Abstract
Exposures to fine particulate matter PM2.5 and ozone O3 are associated with Alzheimer’s disease (AD) risk. Mexico City residents have lifetime exposures to PM2.5 and O3 above annual USEPA standards and their brains contain high redox, combustion, and friction-derived magnetite nanoparticles. AD pathological changes with subcortical pre-tangle stages in infancy and cortical tau pre-tangles, NFT Stages I-II, and amyloid phases 1-2 are identified by the 2nd decade. Given their AD continuum, a reliable identification of cognitive impairment is of utmost importance. The Montreal Cognitive Assessment (MoCA) was administered to 517 urbanites, age 21.60±5.88 years, with 13.69±1.28 formal education years, in Mexican PM2.5 polluted cities. MoCA score was 23.92±2.82, and 24.7% and 30.3% scored ≤24 and ≤22, respectively (MCI≤24, AD≤22). Cognitive deficits progressively targeted Visuospatial, Executive, Language, and Memory domains, body mass index (BMI) impacting total scores negatively (p = 0.0008), aging driving down Executive, Visuospatial, and Language index scores (p < 0.0001, 0.0037, and 0.0045), and males performing better in Executive tasks. Average age for AD MoCA scores was 22.38±7.7 years. Residency in polluted cities is associated with progression of multi-domain cognitive impairment affecting 55% of Mexican seemingly healthy youth. Normal BMI ought to be a neuroprotection goal. MoCA provides guidance for further mandatory neuropsychological testing in young populations. Identifying and lowering key neurotoxicants impacting neural risk trajectories in the developing brain and monitoring cognitive performance would greatly facilitate multidisciplinary early diagnosis and prevention of AD in high risk young populations. Cognitive deficits hinder development of those representing the force moving the country in future years.
Keywords
INTRODUCTION
There is a significant concern regarding cognitive abilities following exposures to fine particulate matter (PM2.5) during fetal and postnatal life [1 –10]. Adolescents’ neurobehavioral performance, particularly affecting sustained attention, short-term memory, and manual motor speed are inversely associated to traffic exposures [11]. The association between Alzheimer’s disease (AD), dementia, traffic, and air pollution is available [12, 13]. There is 138% increased risk of AD per increase of 4.34μg/m3 above the USEPA PM2.5 standard (12.0μg/m3 annual mean averaged over 3 years). Likewise, an estimated 211% risk of increase of AD was observed per increase of 10.91 ppb in O3 over the 10-year follow-up period of 95,690 Taiwanese individuals age ≥65 [12]. Moreover, living closer to high traffic roads is associated with DNA methylation and lower mid-childhood nonverbal cognitive scores [14].
Metropolitan Mexico City (MMC) residents are exposed from conception to death to concentrations of PM2.5 and ozone > USEPA standards and are showing AD neuropathological hallmarks in infancy [9]. Indeed, subcortical pre-tangle stage b [15] was identified in 11-month-old infants, while cortical tau pre-tangles, neurofibrillary tangles (NFT) Stages I-II, and amyloid phases 1-2, were present by the 2nd decade. AD hallmarks were seen in 99% of the 203 consecutive autopsies in seemingly healthy ≤40-year-old subjects and AD pathology was progressive, so by the third and fourth decades, NFT stages III–V were present in ∼25% of subjects [9].
Clinically healthy MMC children and teens showed significant deficits in a combination of fluid and crystallized cognition tasks along with MRI prefrontal white matter hyperintense lesions, brain metabolic abnormalities, and interactive and additive influences of gender, body mass index (BMI), and Apolipoprotein E on cognition [1 , 5–7].
We are very interested in finding a suitable brief screening tool for cognitive impairment knowing: 1) AD hallmarks are evolving in our young seemingly healthy MMC residents [9], 2) PM2.5 and ozone above the USEPA standard affects millions of citizens across Mexico, the USA, and around the world, and 3) two thirds of AD patients are residents in low or middle income countries.
