A High Burden of Cerebral white Matter Lesion: Evidences from 9 Asian Cities

Background Age-related white matter lesion (WML) is considered a manifestation of sporadic cerebral small vessel disease and an important pathological substrate for dementia. Asia is notable for its large population with a looming dementia epidemic. Yet, the burden of WML and its associated risk factors across different Asian societies are unknown. Methods Subjects from 9 Asian cities (Bangkok, Bandung, Beijing, Bengaluru, Hong Kong, Kaohsiung, Manila, Seoul, and Singapore) were recruited (n = 5,701) and classied into i) stroke/transient ischemic attack (TIA), ii) Alzheimer’s disease (AD)/mild cognitive impairment (MCI), or iii) control groups. Data on vascular risk factors and cognitive performance were collected. The severity of WML was visually rated on MRI or CT.


Introduction
Asia had the largest number of people suffering from dementia (22.9 million), which was more than twice the numbers in Europe (10.5 million) or in the Americas (9.4 million), as recorded in the global impact of dementia in the World Alzheimer Report 2015. 1 This number was estimated to triple to 67 million in 2050-2 to 3 times higher than the estimates for Europe (19 million) or the Americas (30 million). 1 Devising and implementing preventive strategies against dementia are of paramount importance, particularly in Asia.
Subclinical sporadic cerebral small vessel disease (CSVD) has been increasingly recognized in recent years to be a prevalent and important pathological substrate for cognitive impairment and dementia. 2 Subclinical sporadic CSVD commonly manifests on neuroimaging as white matter lesion (WML), lacunes, microbleeds, enlarged perivascular space or microinfarcts. 3,4 In patients with stroke, variable severity levels of cognitive impairment are present in more than 80% of patients 5 and the presence of subclinical CSVD lesions is associated with worse cognitive performance. [6][7][8][9][10][11][12] Its presence signi cantly increases the risk of poststroke dementia. 9,13 Its presence signi cantly increases the risk of poststroke dementia. Among those who survive stroke without early-onset poststroke dementia, the presence of CSVD can also increase the risk of delayed-onset poststroke dementia. 9 In patients with Alzheimer's disease (AD), CSVD is associated with worse cognitive performance and more rapid cognitive decline. Among those with mild cognitive impairment (MCI), it also increases the risk of conversion to AD. 14,15 In addition, some studies have even suggested a causative role of CSVD in AD pathologies. 16,17 Overall, population studies showed that the presence of CSVD increases the risk of incident vascular dementia or AD. 18,19 Apart from dementia, subclinical CSVD also increases the risk of incident stroke. 19 Other clinical manifestations of CSVD may include depression and other behavioral problems (e.g. apathy), gait and postural instability, and urinary incontinence, which may occur in isolation or in conjunction with dementia/post-stroke syndrome. Overall, CSVD increases the risk of functional decline and mortality. 19,20 While there are many risk factors that may be associated with sporadic CSVD, age and hypertension are the most consistent factors. 21 In addition, cerebral WML was found to be highly heritable and recent studies have identi ed increasing number of genetic foci associated with CSVD. 22 Given the multifactorial nature of sporadic CSVD, the burden of CSVD may vary across geographical cities and ethnicities. The rst population-based studies comparing the prevalence of sporadic subclinical CSVD between Asians (Chinese) and Caucasians (Australian) suggested that Asians may have a higher prevalence of CSVD than Caucasians, even after adjusting for vascular risk factors. 23 Although few studies had reported the prevalence of subclinical CSVD in stroke patients in different Asia cities, comparison between studies of different cities was di cult because the studies used different scales for the quanti cation of CSVD. [24][25][26] Moreover, studies reporting CSVD prevalence in AD were very few in Asia 11 . Furthermore, there was limited data on CSVD in some cities such as Indonesia, the Philippines, and Thailand. Finally, no studies had compared the prevalence of subclinical CSVD in stroke and AD subjects in different age groups using a standardized CSVD rating method.
The AWARE (Asian White mAtteR lEsion) study group initiated a joint international effort across multiple Asian centres to estimate the burden of CSVD (using WML as a surrogate marker for CSVD) in Asia. The study group reported the largest community (stroke-and dementia-free subjects) study (n = 1,797) on the prevalence of subclinical CSVD previously in Asia, which again showed a high prevalence of CSVD in Asia. 27 Apart from community subjects, the AWARE study group also collected data from subjects with stroke/TIA (n = 1,834) and AD/MCI (n = 2,070). The objective of the present study was to estimate the burden of cerebral WML in stroke/TIA and AD/MCI across 9 cities in Asia. We also included community control groups from the AWARE to compare against the results of stroke/TIA and AD/MCI groups. We also investigated the risk factors and cognitive impact of WML among all subjects and among different clinical groups. The hypotheses were: i) the overall prevalence of WML in Asia is high but there could be variations in the different Asian cities; ii) vascular risk factors are contributors to the WML and WML is associated with worse cognitive function.

