The present study identified distinct CM patterns of TTH through a cluster analysis of 170 TTH participants in a bilingual cross-sectional survey. The results of this study suggest that TTH can be divided into four clusters based on symptoms and signs that are significant to the diagnostic process in Chinese medicine. The four clusters were not only distinguishable in CM patterns, but also differed in aspects of subtypes of TTH (ETTH, frequent ETTH, and CTTH), stress level, pain intensity, and disability level. These findings expand the existing understanding of TTH symptomatology in Western medicine and TTH patterns in Chinese medicine, which may help advance our understanding of the symptoms associated with TTH and subgroups of TTH.
TTH has been shown to be associated with a number of symptoms. The common TTH characteristics and associated symptoms identified in the present study are consistent with the findings of other studies [34–37]. The main similarities are the precipitating factors such as physical activity, stress/tension, when tired, lack of sleep, specific foods/drinks, alcohol, and skipping meals, and some accompanying symptoms such as fatigue, insomnia, and irritability. Emotion-related factors may have impacted on the presence of TTH. The present study found that stress and/or tension (73.6%) was the leading precipitating factors, and the finding is consistent with others (49.4% , 74.5% , 63% in men and 77% in women , 52.5% ). Only a small percentage of anxiety disorders and mood disorders were detected (7.1% respectively). This is probably due to more than three-quarters of the respondents were ETTH sufferers, as it has been shown that psychiatric comorbidities are more common in CTTH patients [40, 41] whereas those having less frequent TTH tend to having a lesser degree of psychiatric comorbidity . These results expand the common understanding of TTH symptomatology in terms of its pain description, trigger factors, and accompanying symptoms, as it provides better understanding of symptomology of TTH. With this knowledge, it is possible that more targeted treatments could be developed.
In summary, a considerable similarity of reported features and associated symptoms on TTH were observed between the present study and studies investigating factors and symptoms associated with TTH from western medical aspects. Inevitably, due to the differences in sampling, methods of studies, and the time points when each study took place, there are some discrepancies in the results reported by the above-mentioned studies and the present studies. For example, specific foods/drinks, as an aggravating factor, varied from 2–35%, skipping meals from 24.8–52.9%, smoking from 8.6 to 38%. In addition, the present study observed that bilateral headache was the most common location of TTH (71%) and followed by pain experience in the forehead (52.7%). Although the study by Li et al.  also listed bilateral and forehead headache being the commonest TTH, their findings were different with bilateral headache being 79.9% and forehead 34.1%.
In CM, TTH is not a recognised disorder. Literature in CM diagnosis of TTH is limited, yet there is no golden CM standard to classify TTH, since none established TTH eligibility criteria in CM diagnosis. Due to this, differentiation diagnostic criteria of pattern identification in treatment varied, which may also indicate a need to establish the CM differentiation diagnosis of TTH in helping identify the significance of clinical outcome led by the variation from practitioners . The CMHQ contains a series of accompanying symptoms that may be similar to other studies, a major difference between our investigation and other studies were that most CMHQ items detecting signs and symptoms were originally set for diagnosis purposes for distinguishing CM patterns. The present study captured a series of accompanying symptoms that are seldom mentioned by others. For instance, sore eyes (43.5%), thirst (40.6%), bloating (30.0%), joint stiffness (40.6%), muscle twitching (31.8%), increased forgetfulness (50.0%), sighing often (41.1%), and, inability to concentrate (47.1%) were common symptoms experienced by TTH suffers in this sample.
The aim of cluster analysis is to differentiate a group of individuals into subgroups with homogeneous attributes that are diverse from other subgroups . For example, using the hierarchical clustering, one study identified subgroups of individuals with headache who self-medicate which could be helpful to tailor intervention strategies for prevention of medication-overuse headache. Another recent study on diabetic peripheral neuropathy (DPN) using cluster and factor analysis identified distinct groups of patients with respect to its clinical impacts on symptom patterns and comorbidities. Such comprehensive approaches could endorse the subgrouping to individualise the evaluation of patients with DPN. In CM syndrome research, exploratory analysis methods were valuable in understanding and verifying the CM patterns in a series of diseases and conditions defined by modern medicine. Those studies covers a range of topics such as Chronic obstructive pulmonary disease (COPD) [46–48], metabolic syndrome , chronic fatigue syndrome [50, 51], diabetic nephropathy , acquired immune deficiency syndrome (AIDS) [53, 54], lung cancer , preoperative colorectal cancer , stroke [57, 58], diabetes mellitus , diabetic retinopathy , excessive menstruation , functional constipation , uterus myoma , acute myocardial infraction , acute pancreatitis , posthepatitic cirrhosis , chronic low back pain , fibromyalgia [66–69], and coronary heart disease [70–72]. Generally, those studies identified explainable CM patterns and interpreted those modern diseases in a reasonable way. In the present study, the use of other measurements enhanced the understandings to the identified patterns in aspects of headache features, severity of headache-related disability, comorbidities, and psychological profiles, which reflect the multidimensional perspectives of TTH. The patterns identified were not only different in symptom manifestations, but also in disability and self-perceived stress and coping.
