Prevalence of Myofascial Trigger Points in Spinal Disorders:
A Systematic Review and Meta-AnalysisThis section is compiled by Frank M. Painter, D.C.
Send all comments or additions to: Frankp@chiro.org
FROM: Arch Phys Med Rehabil. 2016 (Feb); 97 (2): 316–337 ~ FULL TEXT
Alessandro Chiarotto, MSc, Ron Clijsen, PhD, Cesar Fernandez-de-las-Penas, PhD, Marco Barbero, PT OMT
Department of Health Sciences,
Faculty of Earth and Life Sciences,
EMGO(+) Institute for Health and Care Research,
Vrije Universiteit,
Amsterdam, The Netherlands.
OBJECTIVE: To retrieve, appraise, and synthesize the results of studies on the prevalence of active and latent myofascial trigger points (MTrPs) in subjects with spinal pain disorders.
DATA SOURCES: The databases PubMed, Embase, and CINAHL were searched, with no date or language restrictions. Search terms included controlled and free-text terms for spinal disorders and MTrPs. Further searches were conducted in Google Scholar and by contacting 3 experts in the field. Citation tracking of eligible studies was performed.
STUDY SELECTION: Two reviewers independently selected observational studies assessing the prevalence of active and/or latent MTrPs in at least 1 group of adults with a spinal disorder. Twelve studies met the eligibility criteria.
DATA EXTRACTION: Methodologic quality was assessed by 2 reviewers independently using a modified version of the Downs and Black checklist. Two reviewers also used a customized form to extract studies and subjects' characteristics and the proportions of subjects with active and/or latent MTrPs in each muscle assessed.
DATA SYNTHESIS: A meta-analysis was performed when there was sufficient clinical homogeneity in at least 2 studies for the same spinal disorder. The Grading of Recommendations Assessment, Development and Evaluation approach was used to rate the body of evidence in each meta-analysis. A qualitative description of the results of single studies was provided. Low-quality evidence underpinned pooled estimates of MTrPs in the upper-body muscles of subjects with chronic neck pain. The point prevalence of MTrPs in different muscles of other disorders (eg, whiplash-associated disorders, nonspecific low back pain) was extracted from single studies with low methodologic quality and small samples. Active MTrPs were found to be present in all assessed muscles of subjects diagnosed with different spinal pain disorders. Latent MTrPs were not consistently more prevalent in subjects with a spinal disorder than in healthy controls.
CONCLUSIONS: The MTrPs point prevalence estimates in this review should be viewed with caution because future studies with large samples and high methodologic quality are likely to change them substantially.
KEYWORDS: Low back pain; Neck pain; Prevalence; Rehabilitation; Trigger points; Whiplash injuries
From the Full-Text Article:
Introduction
Spinal disorders are among the leading causes of years lived with disability, with low back pain (LBP) ranking first and neck pain (NP) ranking fourth worldwide. [1] The same disorders are also majorly responsible for disability-adjusted life years. [2] The mean lifetime activity-limiting prevalence of LBP is estimated to be approximately 39% and that of NP is 23%, and the point prevalence is approximately 18% and 14%, respectively. [3, 4] Cost-of-illness studies have highlighted that costs associated with these disorders represent a significant burden to society. [5, 6] Considering all these factors together, it is apparent that research to increase our understanding of the etiology of these disorders is critical.