The Montreal Cognitive Assessment (MoCA) is a brief and easy to administer screening test that covers several cognitive domains including episodic memory, language, attention, orientation, visuospatial, and executive functions [16]. In the original paper, Nasreddine and co-authors showed an important pattern: all normal controls age 72.84±7.03 years, scored 26 and above, making them extremely unlikely candidates to meet clinical and neuropsychological criteria for mild cognitive impairment (MCI) [16]. In contrast, 73% of the MCI subjects scored 25 or below [16]. Thus, MoCA provides a quick guidance for referral and further cognitive investigation and has been validated in Mexican populations with a reliability of 0.89, sensitivity of 80%, and 75% specificity, with a cut-off point of 26 points for MCI (area under the curve, 0.886; p < 0.001) [17]. Pugh et al. suggested optimal cutoff scores for distinguishing MCI (≤24) from AD (<22) in 499 adults ages 48–99 enrolled in the Alzheimer’s Disease Neuroimaging Initiative [18]. We selected to apply Pugh et al. [18] MCI and AD cutoff scores and considered scores 25 to 30 as normal cognition.
We have one primary aim for this study: To document in young seemingly healthy Mexican urbanites their performance in MoCA across cities varying in size from >25 million people to <250,000 with complex patterns and sources of air pollutants, including different levels of fine particulate matter (PM2.5) air pollution above current annual USEPA standards. Since the number of formal education years and age are major factors in MoCA performance in Latin-American cohorts [19], selected participants for this study were 21.6±5.8 years old and had 13.67±1.3 years of formal education. The selected cohorts belong to the middle-class socioeconomic level and have similar dietary, exercise, and cultural backgrounds. Since high BMI has been associated with significant cognitive deficits in our teen female populations [7], we included BMI as a key continuous variable.
Overall, only ∼45% of the screened population across the ten cities had normal MoCA scores (25–30). The early identification of cognitive impairment in seemingly healthy young air pollution-exposed urbanites and understanding the relationship between tau and amyloid pathology, cognitive impairment, and AD [9] defined by its underlying pathological processes that can be documented by postmortem examination or in vivo by biomarkers [20] are at the core of our research efforts. We are deeply concerned young MMC residents with a historical documentation of AD pathology progressive changes [9] are showing MoCA scores in the published range [16 –18] of MCI and AD and that similar scores are documented for residents in other polluted cities across the country.
Identifying key neurotoxicants impacting neural risk trajectories in the developing brain, including monitoring cognitive longitudinal performance through brief, easily administered, sensitive, continuous cognitive instruments [16], would greatly facilitate multidisciplinary early prevention and detection AD efforts in high risk young world populations. Progressive cognitive deficits at an early age hinder society development with social, economic, and health serious short-and long-term consequences.
METHODS
Air quality data
Metropolitan Mexico City, Saltillo, Queretaro, Reynosa, Villahermosa, Hermosillo, Zapopan, and San Luis Potosi were selected on the bases of their geographic location, their level of air pollutants and their urban characteristics. For this study, we focused on PM2.5 (≤2.5μm particles) and worked with both the respective 24 h and annual averages. Supplementary Table 1 shows a summary of the main characteristics of the study cities, monitoring sites, and sources of particulate matter (PM). Figure 1 shows a box PM2.5 plot for the study cities. PM2.5 averages were integrated into box plots to facilitate the visualization of the comparison of the medians and annual means. PM2.5 data for MMC, Hermosillo and Saltillo were obtained from direct measurements in these cities, while for Queretaro data were obtained from a monitoring site in Juriquilla 15 km to the north of downtown Queretaro. PM2.5 24 h averages for Villahermosa, Zapopan, and San Luis Potosí were obtained from on-site measured PM10 concentrations assuming the typical percent of the fine fraction in the PM10 for similar cities. On the other hand, particle data for Reynosa was assumed to be proportional to PM2.5 concentration values registered in the Mission monitoring site of the neighbor border city of McAllen, Texas ∼10 km north downwind.
Box PM2.5 plots for 2017:24 hr averages for Reynosa, Hermosillo, Querétaro (Juriquilla), Villahermosa, Zapopan, Saltillo, San Luis Potosí and Northeast Metropolitan Mexico City, and for Zapopan 2015. National Ambient Air Quality Standards NAAQS US EPA, 24 hours 35μg/m3 and annual 12.0μg/m3, averaged over 3 years.
Study population and demographics
The research was done in accord with the ethical standards of the Revised Helsinki Declaration of 2000, the study was approved by ethical and research committees and a written informed consent obtained from all participants and/or their parents. The 517 participants had an average age of 21.60±5.88 years, with 13.69±1.28 years of formal education (Table 1). Recruitment of participants was done through word of mouth. Subjects completed a baseline medical examination and were considered clinically healthy and representative of a Mexican middle socioeconomic class.
