There are several types of brain aneurysms classified by shape. These are saccular (also known as berry aneurysms), which are by far the most common. The less common fusiform and dissecting types. Brain aneurysms may also be classified by size (small, large, and giant), by their location, or by the cause such as a mycotic aneurysm (caused by an infection). Most aneurysms occur where an artery branches, often at the base of the brain. The treatment options vary, depending on the type of aneurysm, its size, and its location.