A clinical sign in subjects with spinal disorders is the presence of myofascial trigger points (MTrPs). [7–9] Expert-based definitions of MTrPs identify these as hypersensitive spots within a taut band of skeletal muscle that are painful on compression and which can evoke referred pain. [10] From a clinical perspective, MTrPs can be differentiated by manual assessment into active and latent. [10] Active MTrPs elicit local and referred pain that reproduce the symptoms that the patient suffered from and are recognized as a familiar complaint, whereas latent MTrPs reproduce local and referred pain that does not reproduce any spontaneous symptoms perceived by the patient. [10]
The clinical distinction between active and latent MTrPs is supported by histochemical findings showing that active MTrPs contain higher levels of algogenic substances and chemical mediators (eg, bradykinin, substance P, serotonin) than latent MTrPs and body areas without MTrPs. [11, 12] Both active and latent MTrPs can be involved in pain sensitization processes involving the central nervous system, [13, 14] and these processes have been shown to be altered in subjects diagnosed with different spinal disorders. [15–18] Further, latent MTrPs can be contributors to musculoskeletal signs and symptoms (eg, muscle imbalance, muscle weakness, fatigability), as reported by recent studies. [19–21]
The presence of active MTrPs in a subject can lead to the diagnosis of myofascial pain syndrome, which is considered to be a major cause of musculoskeletal pain, and its prevalence in adult subjects is reported to be high. [22] Several studies have reported the prevalence of manually assessed active and latent MTrPs in different muscles of subjects diagnosed with spinal disorders. To our knowledge, however, no systematic reviews to date have attempted to retrieve all of these studies to assess their methodologic quality and summarize their findings.
The objective of our study was therefore to conduct a systematic review of the literature with a meta-analysis to synthesize the evidence on the prevalence of active and latent MTrPs in subjects with spinal disorders. The body of evidence on the prevalence of MTrPs was considered and analyzed separately for each spinal pain disorder following diagnoses and definitions used by the authors of the original studies (eg, LBP, NP, whiplashassociated disorder [WAD]).
Three specific aims were defined for each disorder:(1) to estimate the prevalence of active MTrPs in all evaluated muscles;
(2) to compare the prevalence of latent MTrPs in subjects diagnosed with different spinal disorders (eg, NP vs WAD); and
(3) to compare the prevalence of latent MTrPs in both subjects diagnosed with a spinal disorder and healthy controls.We did not aim to compare the prevalence of active MTrPs between subjects diagnosed with different spinal disorders or between subjects with a spinal disorder and healthy controls because, by definition, active MTrPs cannot be present in healthy subjects. [7–10]
Results
Figure 1 is a flowchart of study retrieval, screening, and eligibility. The 3 experts in the field we contacted did not identify any additional studies besides those retrieved from the database searches and citation tracking. Twelve studies met the eligibility criteria and were included in this review [40–51]; 4 of these studies [40, 42, 44, 49] were included in the meta-analyses. All studies were cross-sectional and assessed the point prevalence of MTrPs. The characteristics of these studies (ie, spinal disorders involved, diagnostic criteria, muscles assessed, country, setting) are summarized in table 1. One study compared the point prevalence of MTrPs in subjects with NP and WAD, [40] 8 compared the prevalence of MTrPs in ≥1 group of subjects with a spinal disorder and healthy controls, [42–47, 49, 51] and the remaining 3 assessed the prevalence of MTrPs only in a group of subjects with a spinal disorder. [41, 48, 50]
Five studies were conducted in subjects with NP [40, 42–44, 49]; 3 included subjects with WAD [40, 43, 45]; 1 included a mixed population of subjects with spinal pain [41]; 2 included subjects with NSLBP [46, 47]; 1 included subjects with cervicogenic headache [48]; 1 included subjects with lumbar disk herniation [50]; and 1 included subjects with cervical radiculopathy (CR). [51] Sample size, sex age, and other characteristics of the subjects included in the studies are shown in table 2.
Diagnostic criteria for MTrPs varied widely across the studies. Six studies [40, 42, 44, 45, 49, 51] adopted expert-based definitions for active and latent MTrPs [10]; Iglesias-Gonzalez et al [47] adopted the same criteria but with the exclusion of local twitch response as a diagnostic criterion; Samuel et al [50] used different criteria for active MTrPs; and the other 4 studies [41, 43, 46, 48] did not make a clear distinction between active and latent MTrPs (see table 1).