Demographics, MoCA scores, and BMI in the young cohort age 21.6±5.8 years from urban Mexican cities
MoCA administration and scoring
The Spanish version of MoCA was used in this study. MoCA assesses global cognitive function and contains 10 subtests [16]. Visuospatial abilities are assessed using a clock-drawing task and a three-dimensional cube copy (4 points), short-term memory is tested with learning trials of five nouns followed by a delayed recall task (5 points). Executive functions are assessed using a task adapted from the Trail Making B test, a phonemic fluency task (1 point), and a two-item verbal abstraction task (2 points). Attention, concentration, and working memory are evaluated using an attention task, a serial subtraction task and digits forward and backward (6 points). Language is tested with a naming task with animals (3 points), repetition of two syntactically complex sentences (2 points), and the fluency task (1 point). Orientation is evaluated by time and place (6 points).
MoCA scores were converted into 6 index scores based on the combinations used by Petersen [21] and Julayanont et al. [22]. The Executive Index Score (EIS) was the sum of Trail making, clock drawing, digit span forward and backward, letter A tapping, serial 7’s subtraction, word fluency and abstraction (total score 13); Language Index Scores (LIS): animal naming, sentence repetition and word fluency (total score 6); Visuospatial Index Score (VIS): cube copy, clock drawing and animal naming (total score 7); Attention Index Score (AIS): digit span forward and backward, letter A tapping, serial 7’s subtraction, sentence repetition, the 10 words recalled at both immediate recall trials (total points 18). Petersen original Delayed Recall Score plus VIS, EIS, and LIS was also used [21]. Physicians, psychologists, and health senior students trained in cognitive testing, administered the MoCA to the target population.
Statistical analysis
We investigated the scores of the MoCA test under each of the original categories: Visuospatial/Executive, Identification, Memory, Attention, Language, Abstraction, and Orientation and under the 6 Index Scores [21, 22]. We also focused on each of the following sections of the data: All (cities), Hermosillo (control city with the lowest PM2.5 concentrations > USEPA standard), MMC (megacity), OTHERS (all cities, except the control), and Non-MMC (all cities, except Metropolitan Mexico City). We considered a MoCA score of ≥25 as normal, 23-24 as MCI and ≤22 as dementia according to Pugh and co-workers [18]. In our linear regression models, we first consider the total MoCA score as response, and BMI, age, gender, and years in education as predictors. We run the regression models in each of the aforementioned sections of our data. We performed the statistical analyses using Excel and the statistical software ‘R’ (http://www.r-project.org/).
RESULTS
Air pollution data
All 517 participants were residents in urban areas with concentrations of PM2.5 above the annual USEPA standards (Fig. 1). Hermosillo (HER) was the less polluted city versus MMC NE where the PM2.5 annual mean was ∼2.5 times higher than the annual standard (12.0μg/m3). Most of the sites also exceeded the PM2.5 24 h average USEPA standard in different extent.
MoCA results
The mean MoCA score was 23.92±2.82 for the 517 subjects’ age 21.6±5.88 years with 13.6±1.2 years of formal education. Fifty-five percent of subjects scored 23-24 MCI scores (24.7%) and 30.3% scored ≤22 AD scores. Table 1 shows the demographic characteristics, MoCA scores, and BMIs for each group of participants in each city. MoCA total scores were not statistically different regardless of residency (Table 2). However, significant differences were present between the Control less polluted city (HER) and MMC and OTHERS in cube drawing, language fluency task, abstraction, and orientation (Table 2). Examples of cube, clock, and trail drawings in participants across the country are seen in Fig. 2. In (Supplementary Table 2A), MoCA total scores decrease as BMI increases (p = 0.0041). Age is a major negative variable accounting for lower MoCA scores across all participants (p = 0.0289) and it is most significant for MMC residents (p = 0.0093). The number of formal education years impacts positively the MoCA total scores across all participants (p = 0.0247). The Visuospatial/Executive data analyzed as the original [16] three tasks: Trail Making B test, cube and clock drawing showed a strong negative response as BMI went up (p = 0.0001), while age had a negative impact in the overall population (p = 0.0005) and in MMC (p < 0.0001). In (Supplementary Table 2B), males had higher scores for Visuospatial/Executive tasks (p = 0.0317), and MMC residents scored better than OTHER city counterparts in Visuospatial/Executive and Attention tasks (p = 0.0002 and 0.0008, respectively). However, MMC residents did poorly in Delay Memory when compared with OTHERS (p < 0.0001). MMC residents did better that HER in Attention tasks when the number of education years went up (p = 0.0024). In (Supplementary Table 2C), keeping a normal BMI (18.5 to <25) resulted in higher Visuospatial/Executive scores (p = 0.0110) in males and overall better scores for males in MMC versus OTHERS (p = 0.0008). Being an overweight female in MMC negatively impacted Memory (p = 0.04).
