Are berry aneurysms in the circle of Willis more commonly single or multiple?

• A. Single
• B. Neither; both equally common
• C. Multiple
• D. Cannot be determined

Answer: (C)

Berry aneurysms (saccular aneurysms) form most commonly at branch points in the circle of Willis due to defects in the media and elastic tissue that line the vessel wall. They can present as a single outpouching, but they are not rare in multiple; a substantial minority of patients with intracranial berry aneurysms have two or more lesions. Estimates in practice place the proportion of patients with multiple aneurysms in the range of roughly a quarter to a third, and this tendency is especially noted in certain conditions such as polycystic kidney disease and some connective tissue disorders. Because multiple sites can be involved, finding one aneurysm should prompt careful evaluation of the entire circle of Willis for additional aneurysms. This is why multiple is the best answer: berry aneurysms in the circle of Willis are more commonly found as multiple lesions than as a single one.