Cerebral microhemorrhages: mechanisms, consequences, and prevention

Abstract

The increasing prevalence of multifocal cerebral microhemorrhages (CMHs, also known as “cerebral microbleeds”) is a significant, newly recognized problem in the aging population of the Western world. CMHs are associated with rupture of small intracerebral vessels and are thought to progressively impair neuronal function, potentially contributing to cognitive decline, geriatric psychiatric syndromes, and gait disorders. Clinical studies show that aging and hypertension significantly increase prevalence of CMHs. CMHs are also now recognized by the National Institutes of Health as a major factor in Alzheimer’s disease pathology. Moreover, the presence of CMHs is an independent risk factor for subsequent larger intracerebral hemorrhages. In this article, we review the epidemiology, detection, risk factors, clinical significance, and pathogenesis of CMHs. The potential age-related cellular mechanisms underlying the development of CMHs are discussed, with a focus on the structural determinants of microvascular fragility, age-related alterations in cerebrovascular adaptation to hypertension, the role of oxidative stress and matrix metalloproteinase activation, and the deleterious effects of arterial stiffening, increased pulse pressure, and impaired myogenic autoregulatory protection on the brain microvasculature. Finally, we examine potential treatments for the prevention of CMHs based on the proposed model of aging- and hypertension-dependent activation of the reactive oxygen species-matrix metalloproteinases axis, and we discuss critical questions to be addressed by future studies.

in recent years, significant advances in imaging technologies enabled the detection of previously undetected small chronic intracerebral hemorrhages [<5 to 10 mm in diameter (78)] termed cerebral microhemorrhages (CMHs; also described as microbleeds, multifocal signal loss lesions, petechial hemorrhages). CMHs are due to the rupture of small arteries, arterioles, and/or capillaries [in human CMHs, the diameter of ruptured vessels is estimated to be less than 200 µm, with many bleeds occurring at the arteriole and capillary levels (67)]. CMHs appear as small, oval hypointense lesions corresponding to focal hemosiderin depositions, which can be best detected using T2*-weighted Gradient-Recall Echo (T2*-GRE) MRI sequences (Fig. 1). Because hemosiderin remains present for a long time at the location of a previous bleeding, these methods enable the assessment of chronic, cumulative CMH burden.

Fig. 1.

Cerebral microhemorrhages visible on T2*-GRE MRI sequences in the axial plane (please see white arrowheads). All images were obtained on a 1.5 T field strength scanner (GE Medical Systems). A: 76-yr-old man with chronic untreated hypertension and mild cognitive impairment presenting with transient ischemic attack manifested as right hemiparesis. Imaging was negative for acute cerebral infarction. Although at least one lesion involves deeper brain regions, many of the cerebral microhemorrhages (CMHs) involve the gray-white matter junction. B: 82-yr-old woman with a diagnosis of probable Alzheimer's disease in the moderate stage of diabetes, coronary artery disease, and hypertension. Imaging was obtained as part of the outpatient assessment for memory loss and risk prediction for future risk of intracranial hemorrhage, as it was preceded by a concurrent new diagnosis of deep venous thrombosis. Several lobar CMHs are noted, suggesting amyloid angiopathy. C: 73-yr-old man with amnestic mild cognitive impairment, hypertension, coronary artery disease, and current smoking, presenting with right visual field deficit of unclear duration. Note the large occipital bleed (thick arrow) and additional two left hemispheric CMHs (arrowheads). D: 57-yr-old man with long-standing smoking, hypercholesterolemia, and untreated hypertension. He presented with transient ischemic attack manifested as hemisensory loss and dysarthria. E: 61-yr-old man presenting with new-onset mild aphasia and headache. He had a prior history of chronic anticoagulation for cardiac valvular disease, with therapeutic levels of warfarin upon presentation. Note the large intracerebral hemorrhage (thick arrow) and additional CMH (arrowhead), illustrating the potential association between CMHs and larger intraparenchymal bleeds. F: 86-yr-old woman with prior stroke, atrial fibrillation (not on anticoagulation), and hypertension presenting with acute stroke, treated with intravenous thrombolytics with subsequent development of intraparenchymal hemorrhage (thick arrow). Note the presence of CMH (thin arrow) in the contralateral hemisphere.

In this review, the pathophysiology of CMHs is considered in terms of potential mechanisms involved, known risk factors and their clinical significance. The cellular and molecular mechanisms underlying structural weakening of cerebral vessels, the pathophysiological events promoting rupture of the vascular wall and the impact of risk factors (in particular, the synergistic effects of aging and hypertension) on the mechanical properties of the extracellular matrix and smooth muscle cells, which all contribute to the pathogenesis of CMHs, are critically discussed, and possible targets for intervention for prevention are identified.

Epidemiology and Risk Factors for CMHs

Age is the most significant independent risk factor for CMHs (21, 22, 95). Prevalence of CMHs is low in younger subjects and progressively increases with age (145, 148). The majority of studies report that the prevalence of CMHs is 24% to 56% in elderly patients (92, 104, 143, 145, 175, 235, 242). Because of the rapid development and increased availability of imaging methods, this figure is expected to significantly increase in the near future (232). Approximately half of the patients present with multiple CMHs (92). Despite their clinical importance, the pathogenesis of CMHs and the mechanisms responsible for the dramatic age-related increases in CMH incidence in human studies are not well understood, and there are no effective treatments available for prevention.

Hypertension is another major independent risk factor for CMHs (Fig. 2) (27, 95, 148, 150, 174, 222). The key role of the interaction of aging and hypertension in the development of CMHs is indicated by the findings that hypertension almost exclusively increases CMHs in elderly patients and aged laboratory animals (194).