Summary of MoCA original cognition scores [16] in Control city Hermosillo (HER), Metropolitan Mexico City (MMC), and OTHERS (all cities except HER)
When we analyzed the original subtests [16] in the cohorts with normal cognition NC≥25, MCI 23-24, and AD≤22 (Table 3A) we found significant deficits in targeted subtests from NC versus MCI including the Cube and Clock drawing tests, Delay Memory, Serial 7’s backward, and the repetition of two complex sentences. The comparison between MCI to AD scores showed significant deficits in word fluency task and digit span forward and backward, while when we compared NC versus AD scores everything was compromised except letter A tapping (Table 3A).
MoCA raw scores data, age, and residency. The original MoCA categories [16] are the targets for comparison in the young cohort age 21.60±5.88 years, residing in PM2.5 polluted Mexican cities. Pugh et al. [18] optimal cutoff score for distinguishing normal cognition NC (25–30), MCI (23-24), and AD (≤22) were applied here
The cognitive domain index scores data [21, 22] (Table 3B) showed two main findings: the progression of significant deficits in specific cognitive domains from NC to MCI to AD scores and the presence of multi-domain deficits across cities regardless of PM2.5 concentrations above the USEPA standard. Specifically, subjects with scores in the MCI range showed significant deficits in the Executive (EIS), Language (LIS), Visuospatial (VIS), and the Composite Delay Memory and EIS + VIS + LIS and for scores in the AD range (≤22) all Composite scores were involved except Orientation (Table 3B). To contrast our results versus the NC, MCI, and AD cases from Julayanont et al. [22] paper, Table 3C showed their data in subjects aged 73.82±0.93 years versus our 21.60±5.88-year-old cohort.
Executive Index Score (EIS) is the sum of Trail making, clock drawing, digit span forward and backward, letter A tapping, serial 7’s subtraction, word fluency and abstraction. Language Index Scores (LIS): animal naming, sentence repetition and word fluency. Visuospatial Index Score (VIS): cube copy, clock drawing and animal naming. Attention Index Score (AIS): digit span forward and backward, letter A tapping, serial 7’s subtraction, sentence repetition, the 10 words recalled at both immediate recall trials.
Contrasting data from subjects age 73.825±0.93, Normal Cognition and MCI and AD groups from Julayanont et al. [22]
DISCUSSION
Examples of Visuospatial and Executive tasks (cube and clock drawing and Trail making) in 8 subjects, in parenthesis their MoCA scores. Notice both cube and clock drawings are significantly abnormal in subjects with normal MoCA scores (28, 26, 25 corresponding to 18F, 18M, and 21M). Major abnormalities are seen in subjects with MCI and AD scores (lower two cases).
Fifty-five percent of seemingly healthy 21.6-year-old residents in urban polluted cities are showing cognitive impairment in the MoCA score range for MCI and dementia. Age, BMI, and residency in polluted cities are the critical variables driving MoCA scores down. There is a striking progression of specific domain deficits, from normal cognition to MCI including a combination of Visuospatial, Executive, Language, and Memory domains. The progression to AD cores at age 22.38±7.7 years is extremely worrisome and is characterized by worsening of the initially involved domains like Delay Memory and sentence repetition and new abnormal scores in specific subtests, i.e., word fluency task and digit span forward and backward.
We are essentially seeing the progression of multi-domain cognitive impairment associated with syndromal staging of cognitive continuum [20] in the second and third decades of life. MMC young residents are exposed to lifelong-including intrauterine life-high concentrations of PM2.5 and ozone and historically are showing AD continuum [9, 20]. It is important to point out that residents in the non-MMC polluted cities— for whom we had no neuropathological studies— have no significant differences in MoCA results versus Mexico City.