Subjects and recruitment procedure
The AWARE study included a total of 5,701 subjects. Subjects consisted of patients with stroke/TIA (n = 1,834), AD/MCI (n = 2,070) and controls (n = 1,797) across 9 Eastern and South-Eastern Asian cities (Bangkok, Bandung, Beijing, Bengaluru, Hong Kong, Kaohsiung, Manila, Seoul, Singapore) (see Supplemental Fig. S1). Patients with AD dementia or MCI were assessed and recruited in cognitive disorder clinics. Patients were diagnosed with AD dementia according to the NINCDS-ADRDA criteria. 28 Patients were classi ed as having MCI if the cognitive syndrome mimicked those of AD (e.g. episodic memory impairment), cognitive impairment ful lled that of local standards in de ning cognitive impairment, and caused no or minimal functional impairment in daily living. Patients with mixed dementia cases were excluded, and hence, none of the patients with AD dementia or MCI had history of overt stroke. We included both hemorrhagic and ischemic stroke in the stroke group. We de ned stroke according to clinical evidence of cerebral injury based on symptoms persisting 24 hours or longer, and other etiologies excluded. We de ned TIA based on transient neurological de cits (less than 24 hours) and the absence of infarcts/hemorrhage on neuroimaging. 29 Based on available data from each cohort, we de ned dementia in stroke/TIA patients if they had a clinical dementia rating scale of 1 or above (n = 115), a mini-mental state examinations (MMSE) of 16 or below (n = 1,137) 30 or they were diagnosed by respective dementia experts to have vascular dementia according to the NINDS-AIREN criteria (n = 117). 31 Healthy controls were subjects without dementia, stroke/TIA, or severe medical illnesses and were functionally independent. 27 Ethics approval of each cohort were obtained from the review board of the a liated university.

Clinical and cognitive measures
Clinical and cognitive measures such as age, sex, education years, and MMSE were collected. Patients were de ned as having hypertension if they had a history of hypertension or were using antihypertensive medication. Diabetes mellitus (DM) was de ned as a fasting serum glucose level of 7.0 mmol/L or higher, a postprandial serum glucose level of 11.1 mmol/L or higher, or the use of oral hypoglycemic agents/insulin. Hyperlipidemia was de ned as a total cholesterol level of 5.2 mmol/L or higher, a lowdensity lipoprotein cholesterol level of 2.6 mmol/L or higher, a triglyceride level of 1.70 mmol/L or higher, or the use of lipid-lowering drugs. All patients provided informed consent and the work described was carried out in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki).

Imaging
Subjects with either magnetic resonance imaging (MRI) or computerized tomography (CT) were eligible for this study. For details of the scanner information, please see Supplemental Table S1. The severity of WMLs was rated on axial MRI FLAIR or CT scan. To assess inter-rater agreement between different raters, Hong Kong acted as the central rater and each center provided 30 raw images to Hong Kong for evaluation of inter-rater agreement.

