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Mycotic Aneurysm

Axial noncontrast CT shows focal hyperdensity in the right parietal cortex, associated with a small amount of subarachnoid hemorrhage. MR images show a focal area of signal abnormality in this same region, with hyperintensity/enhancement on T1 and hypointensity on T2 consistent with blood products (intracelular methemoglobin). Cerebral arteriogram shows a small peripheral aneurysm, which has enlarged somewhat on follow up angiography (last two images).

Differential Diagnosis:
The differential diagnosis of a small peripheral hemorrhagic lesion is broad, including trauma, tumor, aneurysm and vascular malformation. Peripheral aneurysms due to atherosclerosis or underlying congenital vascular abnormality are very unusual. Etiologies such as mycotic or oncotic aneurysms should be considered in this location.

Septic emboli from endocarditis may cause brain abscess, mycotic aneurysm, or vasculitis. Septicemia from any cause may lead to endocarditis, but the most common is S. aureus endocarditis in IV drug abusers. Mycotic aneurysms comprise less than 5% of all intracranial aneurysms, and may be multiple in 20%. The aneurysms are commonly located superficially over the convexities. These lesions may respond to antibiotics, but may enlarge on follow up examination and require surgery. Oncotic aneurysm is a similar process caused by embolization of atrial myxoma tissue. Mycotic aneurysms have a higher risk of bleeding than congential aneurysms.