Abstract
Objective:
A cross-sectional study was conducted to describe the prevalence, locations, and risk factors for brain microbleeds (BMBs) in neurodegenerative dementia.
Methods:
The database of the Alzheimer Center Reina Sofía Foundation was searched, BMBs were described, and the potential associations of BMBs were investigated using univariate statistics.
Results:
A total of 148 patients (age 81.6 [standard deviation 6.7], 79.1% female) were studied. Prevalence of BMBs was 44.6%. A group of patients with unusually high (ie, ≥4) number of BMBs were identified, which displayed higher number of vascular risk factors and vascular diseases. Brain microbleeds were also associated with ischemic lesions in the basal ganglia (r = .39), clinical diagnosis of Alzheimer’s disease (AD) and cerebrovascular disease (r = .33), cortical infarction (r = .20), and antiaggregant or anticoagulant treatment duration (r = .20).
Conclusions:
Brain microbleeds are associated with vascular burden and AD diagnosis in old patients with neurodegenerative dementia. More research is warranted regarding the mechanisms and potential clinical implications of these results.
Introduction
The advent of magnetic resonance imaging (MRI) studies brought attention to the presence of very small bleeds in the brain of patients and healthy participants. In people without dementia, brain microbleeds (BMBs) have been associated with aging, 1 hypertension, diabetes, stroke, 2 worse cognitive performance, 3 –5 ∊4 apolipoprotein E gene (APOE) allele, 1 –6 and death. 7 The most frequently reported locations for BMBs are the basal ganglia (BG) and the cerebral lobes 3 and the proposed pathological substrates are, respectively, arteriolosclerosis and amyloid angiopathy. 2–3
Brain microbleeds are very frequently observed in vascular dementia but are also frequent in patients with dementia due to Alzheimer’s disease (AD). 8 However, the physiopathology and the clinical implications of BMBs are not well known, and this gap of knowledge is particularly notorious in old patients with advanced neurodegenerative dementia. The study of BMBs in neurodegenerative dementia is relevant for, at least, 3 reasons. First, description of BMBs may reveal information about etiology and physiopathological mechanisms of dementia. Second, BMBs may predict potential adverse outcomes of treatments. 9 And third, BMBs may be associated with particular clinical features or more rapid deterioration.
The aim of the present investigation was to describe and analyze the frequency, location, and potential risk factors for BMBs in a cohort of institutionalized patients with neurodegenerative dementia. A high prevalence of BMBs was expected on the basis of association of BMBs with aging, vascular, and Alzheimer’s pathology. In addition, we hypothesized that cortical BMBs would be associated with a clinical diagnosis of AD, whereas subcortical BMBs would be linked to the traditional vascular risk factors (ie, hypertension, diabetes, dyslipidemia, and tobacco use).
Materials and Methods
Setting
Data for the present study were collected from the database of the Alzheimer’s Center Reina Sofia Foundation (ACRSF), a center devoted to translational and holistic research in neurodegenerative dementia. 10 The clinical and MRI research protocol of the ACRSF was described elsewhere. 10,11 Briefly, patients with neurodegenerative dementia from the Autonomous Community of Madrid (ACM) are derived to the ACRSF through the usual channels of the ACM Public System of Health. Upon admission to nursing home or to day care unit, persons with dementia and their family caregivers are invited to sign consent to participate in a research program that includes clinical and social assessment, blood determinations, MRI study, and brain donation. The informed consent form for the ACRSF research protocol was previously approved by the ACRSF ethics committee. Clinical assessments and blood extraction are performed every 6 months, whereas MRI is performed annually. All the patients who received the baseline visit (ie, 2-6 weeks after admission), and had also MRI study with systematic evaluation of BMBs performed within the first year upon the date of the baseline visit, were selected for the present investigation.