Measurement of WML
The severity of WML for each subject was rated according to the modi ed Fazekas scale 32 or the Age-Related White Matter Changes Scale (ARWMC) 33 based on the MRI axial FLAIR sequence or CT. We used the operationalized global Fazekas rating, which has a score ranging from 0-3. 34 We also used the operationalized global ARWMC score, with score ranging from 0-3 . 33 We de ned the presence of moderate-to-severe WML by a global score of ≥ 2 for both Fazekas and ARWMC scale. The Hong Kong community dataset with both Fazekas and ARWMC global ratings showed that the correlation between the scales are high (p < 0.0001; r = 0.943). Both visual rating scales of WML showed good agreement between CT and MRI. 35 Statistical analysis Data in continuous variables were rst examined by a test of normality. Group comparisons with clinical variables and demographic information were performed using the Kruskal-Wallis test for continuous variables while the chi-square test was used for categorical variables (p < 0.05). Post-hoc analyses between cities were conducted. Multiple logistic regression models were used to study the association between vascular risk factors and prevalence of moderate-to-severe WML with adjustment for age, sex and level of education, Asian regions, disease groups as appropriate. Multiple linear regression models were used to study the association between prevalence of moderate-to-severe WML and MMSE with adjustment for age, sex and level of education, Asian regions and hypertension. In addition, we strati ed the subjects into age groups among disease groups (≤ 60, 61-70, 71-80 and ≥ 81 years old) to compare the associations between vascular risk factors and prevalence of moderate-to-severe WML.
Pairwise deletion was applied on missing data. Bonferroni correction was used for multiple comparison correction. All analyses were performed using IBM SPSS (IBM SPSS Statistics for Mac OS, Version 24.0). Table 1 shows the clinical demographics of the 3 groups: i) stroke/TIA group; ii) AD/MCI group, and iii) control group. The prevalence of moderate-to-severe WML (de ned as ≥ 2 on visual rating) in stroke/TIA, AD/MCI and control groups were 43.3%, 38.2% and 36.7% respectively (Table 1). Agreement in WML rating standard was rst established before data analysis. The Fazekas scale was used as the tool for inter-rate measurement as most countries adopted this scale. The intra-class correlation range from fair to excellent in different centers (0.59-0.78), please see supplemental results.

Results
Clinical demographics Table 1 illustrated the clinical demographics of the disease groups and controls in the AWARE study.  The prevalence of moderate-to-severe WML in different age groups were examined. Age was divided into four groups and the overall prevalence of moderate-to-severe WML increased with age. The positive trend between age and the prevalence of moderate-to-severe WML was consistent in all groups (Fig. 1). The prevalence of the stroke/TIA group was 32.2% at ≤ 60 years old but increased to 64.2% at ≥ 81 years old. Prevalence of moderate-to-severe WML in the AD/MCI group was 20.0% at ≤ 60 years old and increased at a relatively moderate rate to 49.1% at ≥ 81 years old (Fig. 1). The control group had a relatively lower WML at a younger age (8.3% at ≤ 60 years old) but its prevalence increased proportionally with age and its prevalence reached 65.2%, matching that in the stroke/TIA group at ≥ 81 years old.