Fig. 2.

Distribution and pathogenesis of cerebral microhemorrhages (CMHs) associated with hypertension and Alzheimer’s disease in elderly patients. Left: in elderly patients CMHs associated with hypertensive vasculopathy typically affect the small perforating end-arteries located in the deep gray nuclei, brain stem, cerebellum, and deep white matter. The scheme highlights potential factors determining location and mechanisms involved in microvascular fragility. Accordingly, age-related large conduit artery stiffening increases pulsatile pressure, which can penetrate into the vulnerable portion of the microcirculation due to an impairment of myogenic autoregulatory protection in the proximal resistance arteries. The predilection of brain regions to hypertension-induced CMHs is determined by the branching pattern of penetrating arterioles. Increased pressure-induced vascular reactive oxygen species (ROS) production activates matrix metalloproteinases (MMPs), which degrade collagens and other components of the extracellular matrix (ECM), compromising the structural integrity of the cerebral microvasculature and promoting CMHs (inset). Right: in Alzheimer’s disease, CMHs develop in vessels affected by cerebral amyloid angiopathy (CAA). Location of the CMHs is determined by the predilection of brain regions for CAA pathology, which preferentially affects the small arteries and arterioles located in the cerebral cortex and at the junction of white and gray matter. The scheme depicts that deposition of Aβ in the vascular wall (green) promotes vascular smooth muscle cell atrophy and oxidative stress and exacerbates MMP-mediated degradation of the ECM, compromising the structural integrity of the vessels. The model predicts that because similar cellular mechanisms are involved in the pathogenesis of CMHs in both conditions, similar interventions should be effective for prevention as well.

Cerebral amyloid angiopathy and Alzheimer’s disease (AD) also constitute an important risk factor for CMHs (Fig. 2). The prevalence of CMHs is 26–48% in AD patients (16, 132, 243, 244). AD patients often exhibit multiple CMHs (74, 132). CMHs in AD patients are clinically important, as they may exacerbate cognitive decline, predict risk of future intracerebral hemorrhage, and indicate immunotherapy-related adverse events (74, 165). CMHs in AD patients typically show a cortical/subcortical localization (Fig. 2) (76, 120, 244). Cerebral amyloid angiopathy (CAA) in AD patients show a similar distribution (224), and it is believed that most CMHs in AD patients develop in vessels affected by CAA. Animal models of CAA also exhibit spontaneous CMHs (68).

The link between diabetes mellitus and CMH pathogenesis is not well understood (157). Initial analysis of the data obtained in the Northern Manhattan Study suggests that diabetes medication use is negatively associated with CMHs, but fasting blood glucose is not (21). In type 1 diabetic patients with proliferative diabetic retinopathy, an increased prevalence of CMHs was reported (237), suggesting that generalized microangiopathy may contribute to both the cerebral and retinal microvascular injury. Larger studies are warranted to elucidate the exact role of diabetes in the pathogenesis of CMHs and to determine whether those with medication‐controlled diabetes are, indeed, more protected from microvascular injury.

CMHs are also found in close to half of individuals affected by cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), which is an adult-onset hereditary stroke disorder caused by mutations of the Notch 3 gene (prevalence: 1–9 per 100,000) (219).

Clinical Significance of CMHs

Detection of CMHs using advanced imaging modalities.

Continued advances in MRI technology, including a continued increase in the imaging sequences available for clinical imaging, as well as access to MRI studies, have allowed for an increased detection of CMHs over the last two decades (for an excellent overview, see Ref. 223). These lesions, which typically are not seen on CT, were originally described (124) as flow void area on T2*-GRE MRI sequences and comprise small perivascular hemosiderin deposits surrounded by essentially normal brain tissue (78). After extravasation of erythrocytes into the perivascular space, within hours, polymorphic nuclear leukocytes and scavenger cells appear. Degradation of hemoglobin contained in erythrocytes by these cells leads to the formation of hemosiderin. Hemosiderin is paramagnetic, which allows for its detection by T2*-GRE MRI sequences.

Guidelines proposed by a panel of experts for detecting CMHs take into account the size, shape, and signal characteristics (78). More specifically, CMHs are defined as round or ovoid lesions, that appear black and with a blooming effect on T2*-GRE sequences, devoid of signal hyperintensity on T1-weighted or T2-weighted sequences and are distinct from potential CMH “mimics,” such as vessel flow voids, bone, or iron or calcium deposits. In addition, the clinical history should exclude traumatic diffuse axonal injury. Although there has been a lot of variability in defining cut-off points to classify CMHs, the guidelines recommend the range of 5–10 mm as an upper size limit. Of note, this upper range allows for the best distinction from larger bleeds (macrobleeds). A potential limitation in detecting CMHs is represented by disagreement among raters about the number and presence of lesion, a problem that could be addressed by the future use of validated standardized rating scales (79). The detection of CMHs is sensitive to many MRI imaging variables, such as field strength, echo times, and resolution. Recent studies using the susceptibility-weighted imaging (SWI) approach reported an increased sensitivity of detection of small lesions, resulting in a threefold increase in the number of CMHs detected (119). The development of high-resolution novel imaging methods and prospective studies using serial scans is expected to promote the better understanding of the pathogenesis of CMHs and shed new light on the clinical consequences of this currently underdiagnosed disease entity (81).

Consequences of CMHs.

There is substantial evidence that CMHs are clinically not silent. Here, we provide an overview of the scientific and clinical evidence for the role of CMHs in cognitive decline and gait abnormalities and the link between CMHs and small vessel disease, subsequent stroke, and increased mortality.

role of cmhs in cognitive decline.