Study Variables
Per the ACRSF protocol, medical history was conducted by neurologist through interview with family caregivers and review of all the available medical information (medical records, laboratory tests, and current medications). The following groups of variables were selected from the ACRSF protocol for the present investigation: Demographic variables: age, sex, and education. Dementia-related variables: disease duration, severity of dementia (according to the Global Deterioration Scale),
12
etiological diagnosis, and number of ∊4 APOE alleles. Vascular risk factors and vascular diseases (either past or present): hypertension, diabetes, dyslipidemia, tobacco use, ischemic cardiopathy, atrial fibrillation, and cerebrovascular episodes (including both stroke and transient ischemic attacks). Medications: antiaggregant or anticoagulant agents. These medications were analyzed on the basis of the number of years of treatment and of whether the patient was being treated or not at the time of assessment.
Magnetic Resonance Imaging Study
Magnetic resonance images were acquired in a Signa HDxt (GEHC, Waukesha, USA) 3T scanner at the Neuroimaging Department of the ACRSF. The scanner was equipped with an 8-channel phased array receive coil and a gradient system capable of producing a gradient strength of 50 mT/m. The MRI protocol included the following pulse sequences: (1) oblique reconstructions of a 3-dimensional (3D) fast-spoiled gradient recalled echo-based sequence (matrix 288 × 288, field of view [FOV] 24 × 24 cm, slice thickness 1 mm, repetition time [TR] 7.1 milliseconds, echo time [TE] 3.1 milliseconds, 1 excitation); (2) axial 2-dimensional (2D) gradient-echo echo-planar imaging (matrix: 224 × 192, FOV 24 × 24, slice thickness 2.4 mm, TR 2225 milliseconds, TE 11.5 milliseconds, 2 excitations); (3) axial 2D fluid-attenuated inversion recovery (matrix 256 × 192, FOV 24 × 24 cm, slice thickness 3.4 mm, TR 9000 milliseconds, TE 124 milliseconds, TI 2500 milliseconds), and (4) coronal 2D, T2-weighted fast-spin echo (matrix 384 × 224, FOV 22 × 22 cm, slice thickness 3.4 mm, TR 5340 milliseconds, TE 85 milliseconds).
Brain microbleeds were systematically searched, counted, and located by an experienced neurorradiologist (A.R.) according to a previously elaborated protocol. The BMBs were identified as very hypointense, intra-axial, round lesions of less than 10 mm diameter. The predefined locations of BMBs were as follows: cerebral lobes (with the distinction of frontal, temporal, parietal, and occipital lobes), periventricular and deep white matter (PDWM; for BMBs that were in the intimate vicinity of, or less than 1 cm far from, the ventricular wall), BG (thalamus included), cerebellum, and brain stem (BS; Figure 1). The protocol of evaluation of MRI results also included qualitative ratings of global and hippocampal brain atrophy, using a scale from 0—no atrophy to 4—maximal atrophy 13 as well as ratings of white matter and subcortical small vessel disease from 0—no disease to 3—maximal disease. 14 These assessments were performed by the same neurorradiologist who conducted the evaluation of BMBs, in the same session (A.R.).
Echo-planar imaging (EPI) T2-weighted MRI sequences from a 68-year-old woman with AD and cerebrovascular disease who presented BMBs in virtually all the studied locations. She also had hypertension, diabetes, and dyslipidemia although never treated with antiaggregant or anticoagulant agents. BMBs can be seen (arrows) in the cerebellum (A), the brain stem (B), the temporal lobe, the occipital lobe, and the basal ganglia (C), and the frontal lobe (D). AD indicates Alzheimer’s disease; BMBs, brain microbleeds; MRI, magnetic resonance imaging.
Statistical Analyses
The frequency and location of BMBs, as well as the demographic, clinical, and genetic variables, were presented using descriptive statistics. Study groups were created post hoc according to the number of BMBs, and differences among the study groups were explored by means of unadjusted analysis of variance and chi-square test. The potential correlations of BMBs were further explored using bivariate Spearman’s r coefficients that were interpreted as follows: negligible <.20; weak .20 to .34; moderate .35 to .50; and strong >.50. 15 All the statistical tests were 2-tailed and a level of P < .05 was chosen for statistical significance. Statistical analyses were performed using SPSS version 16.0 software (SPSS, Chicago, Illinois).