Very few studies have examined the differences between ETTH and CTTH beyond headache days. In modern medicine, identification of subtypes of TTH under ICHD-II is mainly distinguished by the frequency of headache attacks on the basis of epidemiological studies . Within ETTH, its infrequent subtype occur at lower frequency (< 1 day a month) than the frequent subtype (≥ 1 day a month). The present study indicated that the infrequent ETTH reported much lower headache intensity (mild, mean of 2.78) than other two (moderate, mean of 5.85 and 5.81 respectively) and showed the lowest level of disability. This is in line with the description of ICHD-II that such infrequent subtype has very little impact on the individual whereas the chronic subtype in the present study is associated with a high level of disability. Since ETTH and CTTH also differ in the level of disability and some symptomatology.it is possible to sub-categorise TTH from a multi-dimensional perspective, but not just limited to the frequency of headache.
In the present study, the four CM patterns differed from the current TTH subtypes. The three patterns not only differ in headache frequency, but also in headache intensity and disability. Over three-quarters of participants in Clusters 1 and 2 had frequent ETTH and about one-fifth had CTTH, whereas one-third in Cluster 3 had CTTH, and half had frequent ETTH. All these three clusters had very few participants with infrequent ETTH, whereas one-third of Cluster 4 was having infrequent ETTH (< 1 day). Those results indicate that the CM pattern identification goes beyond headache frequency as it focuses on symptoms and signs that TTH sufferers experience in addition to their headache frequency.
Currently, there is a significant gap in understanding sub-types of TTH. On one hand, the IHS diagnostic criteria for TTH are designed to distinguish TTH from other types of headaches to some degree, and to classify TTH into three subtypes based upon attack frequency only. Non-headache symptoms associated with TTH are, however, not explained or accounted for. On the other hand, despite several epidemiological studies observing a series of aggravating and relieving factors and accompanying symptoms of TTH, clinical practice to date has not given adequate attention to TTH symptoms. The current study fills those gaps by further understanding non-headache symptoms in TTH and using knowledge of pattern identification and advanced statistical methods to identify three clinically-meaningful subgroups of TTH. The presence of these subgroups of TTH sufferers indicates that there is a need to go beyond frequency and relieving factors of TTH. Addressing headache as well as accompanying non-headache symptoms may lead to more efficient treatment strategies.
This study has important strengths. To the best of authors’ knowledge, the present investigation is the first study using exploratory statistical method to research TTH-related symptoms as well as identifying CM patterns of TTH. Our study presented an original statistical methodology that allowed the identification of clinical CM patterns. The method applied, that is using objective exploratory analytic approaches to the symptom-based clinical variables of TTH participants, provides an alternative to current modern medicine approaches in understanding the symptoms associated with TTH and subgroups of TTH. The survey was both hospital-based and general population-based. As a result, it should be applicable to the majority of TTH population.
In summary, the findings expand the existing understanding of TTH symptomatology and TTH patterns. They provide essential information for future research on subgroups of TTH. Nevertheless, several limitations of the current study should be considered. Firstly, the present results could be limited due to its sample size, as some other possible patterns may be observed with a larger sample size. Secondly, relying on exploratory analysis has its drawbacks, since statistically-determined clusters can be affected by many factors. Although we conducted exploratory analysis, we relied on experts’ opinions when interpreting the generated factors and labelling the grouped symptoms and signs. However, expert opinions may be subjective. The present study minimised this potential limitation by a combined approach of exploratory analysis and expert opinions. Both internal and external experts were consulted during the processes of evaluation, determination and labelling of clusters. Finally, this study is a cross-sectional study, which only analysed the symptom distribution collected at a specific duration over the last 3 months. The presence and the severity of symptoms observed may change over time. Future studies may use longitudinal cohort approaches to evaluate the stability of the identified CM patterns over time, and to assess the effect of interventions.