Patients with CMHs have higher incidence of cognitive dysfunction (22, 92, 144, 220, 233, 234, 238, 240, 241). The behavioral consequences of CMHs likely depend on multiple factors, including CMH number and size, location, and/or cooccurrence of other diseases. There is strong evidence that a higher number of CMHs is associated with more severe cognitive dysfunction (238). Cross-sectional studies of the cognitive consequences of CMHs suggest that CMHs are associated with impaired executive function, decreased attention and processing speed, and impaired global cognition (245). CMHs have also been linked to significant behavioral problems, such as post-stroke emotional lability (181), depression (177, 179, 180, 184), suicidality (178), and fatigue (183), as well as impaired post-stroke cognitive recovery (182). Thus, further studies are needed to elucidate how CMHs affect the clinical presentation, treatment response, and outcome of geriatric psychological disorders. Future studies should also determine whether prevention of CMHs in high-risk patients would prevent or delay cognitive decline and/or alter the incidence of psychological disorders in the elderly.

The behavioral consequences of CMHs clearly depend on their locations (220). A recent meta-analysis shows that CMHs in deep brain regions, lobar regions, basal ganglia, and thalamus are associated with significant cognitive decline (238). Temporal lobe-located CMHs were reported to be associated with memory and attention impairment, whereas frontal lobe CMHs results in impairment of memory, concept shifting, psychomotor speed, and attention (220). CMHs in deep and lobar regions were shown to associate with attention/executive function and fluency domains (218). In other studies, lobar CMHs and CMHs located predominantly in the left hemisphere were shown to associate with late-onset depression (64). Despite these advances, it should be noted that there are several limitations to establishing direct cognitive associations, according to location of CMHs, including limitations inherent to imaging modalities used, study demographics, and presence of confounding variables (e.g., education) that affect patient performance on commonly used cognitive tests, and limited sensitivity of available cognitive measures. In many cross-sectional clinical studies, cognitive tests and sample size lack the sensitivity to measure subtle changes in performance due to a small number of CMHs. Cognitive consequences of CMHs are also influenced by other brain pathologies, such as the presence of periventricular leukoaraiosis (59).

The mechanisms by which CMHs impair cognitive function are likely to include focal brain damage and secondary disruption of neuronal and astrocytic communication in neighboring brain regions. CMHs likely cause a persisting local inflammatory response, focal blood-brain barrier disruption, activation of microglia, and perivascular macrophages (152). Recent studies propose that multiple CMHs lead to disruption of structural networks in the brain (90). It is less understood whether CMHs can resolve, although disappearance of imaging signs of CMHs has been documented in the literature (245).

Finally, the lack of agreement regarding cognitive assessment methods is likely responsible for the variability in the reported rate of occurrence and/or severity of cognitive impairment in patients presenting with CMHs. Studies using complex batteries of neuropsychological tests (including tests to assess intellectual function, verbal and visual memory, naming and perceptual skills, speed and attention, and executive function) demonstrate significant association between CMHs and cognitive decline (233). Although there are studies that fail to demonstrate a clear association between CMHs and cognitive impairment, this is usually due to the lack of sensitivity of the memory assessment used. For example, many studies rely on the mini-mental state examination (MMSE) (220), which is known to lack sensitivity to mild cognitive impairment. In addition, MMSE scores are affected by educational level, such that higher education may lead to false-negatives among patients with CMHs. Thus, we would like to emphasize the pressing need for the development and use of validated, sensitive, comprehensive cognitive neurological assessment methods in studies that attempt to evaluate the pathogenic role of CMHs in cognitive impairment.

role of cmhs in gait abnormalities.

There are clinical studies extant linking CMHs with gait dysfunction (25, 46, 134). Using an animal model of CMHs, we provided proof-of-concept showing that CMHs in aged mice cause gait disorder, which correlates with the severity and number of the CMHs (194). Gait and balance are governed by an interaction of the brain stem, sensory systems (vision, touch, vestibular input), somatosensory cortex, basal ganglia system, frontal subcortex, and the cerebellum. Because of these complex regulatory interactions, a large variety of motor and sensory problems associated with CMHs in multiple brain regions may lead to gait and balance disorders. High-level gait disorders may be caused by CMHs affecting cognitive and attentional processes governing balance and gait. The clinical importance of identifying potentially preventable microvascular mechanisms of gait abnormalities in the elderly cannot be overestimated. Gait abnormalities, which are present in more than one-third of older adults, increase the risk of falls with often tragic consequences. Falls are the fifth most common cause of death in the elderly, and only 25% of patients who suffer hip fractures regain their former level of function. Further studies are needed to adopt sensitive methods of gait analysis to detect even slight, subclinical gait abnormalities to be able to predict CMHs in high-risk patients. Incorporating gait as an end point in future studies designed to evaluate treatments for CMH prevention will also be highly informative.

cmhs, subsequent stroke, and mortality.

The presence of CMHs appears to predict subsequent intracerebral hemorrhage in elderly patients (198), or in patients with ischemic stroke (58, 121), AD, or those receiving anticoagulants (107, 110) (17). These associations suggest that similar cellular and molecular mechanisms may underlie vascular fragility, leading to both CMHs and ensuing larger intracerebral hemorrhages with significant clinical impact. There are several studies extant showing that the presence of CMHs is associated with an increased risk for cardiovascular mortality (2, 17, 77).

cmhs and small vessel disease.

