Subclinical Microinfarctions on Brain MRI
Subclinical microinfarctions are small areas of brain tissue death (typically <1.5 cm) detected on MRI that occur without causing recognizable neurological symptoms at the time of the event, representing "covert" or "silent" brain injury that is far more common than symptomatic stroke. 1, 2
Definition and Imaging Characteristics
Subclinical microinfarctions are defined as imaging or neuropathological evidence of central nervous system infarction without a history of acute neurological dysfunction attributable to the lesion 1
The term "covert" brain lesions is preferred over "silent" lesions because these injuries are linked to long-term clinical and cognitive deficits despite the absence of acute symptoms 1
These lesions are typically detected only on brain MRI using diffusion-weighted imaging (DWI) sequences for acute ischemic lesions 1
Pathologically, microinfarcts are minute foci with neuronal loss, gliosis, pallor, or more cystic lesions, ranging from 50 μm to a few millimeters in size 3
Lacunar infarcts specifically refer to small subcortical infarcts (<1.5 cm) located in the basal ganglia, brain stem, or deep white matter supplied by penetrating arteries 4, 5
Prevalence and Epidemiology
The incidence of covert brain lesions markedly exceeds the incidence of clinically symptomatic stroke by more than tenfold 1
The prevalence of silent cerebral infarction increases dramatically with age: approximately 11% between ages 55-64 years, 22% between ages 65-69,28% between ages 70-74,32% between ages 75-79,40% between ages 80-85, and 43% beyond age 85 1
Application of these prevalence rates to population estimates suggests approximately 13 million people have silent stroke 1
In autopsy studies, microinfarcts are found in 24% of nondemented older individuals, 43% of patients with Alzheimer's disease, and 62% of patients with vascular dementia 3
Risk Factors and Associated Conditions
Major vascular risk factors include hypertension, diabetes mellitus, hyperlipidemia, and smoking 6, 7
The combination of hypertension and hyperlipidemia results in more severe subclinical vascular impairment than either risk factor alone 7
Advanced carotid atherosclerosis with intraplaque hemorrhage and plaque ulceration is strongly associated with covert brain infarctions (prevalence ratio 3.33 and 1.91, respectively) 8
Small vessel disease pathology, including arterial media thickening from fibrinoid deposition and smooth muscle hypertrophy, underlies many microinfarctions 5
Clinical Significance and Long-Term Outcomes
Population-based studies demonstrate that covert brain lesions are associated with approximately 4-fold increased risk of incident symptomatic stroke and 2-fold increased risk of dementia during long-term follow-up 1
Subclinical ischemic vascular dementia, characterized by multiple lacunar infarctions, is a recognized clinical pattern of vascular cognitive impairment 2
Even in the absence of acute symptoms, these lesions contribute to cognitive decline, particularly affecting memory, attention, and reaction time 7
Mixed pathology is extremely common, with vascular disease coexisting with Alzheimer's disease pathology in up to 38% of cases 2
Pathophysiology
Two major mechanisms underlie small vessel disease: (1) brain ischemia in regions supplied by affected arteries causing deep small infarcts, and (2) leakage of fluid causing edema and gliosis in white matter tracts 5
Blood-brain barrier dysfunction and endothelial impairment play pivotal roles in small vessel disease pathophysiology 6
Mechanisms include thrombosis, embolism (from atherosclerotic plaque or cardiac sources), hypoperfusion, and vasospasm 1
Distribution and Location
Microinfarcts are found in all brain regions, possibly more frequently in the cerebral cortex, particularly in watershed areas 3
Lacunar infarcts specifically involve the basal ganglia, brain stem, thalami, and deep cerebral white matter 2, 5
Multiple microinfarcts are common, and their presence combined with white matter signal changes may indicate specific diagnoses like CADASIL 2
Clinical Implications for Patients with Risk Factors
Patients with hypertension, diabetes, or hyperlipidemia should be considered at high risk for harboring subclinical microinfarctions even without neurological symptoms 6, 7
Aggressive management of vascular risk factors including blood pressure control, glycemic control, lipid management, and smoking cessation is crucial for prevention 6
The presence of subclinical microinfarctions identifies patients requiring more intensive secondary prevention strategies 1, 6
Brain MRI with DWI sequences should be obtained in patients with acute retinal ischemia, as up to 31% have concurrent silent cerebral infarctions 1
Important Caveats
Standard clinical neurological examination has limitations in detecting these lesions, which is why advanced imaging is essential 1
Whether procedure-related covert brain lesions (such as from cardiac catheterization) confer the same long-term risk as spontaneously occurring lesions remains to be established 1
The presence of multiple lacunar infarcts does not exclude concurrent neurodegenerative processes, and comprehensive evaluation is essential 2