etiology
This CT cross section shows a mycotic aneursym in the descending thoracic aorta. Note the air within the thickened vessel wall. Mycotic aneurysm refers to the growth of fungi within the vascular wall, usually following impaction of a septic embolus or from primary infection of the vessel wall. This term is also used to refer to the growth of bacteria within the vascular wall.
Up to 15% of patients with infective endocarditis
have septic intracranial aneurysms. Mycotic aneurysms comprise 2.5-6.2% of all
cerebral aneurysms. They may occur by direct extension of infection from the
lumen through the arterial wall. Alternatively, infected emboli may lodge in the
vasa vasorum, causing destruction of the vessel wall. A third mode of aneurysm
formation is contiguous extension to the vessel from an external focus. Days or
weeks may be necessary for the evolution of these lesions. Mycotic aneurysms
tend to undergo rapid increases in size and false aneurysms may arise in the
adjacent brain parenchyma. Films .4-.6 are serial angiograms from a patient with
SBE demonstrating rapid increase in size of a mycotic aneurysm from an initial
site of luminal irregularity. Film .6 (angiogram and enhanced CT) demonstrates
the characteristic target appearance of a partially thrombosed "giant" mycotic
aneurysm (see file on giant aneurysms). Several authors have advocated the use
of serial angiography to follow patients undergoing medical therapy for giant
aneurysms. Evolution of mycotic aneurysms may be delayed by appropriate
antibiotic therapy, which may also produce their complete resolution. Surgery or
embolization is considered for rapidly growing lesions. This giant aneurysm was
embolized with tissue adhesive.
Mycotic cerebral aneurysms usually occur
at sites of vessel branching, most commonly in the middle cerebral artery
territory (Film .7). One or more aneurysms in peripheral branches of cerebral
arteries suggest an infectious etiology. Adhesions may form between the
subarachnoid space and brain in the region of the aneurysm. These adhesions may
prevent free escape of blood into the subarachnoid space. Mycotic aneurysms,
which have a tendency to bleed, due to their fragile, friable walls, therefore
tend to bleed into the brain parenchyma or subdural space. Hematomas tend to be
infected.
A CT scan of the thorax with intravenous contrast is shown below. Note the thickened density around the aortic root and the contrast-containing cavity (arrow).
A mycotic aneurysm was found at surgery. The prosthetic aortic valve, aortic root and ascending aorta were removed and an aortic root homograft inserted. The patient was discharged from the hospital two weeks later. He was kept on chronic antibiotic suppression therapy.
Mycotic aneurysm of the SMA
This patient presented with bacterial endocarditis, for which he had just begun antibiotic treatment. He also complained of abdominal pain, and an intravenous contrast enhanced CT of the abdomen was obtained.
The two sample CT images demonstrate absence of contrast within the superior mesenteric artery. The artery is enlarged and contains low attenuation thrombus. Given the known history of bacterial endocarditis, the likely diagnosis is of a mycotic aneurysm.
Mycotic aneurysm of the SMA
The patient was treated with appropriate antibiotics and the aneurysm resolved, as demonstrated on sequential ultrasound scans (not shown).