White matter hyperintensities (WMHs; or leukoaraiosis) are nonspecific hyperintense changes localized to the cerebral white matter, which are frequently seen on T2/FLAIR sequences in elderly individuals. White matter damage in leukoaraiosis develops as a consequence of small vessel disease, and there is growing evidence from imaging studies, suggesting that CMHs contribute to the pathogenesis of leukoaraiosis and its functional consequences (16, 70, 93, 217). Presence and progression of small-vessel disease are strong predictors of new CMHs (73).

Pathophysiology of CMHs

The cellular and molecular mechanisms underlying microvascular fragility associated with the development of CMHs are not well understood. Here, we provide an overview of possible mechanisms inferred from studies on both animal models of CMHs (Fig. 2) and studies on pathological structural alterations in the peripheral vasculature induced by known risk factors of CMHs in the cerebral circulation (Fig. 3). Special emphasis is placed on age-related structural and functional maladaptation to hypertension and the underlying mechanisms.

Fig. 3.

Cellular mechanisms by which aging exacerbates development of CMHs. The scheme highlights the role of age-related exacerbation of pressure-induced vascular oxidative stress, MMP activation, and structural maladaptation to hypertension in pathogenesis of CMHs. Accordingly, the increased intraluminal pressure and consequential increases in wall tension activate NADPH oxidases and promote mitochondria-derived production of ROS (mtROS) in the aged vascular smooth muscle cells. Pressure-induced vascular oxidative stress is exacerbated in aging due to a homeostatic failure due to dysregulation of NRF2-mediated antioxidant response. Vascular oxidative stress is responsible for endothelial dysfunction and the increase in MMP activity, which is implicated in collagen degradation, smooth muscle cell atrophy, and degradation of elastic components of the basement membrane. These structural changes weaken the microvascular wall and increase vulnerability to the formation of CMHs. Arrows indicate the effects of aging.

Structural determinants of microvascular fragility.

role of alterations in the extracellular matrix of cerebral vasculature in the pathogenesis of cmhs.

The extracellular matrix contributes significantly to the tensile strength of the vascular wall. The components of the extracellular matrix that account for the majority of the mechanical properties of cerebral vessels are collagen and elastin deposited by smooth muscle cells in the medial layer. The extracellular matrix is organized in a three-dimensional network connected to smooth muscle cells and distributes tensile stress in the vascular wall. Originally, detection of localized microaneurysms on brain vessels that were associated with spontaneous cerebral hemorrhages in early studies (23) led to the hypothesis that degenerative or inflammatory changes affecting small arterioles cause structural weakening and eventually rupture of the vascular wall. It is believed that disruption/weakening of the extracellular matrix network is also a critical factor in the development of CMHs. In experimental animals injection of extracellular matrix-degrading enzymes [e.g., collagenases (229)] result in the development of intracerebral hemorrhages. Further, human patients with genetic disorders affecting components of the extracellular matrix often present with intracerebral hemorrhages, including CMHs (221). These include both collagenopathies such as COL4A1 gene mutations (1, 101, 141, 215), COL3A1 gene mutation/Ehlers-Danlos syndrome (122, 131), mutations of COL1A1 or COL1A2 [osteogenesis imperfecta (60)], and elastinopathies, such as the Marfan syndrome [mutation in the FBN1 gene encoding the ECM protein fibrillin 1 (128)] and mutation in the ELN gene-encoding elastin [supravalvular aortic stenosis/Williams-Beuren syndrome (97)].

In the vascular wall, matrix metalloproteinases (MMPs) are known to degrade collagen and elastin and other components of the basal lamina and extracellular matrix, compromising the structural integrity of the cerebral vasculature. Cerebral arteries express MMP2 (collagens I, II, III, IV, VII, X), MMP3 (collagens II, IV, IX, X, X), MMP8 (collagens I, II, III, VII, VIII, X), MMP9 (collagens IV, V), MMP12 (elastin, fibronectin, collagen IV) and MMP13 (collagens I, II, III, IV, IX, X, XIV). Studies on mouse models of hypertension-induced CMHs demonstrate that the hypertension-related microvascular fragility and pathogenesis of CMHs involve weakening of the vessel wall by upregulation and/or oxidative stress-dependent activation of MMPs, such as MMP2 and MMP9 (226, 227). Importantly, aging was shown to promote the development of CMHs in mouse models by exacerbating hypertension-induced oxidative stress and redox-sensitive activation of MMPs in the cerebrovasculature (Fig. 3). The mechanisms by which hypertension promotes vascular oxidative stress in the cerebral vasculature are multifaceted (194). Multiple lines of evidence suggest that in addition to circulating factors (e.g., angiotensin II), changes in the hemodynamic environment (high pressure, increased circumferential wall tension) associated with hypertension activate ROS production in the vascular smooth muscle cells (166, 194, 205, 206). Recent studies demonstrate that high pressure elicits increased mitochondrial production of ROS, as well as activation of NADPH oxidases in smooth muscle cells located in the cerebral arteries and that these effects are significantly exacerbated in aging (166, 194). Aging per se is known to promote vascular oxidative stress (7–12, 30–33, 35–45, 99, 100, 111, 130, 166, 194, 196, 200, 201, 203, 204, 207, 209–212, 216) and to impair adaptive activation of Nrf2-driven antioxidant response pathways (7, 30, 37, 40, 203). We have recently reported that treatment of aged mice with the naturally occurring polyphenol resveratrol inhibits the development of hypertension-induced CMHs by attenuating oxidative stress-induced MMP activation (194). Importantly, resveratrol inhibits vascular oxidative stress by attenuating mitochondrial ROS production, inhibiting NADPH oxidases, and by upregulating Nrf2-dependent pathways (34, 37, 40, 130, 194, 195, 202, 208, 209, 247). Multiple lines of evidence suggest that activation/expression of MMPs are suppressed by NO and that decreased NO bioavailability promotes MMP activation in the vascular wall (47, 51, 80, 83, 214). Thus, it is important that hypertension in young mice can induce CMHs only after the inhibition of NO synthesis (226–228). Because aging is known to significantly decrease bioavailability of NO (9, 11, 30, 31, 42, 195, 199), we predict that age-related endothelial dysfunction may contribute to the pathogenesis of CMHs in the elderly.