Prevalence of WML in speci c disease groups
Clinical demographics of various Asian cohorts in speci c disease groups were presented in Tables 2a,  2b & 2c. Hong Kong, Kaohsiung, Bandung, Bangkok, Singapore, Bengaluru and Manila contributed data to the stroke/TIA group. When assessing the prevalence of moderate-to-severe WML using the ≥2-cutoff, Bandung showed the highest prevalence of moderate-to-severe WML (88.9%) while the Kaohsiung had the lowest prevalence of 30.0% (see Table 2a). Data from Bengaluru (n = 31) and Manila (n = 6) had a relatively small sample size and were excluded from the ANOVA and the subsequent regression analyses. Brief information about the Bengaluru group was speci ed in the supplemental results.   Bonferroni correction is applied on the alpha (α = 0.005).
Seoul, Kaohsiung, Singapore, Beijing, Manila, and Bengaluru contributed data to the AD/MCI group. Among this group, both Kaohsiung and Singapore showed a very high prevalence of WML with gradings 2 or above (58.4% and 57.9% respectively) while Manilla showed the lowest (18.9%) (see Table 2b). Data from Bengaluru had a relatively small sample size (n = 24) and were excluded from the ANOVA analysis, as well as the subsequent regression analyses. Brief information about the Bengaluru group was included in the supplemental results.  Clinical demographics of the AD/MCI group * Bonferroni correction is applied on the alpha (α = 0.005).
The control group consists of data from Hong Kong, Singapore, and Seoul. Singapore showed the highest prevalence of moderate-to-severe WML (47.5%), followed by Hong Kong (28.8%) while Seoul showed the lowest (16.5%) (see Table 2c).   Table 3). Strength of association between hypertension and moderate-to-severe WML decreased with increasing age. Similar pattern was observed in the stroke/TIA, AD/MCI and control groups (see Supplemental Tables S2 -S5).
The association between hyperlipidemia and moderate-to-severe WML was signi cant in all subjects (p = 0.009; OR = 1.20, 95% CI = 1.05-1.37), as well as in those in the 61-70 age range (p = 0.048; OR = 1.28, 95% CI = 1.00-1.64), (Table 3 & Supplemental Table S2). However, the association in the 61-70 age group did not survive Bonferroni correction for multiple comparison. There was no signi cant association between diabetes mellitus and moderate-to-severe WML in all disease groups.

The risk of moderate-to-severe WML in different Asian regions
In order to examine the variations of WML in different regions and accounting of sampling bias in each region, the regression was performed with adjustment for age, sex and level of education, Asian regions and disease groups (see Table 3).   Table 3 shows the vascular risk factors contributing to moderate-to-severe white matter lesions. Age, sex, level of education, Asian cities and disease groups are entered as covariates in each regression model as appropriate.
Multiple linear regression analyses showed that the presence of moderate-to-severe WML was signi cantly associated with lower MMSE scores among all subjects and among each disease group (see Table 4). Overall, the MMSE was 20.6 ± 6.2 in those with moderate-to-severe WML (n = 2,529) compared to 22.5 ± 6.5 in those none-or-mild WML (n = 3,621). Further, subgroup analysis showed that MMSE was approximately 2 points lower in those with moderate-to-severe WML compared to those without. The MMSE scores in the moderate-to-severe WML and none-or-mild WML were 20.4 ± 6.9 and 22.6 ± 7.0 respectively in the stroke/TIA group; 17.5 ± 6.2 and 19.7 ± 5.9 respectively in the AD/MCI group; and 24.5 ± 3.9 and 26.3 ± 3.0 respectively in the control group. Subjects with stroke/TIA and dementia had a higher prevalence of moderate-to-severe WML than those with stroke/TIA but without dementia (51.7% vs 40.7%). Subjects with AD dementia also had a higher prevalence of moderate-to-severe WML than those with MCI (41.2% vs 25.3%).  Table 4 shows the association between moderate-to-severe white matter lesions contributing to cognition. Age, sex, level of education, Asian cities and hypertension are entered as covariates in each regression model.

