Through this observational study, we aimed to clarify if there is an association between DWMHs and RLS in migraine patients, especially in those with large or permanent shunts. The prevalence rate of DWMHs in our study was 58.2%, which is slightly higher than that reported in previous studies (7), despite the younger age (39.3 ± 11.7 years) of our participants. The overall prevalence of RLS in our study (48.5%) was similar to that of our previous work (46.1%) (10), but lower compared to other studies (9, 18). Also, a higher prevalence rate of RLS in patients diagnosed with MwA, as reported in the literature (10, 19), was not found in our study. This was probably because of the limited sample size of patients with MwA. Based on our findings, DWMHs in migraine patients have no association with RLS, regardless of the subtypes or grades. Neither patients with permanent RLS nor those with large shunts have an increased risk of presenting a higher prevalence of DWMHs. After controlling for other variables, age and aura seem to be independent risk factors for an increased prevalence of DWMHs.
RLS and DWMHs in migraine
WMHs have always been considered as a phenomenon of small vessel disease (20), and the most consistent risk factors for WMHs are age and hypertension (21). Current literature suggests that migraine is a risk factor for WMHs (5, 22), raising questions regarding whether migraine patients are more vulnerable to the development of WMHs and how these may affect them.
The association between RLS and WMHs in migraine patients has long been debated. The majority of published papers have reported no association between RLS and WMHs (7, 9, 23); however, some research has shown a positive association (24, 25). There is evidence that migraine patients have a higher prevalence of RLS (9, 10), and it is believed that paradoxical embolism caused by RLS is related to cryptogenic stroke and silent brain infarcts in migraine (26). Therefore, some authors suggest that RLS is involved in WMHs through a similar mechanism (27).
However, as no widely accepted theory on the pathology of WMHs has been proposed, it is possible that periventricular WMHs (PVWMHs) and DWMHs have differences in pathology, according to neuroimaging studies (28). High burdens of PVWMHs are more often associated with clinically isolated syndromes and cognitive impairment diseases (28, 29), whereas DWMHs have a higher prevalence among migraine patients. In the CAMERA series studies, only DWMHs in women were found to have a different prevalence between migraineurs and controls (3, 5). Furthermore, DWMH lesions that seem to progress in migraine patients as pre-existing microstructural white matter changes have been observed years before the emergence of visible focal WMHs on conventional MRI (30). Furthermore, a study on migraine and tension-type headache patients suggested that small DWMHs are associated with RLS in young migraineurs (25). Therefore, we may assume that if paradoxical embolisms caused by RLS play a part in WMHs in migraine patients, it is most likely associated with DWMHs, rather than PVWMHs. In addition, similar to the case in cryptogenic stroke, patients with permanent RLS and large shunts should be at a higher risk (31, 32).
In this study, c-TCD instead of heart sonography is used to detect RLS due to its higher sensitivity (33). Although c-TCD cannot distinguish the different levels of RLS, both intracardiac and extracardiac shunts can cause paradoxical embolism. Moreover, compared to heart sonography, c-TCD has a better detection rate on extracardiac shunts (34). A different system for rating RLS grades was used in this study instead of the one that most former studies on this topic have used, mainly dividing RLS into three grades: negative, less than 10 MBs, and more than 10 MBs. This decision was based on our former research, which has shown that the prevalence rate of large shunts (defined as > 25 MBs or curtain on TCD spectrum) in migraineurs is significantly higher than that in healthy controls (10). Thus, when it comes to a possible paradoxical embolism caused by RLS, we believe that large shunts under our category system should more likely be deemed a probable cause. However, results from our study add further evidence to the lack of association between RLS and DWMHs, even in patients with large or permanent shunts. Therefore, it is not likely that RLS contributes to the high prevalence of DWMHs in the migraine population through paradoxical embolism.
Age, sex, and cognitive impairment
In concordance with previous findings (7, 35), a positive association between age and DWMH prevalence was found in our study (OR: 1.067, 95% CI: 1.035–1.101, p < 0.001; per year).
Whether this higher burden of DWMHs is a consequence of normal aging or migraine as a risk factor remains unclear. There is consistent evidence of age-related accumulation of both PVWMH and DWMH burden (21, 36), suggesting that WMHs might be part of the normal aging pattern in brain tissue. However, an imaging pattern study on brain aging in the general population has shown that WMHs contribute to beyond-normal brain aging imaging findings (36). DWMH progressions in migraine patients have been reported in some longitudinal studies, suggesting that female migraineurs or patients with recurrent headache attacks may develop heavier DWMH burdens (3, 4). Therefore, it is possible that this association between age and DWMHs in migraine patients is an outcome of both aging and the impacts of migraine-related factors over the years.
The difference between sexes, as reported in several studies (6, 30), was not detected in our study. A possible reason is the different proportion of women on oral contraceptives in different study populations. In our study, only 8 out of the 185 women reported a history of oral contraceptive use, compared with 25% of women who had a history of over 15 years of oral contraceptive use in the CAMERA study (30). Considering that the use of oral contraceptives can be a risk factor for vascular incidents, the low rate of oral contraceptive usage in our study group could account for this difference.
We did not find any association between DWMH prevalence and cognitive impairment, which may be explained by the mild load of DWMHs in migraine, in line with a previous study (3). Furthermore, a recent study on neuroimaging suggests that instead of DWMHs, PVWMHs are associated with cognitive impairment (28).
Migraine features
With regard to migraine features, we found a positive association between DWMHs and aura, which is consistent with a previous finding (37). Cortical hyper-excitability is thought to lower the threshold for cortical spreading depression, which could be responsible for aura in migraine. This cortical spreading depression might be caused by ischemic events, which possibly links the association of aura with DWMHs (38).
Disease duration and attack frequency, as reported in the literature (9, 24), were found to have no association with DWMHs in our study. Contrary to what we expected, we found a negative association with family history. However, it must be mentioned that patients with a self-reported positive family history in our study were associated with younger age and longer disease duration. Therefore, it would be too early to draw a conclusion based on our current data.
Strengths and limitations
This is a cross-sectional multicenter study with 14 participating hospitals, which to some extent makes our data representative of Chinese migraine patients from headache clinics. To the best of our knowledge, few reports on this issue have focused on patients with large shunts. Therefore, our study adds new evidence clarifying that RLS was found to have no association with DWMHs in migraine even in patients with large shunts (> 25 MBs or curtain on spectrum).
Our study presents a series of limitations. First, this study is lacking in healthy controls due to the difficulty in obtaining MRI images from healthy populations. Since we focused on the association between RLS and DWMH in migraine patients, our conclusion still has some reference value in clinical practice. Second, because of our limited sample size, only 13.5% of the patients were diagnosed with MwA. In addition, as 45% of our participants were diagnosed with migraine for the first time, participants in our study have suffered from severe migraine attacks less often. Therefore, conclusions from our study may not be applicable to severe migraine patients. For future research, work on DWMH progression in different subtypes of migraine and in patients and controls who have undergone patent foramen ovale closure surgery should be conducted.