11 out of 18 standardized bilateral tender points (31). Over the next twenty years, it became apparent that these criteria were inadequate, as they did not address the complete clinical presentation of patients with FM and the identification of 11/18 tender points was deemed to be arbitrary (32). The 2010 criteria were intended to provide an alternative diagnostic approach and included a Symptom Severity Score (SS) and a Widespread Pain Index (WPI) (32). The SS aimed to address pain and secondary symptoms, and the WPI includes a questionnaire and body pain diagram for patients to document their pattern of pain (6). A combined SS and WPI score (known as the polysymptomatic distress scale [PSD]) ≥ 13 is the threshold for a diagnosis of FM (32). These criteria have been reported to have a sensitivity of 96.6% and specificity of 91.8% (32). The 2016 revision to these criteria emphasizes the chronic widespread pain aspect of FM, requiring it for diagnosis (34, 35).
Myofascial Pain Syndrome:
The diagnosis of MPS was originally based in the “Trigger Point Manual” by Travell and Simons, which included the presence of a MTrP, local twitch response, referred pain and weakness, without atrophy, autonomic symptoms or restricted range of motion (15, 16). However, these criteria rely heavily on clinical judgement (36, 37). More recent reviews have examined the role of the physical exam in detecting MTrPs and have found it to be inadequate (36). The current research regarding identification of MTrPs is highly variable and significantly limited, and the current consensus is that physical examination should not be used in isolation when diagnosing patients with MPS (36-39).
Given the abovementioned challenges in diagnosing FM or MPS based on history or physical examination, the need for objective diagnostic criteria is clear. Emerging research on the role of biomarkers to indicate normal or pathological processes suggests a promising role in the differential diagnosis of patients with chronic MSK pain (11, 17). Diagnostic ultrasound and MRI studies have demonstrated changes in local muscle tissue thought to represent MTrPs (25, 40). Needle electromyography has been used to identify abnormal motor neuron activity associated with MTrPs (41). While these tools have excellent application in research, it must be acknowledged that their clinical utility is limited at this time.
Clinical Application & Conclusions:
This review outlined the high prevalence and heavy financial burden of chronic MSK pain, particularly that resulting from both FM and MPS. Although they may be similar in clinical presentation, understanding the underlying pathophysiology and associated symptoms is important for clinicians to understand why and how these patients may respond to treatment. Those with MPS typically respond well to manual and physical therapies, while patients with FM generally require a more multidisciplinary approach.
Although differentially diagnosing these conditions can be challenging and the diagnostic criteria are limited, the current best practice is a thorough history and physical examination, including manual palpation to attempt to identify and distinguish the difference between MTrPs or TPs. Hopefully future research will identify clinically useful tools to objectively diagnose patients with chronic MSK pain with particular assistance for these two conditions.
Although differentially diagnosing these conditions can be challenging and the diagnostic criteria are limited, the current best practice is a thorough history and physical examination, including manual palpation to attempt to identify and distinguish the difference between MTrPs or TPs. Hopefully future research will identify clinically useful tools to objectively diagnose patients with chronic MSK pain with particular assistance for these two conditions.
Study Methods:
This scoping, narrative review did not report on complete methodology. The PubMed database was searched with key terms and inclusion was determined by relevance to the objective.
Study Strengths / Weaknesses:
Strengths:
- This review compares and contrasts the pathophysiology, epidemiology, clinical presentation and diagnostic approach for FM and MPS, a worthwhile endeavor given the prevalence of these conditions and the confusion that often surrounds them clinically.
- The authors review emerging research regarding novel diagnostic tools that may assist with objective diagnosis of these conditions.
- The authors acknowledge that while emerging diagnostic tools are helpful in the research setting, their clinical utility is limited at this point.
Weaknesses:
- The greatest weakness of this study is the lack of methodology reported. It may be noted that the authors merely presented a comparison of two conditions, rather than appraising evidence to summarize the literature, however, methodology beyond a search strategy would have improved the quality of this review paper.
- The authors did not report if or how the included articles were included or appraised.
- While this article provides a summary of the literature included, there are few comments on the participants or clinical setting of the included studies, thus limiting the external validity of the review.