Results
At the time of database consultation (January 31, 2013), 245 patients were included in the ACRSF database, but 97 (39.4%) patients did not have MRI study. Patients without MRI study had history of dyslipidemia (54.3% vs 35.2%; P = .004) and of ischemic cardiopathy (14.1% vs 5.6%; P = .026) more frequently. There were no other differences between patients with and without MRI (all other P values > .05, data not shown). Hence, 148 patients are described and analyzed in the present investigation.
The mean age (standard deviation [SD], range) of the sample was 81.6 years (SD 6.7, range 56-98) and 79.1% of the patients were female. The educational attainment of the sample was as follows: 11.5% illiterate, 14.2% incomplete primary education, 70.8% primary education, and 3.4% secondary education. The mean duration of symptoms was 6.9 years (SD 3.2, range 0-18 years) and the severity of dementia was as follows: 6.2% mild dementia, 20.5% moderate dementia, 52.7% moderately severe dementia, and 20.5% severe dementia. The most frequent dementia etiology was (either probable or possible) AD (53.3%), 16 followed by AD plus cerebrovascular disease (17.6%), and non-AD neurodegenerative dementia (8.8% 17,18 ; the etiology of dementia was uncertain in 20.3% of the patients). The mean number of vascular diseases was 1.5 (SD 1.2, range 0-5) and 23.2% of the patients were on antiaggregant or anticoagulant medications.
Brain microbleeds were observed in 66 (44.6%) of 148 patients, with a range from 1 to >100 BMBs per patient. The number of BMBs per patient fitted an exponential curve, with most patients displaying none or very few BMBs, but a small number of patients presenting an unusually high number of BMBs (22 patients [14.9%] displayed 4 to >100 BMBs; Figure 2). The 2 extreme groups (ie, patients without BMBs and patients with high number of BMBs) were deemed to be potentially illustrative in the study of BMB risk factors and, for that reason, 3 groups were created post hoc and described (Table 1). Compared to the groups with few or no BMBs, the group with many BMBs displayed a more frequent history of hypertension, dyslipidemia, diabetes, ischemic cardiopathy, and cerebrovascular disease. The etiological diagnosis of probable AD was less frequent in patients with high number of BMBs, which received more frequently the diagnoses of AD and cerebrovascular disease and of dementia of uncertain etiology.
Frequency of BMBs in the study sample (N = 148). BMBs indicates brain microbleeds.
Demographic and Medical Variables in the 3 Post Hoc Study Groups.a
Abbreviations: AD, Alzheimer’s disease; APOE, apolipoprotein E gene; BMBs, brain microbleeds; GDS, Global Deterioration Scale; SD, standard deviation.
a Values represent mean (SD) unless % is indicated.
b Statistically significant differences between groups (P < .05, Scheffé’s test).
c APOE haplotype was not available for 30 patients (20.3%). 12
The prevalence of BMBs in the studied brain regions is represented in Figure 3. Lobar BMBs were observed in 46 (31.1%) patients, PDWM BMBs were observed in 12 (8.1%) patients, 36 (24.3%) patients presented BMBs in the BG, the cerebellum displayed BMBs in 15 (10.1%) patients, and there were BS BMBs in 10 (6.8%) patients. Those figures exceeded the total prevalence of BMBs of 43.6% because there were 34 (23.0%) patients who presented BMBs in more than 1 location. The most frequent (13.5%) co-occurence of BMBs was cerebral lobes and BG. In contrast, no patient displayed BMBs in the BS or in the cerebellum only (ie, BMBs in the BS and cerebellum appeared always in combination with BMBs in other locations, usually the cerebral lobes and/or the BG). Regarding the lobar BMBs, the most frequent location was the frontal lobe (25 patients, 16.9%), followed by the occipital lobe (22 patients, 14.9%; Figure 3B). Since the frequency of occipital BMBs was higher than expected (given the small size of that lobe), a post hoc analysis was conducted to specifically explore the correlates of the occipital BMBs.
