Aneurysms are blister-like dilatations that may develop on any blood vessel in the body. When they occur on the arteries supplying blood to the brain they are termed cerebral aneurysms or berry aneurysms, a reference to an early description of them resembling berries on a stalk. Some aneurysms rupture and bleed resulting in a disease called aneurysmal subarachnoid haemorrhage (aSAH). Not every aneurysm will bleed however. Aneurysmal SAH affects about 9 in every 100,000 people each year.
Aneurysms Come in Different Shapes and Sizes
Aneurysms may broadly be described as saccular or fusiform. Most intracranial aneurysms are saccular meaning they resemble a bag or balloon hanging off the side of the blood vessel. For the same reason they are sometimes described as berry aneurysms. Such aneurysms will most often develop at branch points of normal arteries at the base of the brain.
A rarer variety of aneurysm is the Blood Blister Like Aneurysm (BBLA). These characteristically develop on the sidewall of vessels, most often the anterior surface of the internal carotid artery. These are aggressive lesions highly prone to bleeding as they lack elements of the blood vessel wall leaving only a tenuous covering of a hole in the artery.
Fusiform aneurysms are so called because the artery dilates circumferentially. That is more characteristic of aneurysm that result from tearing int the vessel wall (dissection).
An aneurysm in the cranial cavity will be termed giant when they exceed 25mm in size. The risk of both spontaneous rupture and treatment increase as aneurysms grow larger. Giant aneurysms are distinguished by their especially adverse natural history (i.e. what occurs if they are left untreated) and may behave as brain tumours can do by exerting pressure effects against the normal structures of the brain
Why Do Aneurysms Develop?
The most common type of saccular aneurysm probably reflects a wear and tear phenomenon in the wall of the blood vessel. "Hardening" of the arteries (atherosclerosis) contributes to this and the factors associated with such hardening are more commonly seen in people with cerebral aneurysms.
The strongest associations are tobacco smoking and uncontrolled elevation in blood pressure. Stopping smoking and treating hypertension reduce the likelihood of aneurysm formation and rupture.
There are systemic diseases where brain aneurysms are found more commonly. These include adult polycystic kidney disease (APCKD), sickle cell disease and diseases of the body's collagen e.g. Marfan's disease. Infections of the heart valves (Endocarditis)can cause small pockets of infection to spread through the blood stream and into the walls of the arteries in the brain. These pockets can weaken the wall of the artery causing aneurysms to form. Traumatic injuries to the blood vessels e.g. stab wounds to the head or injuries to vessels during surgery on the brain or sinuses are very rare causes of certain types of aneurysms.
Associated Diseases
There are systemic diseases where brain aneurysms are found more commonly. These include adult polycystic kidney disease (APCKD), sickle cell disease and diseases of the body's collagen e.g. Marfan's disease or Ehlers Danlos Syndrome. Infections of the heart valves (Endocarditis)can cause small pockets of infection to spread through the blood stream and into the walls of the arteries in the brain. These pockets can weaken the wall of the artery causing aneurysms to form.
Traumatic injuries to the blood vessels e.g. stab wounds to the head or injuries to vessels during surgery on the brain or sinuses are very rare causes of certain types of aneurysms.
Genetic Susceptibility
Nonetheless saccular aneurysms do develop in people without these risk factors and it Is recognised that an individual's genetic make -up, defining them as an individual, may render some more vulnerable to developing aneurysms than others.
However it is relatively uncommon for that vulnerability to passed on to the next generation. Current data suggest that a person is no more likely to harbour a brain aneurysm than the average unless two or more first-degree relatives (Mother/Father/Brother/Sister/Son/Daughter) have a brain aneurysm.
Consequently UK guidelines are that screening for cerebral aneurysms need not be undertaken unless two or more first-degree family members have been affected.
How Common Are Brain Aneurysms?
Studies suggest that unruptured aneurysms are present in about 3.2% of the population- or more than one person in fifty. Only a tiny minority of those will ever cause bleeding- approximately 0.25% of those are believed to ever rupture.
This therefore accounts for about 3-5% of stroke in the United Kingdom annually but is relatively more frequent in younger people than other types of stroke, most often between 20 and 50 years.
20-30% of people diagnosed with an aneurysm have more than one. Less than 5% occur in children and they are more commonly found in women.
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