Commentary from Dr. John Srbely DC, PhD (leading expert in this area):
Chronic musculoskeletal pain (MSK) is a significant health concern in today’s society, with a reported prevalence as high as 80% in the aging demographic. Given that the over 65 age group is the fastest growing sector of Canada’s population, chronic MSK pain is poised to become Canadian healthcare’s greatest challenge.
Myofascial pain syndrome (MPS) is the most common manifestation of chronic musculoskeletal pain. While its clinical management is not exclusive to chiropractors, emerging research points to the potentially leading role chiropractors could play in the future management of this clinical enigma. As emerging research highlights the foundational role of central sensitization in the pathophysiology of chronic MPS, mounting evidence also suggests that spinal manipulative therapy (SMT) mediates its therapeutic benefits by modulating central sensitization. This evolving body of literature underscores the potentially pivotal role SMT may hold in the future management of chronic MPS, positioning chiropractors as gatekeepers for chronic musculoskeletal pain within the future health delivery landscape.
This review presents a comprehensive, current account of the published literature in the field of MPS. It relates to chiropractors by aiming to update current knowledge and addressing central, outstanding gaps in the pathophysiology and clinical manifestation of MPS, as well as its close counterpart fibromyalgia. The need for advancing cost-effective, long-term management strategies for chronic musculoskeletal pain has never been more timely and it is our hope that this review informs future practice strategies and research within the chiropractic profession.
Dr. John Z Srbely DC PhD graduated from the Canadian Memorial Chiropractic College (1992) and certified in Clinical Acupuncture (2000). He received his PhD degree in biomechanics and neurophysiology in 2008 from the University of Guelph and accepted his first faculty appointment (Assistant Professor) in September 2008 in the Department of Human Health and Nutritional Science (HHNS), College of Biological Sciences, University of Guelph (Guelph, Ontario, Canada). He held a prestigious Canadian Chiropractic Research Foundation Research Chair in Spine Mechanics and Neurophysiology (2008-2013) and was recently promoted to Associate Professor with tenure in July, 2018. He is a member of the Editorial Board of the Journal of the Canadian Chiropractic Association and Associate Chair of the Natural, Physical and Engineering Science Research Ethics Board at the University of Guelph. He is currently the Director of the HHNS Neuromuscular Health and Chronic Pain clinical-research facility where he is pursuing his primary research interests in the study of the neurophysiologic mechanisms of chronic pain. The central theme of his research program is the study of central sensitization and its role in the clinical manifestation of chronic musculoskeletal disease including myofascial pain, osteoarthritis, and chronic degenerative joint/spine disease.
Myofascial pain syndrome (MPS) is the most common manifestation of chronic musculoskeletal pain. While its clinical management is not exclusive to chiropractors, emerging research points to the potentially leading role chiropractors could play in the future management of this clinical enigma. As emerging research highlights the foundational role of central sensitization in the pathophysiology of chronic MPS, mounting evidence also suggests that spinal manipulative therapy (SMT) mediates its therapeutic benefits by modulating central sensitization. This evolving body of literature underscores the potentially pivotal role SMT may hold in the future management of chronic MPS, positioning chiropractors as gatekeepers for chronic musculoskeletal pain within the future health delivery landscape.
This review presents a comprehensive, current account of the published literature in the field of MPS. It relates to chiropractors by aiming to update current knowledge and addressing central, outstanding gaps in the pathophysiology and clinical manifestation of MPS, as well as its close counterpart fibromyalgia. The need for advancing cost-effective, long-term management strategies for chronic musculoskeletal pain has never been more timely and it is our hope that this review informs future practice strategies and research within the chiropractic profession.
Dr. John Z Srbely DC PhD graduated from the Canadian Memorial Chiropractic College (1992) and certified in Clinical Acupuncture (2000). He received his PhD degree in biomechanics and neurophysiology in 2008 from the University of Guelph and accepted his first faculty appointment (Assistant Professor) in September 2008 in the Department of Human Health and Nutritional Science (HHNS), College of Biological Sciences, University of Guelph (Guelph, Ontario, Canada). He held a prestigious Canadian Chiropractic Research Foundation Research Chair in Spine Mechanics and Neurophysiology (2008-2013) and was recently promoted to Associate Professor with tenure in July, 2018. He is a member of the Editorial Board of the Journal of the Canadian Chiropractic Association and Associate Chair of the Natural, Physical and Engineering Science Research Ethics Board at the University of Guelph. He is currently the Director of the HHNS Neuromuscular Health and Chronic Pain clinical-research facility where he is pursuing his primary research interests in the study of the neurophysiologic mechanisms of chronic pain. The central theme of his research program is the study of central sensitization and its role in the clinical manifestation of chronic musculoskeletal disease including myofascial pain, osteoarthritis, and chronic degenerative joint/spine disease.