Prevalence of BMBs in the studied brain regions (A) and in the different cerebral lobes (B; N = 148). BMBs indicates brain microbleeds; LOB, cerebral lobes (F, frontal, P, parietal, T, temporal, O, occipital); PDWM, periventricular and deep white matter; BG, basal ganglia; CBL, cerebellum; BS: brain stem.
The total number of BMBs was moderately associated with the presence of ischemic lesions in the BG (r = .39) and with the diagnosis of AD and cerebrovascular disease (r = .33 for the total number of BMBs, r = .35 for BS BMBs; Table 2). Weak association was found between the number of BMBs and the number of (either ischemic or hemorrhagic) cortical infarcts (r = .20). Hemorrhagic infarctions were not observed in the patients without BMBs (there were only 2 patients with a total of 3 cortical ischemic infarctions in that group). In contrast, 10 patients with BMBs displayed a total of 14 cortical infarctions (5 patients with 9 ischemic infarctions and 5 patients with 5 hemorrhagic infarctions). Weak association was also observed between years of use of antiaggregant or anticoagulant medications and BMBs (r = .20 for the total number of BMBs, r = .25 for BG BMBs). In the post hoc analysis of the occipital BMBs, only mild correlation was found between occipital BMBs and occipital infarction (r = .21; all other r values <.18, data not shown).
Correlates of BMBs.a
Abbreviations: AD, Alzheimer’s disease; APOE, apolipoprotein E gene; BG, basal ganglia; BMBs, brain microbleeds; BS, brain stem; CBL, cerebellar; MRI, magnetic resonance imaging; n, number; PDWM, periventricular and deep white matter.
a The values represent the Spearman’s correlation coefficient; BMBs were measured using a scale from 0 to 2 (0, no lesions; 1, unilateral lesions; 2, bilateral lesions), except for lobar BMBs that were measured from 0 to 8 (that score was obtained from the sum of the BMB scores in the four cerebral lobes); BG ischemic lesions were measured using the scale of Wahlund et al. 14 ; atrophy was measured using a scale from 0 to 4. 13
b P < .05.
c P < .0005.
d P < .005.
Discussion
The research protocol of the ACRSF, which included performance of MRI study with systematic evaluation of BMBs, gave us the opportunity to investigate the frequency, location, and potential risk factors for BMBs in a sample of very old institutionalized patients with neurodegenerative dementia. As it was expected, a high prevalence (44.6%) of BMBs was observed, which was higher than prevalence of 23% recently reported in a meta-analysis of patients with AD. 8 We possibly found more BMBs due to older age and more frequent cerebrovascular disease, but it must also be noted that all the previous reports were based on 1.0-T or 1.5-T magnetic field strengths. The current study was the first to use 3.0-T equipment that is known to improve the sensitivity for BMB detection. 19
An exponential distribution of BMB frequency was observed (Figure 2), which led us to defining a group of patients with unusually high number of BMBs. That group, compared to patients without or with few BMBs, had more frequent medical history of dyslipidemia and also more number of vascular risk factors and vascular diseases (Table 1). However, the observed distribution of BMBs, with a small number of patients displaying many BMBs, did not fit easily into a model of sum of effects of the analyzed vascular factors. Rather, the highly skewed distribution of BMBs should be accounted for by some other uncontrolled factors (eg, genetic predisposition to amyloid angiopathy) and/or by different types of factor interaction. A meta-analysis of studies in healthy adults demonstrated an effect of hypertension and diabetes in the prevalence of BMBs, but the potential interactions between factors were not addressed. 2
An interesting exception to the positive association between vascular factors and BMBs was smoking, which never occurred in the patients who displayed high number of BMBs. Protective effect of tobacco use on BMBs was previously reported in adults with cerebrovascular disease 2 and that finding might be interpreted in terms of selective survival of smokers not bearing the ∊4 APOE allele. 20 However, that explanation is very unlikely in our patients since there was no association found between ∊4 APOE allele and BMBs (Table 1).
