A Cerebral Aneurysm is a balloon-shaped bulge with a neck at its base, which originates from the weakening of the wall of an artery in the brain. As the aneurysm grows, it exerts pressure on the surrounding structures, causing, among other symptoms, headaches (headache) or vision problems, in addition to the possibility that it may rupture. When the aneurysm ruptures, it releases blood into the spaces surrounding the brain, which is called subarachnoid hemorrhage (SAH), an adverse cerebrovascular event that can be life-threatening.
According to studies, about 2% of the adult population in the US has intracranial aneurysms, representing an incidence of 4 million people. These aneurysms rupture causing intracranial hemorrhage in 25,000 to 30,000 patients each year.
Cerebral blood flow
To get a clear picture of aneurysms, it is necessary to understand the circulatory system of the brain. Blood reaches the brain through two paired arteries, the internal carotid arteries and the vertebral arteries. The internal carotid arteries carry blood to the anterior areas and the vertebral arteries supply blood to the posterior areas of the brain. Once they cross the skull, the right and left vertebral arteries join to give rise to the basilar artery. This artery and the internal carotid arteries communicate with each other forming a ring at the base of the skull called the Circle of Willis.
Aneurysms that have not ruptured and have no symptoms are less dangerous than an aneurysm that has recently ruptured. Aneurysms <10 mm have an annual probability of tearing of 0.1%, and those ≥10 mm of 0.5 – 1%. In young people, in whom the time of exposure to tearing is longer, preventive treatment to avoid aneurysm rupture is more beneficial.
Sites where aneurysms form
Approximately 80% of aneurysms form in the anterior part of the brain (anterior circulation), while the remaining 20% form in the posterior part of the brain (posterior circulation).
Types of Aneurysms
The main types of aneurysms include:
Saccular. This is the most common type and they usually form at the bifurcation of large caliber arteries located at the base of the brain. The aneurysm protrudes from one side of the artery and has a distinctive neck at its base.
Fusiform. The aneurysm protrudes in all directions and has no distinct neck.
Giant. Are those measuring > 2.5 cm in diameter; the neck of the aneurysm is usually wide and may involve more than one artery. They can be saccular or fusiform. They are usually located in the terminal part of the internal carotid artery, the bifurcation of the middle cerebral artery and in the upper part of the basilar artery. The risk of rupture of giant aneurysms is 6% in the first year of diagnosis, although they can remain the same size for a long time.
Traumatic. They are caused by a closed head injury or penetrating trauma.
Most aneurysms have no symptoms until they rupture. Ruptured aneurysms release blood into the spaces around the brain. This is called a subarachnoid hemorrhage (SAH). Unruptured aneurysms rarely show symptoms until they grow large enough to put pressure on surrounding vital structures. Rupture usually occurs while the patient is active rather than when asleep. At the time the aneurysm ruptures and intracranial hemorrhage occurs, intracranial pressure rises suddenly, accounting for the transient loss of consciousness seen in approximately 50% of patients.
In approximately 45% of cases, the first symptom is severe headache on exercise. Occasionally, tears are manifested at the onset by moderate headache or a change in the usual characteristics of the pain. The headache is usually generalized and is accompanied by neck stiffness and vomiting.
Signs and Symptoms of a ruptured aneurysm
Among the most frequent signs and symptoms of a ruptured aneurysm are:
1. Sudden onset of a severe headache (often described by the patient as «the worst headache of my life»).
2. Nausea and vomiting
3. Stiff neck
4. Transient loss of vision or consciousness
Some of the signs and symptoms of an unruptured aneurysm include:
-Pain above or behind the eyes
-New unexplained headaches (rare)
Causes of aneurysms
Studies have shown a strong link to family history. If a first-degree family member has had an aneurysm, the likelihood of having an aneurysm is four times greater. There are some inherited medical conditions associated with the occurrence of an aneurysm such as Ehlers-Danlos disease, Marfan syndrome, neurofibromatosis and polycystic kidney disease. Individuals with a family history of cerebral aneurysm may benefit from early diagnosis by MRI if they have suspicious symptoms.
In many people, an unruptured aneurysm may be discovered incidentally during a screening for some other medical condition. Diagnostic tests are used to determine the location, size, type and involvement of the aneurysm with other structures.
Imaging studies used to diagnose a cerebral aneurysm include:
Computed tomography angiography. It involves the use of a contrast medium to visualize the cerebral arteries. This type of study provides the best images of the brain’s blood vessels.
Magnetic resonance angiography. It allows detailed visualization of the cerebral circulation, as well as other brain structures by means of a magnetic field and radiofrequency waves. Although it does not require contrast medium, it also examines the cerebral arteries.
Risk of rupture
The risk of rupture of an aneurysm will be higher or lower, depending on the size and location; however, when a rupture occurs there is a 50% risk of death. Risk factors for rupture include smoking, high blood pressure, alcohol consumption, family history, atherosclerosis, oral contraceptive use and lifestyle. As for the size and location of the aneurysm; the larger they are, the higher the risk of rupture; as for location, aneurysms in the posterior circulation (basilar artery, vertebral and posterior communicating arteries) have a higher risk of rupture.
Treatment options include:
Sometimes the best treatment may be to simply watch and reduce the risk of rupture (stop smoking, control blood pressure). Unruptured aneurysms that are small and symptom-free can be observed with annual imaging scans until growth or the appearance of symptoms warrants a surgical procedure.
One of the most commonly used treatments for aneurysms is surgical clipping. Under general anesthesia, an opening is made in the skull, called a craniotomy. The brain is gently retracted so that the artery with the aneurysm can be located. The clip is placed on the neck of the aneurysm to prevent normal blood flow from entering the aneurysm. The clip is made of titanium and is permanently in place.
Arterial occlusion and bypass
If a surgical clip cannot be placed or the artery is too damaged, the neurosurgeon may completely block the artery that has the aneurysm. Blood flow is diverted around the occluded section of the artery by inserting a blood vessel. The graft is usually a small artery, usually taken from the leg that is connected above and below the occluded artery so that blood flow is diverted (bypassed or bridged) through the graft.
In contrast to surgery, another form of treatment is endovascular coiling, a procedure used to obstruct blood flow in an aneurysm. This intervention is performed in the angiography room by a neurointerventional physician and sometimes requires general anesthesia. In this procedure, a catheter (where the coil is placed) is inserted into an artery in the groin and then passed through the blood vessels to the aneurysm.
The physician guides the catheter through the bloodstream while watching a fluroscopy monitor (a type of x-ray). Through the catheter, the aneurysm is filled as much as possible with the coil, which is made of soft platinum and shaped like a spring, to help it coagulate and eventually be excluded from circulation.