Methodologic quality assessment
Table 3 presents the methodologic quality assessment of the 12 included studies. Because 1 reviewer (A.C.) was an author of an eligible study, he was not involved in the quality assessment of that study, which was assessed by the third reviewer (R.C.). Overall, the studies presented low methodologic quality, with only 2 meeting >50% of the applicable criteria of the checklist. [40, 42] Seven criteria were not applicable to 3 studies because they included only 1 group of subjects. [41, 48, 50] All but 1 study [41] had a clear description of the hypothesis/aim/objective (ie, item 1), and all but 2 studies [41, 43] clearly presented the inclusion and exclusion criteria of the subjects included (ie, item 3). Item 8 regarding the description of the sampling method for the recruitment of subjects from the source population was not met by any study, likewise for item 13 on the accuracy of the main outcome measures (ie, MTrPs diagnosis) and item 16 on the sample size calculation. Only Hua et al [46] reported on the proportion of recruited subjects who agreed to participate (ie, item 9). Items 14 and 15 on selection bias regarding the recruitment of subjects and controls from the same population and over the same time period were met by a small minority of the studies (see table 3). The distribution of principal confounders in each study group (ie, item 4) was met by De-la- Llave-Rincon et al, [42] but the other studies did not report on some key potential confounders (ie, body mass index, anxiety, depression, pain hypersensitivity).
Neck pain
Among the 5 studies that reported on the point prevalence of MTrPs in subjects with NP, 4 studies included subjects with chronic NP; of these, 2 studies [40, 42] had high methodologic quality and the other 2 studies [44, 49] had low quality (see table 3). Ettlin et al [43] did not report the inclusion criteria or clinical characteristics of the included subjects with NP, had low methodologic quality, did not make a distinction between active and latent MTrPs, and did not make a distinction between the diagnosis of myofascial pain syndrome and MTrPs. For these reasons, this study was excluded from the meta-analyses, and its results are not presented here because they were poorly informative.
There was low-quality evidence that the point prevalence of active MTrPs in 91 subjects with chronic NP is 38.5% (95% CI, 29.1–48.9) on the right upper trapezius, 29.8% (95% CI, 21.3– 40.0) on the left trapezius, 16.9% (95% CI, 10.4–26.2) on the right levator scapulae, 14.8% (95% CI, 7.7–26.7) on the left levator scapulae, 22.0% (95% CI, 14.5–32.1) on the right sternocleidomastoid, and 19.8% (95% CI, 12.6–29.6) on the left sternocleidomastoid (fig 2). Downgrading of the quality of evidence was caused by low methodologic quality and imprecision. Low-quality evidence (because of inconsistency and lack of precision) was also found on the point prevalence of active MTrPs on the temporalis muscle of 76 subjects with chronic NP, resulting in pooled estimates of 11.9% (95% CI, 1.0–63.6) on the right side and 11.2% (95% CI, 1.2–56.9) on the left side (fig 3). The point prevalence for active MTrPs on other muscles is summarized in table 4, where it can also be seen that all results were extracted from studies with small samples.
ORs with 95% CIs were calculated for a high-quality study [40] that compared the presence of latent MTrPs in subjects with chronic NP and chronic WAD (table 5). A significant difference was found for the left sternocleidomastoid, indicating that subjects with chronic NP had lower odds than those with WAD to display a latent MTrP on this muscle. For all other muscles, no significant differences were found regarding the presence of latent MTrPs between subjects with NP and those with WAD (see table 5).
There was low-quality evidence showing that pooled ORs for the presence of latent MTrPs on both sides of the upper trapezius and levator scapulae were not statistically different between subjects with chronic NP and healthy controls (fig 4). Reasons for downgrading evidence quality were low methodologic quality and small sample sizes. The same level of evidence was found for pooled ORs showing that subjects with chronic NP have significantly higher odds than healthy controls to have latent MTrPs on both sides of the sternocleidomastoid (see fig 4).
One study40 of high methodologic quality found that subjects with chronic NP had significantly higher odds than healthy controls to have latent MTrPs on the masseter and temporalis muscles (see table 5). Munoz-Munoz et al [49] (low quality) did not find a significant difference for latent MTrPs on other muscles between subjects with chronic NP and healthy subjects (see table 5). All of these results were extracted from studies with small samples (see table 2).
