On the origin, natural history and treatment of cerebral aneurysms: A brain aneurysm starts as a focal point of weakness on an arterial blood vessel. This area is exposed to the stresses of normal arterial blood pressure which expand the aneurysm in size over time usually over many years. Aneurysms typically form in the part of the artery that is exposed to turbulent blood flow-the branch-points of arteries (figure 1) and have an association with a history of smoking and high blood pressure. As aneurysms grow over time, they have the capacity to rupture, resulting in bleeding around or under the brain (figure 2), within the brain (Figure 3), or within the fluid filled cavitiesventricles of the brain (IVH) (Figure 4).
Cerebral angiogram AP view of a basilar tip aneurysm. White arrows indicate vector of blood flow. Yellow circle identifies the aneurysm neck and red line demarcates aneurysmal dome and origin of bleeding.
Left: Axial CT images show subarachnoid hemorrhage in the basal cisterns (white arrows). Right: Patient two weeks after event with reabsorption of blood in the subarachnoid space.
Axial CT shows intracerebral hemorrhage originating from a ruptured middle cerebral artery aneurysm.
Axial CT shows intraventricular hemorrhage (white signal) localized to the right lateral ventricle (white arrows). Blood originates from aneurysmal intracerebral hemorrhage which has dissected into the lateral ventricle (black arrow)
Brain aneurysms are often detected incidentally before they have ruptured or have caused symptoms. Larger unruptured aneurysms, however, may cause neurological problems as a result of physical distortion of surrounding critical neural structures. Diagnosis is typically established by either CT angiography or MR angiography and may be further confirmed and defined with cerebral angiography. The treatment of asymptomatic, unruptured aneurysms is based on a careful consideration of the risks associated with future rupture counterbalanced with the risks associated with treatment. Patient age, general medical condition, location of the aneurysm, aneurysm size, and growth of the aneurysm over time are some of the factors that inform the surgeon's recommendation.
Ruptured Aneurysms and Subarachnoid Hemorrhage
Bleeding or leakage from a ruptured aneurysm most commonly develops underneath the brain where cerebral blood vessels emerge from the base of the skull. This area known as the subarachnoid space is a fluid filled enclosure that allows the brain to be suspended in the confines of the cranial cavity. Bleeding in this area is known as subarachnoid hemorrhage and is an extremely serious condition that is associated with a substantial possibility of immediate mortality. Other symptoms that imply the diagnosis include severe headache (often described as 'the worst headache of my life'), vomiting, fainting, seizures, double vision, lethargy, paralysis, stroke-like symptoms and coma. Bleeding typically occurs in discrete self-limiting episodes; patient management therefore focuses first and most importantly eliminating the possibility of rebleeding. It is well recognized that rebleeding may cause death or further decline in function and occurs in up to 20% of patients within the first two weeks and in 50% of patients within the first six months after initial ictus.
Bleeding risk is addressed by excluding the flow of blood into the aneurysmal sac from the parent artery (Figure 1) and may be adressed either a by surgical operation, craniotomy with application of a titanium clip (Figure 5) across the base of the aneurysm (Figure 6), or by endovascular embolization (Figure 7, 8 ) where the aneurysm is packed and excluded from the circulation with fine titanium fibrils or, coils (Endovascular Case Study 2B) or a flow-diverting stent (Figure 9).
Aneurysm clips are manufactured in many different shapes and sizes to accommodate complex vascular anatomy.
Intra-operative photo showing aneurysm in place securing a posterior communicating artery aneurysm.
Left: Cerebral angiogram demonstrating direction of carotid artery bloodflow (white arrows), location of aneurysmal neck (yellow hashed line) and turbulent blood flow within aneurysm (red arrows) dome (perimeter marked by red line). Right: Aneurysm has been treated with a full and complete packing of platinum coils. Coils do not extend beyond aneurysm neck (yellow hashed line) and there is no obstruction or narrowing of parent vessel. Notice that dome of aneurysm no longer fills with contrast dye. A microcatheter in the carotid artery is indicated (white arrows).
Cerebral angiogram demonstrating effective embolization of anterior cerebral artery aneurysm.
Endovascular aneurysm stent.
Other complications that may develop in the patient's clinical course are referable to the inflammatory (vasospasm) and obstructive (hydrocephalus) effect of blood in the subarachnoid space and the patient's physiological response to stress including (cardiac ischemia, pulmonary edema). These complications may develop days or weeks after what may have been a successful operation to prevent rebleeding; prevention of permanent complications requires the vigilance and prompt action of the neurosurgical team.
Brain aneurysms sometimes run in the family. If there are more than two first-degree relatives (i.e. brother, sister, parents, children) who have a brain aneurysm, there is approximately a 25% risk that another family member also has a brain aneurysm. There are also genetic disorders that are associated with an increased risk of developing a brain aneurysm. These genetic disorders include: autosomal recessive polycystic kidney disease, fibromuscular dysplasia, Ehlers-Danlos syndrome and Marfan syndrome. Patients with this type of medical history are usually recommended to undergo MRI or CT angiography to screen for aneurysms.
These aneurysms develop as a result of an infection in the blood vessel wall from either a fungal or bacterial organism. Typically, mycotic aneurysms develop in cerebral vessels that are on the surface of the brain thereby resulting in a bleeding pattern that differs from saccular aneurysms. These lesions may be treated surgically, with endovascular techniques although healing may be achieved with antibiotic therapy alone.