Additional References:
- Gerwin RD. Classification, epidemiology, and natural history of myofascial pain syndrome. Curr Pain Headache Rep 2001; 5: 412–420.
- Fleckenstein J, Zaps D, Ruger LJ, et al. Discrepancy between prevalence and perceived effectiveness of treatment methods in myofascial pain syndrome: results of a cross-sectional, nationwide survey. BMC Musculoskelet Disord 2010; 11(32): 1-9.
- Public Health Agency of Canada. Economic Burden of Illness in Canada. Ottawa, ON: Health Canada; 2005-2008. Available at: http://www.phac-aspc.gc.ca/publicat/ebic-femc/2005-2008/assets/pdf/ebic-femc-2005-2008- eng.pdf Accessed November, 2016.
- Buskila D, Neumann L, Odes LR, et al. The prevalence of musculoskeletal pain and fibromyalgia in patients hospitalized on internal medicine wards. Semin Arthritis Rheum 2001; 30(6): 411–417.
- Borg-Stein J, Iaccarino MA. Myofascial pain syndrome treatments. Phys Med Rehabil Clin N Am. 2014; 25(2): 357–374.
- Rivers WE, Garrigues D, Graciosa J, et al. Signs and symptoms of myofascial pain: An international survey of pain management providers and proposed preliminary set of diagnostic criteria. Pain Med. 2015; 16(9): 1794-1805.
- Skootsky SA, Jaeger B, Oye RK. Prevalence of myofascial pain in general internal medicine practice. West J Med 1989; 151(2): 157–160.
- Leventhal LJ. Management of Fibromyalgia. Ann Intern Med 1999; 131(11): 850–858.
- Fitzcharles MA, Boulos P. Inaccuracy in the diagnosis of fibromyalgia syndrome: Analysis of referrals. Rheumatology 2003; 42(2): 263–267.
- Fitzcharles MA, Esdaile JM. The overdiagnosis of fibromyalgia syndrome. Am J Med 1997; 103(1): 44–50.
- Schneider MJ. Tender Points / Fibromyalgia vs. Trigger Points / Myofascial Pain Syndrome: A need for clarity in terminology and differential diagnosis. J Manipulative Physiol Ther 1995; 18(6): 398-406.
- Fitzcharles M, Ste-Marie PA, Goldenberg DL, et al. 2012 Canadian guidelines for the diagnosis and management of fibromyalgia syndrome: Executive summary. Canadian Pain Society 2013; 18(3): 119-126.
- Gerwin RD. Diagnosis of myofascial pain syndrome. Phys Med Rehabil Clin N Am 2014; 25(2): 341–355.
- Blotman F, Thomas E, Myon E, et al. Awareness and knowledge of fibromyalgia among French rheumatologists and general practitioners. Clin Exp Rheumatol 2005; 23(5): 697–700.
- Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1983.
- Travell JG, Simons DG, Simons LS. Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999.
- Shah JP, Danoff JV, Desai MJ, et al. Biochemicals associated with pain and inflammation are elevated in sites near to and remote from active myofascial trigger points. Arch Phys Med Rehabil 2008; 89(1): 16–23.
- Rickards LD. The effectiveness of non-invasive treatments for active myofascial trigger point pain: A systematic review of the literature. Int J Osteopath Med 2006; 9(4): 120–136.
- Latremoliere A, Woolf C. Central sensitization: a generator of pain hypersensitivity by central neural plasticity. J Pain 2009; 10(9): 895–926.
- Srbely JZ, Dickey JP, Lee D, et al. Dry needle stimulation of myofascial trigger points evokes segmental antinociceptive effects. J Rehabil Med 2010; 42(5): 463-468.
- Woolf CJ, Salter MW. Neuronal plasticity: increasing the gain in pain. Science 2000; 288(5472): 1765–1769.