Whiplash-associated disorder
One study [40] of high quality and 1 study [43] of low quality reported on the point prevalence of MTrPs in subjects with chronic WAD; another study [45] with low quality assessed subjects with acute WAD. The results of Ettlin et al [43] are not presented here because they made no distinction between active and latent MTrPs and the diagnosis of myofascial pain syndrome and MTrPs. Statistical pooling of the other 2 studies [40, 45] was not feasible because of ample heterogeneity in the symptom duration of subjects with WAD (see table 2).
ORs with 95% CIs were calculated for the comparisons of active MTrPs in subjects with chronic WAD and chronic NP and the comparisons of latent MTrPs in subjects with acute WAD and healthy controls (table 6). The point prevalence of active MTrPs in subjects with WAD is also shown in table 6. Table 5 presents the ORs with 95% CIs for the study [40] that compared the presence of latent MTrPs between subjects with chronic NP and chronic WAD. Fernandez-Perez et al, [45] in a study of low quality and small sample size, showed significantly greater odds of displaying latent MTrPs on the right upper trapezius, right sternocleidomastoid, and right scalene in subjects with WAD than in healthy controls (see table 6).
Cervicogenic headache
One study [48] with low methodologic quality reported on the prevalence of MTrPs in 11 subjects with cervicogenic headache. This study did not make a distinction between active and latent MTrPs, and it was not possible to extract this difference from the article because it was not clear whether symptom recognition was used for the diagnosis of MTrPs. For this reason, the results of this study are not presented here.
Cervical radiculopathy
Sari et al [51] reported on the prevalence ofMTrPs in 244 subjects with CR compared with 122 healthy controls. This study was low quality and did not make a distinction between right/left or symptomatic/ asymptomatic muscle sides. Point prevalences for activeMTrPs were as follows: 51.2%on the total sample, 16.3% on the levator scapulae, 14.7% on the splenius capitis, 14.3% on the rhomboid minor, 13.5% on the upper trapezius, 10.2% on the rhomboid major, and 8.6% on the multifidus. No significant differences between subjects with CR and controls were found for latent MTrPs on the levator scapulae (OR = .80; 95% CI, .52–1.26), splenius capitis (OR = 1.02; 95% CI, .58–1.83), rhomboid minor (OR = 1.15; 95% CI, .69–1.92), upper trapezius (OR = .83; 95% CI, 0.55–1.25), and rhomboid major (OR = 1.37; 95% CI, 0.59–3.18). Subjects with CR displayed significantly lower odds than controls with latent MTrPs on the multifidus (OR = .48; 95% CI, .25–.91).
Nonspecific low back pain
Two studies [46, 47] with low methodologic quality reported on the prevalence of MTrPs in subjects with NSLBP: one included only subjects with chronic NSLBP, [47] and the other included only acute and subacute NSLBP. [46] The results of the latter are not presented because a distinction between active and latent MTrPs could not be established from the diagnostic criteria used (see table 1). The point prevalences of active MTrPs from the study of Iglesias-Gonzalez [47] in 42 subjects with chronic NSLBP are shown in table 7. No ORs were calculated for latent MTrPs in subjects with NSLBP and healthy controls because of the differences in assessing more/less painful or dominant/nondominant muscle sides. Overall, latent MTrPs tended to be more prevalent in subjects with chronic NSLBP than in healthy subjects (see table 7).
Lumbar disk herniation
Samuel [50] (low methodologic quality) assessed the point prevalence of active MTrPs in 60 subjects diagnosed with lumbar disk herniation. In this study, no distinction between right/left or symptomatic/asymptomatic muscle sides was made, and some criteria used in other studies for the diagnosis of active MTrPs were not used (see table 1). Point prevalences were as follows: 50.0% tibialis anterior, 67.7% extensor hallucis longus, 3.3% extensor digitorum longus, 81.7% gluteus maximus, 13.3% gluteus minimus, 48.3% hamstring, 80.0% gastrocnemius, and 1.7% quadriceps.
Mixed population of spinal disorders
One study [41] of low quality assessed the presence of MTrPs in a mixed sample of subjects with chronic spinal pain. In this study no distinction between active and latent MTrPs was made, and its results are not presented here.