Discussion
This is the rst multi-center study to formally assess the prevalence of moderate-to-severe WML in stroke/TIA and AD/MCI, in different age groups, and from 9 Asian cohorts. The prevalence of moderateto-severe WML was higher in stroke/TIA (43.3%) than that in AD/MCI (38.2%), X 2 (1, n = 3904) = 10.6, p = 0.001. Moderate-to-severe WML was associated with hypertension, hyperlipidemia and a lower MMSE score.
In this study we primarily reported the prevalence of moderate-to-severe WML, rather than any presence of WML. Previous studies that included any presence of WML (i.e. including those with mild WML as well) reported a much higher prevalence of WML, reaching 81.4% in the community. 36 We recorded only moderate-to-severe WML because previous studies showed that only those with moderate-to-severe WML as de ned by a grade 2 or above in the global rating of Fazekas or ARMCW scale were associated with increased risk of incident cognitive decline. Although mild or focal/punctate WML may still represent early or minor SVD, previous studies showed that it had no or minimal clinical relevance, while longitudinal studies revealed no progression of mild WML over time. Whereas, early con uent to con uent WML will likely progress in size and is clinically relevant. 15,18,20,34 In cohorts where the prevalence of mild WML is high and that of moderate-to-severe WML is low, reporting any presence of WML may overestimate the severity of CSVD.
WML in stroke/TIA Despite the mean age of the stroke/TIA group was the youngest (67.7 ± 12.2 years old) when compared with AD/MCI group (76.3 ± 8.7 years old) and controls (70.3 ± 6.0 years old), stroke/TIA group had a higher prevalence of moderate-to-severe WML to that of AD/MCI group as a whole and at each age group, with a prevalence of 32.2% for age ≤ 60 that increased to 64.2% for subjects older than 80 years (Fig. 1).
The high prevalence of moderate-to-severe WML in stroke/TIA was most probably explained to a large extent by the fact that both stroke/TIA and WML shared a strong association with cardiovascular risk factors, in particular hypertension. 37 The most alarming nding was observed in Bandung, Indonesia where the prevalence of moderate-to-severe WML is highest among all cities. In particular, hypertension (81.5%) in this group was high despite the relatively young mean age (59 years old). Note further that the Indonesian cohort provided only CT imaging and hence we might have even underestimated the prevalence of moderate-to-severe WML, as CT is less sensitive in detection of WML relative to MRI. This is indeed the rst study investigating the prevalence of WML in Indonesia, which showed a very high prevalence of WML in this region, alongside a high prevalence of hypertension.

WML in AD/MCI
The prevalence of moderate-to-severe WML in AD/MCI group was lower than that in stroke/TIA across all age distributions by about 12-15%. Still, the prevalence ranged from 20% for age ≤ 60 to almost 50% for subjects older than 80 year old. The prevalence of HT and other cardiovascular risk factors were lower in AD/MCI than that in the stroke/TIA group, which might at least partially explain the lower prevalence of WML in AD/MCI than in the stroke/TIA group.

WML in the control group
Our previous report on the prevalence of CSVD in Asian communities did not report speci cally the prevalence of moderate-to-severe WML according to different age groups. 27 In the current study, the prevalence of moderate-to-severe WML rose from 28.1% at 61-70 years old to 65.2% at ≥ 81 years old. Of note, this prevalence of WML is even slightly higher than the prevalence rates of amyloid positivity reported previously among subjects with normal cognition, which vary from around 20% at 70 years old to slightly above 40% at 90 years old. 39 Among the three cities with community subjects, Singapore recorded the highest prevalence of moderate-to-severe WML despite a mean age of only 69.96 years old, which could again be related to the fact that Singapore group had the highest prevalence of hypertension (80.4%) and hyperlipidemia (75.4%) relative to other cities.

Association between WML, vascular risk factors and global cognition
We found that the strength of association between hypertension and moderate-to-severe WML decreased with increasing age, which suggested that the effects of blood pressure-lowering therapy may be more effective in the younger age group. While among older patients, factors other than high blood pressure (e.g. impaired autoregulation) may have a greater contribution to WML. 40  Previous studies showed con icting results with respect to the association between hyperlipidemia and WML. 41,42 The present study with a large sample size of approximately 6000 subjects did show a small yet signi cant association between hyperlipidemia and moderate-to-severe WML. The strength of association was less than that for hypertension. Similar to most previous studies, we could not demonstrate an association between DM and WML. Note, however, that other studies showed that DM was related to measures of lacunes, rather than to WML. [43][44][45] Above all, we have shown that moderate-to-severe WML was associated with worse cognitive performance in stroke/TIA, AD/MCI and controls. MMSE of those with moderate-to-severe WML was approximately 2 points lower compared to those without moderate-to-severe WML among all subjects and among each of the clinical groups.