- Woolf CJ. Central sensitization: Implications for the diagnosis and treatment of pain. Pain 2011; 152(3): S2-15.
- Russell IJ, Vaeroy H, Javors M, et al. Cerebrospinal fluid biogenic amine metabolites in fibromyalgia/fibrositis syndrome and rheumatoid arthritis. Arthritis Rheum 1992; 35(5): 550-556.
- Woolf CJ. Pain: moving from symptom control toward mechanism-specific pharmacologic management. Ann Intern Med 2004; 140(6): 441-451.
- Chen Q, Bensamoun S, Basford J R, et al. Identification and quantification of myofascial taut bands with magnetic resonance elastography. Arch Phys Med Rehabil 2007; 88(12): 1658–1661.
- Wolfe F, Brähler E, Hinz A, et al. Fibromyalgia prevalence, somatic symptom reporting, and the dimensionality of polysymptomatic distress: Results from a survey of the general population. Arthritis Care Res 2013; 65(5): 777–785.
- Riley JL, Robinson ME, Wise EA, et al. Sex differences in the perception of noxious experimental stimuli: A meta-analysis. Pain 1998; 74(2–3): 181–187.
- Rollman GB, Lautenbacher S. Sex differences in musculoskeletal pain. Clin J Pain 2001; 17(1): 20–24.
- Bergman S, Herrström P, Högström K, et al. Chronic musculoskeletal pain, prevalence rates, and sociodemographic associations in a Swedish population study. J Rheumatol 2001; 28(6): 1369-1377.
- Tunks E, McCain GA, Hart LE, et al. The reliability of examination for tenderness in patients with myofascial pain, chronic fibromyalgia and controls. J Rheumatol 1995; 22(5): 944-952.
- Wolfe F, Smythe HA, Yunus MB, et al. The American College of Rheumatology 1990 criteria for the classification of fibromyalgia. Arthritis Rheum 1990; 33(2): 160-172.
- Wolfe F, Clauw DJ, Fitzcharles MA, et al. The American College of Rheumatology preliminary diagnostic criteria for fibromyalgia and measurement of symptom severity. Arthritis Care Res 2010; 62(5): 600–610.
- Wolfe F, Clauw DJ, Fitzcharles MA, et al. Fibromyalgia criteria and severity scales for clinical and epidemiological studies: A modification of the ACR Preliminary Diagnostic Criteria for Fibromyalgia. J Rheumatol 2011; 38(6): 1113-1122.
- Wolfe F, Egloff N, Häuser W. Widespread pain index scores among fibromyalgia-positive cases assessed with the 2010/2011 fibromyalgia criteria. J Rheumatol 2016; 43(9): 1743-1748.
- Wolfe F, Clauw DJ, Fitzcharles M, et al. 2016 Revisions to the 2010/2011 Fibromyalgia Diagnostic Criteria. Semin Arthritis Rheum 2016; 46(3): 319-329.
- Lucas N, Macaskill P, Irwig L, et al. Reliability of physical examination for diagnosis of myofascial trigger points: a systematic review of the literature. Clin J Pain. 2009; 25(1): 80–89.
- Tough EA, White AR, Richards S, et al. Variability of criteria used to diagnose myofascial trigger point pain syndrome – evidence from a review of the literature. Clin J Pain 2007; 23(3): 278–286.
- Myburgh C, Larsen AH, Hartvigsen J. A systematic, critical review of manual palpation for identifying myofascial trigger points: Evidence and clinical significance. Arch Phys Med Rehabil 2008; 89(6): 1169–1176.
- Ohrbach R, Gale EN. Pressure pain thresholds, clinical assessment, and differential diagnosis: reliability and validity in patients with myogenic pain. Pain. 1989; 39(2): 157–169.
- Sikdar S, Shah JP, Gebreab T, et al. Novel applications of ultrasound technology to visualize and characterize myofascial trigger points and surrounding soft tissue. Arch Phys Med Rehabil 2009; 90(11): 1829–1838.
- Couppe C, Midttun A, Hilden J, et al. Spontaneous needle electromyographic activity in myofascial trigger points in the infraspinatus muscle: A blinded assessment. J Musculoskelet Pain 2001; 9(3): 7–16.