MMP activation may provide a potential common mechanism of CMHs associated with multiple diverse pathophysiological conditions. In mouse models increased MMP9 activity was implicated in cerebral amyloid angiopathy-related hemorrhages (102, 103). Recent findings also suggest involvement of MMPs and lower levels of tissue inhibitors of matrix metalloproteinases (or TIMPs) in the pathogenesis of CMHs associated with AD (50, 248) in human patients. Interestingly, increased activity of the MMP system was also proposed to be a major contributing factor to increased CMH occurrence associated with anti-Aβ immunotherapy (236). High methionine diet and/or the resulting hyperhomocysteinemia were also reported to increase CMHs in mouse models, which are associated with increased expression and MMP2 and MMP9 (172). On the basis of the available evidence, future studies are warranted to test the efficacy of MMP inhibitors in animal models of various disorders that are characterized by increased incidence of CMHs.

In response to hypertension, there is a significant mechanical/structural adaptation process of the vascular wall (49, 135–140, 146). The vascular smooth muscle cells sense the increases in wall tensile stress. In healthy young organisms, by changing the synthesis and/or turnover of components of the extracellular matrix, stress distribution and stress concentration factor within the vascular wall can be attenuated, decreasing the susceptibility for rupture. However, in aging or under pathological conditions, these mechanical/structural adaptation processes are likely impaired (Tarantini S and Ungvari Z, unpublished observation), and intracerebral hemorrhages/CMHs thus ensue. Similar remodeling processes are thought to determine fragility of cerebral microvessels and vessels of the peripheral circulation. Thus, potential mechanisms that may be involved in the pathogenesis of CMHs are inferred here from the studies on the age-related changes in the cerebral circulation and the studies on hypertension-induced remodeling in diverse vascular beds. There is evidence that with aging, cerebral arterioles undergo atrophy, which is associated with a decline in elastin content (82). Age-related elastin degradation is also evident in the human cerebral vasculature (69). Further, cerebral amyloid angiopathy is also associated with significant elastin degradation, which is attributed to an upregulation of elastase expression in the smooth muscle cells (116). Collagen fibers in cerebral arterioles play a critical role in preventing damaging overextension of elastic components of the vascular wall. Despite their importance, it is not well understood how hypertension and aging affect the expression/synthesis/organization of various collagens in the wall of cerebral vessels (82, 89). As both injection of exogenous elastase and collagenase is known to induce intracerebral hemorrhages and activated MMPs degrade both elastin and collagens, the specific roles of altered elastin and collagen content/functionality in increased susceptibility of aged cerebral arterioles to rupture should be elucidated in future studies.

In angiotensin II-treated mice lysyl oxidase mediated ECM cross-linking was shown to contribute significantly to hypertension-induced remodeling and stiffening of arteries (52). Other studies demonstrate that the enhanced sensitivity to stroke in the stroke-prone spontaneously hypertensive rat is associated with an increased susceptibility of the arterial internal elastic lamina to rupture and dysregulation of lysyl oxidase (28). Importantly, β-aminopropionitrile, an inhibitor of lysyl oxidase-mediated cross-link formation in collagen and elastic fibers, exacerbates vascular rupture in spontaneously hypertensive rats (28). Thus, further studies are warranted to elucidate the role of lysyl oxidase also in the pathogenesis of CMHs, determining whether aging and other conditions that promote CMHs are associated with altered expression/activity of lysyl oxidases.

role of alterations in vascular smooth muscle cells in the pathogenesis of cmhs.

Unlike in elastic conduit arteries, in small cerebral arteries and arterioles, the smooth muscle cells significantly contribute to the passive mechanical properties of the vascular wall. Advanced age is associated with atrophy in cerebral arterioles, which results in a decreased cross-sectional area of smooth muscle cells (82), likely weakening the vessel wall. Smooth muscle cell loss is also prevalent in vessels affected by cerebral amyloid angiopathy (116). Possible mechanisms that may contribute to cerebral arteriolar atrophy involve circulating IGF-1 deficiency, which is the most prominent endocrine change with aging. Importantly, mouse models of circulating IGF-1 deficiency exhibit arteriolar atrophy, mimicking the aging phenotype (185). In healthy subjects in response to hypertension, cerebral vessels exhibit adaptive media hypertrophy (135), which likely increase the resistance of the vascular wall to rupture. There is evidence that in aging and IGF-1 deficiency, this adaptive response is impaired, which likely exacerbates CMHs (185). The concept that smooth muscle atrophy in cerebral arterioles contributes significantly to their increased susceptibility to rupture is supported by the histological findings from patients affected by CADASIL and animal models of CADASIL, showing that the underlying pathology for CADASIL-related CMHs is a progressive degeneration of the smooth muscle cells and the consequential structural weakening of cerebral vessels (4, 6, 29, 225). In addition to affecting the smooth muscle coverage of arterioles and activating ROS/MMP axis, age-related alterations in smooth muscle phenotype can exert multifaceted effects on the pathogenesis of CMHs. On the basis of the findings that radiation-induced vasculopathy is also associated with CMHs, further studies should elucidate the role of cellular senescence-related pathways age-related CMHs (19, 160). New research is also needed to address the role of proinflammatory changes and the interaction of vascular smooth muscle cells and MMP-expressing perivascular macrophages (3) as well.

