Background
Tension-type headache (TTH) is the most common type of primary headache disorder and is the third most common disease in the world [
1]. TTH has been considered a non-serious disorder compared to migraine. However, it is a major health problem and leads to a severe socioeconomic burden owing to its high prevalence and comorbidities such as anxiety, depression, temporo-mandibular disorders, fibromyalgia, and obstructive sleep apnoea [
2‐
6]. Patients with TTH with comorbidities have more severe symptoms than those without comorbidities [
2]. Therefore, identification of the associated comorbidities of TTH is important for better management of TTH and may reduce the burden of the disease.
Restless legs syndrome (RLS) is a sleep-related sensory-motor disorder characterized by unpleasant feelings in the legs, especially during rest or at bedtime. The unpleasant feelings are relieved by voluntary leg movement [
7]. Previous population-based and clinic-based studies have consistently reported a significant association between RLS and migraine [
8‐
14].
The mechanism for the association between migraine and RLS is still uncertain. Dopamine and iron dysregulation have been proposed to be mechanism both for migraine and RLS [
7,
15‐
17]. Therefore, they were considered as mechanisms for the comorbidity of two disorders. Furthermore, genetic associations between RLS and migraine have been suggested [
18]. Although numerous reports have suggested a close association between migraine and RLS, population-based information regarding the association between RLS and TTH, which is the most common type of headache, is limited. Nevertheless, a few studies have suggested the presence of significant associations between RLS and TTH. A population-based report indicates that RLS has a significant association with non-migraine headache [
14]. A health-insurance database study in Taiwan revealed that individuals with TTH have an increased risk for developing RLS [
19].
We hypothesized that RLS and TTH would have a significant association in a general population-based sample. The Korean Headache-Sleep Study (KHSS) is a cross-sectional nationwide population-based survey on sleep and headache in Korean adults aged 19–69 years and may provide an opportunity for us to assess the association between RLS and TTH. Therefore, we estimated the prevalence of TTH and RLS in a Korean general population-based sample, compared the association between TTH and RLS in comparison with individuals with non-headache, and assessed the clinical impact of RLS in individuals with TTH using data from the KHSS.
Discussion
The main findings of the present study are as follows: 1) The prevalence of TTH and RLS are 21.1% and 5.3%, respectively, in the Korean general population sample used; 2) The prevalence of RLS among subjects with TTH was significantly higher than among participants with non-headache; and 3) TTH participants with RLS had more severe headache intensity and higher prevalence of anxiety and depression than those without RLS.
While a significant association between RLS and migraine has been consistently reported, information regarding the association between RLS and TTH, which is another common primary headache, is limited. Recently, a retrospective cohort study using a health insurance database suggested that patients with TTH had an increased risk of developing RLS [
19]. Nevertheless, the study did not provide cross-sectional evidence for an association between RLS and TTH, identified TTH and RLS cases on the basis of diagnostic codes rather than diagnostic criteria and did not assess the impact of RLS on the clinical presentation of TTH. Here, we identified RLS and TTH cases on the basis of validated criteria and found that RLS was more prevalent among individuals with TTH than among those with non-headache in a general population-based sample. In addition, we found that participants with TTH with RLS had more severe headache intensity than those with TTH without RLS.
In the present study, TTH individuals with anxiety and depression had higher RLS prevalence than those without anxiety and depression. Multivariable regression analyses revealed that depression is an independent predictor of RLS. These findings are concurrent with those of previous studies of the association between RLS and mood symptoms. Individuals with anxiety or depression have been shown to have a higher risk of RLS and vice versa [
29‐
31]. Anxiety and depression are also common comorbid conditions of TTH [
3]. Therefore, it is plausible that mood symptoms, such as anxiety and depression, are important factors linking RLS to TTH. Further studies are required among TTH subjects regarding mood symptoms and RLS to elucidate the associations among RLS, mood symptoms, and TTH.
We classified 113 participants fulfilling both TTH and PM criteria as subjects with TTH in accordance with the general rule of ICHD-3 beta. Although RLS prevalence in participants with TTH fulfilling PM criteria did not significantly differ from that in those with TTH not fulfilling PM criteria, RLS prevalence in participants with TTH fulfilling PM criteria was significantly higher than in those with non-headache. However, RLS prevalence among participants with TTH not fulfilling PM criteria was not different from that in participants with non-headache (Fig.