Discussion
In this systematic review we aimed to synthesize the evidence on the prevalence of active and latent MTrPs in spinal disorders by including 12 cross-sectional studies assessing point prevalence in 6 different spinal pain disorders. Low-quality evidence was found for the pooled point prevalence of active MTrPs of different muscles in subjects with chronic NP. Point prevalences of active MTrPs in subjects with WAD and NSLBP were extracted from single studies that had low quality and/or very small samples. In 1 study of high methodologic quality, [40] no significant differences were found in the prevalence of latent MTrPs between subjects with chronic NP and those with chronic WAD. There was low-quality evidence that subjects with chronic NP had higher odds than healthy controls to display latent MTrPs on the sternocleidomastoid, but not on upper trapezius and levator scapulae. Small sample sizes and/or low methodologic quality were a constant characteristic of the findings on latent MTrPs taken from individual studies on different spinal disorders (ie, NP, WAD, NSLBP, CR). Future studies of high methodologic quality and with larger samples are very likely to substantially change the point prevalence estimates of MTrPs presented here.
Of the 12 studies included in this review, 10 studies [41, 43–51] had low methodologic quality (see table 3). Several items were not met by any study, and this emphasizes that future studies on the prevalence of MTrPs in spinal disorders should attempt to meet these criteria. For example, a lack of information on the sampling method used to recruit subjects from the source population, and on the proportion of those recruited agreeing to participate, might strongly limit the generalizability of findings. Information regarding the recruitment of subjects and controls from the same source population should also be reported because a failure to do so might hinder the assessment of selection bias in these studies. The measurement properties of active and latent MTrPs diagnostic criteria were not presented in any of the studies. This lack is even more important if we consider that 2 systematic reviews [52, 53] reported limited reliability for MTrPs criteria. However, in line with more recent individual studies, [54, 55] these reviews also highlighted that reliability was strongly dependent on the selected muscles and on other variables (eg, examiners’ training and experience). Detailed reference to this information should be provided in future studies because this would confirm that researchers are at least aware of this potential limitation of MTrPs assessment.
Future studies comparing the prevalence of latent MTrPs in 2 distinct groups should calculate their sample size beforehand to reduce the chances of finding false significant results as a result of poor statistical power (ie, type I error). Another crucial aspect that should be better addressed by future studies is the reporting of potential key confounders when comparing the prevalence of MTrPs in 2 distinct groups of subjects. In fact, patients’ morphologic features or impairments of the nociceptive system may limit the applicability of the diagnostic criteria for MTrPs. Detection of the taut band can be difficult in subjects with a high body mass index because of the presence of subcutaneous fat. Moreover, criteria based on pain provocation (ie, spot tenderness, pain recognition, referred pain) may be affected by general hypersensitivity or the presence of comorbid conditions that may alter the perception or reporting of pain (ie, depression, somatization). [45]
In 4 studies, [41, 43, 46, 48] a clear distinction between active and latent MTrPs was not made, making it difficult to extract meaningful data from these studies. The distinction between active and latent MTrPs, initially made for clinical purposes by experts in the field, [10] has been supported by studies showing that the 2 phases/stages of MTrPs present different histochemical findings. [11, 12] Two other studies [50, 51] did not present the assessment of MTrPs on each side of each muscle (or did not distinguish between painful sides), making the interpretation of the results more difficult. This variability in the diagnostic criteria for MTrPs is not new [57] and can strongly hinder the synthesis of data from different studies. Such variability is probably the result of lack of data on the criterion validity of MTrPs diagnostic criteria because of the absence of a feasible and accepted reference standard to estimate indices (eg, sensitivity, specificity). In this regard, a future study might focus on reaching an international and multidisciplinary consensus on the diagnostic criteria for MTrPs (eg, Delphi survey), with the goal of standardizing their assessment in clinical practice and research. Overall, good quality studies of the reliability of different sets of criteria are needed to further strengthen or weaken the findings of the available literature. [52–55, 57]