Lipohyalinosis (also known as “fibrinoid necrosis”) is a complex form of small-vessel disease prevalent in hypertensive elderly patients affecting deep-perforating arteries, which are small arteries branching off directly from relatively large arteries in the basal ganglia, white matter, the brain stem, and the cerebellum (65). The “fibrinoid deposition” in the vessel wall characteristic to this disease is attributed to the effusion of plasma components due to inflammation and consequential blood-brain barrier disruption (the histological term “fibrinoid” refers to the positive fibrin staining of the amorphous eosinophilic material). Because of the phenotypic changes in smooth muscles and remodeling of the ECM associated with lipohyalinosis, the structural integrity of the vascular wall is compromised, promoting the formation of microaneurysms and intracerebral hemorrhages (151). Hyaline arteriolosclerosis (also known as arteriolar hyalinosis) is a simple form of small-vessel disease manifested in hypertensive elderly individuals, which is characterized by thickening of the arteriolar wall by massive collagen deposits around the basement membrane. Arteriosclerosis associates with CMHs (161), as arteriolar walls affected by hyalinosis are presumably weakened and are, therefore, more vulnerable to rupture.

Effects of arterial stiffening and increased pulse pressure on the brain microvasculature.

In healthy young individuals, the Windkessel effect of the elastic conduit arteries (aorta and carotid arteries) helps in damping hemodynamic pulsatility over the cardiac cycle (18, 189). Both the vascular aging process and exposure to vascular risk factors (including hypertension, hyperlipidemia, and diabetes) in the elderly are associated with increases in aortic stiffness, which impairs the Windkessel function, resulting in consequential increases in the amplitude of systolic pressure and amplification of pressure pulsatility in the elderly (48, 133). Cerebromicrovascular injury has long been hypothesized to result from the transmission of increased pulsatile pressure into the thinned-walled distal portion of the aging cerebral microcirculation (reviewed in Refs. 123, 164). Excessive arterial pressure wave reflections returning from the peripheral resistance arteries may augment pressure pulsatility transmitted to the cerebral microcirculation in the elderly. There is now increasing experimental and clinical evidence that pressure pulsatility, indeed, penetrates the cerebral microcirculation in aging (155, 187), which likely represents an important contributing factor to the pathogenesis of CMHs, as well as microvascular injury leading to blood brain barrier disruption, neuroinflammation, and white matter disease. The existing data are consistent with the hypothesis that age-related autoregulatory dysfunction of proximal resistance arteries allow the pressure waves to travel without substantial attenuation within the cerebral circulation and reach the vulnerable portion of the microcirculatory network, exacerbating microvascular damage (see below).

Role of impaired myogenic protection in the pathogenesis of CMHs.

Cerebral autoregulatory mechanisms are vested with the responsibility of protecting the fragile cerebral microvessels from transmission of increased pulsatile pressure (Fig. 2) (for a review, see Ref. 193). Particularly, the myogenic response of proximal resistance arteries plays a crucial role in dampening of high-pressure waves and reducing the likelihood of pressure-induced microvascular injury (14, 61, 62, 84, 125). Myogenic responses in healthy individuals involve powerful pressure/wall tension-induced constriction of small cerebral arteries that significantly increases segmental hydrodynamic resistance. Because of a larger pressure drop along the proximal resistance arteries, transmission of transient or chronic surges in pulsatile pressure is prevented from reaching the distal brain microcirculation (Fig. 2). Recently, we have shown that in laboratory animals, advanced age is associated with impaired myogenic adaptation to pulsatile pressure in isolated middle cerebral arteries (167). If this newly found evidence holds true for proximal resistance arteries in aged patients, then the age-related decrease of cerebral myogenic protection from hypertensive events could potentially increase the pressure-induced stress imposed on the thin-walled cerebral microvessels of elder individuals. The observation that pressure pulsatility penetrates the cerebral microcirculation in aging (155, 187, 187) supports this concept.

Anecdotal evidence indicates that certain otherwise common activities may impose an undue burden upon the aged cerebral circulation with impaired autoregulatory mechanisms. For example the transient arterial pressure increase caused by the Valsalva maneuver (defined as a forced expiratory blow against a closed glottis) is fairly common in many activities that involve moderate exertion. Such activities include weight lifting, blowing air into inflatable devices, intense coughing, vomiting, nose blowing, and strain during defecation. Depending on the levels of expiratory strain, intrathoracic pressure in these conditions may increase well over 150 mmHg. For instance, strain associated with coughing may increase intrathoracic pressure to ~300 mmHg for several seconds (159). The intrathoracic pressure is transmitted to the circulatory system, and the resulting pressure wave readily reaches the brain. While normally harmless, such transient rises in cerebral arterial pressure along with anger, sexual intercourse, and startling may impose excessive pressure to the brain microvessels of elder individuals due to the impaired myogenic responses of aged proximal arterioles, which allows the pressure wave to penetrate to the distal microvasculature, contributing to the development of microhemorrhages. We posit that in elderly patients, when microvascular pressure exceeds the threshold for structural injury in weakened microvessels, CMHs ensue (Fig. 2). Genetic mouse models of microvascular fragility provide proof-of-concept showing that short-term exercise (which is known to increase blood pressure) can significantly exacerbate CMHs (94). Rapid development of severe hypertension in cats with reduced renal mass results in autoregulatory dysfunction (as suggested by the marked brain edema), promoting CMHs (20). It should be noted that during acute hypertensive episodes, venular pressure can also increase significantly (112, 114), which has been linked to disruption of the blood-brain-barrier. For example, while during phenylephrine-induced acute hypertension in rats, pial arteriolar pressure increases by ~47 mmHg and pial venous pressure increases by ~20 mmHg (113). Given the much thinner wall of small veins, it is logical to predict that this pressure increase may predispose to venular CMHs as well.

