A berry aneurysm is a congenital vascular anomaly.

A berry aneurysm is a balloon-like outpouching of a cerebral arterial wall that is berry shaped (hence, the name). This outpouching most often reflects a gradual weakening of the arterial wall as a result of chronic hypertension or arteriosclerosis and places the artery at risk to rupture, causing a stroke. Some cerebral vessels are inherently weak and susceptible to berry aneurysms, such as the arteries associated with the circle of Willis, where small communicating arteries connect larger cerebral arteries (internal carotid, vertebral, and basilar arteries). A ruptured berry aneurysm bleeds into the subarachoid space.

However, such congenital abnormalities are not necessarily genetically transmitted.

Berry Aneurysm

The berry aneurysm is a saccular aneurysm (a dilated outpouching of a blood vessel that forms a sac-like structure

The abnormality is most commonly found intracranially. At birth, the arterial medial layer is abnormal creating a focal weakness in the arterial wall. The dilated outpouching is not present at birth, but forms over time with increased vascular pressure and various risk factors including cocaine use and cigarette smoking. The majority (90%) of berry aneurysms are found in the anterior cerebral circulation of the circle of Willis at its major branch points (internal carotid arteries, anterior communicating arteries, etc.), where there is significant turbulence and shear forces. Multiple aneurysms in a single person are seen in 20% to30% of cases. Annual rupture rates in one large study showed that aneurysms <10 mm had a rupture rate of 0.05%. Rates of rupture were higher (~1%) for aneurysms >10 mm in diameter, those that were symptomatic, or those located in the posterior circulation.

Most aneurysms are clinically silent and asymptomatic, remaining undetected until rupture. Clinical presentation of a ruptured berry aneurysm (the first presentation in over 50% of cases) is most commonly "the worst headache of my life" with nausea and vomiting. There may be a loss of consciousness. The ruptured aneurysm leads to a diffuse subarachnoid hemorrhage (blood over the surface of the brain below the arachnoid) (Figure 7-4A). The hemorrhage leads to contraction of cerebral vasculature (vasospasm) and decreased blood flow, which leads in turn to decreased oxygenation of the brain or cerebral ischemia. Rupture of a cerebral aneurysm is a medical emergency due to the significant morbidity and mortality. With the first rupture, 25% to 50% of patients will die. Some patients may regain consciousness while others will not.

Gross pathology of the aneurysm is a sac-like structure protruding from its artery. Histologically, the aneurysm wall is made up of a thickened, hyalinized intimal layer and adventitia. The media in the area of the aneurysm is absent (Figure 7-4B). The frequency of berry aneurysms varies by anatomic location (Figure 7-5).

While the majority of berry aneurysms are sporadic and not genetically transmitted, they may be associated with polycystic kidney disease (autosomal dominant polycystic kidney [ADPK] disease), Ehlers–Danlos and Marfan syndromes, neurofibromatosis type I, fibromuscular dysplasia, and coarctation of the aorta.