2). Among migrainous features, aggravation by movement was more frequently found in TTH participants with RLS than in those without RLS (Table
2). Univariable and multivariable analyses indicated that headache aggravation by movement is an independent predictor of RLS among participants with TTH (Table
3). These findings were concurrent with previous findings that the frequency of RLS may increase with an increase in the number of migrainous symptoms among migraineurs [
8]. However, other migrainous features, such as unilateral pain, pulsating quality, moderate-to-severe headache intensity, photophobia, and phonophobia were not significantly different between individuals with and those without RLS in the present study. Further studies including various headache populations will validate the association between migrainous features and RLS among headache sufferers.
The 1-year TTH prevalence rate (21.2%) in the present study was somewhat lower than that previously observed in Western countries (20–78%) [
32]. The 1-year prevalence of TTH in Asian countries ranges from 10.3 to 33.3% in most studies of Asian populations, which is somewhat lower than those reported in studies of European and North American countries [
32,
33]. RLS prevalence (5.3%) in the present study was similar to those reported in previous studies from Korea and other Asian countries, which range from 1.8 to 8.3%. Similar to TTH, the reported RLS prevalence in Asian populations is lower than that observed in Western countries [
5,
23,
30,
34]. Therefore, the prevalence of RLS and TTH in the present study is similar to the reported prevalence in Asian countries. The similarities between the prevalence of RLS and TTH in the present study and in previous reports from Asian countries suggest that our study properly evaluated RLS and TTH.
Restless leg syndrome has long been considered to be caused by dopaminergic system dysfunction and disturbed iron metabolism [
16,
35]. Unbalanced level of dopaminergic neurotransmitter might also be involved in migraine pathogenesis and dysfunctional iron metabolism in the brain of migraine has been suggested [
15,
17,
18]. Therefore, it is plausible that some underlying migraine characteristics may be a substrate for linking TTH and RLS in consideration of more strong association between TTH fulfilling PM criteria and RLS than between TTH not fulfilling PM criteria and RLS in the present study.
In the present study, depression was significant contributing factor for RLS among individuals with TTH in univariable and multivariable analyses. Association of depression and RLS has been documented in population-based studies [
34,
36]. Dopaminergic dysfunction was noted in depression [
37]. It also has been considered as a key mechanism of RLS as mentioned above [
35]. Dopaminergic dysregulation in TTH has been proposed in previous studies [
38,
39]. Accordingly, dopaminergic dysfunction could be a mechanism for connecting depression and RLS among individuals with TTH. Further experimental and clinical studies were needed for the association of depression, RLS and TTH.
There are several limitations to the present study. Firstly, we diagnosed RLS by using the Korean version paradigm of questions for the epidemiology studies of RLS based on participant’s report [
23]. Therefore, some conditions similar to RLS may have been included. Akathisia, meralgia paresthetica, peripheral neuropathy, and nocturnal cramping can mimic RLS. Nevertheless, the Korean version paradigm of questions for the epidemiology studies of RLS showed high sensitivity and specificity for the diagnosis of RLS when comparing doctors’ diagnosis of RLS [
7,
23]. Secondly, we did not assess the severity of RLS owing to limitations on questionnaire length. An attempt to quantitatively correlate RLS and headache may add more insight to the association between TTH and RLS. Thirdly, although the current study used a population-based sample with low sampling error, its statistical power was limited in terms of examining the subgroups of interest. In other words, the lack of significant findings in the subgroup analyses might be the result of the limited sample size.
Our study has several strengths. Firstly, we used the data of KHSS which was based on clustered random sampling proportional to the Korean population distribution with low sampling error. This condition allowed us to precisely investigate the association between RLS and TTH in a population-base setting. Secondly, we investigated anxiety and depression, which are common comorbidities among individuals of RLS and assessed the effect of anxiety and depression in the association between RLS and TTH. Thirdly, we assessed the prevalence of RLS among individuals with TTH according to migrainous features.
Acknowledgements
The authors would like to thank Gallup Korea for providing technical support for the Korean Headache-Sleep study.