Importantly, during the Valsalva maneuver, the central venous pressure also increases considerably (239); thus, a retrograde venous pressure wave may also reach the vulnerable cerebral microcirculation, exacerbating damage. In the internal jugular vein, valves are present to prevent the penetration of backward venous pressure into the cerebral venous system during increases in intrathoracic pressure (and consequentially, central venous pressure) associated with the Valsalva maneuver (26, 66, 249, 250). However, in aged individuals, the vein valves are often insufficient, and without competent vein valves, retrograde transmission of venous pressure occurs. Indirect evidence for a potentially important link between venous pressure increase and CMHs comes from studies on victims of domestic violence who survived manual strangulation (5). In these patients, venous obstruction (and consequential increase in venous and microvascular pressure) leads to petechial hemorrhages in the skin, conjunctiva of the eyes, and deep internal organs, including the brain (142, 246).

There is ample evidence that in hypertension, young cerebral arterial vessels exhibit functional adaptation to the altered hemodynamic environment, which includes augmentation of pressure-induced myogenic constriction (87, 98, 125, 190, 196). The hemodynamic effect of this functional adaptation is synergistic with the structural adaptation of resistance vessels to hypertension [hypertrophic inward remodeling (15)]. Together, these adaptive structural changes and functional changes in autoregulatory mechanisms increase segmental resistance in proximal resistance vessels, protecting the injury-prone distal portion of the cerebral microcirculation from pressure overload (115, 118, 129, 153, 154, 168–170, 191, 196, 231). Because of these adaptive functional changes, pressure in the thin-walled microvessels is maintained relatively constant, protecting them from pressure-induced injury (Fig. 2). The cellular mechanisms that underlie development of pressure-induced myogenic constriction of cerebral resistance arteries include increased production of the vasoconstrictor eicosanoid 20-HETE, inhibition of BK_Ca channels, and activation of L_Ca and TRPC channels (53–55, 57, 71, 72, 85, 86, 117, 126, 127, 147). There is strong evidence that aging cerebral arteries do not exhibit a hypertension-induced adaptive increase in myogenic constriction due to the dysregulation of the 20-HETE/TRPC6-mediated pathway of mechanotransduction in the vascular smooth muscle cells (190, 191, 196). We posit that loss of myogenic autoregulatory protection in the brain of hypertensive elderly subjects likely allows high blood pressure to penetrate the distal, injury-prone portion of the cerebral microcirculation, contributing to the pathogenesis of CMHs (Fig. 2). Direct experimental evidence in support for this concept comes from the recent elegant series of studies by Drs. Fan Fan and Richard Roman, who demonstrated that rats with impaired myogenic constriction (due to genetic deficiencies in 20-HETE production) exhibit increased penetration of pressure to the distal portion of the microcirculation, resulting in increased propensity to microvascular injury (56). Specifically, because of deficient myogenic constriction in the proximal arteries of the Fawn Hooded Hypertensive rat model, the pressure in terminal cerebral arteries was shown to be substantially increased (from ~37 to ~63 mmHg) when mean arterial pressure was increased from 100 to 160 mmHg (56). In contrast, in animals with intact myogenic constriction, the same increases in mean arterial pressure were not associated with significant changes in the pressure in terminal cerebral arteries (56). In general, these data correspond well to those obtained in earlier studies, demonstrating that pressure in pial arteries is ~50% of systemic pressure, as proximal larger intracranial vessels contribute significantly to total cerebral vascular resistance (13, 14, 61–63, 87, 88, 158, 171, 176). It should be noted that some earlier studies demonstrated larger increases in pressure in small pial arteries during hypertension (176), which may be attributed to the differences in the experimental models used [e.g., anesthetized cat vs. rat (13), methods for induction of hypertension etc.].

Novel findings implicate endocrine changes that occur during aging in functional and structural alterations in the aged cerebral circulation (163, 204). Of particular interest is the age-related decline in circulating levels of insulin-like growth factor-1 (IGF-1), which have been causally linked to vascular aging phenotypes, including the pathophysiological alterations in the brain microvasculature occurring with age (7, 163, 192, 197, 204). Experimental evidence suggesting that IGF-1 deficiency promotes autoregulatory dysfunction was recently obtained using a novel mouse model of circulating IGF-1 deficiency (adeno-associated viral knockdown of IGF-1 specifically in the mouse liver using Cre-lox technology; Igf1^f/f + TBG-Cre-AAV8) (7). Specifically, studies using this novel murine model have shown that low IGF-1 levels in the circulation impair myogenic autoregulatory adaptation of the cerebral resistance arteries to hypertension, which likely exacerbate hypertension-induced cerebromiscrovascular damage and injury (197). It appears that IGF-1-deficient mice fail to adapt to hypertension by upregulating vascular TRPC6 expression, augmenting the 20-HETE/TRPC-mediated component of pressure-induced myogenic constriction (197), which mimics the aging phenotype. These findings have important clinical relevance, as in population-based studies, lower circulating IGF-1 levels were also shown to associate with aorta stiffening and higher aortic pulse wave velocity (91, 156). Further, experimentally induced IGF-1 deficiency was demonstrated to mimic additional aspects of cerebromicrovascular aging as well (7, 8, 163, 186, 192).