Strengths and limitations
The strength of this study includes the large sample size, the inclusion of the AD/MCI and stroke/TIA groups that are known with a high prevalence of moderate-to-severe WML, as well as a large control group for comparison. Further, this is the rst joint international effort with standardized measurement of the WML burden across multiple clinical cohorts in different Asian cities. In addition, analyses in this study were also adjusted with confounders such as age, sex, education, vascular risk factors, disease groups, and different Asian cities.
However, there are certain limitations in this study. First, we acknowledge that certain biases were introduced in the study. Similar to any multi-center study, the pooled sample may contain a more heterogeneous dataset across centers, as well as a potential source of sampling bias from non-random recruitment. However, we have set the study-speci c inclusion and exclusion criteria, and used the same neuropsychological screening test (MMSE). Survival bias may be introduced as the patients enrolled into study could have been biased to the younger group who had better cognitive function, and milder chronic brain changes. This could lead to an underestimation of the true magnitude of the prevalence of WML in each city, as well as the association between WML, vascular risk factors and cognition. Hypertension was based on patient history and any use of antihypertensive medication. This can induce systematic bias into the study. However, clinicians and researchers have ensured accurate reporting and entry during the data collection period to reduce such bias.
Second, we have only used a brief screening test to measure the overall cognitive function. MMSE is not sensitive in detecting executive dysfunction or slow processing speed that are predominantly affected by CSVD. Further, the MMSE was administered in various languages and cultures speci c to the local population and there may be characteristics differences in each version. 46 Third, we acknowledge that Fazekas and ARWMC are two different visual rating scales for WML. Although the scales are different, a cut-off at ≥ 2 in both scales was selected as they both indicated a certain severity of WML that are progressive, and thus, malignant, 47 and lead to devastating outcomes such as global functional decline or delayed-onset dementia. 20,48,49 The Hong Kong community dataset with both Fazekas and ARWMC global ratings showed that the correlation between the scales is high (p < 0.0001; r = 0.943).
Forth, WML can be associated with in ammation, cerebral amyloid angiopathy, or neurodegenerative disease secondary to Wallerian degeneration, which we were unable to differentiate in this study. Fifth, although we had included data from Bengaluru India to investigate the prevalence and vascular risk factors of WML, the small sample size prevented us from making a comparison on its prevalence of WML with other cities/ethnicities. Finally, other CSVD imaging markers such as lacunes, cerebral microbleeds, enlarged perivascular space and microinfarcts were not assessed and may have impact on the overall clinical manifestation.

Implications
Our ndings that there was a high burden of CSVD/WML in stroke/TIA, AD/MCI, community controls and speci c cities (e.g. Indonesian, Singaporean), and that it was associated with hypertension, hyperlipidemia and poor global cognition. These ndings have huge implications in the management, research and public health strategies for preventing dementia in Asia. Strategies that can improve management of high blood pressure and lipids at both population and individual levels cannot only prevent stroke, but also potentially prevent or delay the development and progression of subclinical CSVD/WML. Recent clinical trials of large sample sizes and long durations did show positive effects of aggressive blood pressure lowering and use of statins upon reducing the progression of WML, along with possible bene ts upon cognition. 50-53 54 Note that any strategies targeting CSVD should also incorporate other measures relevant to dementia prevention (e.g. reduce air pollution, limit alcohol, avoid smoking, provide primary and secondary education, reduce obesity and diabetes, increase physical exercise, improve sleep) so as to achieve the maximal effect in reducing dementia burden. Note further that control in hypertension, in particular, should start in mid-life as the evolution of WML from no to severe WML may take more than a decade. Such comprehensive strategies if can be implemented aggressively and e ciently in cities with high burden of vascular risk factors and CSVD (e.g. Indonesia), the effect size in preventing dementia cases in these cities may be more pronounced. In addition to primary preventive strategies, we propose that more clinical trials should target individuals who already harbor moderate-tosevere CSVD/WML at different clinical contexts, e.g. poststroke/TIA with cognitive impairment (without dementia), MCI (prodromal AD) or even in "preclinical CSVD" dementia-and stroke-free individuals, as they are at high risk of further cognitive decline or dementia. Preferably, such trials should include Asians so that ndings from these trials can be generalized to Asians as well. Declarations