Main findings
This is the first study evaluating clustering of MTrP diagnostic criteria identifiable by manual physical examination to assess their relevance.
Our findings provide a purely data driven justification and modification for the proposed expert consensus on MTrP diagnosis [28]. Concordantly, taut band, hypersensitive spot and referred pain were identified as the most essential MTrP diagnostic criteria, but importantly and in line with the definition by Travell and Simons [31], our results suggest that a hypersensitive spot needs to be located within a taut band, and that the simultaneous occurrence of referred pain and/or complementary diagnostic criteria are necessary for a definite MTrP diagnosis. Consequently, palpation of a taut band alone does lead to the diagnosis of an MTrP.
MCA clearly separated cases with at least two classical diagnostic criteria with either few (cluster 3) or many complementary diagnostic criteria (cluster 4) from cases without any diagnostic criterion or just a taut band (cluster 1) and cases with hypersensitive spots or nodules outside of a taut band (cluster 2). Accordingly, clinicians classified the majority of cluster 3 and all cluster 4 but few cluster 1 and 2 cases as MTrPs. A hypersensitive spot within a taut band was by far the most prominent finding in cluster 3 and 4, and referred pain was unexceptionally restricted to these clusters. This reflects the eminent role of these two criteria in MTrP diagnosis. Clinicians classified all cases exhibiting a hypersensitive spot within a taut band and referred pain as MTrPs.
Conversely, among classical diagnostic criteria, palpable nodules within taut bands contributed least to data representation. Nodules within a taut band alone constituted few cases in cluster 2 and 3. Thus, their role in MTrP diagnosis by purely manual examination seems rather subordinate or just confirmative which corresponds to a prevalent expert opinion [26, 28]. This goes without contradicting their proven presence in MTrPs [18], as nodules might be un-identifiable by palpation in muscles located in deeper tissue layers.
Hypersensitive spots and nodules outside of a taut band (cluster 2) can represent anomalies other than MTrPs and should prompt further diagnosis to identify potential significant causes, such as tumors or swollen lymph nodes. Cluster 2 cases did not exhibit referred pain but several complementary diagnostic criteria and were rarely classified as MTrP. Cluster 2 was the smallest, but emerged in all four muscles and was not particular to certain examiners.
Complementary diagnostic criteria generally should entail muscle examination, as they occurred almost exclusively in combination with classical diagnostic criteria and with hypersensitive spots or nodules outside of a taut band. Accumulation of complementary diagnostic criteria, in particular those of muscular dysfunction, separated cluster 3 from cluster 4 reflecting their important role for increasing MTrP diagnostic certainty. Restricted range of motion and pain during contraction might especially substantiate MTrP diagnosis, as they contributed substantially to MCA data representation and were the most frequent in cluster 4. Muscular weakness contributed less to representation of the data and was found also in the absence of other MTrP diagnostic criteria. Pain exacerbation during emotional stress was associated with MTrPs in our study population, but from a clinical perspective, it represents a general phenomenon in pain conditions [37]. Jump signs occurred only in few cases. Taking into account that it represents a strong reaction to palpation of a hypersensitive spot, it might not contribute substantially to MTrP diagnosis in line with Simons and Travell [31]. The rareness of a local twitch response and autonomic phenomena renders them least important in MTrP diagnosis. Nevertheless, there is agreement about the high specificity of the local twitch response for MTrP diagnosis [23]. In clinical practice, its elicitation during dry needing may also assist MTrP diagnosis ex juvantibus. However, interrater reliability of the local twitch response has been shown to be low in palpatory examinations [38].
Clinical implications for MTrP diagnosis
Based on the considerations outlined above, we propose an MTrP diagnostic algorithm (Munich Myofascial Trigger Point Score, MMTS, Fig. 6). Identification of a taut band containing a hypersensitive spot potentially felt as a nodule appears most decisive for MTrP diagnosis which, according to our results, is only confirmed in combination with either referred pain, a local twitch response and/or at least two complementary diagnostic criteria (with restricted range of motion and pain during contraction taking on greater importance).
Reliability for the identification of hypersensitive spots within a taut band and referred pain appears good in comparison to other MTrP diagnostic criteria [24], but still varies largely between muscles and between studies, pointing to the need for examination standards. Elicitation of referred pain requires strong pressure stimulation, either perpendicularly to the taut band or by pincer grip, for several seconds. Pain can refer to loco-regional or distant sites. The referred pain patterns follow typically but not necessarily those described by Simons and Travell [28].
Our findings apply in particular to superficial muscles that are easy to access, as clusters of MTrP diagnostic criteria emerged particularly clearly in the M. trapezius. Examiners should also be trained in assessing sensory, functional and autonomic signs and symptoms. Obtained findings need to be categorized diagnostically e.g. by ruling out differential diagnosis. Improving accuracy of MTrP diagnosis is a prerequisite for treatments that aim at resolving MTrPs, such as dry needling and MTrP injection techniques, for which promising evidence exists [4, 5].
Diagnosis of MPS
There is consensus that the MTrP is the morphological correlate of an MPS. However MPS definitions vary in preciseness with regard to differential diagnosis e.g. confined pain region versus widespread pain and recognition of pain or other symptoms [1,2,3, 28]. In our study the diagnosis MPS was mainly based on an identified MTrP with recognition of local pain upon pressure and/or recognition of referred pain, but additional information about co-morbidities have been considered by physicians. It seems appropriate to differentiate alternative reasons for pressure pain in muscle (e.g. myositis) and generalized pathologies causing wide spread pain including pressure pain in soft tissues (e.g. fibromyalgia). Therefore future research aiming to define and standardize MPS diagnosis should address differential diagnosis to assure adequate pain treatment.
Strengths & limitations
Unlike previous studies, we investigated clusters of MTrP diagnostic criteria resulting from an MCA without a priori implications on clinical interpretation. Similarity between clusters emerging in the four muscles support generalizability of our finding. Physical examinations were standardized to reflect procedures recommended in standard text books. Furthermore, the examined MTrP diagnostic criteria were based on relevant up-to-date literature. Results were documented by an independent observer. Despite these strengths, our study has limitations: First, it was conducted in a mixed sample of consecutive chronic pain patients undergoing an interdisciplinary pain assessment, and documentation of diagnostic criteria were restricted to the trapezius and levator scapulae muscles. Future research needs to evaluate generalizability of our results to other muscles, e.g. deeper muscles and muscles in different body regions, as well as to different populations. Second, the sample size was comparable to e.g. studies on reliability of MTrP diagnosis [24], but larger samples are needed to confirm the proposed diagnostic algorithm. Third, differences in skills and examination styles of physicians are general challenges in research about manual techniques. This bias was minimized by standardized examination instructions.