Background: Alzheimer's disease (AD) can often be associated with the presence of small hemosiderin deposits, or microbleeds, in the brain. Subjects with AD typically present with episodic memory deficits, although a large proportion present with other cognitive complaints, including difficulty with language or visuospatial/perceptual function. It is unknown whether the proportion of subjects affected by microbleeds, or the regional distribution of microbleeds, differs between subjects with typical and atypical presentations of AD.
      Methods: A cohort of 45 subjects that had amyloid deposition on Pittsburgh compound B (PiB) PET and presented with predominant language (n=31) or visuospatial/perceptual (n=14) deficits were prospectively recruited and underwent a T2* weighted MRI. These subjects were compared to a cohort of 40 subjects with typical dementia of the Alzheimer's type from the Alzheimer's Disease Neuroimaging Initiative (ADNI). Each microbleed was identified by an expert rater and assigned a lobar location.
      Results: Prevalence of microbleeds was 40% in atypical AD and 32% in typical AD. Within the subjects with microbleeds, the mean (SD) number of microbleeds per subject was 14.2 (35.3) in atypical AD and 3.2 (4.6) in typical AD. The topographic density of microbleeds was highest in the frontal and temporal lobes in atypical AD. In contrast, density in typical AD was highest in the temporal and occipital lobes, with lowest density observed in frontal lobes. Subjects with atypical AD had a higher proportion of deep grey and white matter microbleeds than typical AD (13% vs. 0%, p=0.02), with similar proportions of microbleeds observed in infratentorial regions (7% vs. 5%, p=0.74). The total number of microbleeds was associated with age (p=0.07), but not PiB-PET burden, in atypical AD. Within atypical AD, the prevalence of microbleeds was higher in subjects with language (52%) versus visuospatial/perceptual deficits (14%, p=0.02).
      Conclusions: Microbleeds affect a large proportion of subjects with atypical clinical presentations of AD, particularly those with language deficits. The topographic distribution of microbleeds differs from typical AD, with greater burden in the frontal lobes and deep brain regions suggesting possible differences in underlying etiology.