For MMC residents, the initial cognitive domains involved and their progression in a relatively short period of time are extremely relevant to their neuropathology findings: cortical tau pre-tangles, NFT Stages I–II, and amyloid phases 1-2 in the second decade, and progression to NFT III–V in 24.8% of subjects in the third and fourth decades [9]. Moreover, APOE4 carriers have 23.6 times higher odds of NFT V (p < 0.0001) versus APOE4 non-carriers having similar cumulative PM2.5 exposure and age [9]. Key for this MoCA study, age and cumulative PM2.5 (CPM2.5) doses over a lifetime, including pregnancy time, were significant for developing NFT V in the autopsy MMC study [9]. Thus, deficits in Executive, Memory, Visuospatial, and Language functions [22 –24] could be plausibly connected to the extensive neurovascular, white and gray matter pathology, and brain metabolic, volumetric, and structural changes described in clinically healthy MMC children and teens [1–5 , 25]. Copying a cube and drawing a clock represent complex tasks [26]: the initial conversion from a two-dimensional contour to a three-dimensional cube, followed by visuomotor coordination involves visual perception in the parieto-occipital lobe, planning in the frontal lobe, and integration of visual and fine motor sequences in the fronto-parieto-occipital cortices. The Clock Drawing Test requires planning, conceptualization, and symbolic representation to draw a clock’s face and for placing all the numbers correctly [26]. Intact visuoconstructive skills and inhibitory responses are required when placing each hand to tell the time of “ten past eleven” and for a self-initiated-clock-drawing mainly represented in the parietal lobe [27].
This work showed two key pieces of information: for a young healthy population, keeping your BMI between 18.5 and <25 even under severe PM2.5 exposures, will significantly increase Visuospatial scores, and gender is key for the differential in cognitive responses to higher BMI values: being an overweight female had a negative impact on Delay Memory.
An area of utmost importance for highly exposed young residents, especially if we want to intervene early in the development of cognitive deficits, is the issue of optimal working memory and episodic memory for food-related decision-making, and how any alterations contribute to problems with appetite control and weight gain as described by Higgs and Spetter [28]. The complexity of cognitive deficits in urban pollution exposed cohorts ought to be discussed also in relationship with reversible causes of cognitive impairment in young adults, including hypothyroidism, sleep disorders, drugs, vitamin deficiencies, etc. [29]. Relevant to the issue of levels of PM2.5 exposures above USEPA standards, is the fact that regardless of residency and across different levels, patterns, and sources of PM2.5, Executive, Visuospatial Index, Language, and Memory scores are targeted in the first decades of life— a key time for the developing brain— a major finding that ought to be addressed by health and environmental authorities all over the world.
Advantages and shortcomings of this study relative to other studies
Our research design uses data from 20-year-olds with the same number of education years, socioeconomic status and ethnicity, enabling us to rule out the possibility that these key variables will modify our results across different urban areas [19]. We selected healthy individuals and we have a detailed description of AD pathology in 203 consecutive autopsy MMC cases, ages 11 months to 40 years with no extra-neural pathology, that allow us to put forward potential direct correlations with the MoCA results in MMC residents and indirectly in residents across the country. The study has shortcomings. First, although it is clear that MoCA scores across cities are 55% below the expected normal range in clinically healthy subjects, we are not including APOE genotyping, laboratory, or brain imaging results to complement the MoCA testing. Future research ought to include APOE status, a comprehensive metabolic panel with insulin levels, and imaging brain studies.
In summary, exposures to PM2.5≥USEPA standards are associated with progression of multi-domain cognitive impairment affecting 55% of Mexican young urbanites. Since AD is seen as a continuum and cognitive staging may be accomplished using continuous measures [20], MoCA— a brief test covering key cognitive domains— provides quick guidance for further mandatory neuropsychological testing in young urbanites. Simple cognitive testing could be useful for an early identification of individuals with a high risk of developing AD pathological changes [30].
It is also clear, that in the scenario of highly polluted Metropolitan Mexico City, AD pathology starts in infancy, progresses relentlessly in the first two decades of life, and negatively impacts the brain in development [1 –9]. The ill-faded interplay of air pollution, gender, BMI, systemic and neural inflammation, insulin resistance, hyperleptinemia, diabetes, APOE4, lifestyle, and socioeconomic factors could signal the future trajectory of young people toward the development of progressively worse cognitive impairment and AD [2 , 32].
Finding than more than half of the young adult population under study shows cognitive deficits is a serious call about the future development of human capital in any country, imposing a grim impact on health, education, competence, resolving high-stakes decisions optimally, acquiring trained and labor skills, and essentially hinders the short-and long-term development of those who are going to be the force moving the country during the years to come. Identifying and lowering key air pollutants impacting neural risk trajectories in the developing brain and monitoring cognitive longitudinal performance through brief, sensitive, continuous cognitive instruments, would greatly facilitate multidisciplinary early diagnosis and prevention of AD in high risk young world populations.