Pooled estimates of the point prevalence of active MTrPs in subjects with chronic NP on the upper trapezius, levator scapulae, sternocleidomastoid, and temporalis showed that the highest prevalence was on the upper trapezius (see figs 2 and 3). Despite low-quality evidence, these estimates may be to some extent accurate if we consider the high prevalence of NP and shoulder pain in both the working and general population. [58, 59] These painful syndromes are common among office workers, and upper trapezius myalgia is the most frequent neck complaint in occupational groups. [60–62] Results from single studies showed the highest point prevalence for active MTrPs on the suboccipital muscles (see table 4), but all estimates were extracted from studies with very small samples. The significant difference in the prevalence of latent MTrPs on the left sternocleidomastoid between subjects with chronic WAD and chronic NP can be considered a spurious finding, considering that no differences were found for all other muscles and that this study included a small sample. [40] Future studies with larger samples are needed to confirm or refute this peculiar finding for the left sternocleidomastoid. The same applies for meta-analyses or results of individual studies comparing the prevalence of latent MTrPs in subjects with NP and healthy controls (see fig 4 and table 5) because these were extracted from very small samples. Future observational studies should adopt large sample sizes and have a higher methodologic quality to provide more precise estimates of the point prevalence of both active and latent MTrPs in subjects with NP.
Castaldo et al [40] showed that active MTrPs appear to have a higher point prevalence in subjects with chronic WAD than in those with chronic NP (see table 6). If the presence of MTrPs is considered a possible indicator of central sensitization, [13, 14] these results could be explained by the enhanced degree of central sensitization of subjects with WAD compared with those with NP. [18, 28, 63] However, more studies are needed to confirm or reject the findings obtained from the relatively small sample of this study. The same consideration applies to the study of Fernandez-Perez, [45] in which latent MTrPs were compared between subjects with acute WAD and healthy controls; although some differences between groups were found (see table 6), more studies with larger samples are clearly needed.
Several health care providers recognize the clinical relevance of the MTrP concept; however, the validity of this concept is discussed in the scientific community, and controversies still exist. [64–66] To add to this, the aforementioned methodologic limitations of existing studies on spinal disorders strongly influence their findings. For this reason, the apparent and constant presence of the MTrP phenomenon in different spinal disorders emerging from this review (see figs 2–4 and tables 4–7) should be considered with caution at this stage.
All of the elements described here highlight the need to establish the relevance of MTrPs for clinical practice. Nevertheless, we do not know whether MTrPs should be considered an epiphenomenon of the painful conditions or a relevant comorbidity. Clinical guidelines for spinal disorders do not usually consider either MTrP evaluation or MTrP treatment. To improve rehabilitation for spinal disorders, it may be important to establish whether MTrP treatment should be taken into account or not; more specifically, it appears crucial to strengthen the construct validity of the MTrP diagnosis and to reach a consensus on a set of MTrP diagnostic procedures.
Study limitations
We acknowledge as a limitation of this systematic review that the reviewer (A.C.), who was an author of an eligible study, although he was excluded from the quality assessment of that particular study, could have influenced the assessment of the other studies. However, given the circumstances, it was not practical for this review to find a different reviewer from the author originally designed for quality assessment of all of the studies. It should also be considered that the approach of excluding a reviewer only from rating his own study is not new in the literature, and it has been adopted also in Cochrane Reviews on the effectiveness of health interventions. [67, 68]
Conclusions
This systematic review shows that active and latent MTrPs can be present in different spinal disorders (eg, NP, WAD, NSLBP). However, these findings are at best underpinned by pooled estimates of point prevalence that are based on low-quality evidence, according to the Grading of Recommendations Assessment, Development and Evaluation approach. Most of the estimates for both active and latent MTrPs are based on individual studies with very small sample sizes and low methodologic quality. Future studies with large samples and high methodologic quality are needed to provide more reliable and precise estimates on the point prevalence of MTrPs in spinal disorders. Moreover, to facilitate comparison of findings and data pooling, there is an urgent need to standardize the assessment of MTrPs across clinical studies.
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