The most frequent location of BMBs was the cerebral lobes, with particular predisposition for the occipital lobes, followed by the BG (Figure 3). The relatively high prevalence of BMBs observed in the occipital lobe is explained by the traditionally described predisposition of amyloid angiopathy for that lobe. 21 Co-occurrence of BMBs in cerebral lobes and BG was also frequent, with prevalence apparently higher than expected by chance (cerebral lobes 31.1%, BG 24.3%, both 13.5%). That finding suggests the existence of common mechanisms leading to, or even some type of interaction between, amyloid angiopathy and arteriolosclerosis. A possible interaction between amyloid angiopathy and arteriolosclerosis leading to more BMBs received further support from the unexpected finding of lack of correlation between cortical BMBs and probable AD diagnosis (r = −.12; Table 2).
Moderate correlations were observed between total number of BMBs and BG ischemic disease (r = .39) and between BMBs in the BG and the clinical diagnosis of AD and cerebrovascular disease (Table 2). Again, those findings suggest the existence of common mechanisms or links between vascular and AD. In fact, colocation of BMBs and amyloid deposits was described in a single, not replicated, pathological study, 22 which led to the hypothesis that amyloid plaques are initiated by BMBs. 23 More recently, BMBs were proposed as the hypothetical final pathway of 2 main pathophysiological routes for AD (ie, amyloid and vascular routes), which could trigger neuronal dysfunction and death. 8
Brain microbleeds were clearly associated with cortical infarcts but not particularly with hemorrhagic infarctions (9 ischemic infarctions vs 5 hemorrhagic infarctions in the patients with BMBs). On the other hand, a weak association was observed between BMBs and use of antiaggregant or anticoagulant medication (Table 2). With the necessary caution derived from the specific characteristics of the present sample (namely, neurodegenerative dementia and small number of infarctions), our data support the prescription of antiaggregant or anticoagulant medication in patients who are at risk of ischemic vascular disease, even if BMBs are observed.
Apart from a possible predisposition to ischemic cerebrovascular disease, no other clinical implications of BMBs could be derived from the present investigation. Dementia severity and duration looked similar across the 3 BMB study groups (Table 1), which is in agreement with a recent longitudinal study where no clinical implications of BMBs could be found. 24 It must be noted, though, that our study was not designed to investigate the clinical consequences of BMBs and that, in the mentioned study, only a test of general cognition was implemented. 24 Elevated prevalence of dementia of uncertain etiology (31.8%) was observed in the group of high number of BMBs, which could be reflecting some effect of BMBs blurring the clinical typical picture of the underlying dementia (Table 1).
The present study had several limitations. First, it was a cross-sectional design, which precluded a definitive establishment of causality. Second, the evaluation of BMBs was qualitatively performed and not checked for reliability, with the consequent risks of low sensibility and bias. And third, correction for multiple statistical comparisons was not implemented, which was justified by the convenienence of a mainly descriptive and exploratory approach, given the lack of previous studies of BMBs in samples with predominantly advanced neurodegenerative dementia. 25 Hence, the results and explanations derived from the present investigation should be regarded as provisional, awaiting for future confirmation. Clearly, further research is needed to clarify the topographical distribution, the progression, the involved mechanisms, and the clinical implications of BMBs in patients with neurodegenerative dementia. Desirably, that research will include longitudinal studies with automatized assessment of BMBs and evaluation of cognition, executive functions, and other relevant clinical domains, such as activities of daily living and behavior. The new, more sensitive, T2 susceptibility-weighted, MRI techniques will probably contribute to clarify the physiopathological mechanism and the clinical implications of BMBs in AD and other neurodegenerative dementias. 19,26
Footnotes
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.