CMHs in patients with and without hypertension have different distribution patterns (173). It appears that hypertension predominantly promotes deep or infratentorial CMHs (96, 105, 106, 157, 173, 222). Recent studies lead to the hypothesis that regional differences in vascular anatomy and myogenic autoregulatory protection determine the predilection site of CMHs associated with hypertensive vasculopathy. Specifically, it was proposed that in deep and infratentorial regions, penetrating branches arise directly from the posterior cerebral artery and the middle cerebral artery and because of their branching pattern, the segment of proximal vessels, whose resistance protects the microcirculation, is shorter than in other brain regions (109). These anatomical properties would render these vascular supply areas more vulnerable to sudden changes in blood pressure (109). In addition, there are known regional differences in the cellular and molecular mechanisms involved in regulation of myogenic constriction of cerebral arteries. Future studies should elucidate regional variation in the combined effects of age and hypertension in cerebral arteries as well.

Perspectives: From Diagnosis to Prevention

Despite the dramatic increase of publications on the detection and diagnostic and prognostic values of CMHs, several fundamental questions relayed to their pathogenesis and prevention remain to be clarified. Future studies should characterize what percentage of CMHs occurs in arterioles, in capillaries or in venules and determine how this pattern changes in hypertension, aging, and other risk factors for CMHs. Developing novel genetic models with smooth muscle cell-specific depletion of factors involved in microvascular fragility would greatly facilitate the understanding of the pathogenesis of CMHs in aging and in AD. Future work should focus on specific treatments to inhibit or reduce the development of CMHs. Many studies have pointed toward cellular ROS production as an attractive cellular target for intervention to increase microvascular resilience. New research using novel approaches to attenuate age-related microvascular oxidative stress and disrupt oxidative stress-driven pathways involved in the formation of CMHs is very much needed. A growing body of evidence indicates that targeting redox-sensitive MMP activation could be an important treatment option for the prevention of CMHs. The role of cellular senescence programs, contribution of pericytes and perivascular macrophages, as well as inflammatory factors in the pathogenesis of CMHs are still to be fully characterized (67, 94, 149, 162). Alterations in myogenic autoregulatory protection and changes in endothelial tight junctions are likely to exacerbate CMHs at the capillary level. Future studies should determine additional structural factors promoting capillary bleeds (including alterations in biomechanical properties of the basal membrane) and identify targets for intervention (94). In addition to capillaries, tight junctions are also present in arterioles and venules. It is known that different pathophysiological conditions (including diabetes mellitus) result in dysregulation of tight junction proteins, but it is yet unclear how this affects pressure-induced injury in those segments of the vasculature. It is also unclear how retinal microvascular alterations visible on ophthalmoscopy (e.g., hypertensive retinopathy) and CMHs relate. Mechanisms involved in resolution/repair of CMHs, such as a potential role of macrophages suggested by studies on zebrafish models of cerebrovascular injury (108), are not well understood and require further investigation. Randomized controlled trials have been designed to determine the long-term effects of established medications (230) on CMHs, and it is expected that, using similar approaches, application of a mechanism-based therapy targeting pathways identified in experimental studies [e.g., polyphenol compounds (194)] will be also tested in the future. Several compounds that have potentially relevant pharmacological action are approved for other disorders, offering the possibility of drug repurposing for novel prophylactic strategies for CMHs. In the clinical practice, the importance of blood pressure control (determining optimal intensity of treatment; elucidating differences between the effects of various antihypertensive agents with different mechanisms of actions) and lifestyle interventions controlling modifiable risk factors (e.g., to limit temporal blood pressure surges, dietary interventions) should also be emphasized. Since influencing multiple targets (e.g., disrupting the ROS-MMP axis, improving endothelial function, attenuating inflammation, preventing pressure overload) may be most effective, using a combination of vasoprotective drugs and other interventions may be preferable. Clinical studies should also elucidate the link between side-effects of commonly used medications [e.g., statins (148), anticoagulants, aspirin (75)] or even thrombolytics (24) and CMHs. Finally, longitudinal studies should improve our understanding of the prognostic value of CMHs in various pathological conditions associated with aging and the causal link among CMHs, cognitive performance, and gait abnormalities.

GRANTS

This work was supported by grants from the American Heart Association (to S. Tarantini, A. Csiszar, A. Kirkpatrick, and Z. Ungvari), National Institutes of Health (NIH) Grants R01-AT-006526 (to Z. Ungvari), R01-AG-047879, R01-AG-038747, and U54-GM-104938 (to A. Kirkpatrick), R01-NS-056218 (to A. Csiszar), the Oklahoma Nathan Shock Center (NIH Grant 3-P30-AG-050911-02S1), the Arkansas Claude Pepper Older Americans Independence Center at University of Arkansas Medical Center (NIH Grant P30-AG-028718, to Z. Ungvari), the Oklahoma Center for the Advancement of Science and Technology (to A. Csiszar and Z. Ungvari), the Oklahoma IDeA Network for Biomedical Research Excellence (to A. Csiszar), Department of Veterans Affairs Grant CX000340 (to C. Prodan), and the Reynolds Foundation (to Z. Ungvari and A. Csiszar).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

Z.I.U., S.T., A.C., and C.I.P. prepared figures; Z.I.U., A.C., and C.I.P. drafted manuscript; Z.I.U., S.T., A.C.K., A.C., and C.I.P. edited and revised manuscript; Z.I.U., S.T., A.C.K., A.C., and C.I.P. approved final version of manuscript.