A mycotic aneurysm is a type of aneurysm that develops as a result of an infection weakening the vessel wall. Its name can be misleading: although the word “mycotic” evokes fungus, the vast majority of these aneurysms develop due to bacterial rather than fungal infections. For this reason, many physicians today prefer the more accurate term “infectious aneurysm.” The term originally arose from the resemblance of the infected lesions in the vessel wall to the cap of a mushroom.

Its mechanism is as follows: microorganisms (usually bacteria) that enter the bloodstream settle in the vessel wall, where they cause inflammation and destruction. This destruction disrupts the structural integrity of the vessel wall, and a ballooning, that is, an aneurysm, develops in the region where the wall has weakened. The most commonly encountered source is infective endocarditis, an infection of the heart valves; in this case, infected fragments breaking off from the valve (septic emboli) can reach the brain vessels and form an infectious aneurysm there.

It has some important differences from classic brain aneurysms. While classic (saccular) aneurysms are typically located at the main branching points of the vessels and at the base of the brain, mycotic aneurysms are seen more often in the terminal (distal) branches of the vessels, in regions close to the surface of the brain. In addition, their shape is generally more irregular and fusiform, and they do not have a narrow neck. Another important feature is that, because they are related to infection, their size and number can change rapidly; they may shrink and disappear with treatment, or they may grow within a short time.

The fundamental approach in treatment is, first of all, to bring the underlying infection under control with appropriate and sufficiently long antibiotic therapy. Many infectious aneurysms can regress with effective antibiotic treatment. However, if the aneurysm has ruptured, continues to grow despite antibiotics, or carries a high bleeding risk, surgical or endovascular intervention is required. The treatment plan is determined individually, together with the patient’s general condition, the location of the aneurysm, and the course of the infection.

In summary, a mycotic aneurysm is a secondary type of aneurysm arising not directly from the vessel wall itself, but from an infection. For this reason, its evaluation and treatment require a multidisciplinary approach from both a neurosurgical and an infectious diseases perspective.