| Intracranial aneurysm | |
|---|---|
| Other names | Cerhttps://en.wikipedia.org/wiki/Intracranial_aneurysmebral aneurysm |
 | |
| Aneurysm of the basilar artery, and the vertebral arteries. | |
| Specialty | Interventional neuroradiology, neurosurgery  |
Intracranial aneurysm, also known as brain aneurysm, is a cerebrovascular disorder in which weakness in the wall of a cerebral artery or vein causes a localized dilation or ballooning of the blood vessel.
Aneurysms in the posterior circulation (basilar artery, vertebral arteries and posterior communicating artery) have a higher risk of rupture. Basilar artery aneurysms represent only 3–5% of all intracranial aneurysms but are the most common aneurysms in the posterior circulation.
Classification
Diagram of cerebral aneurysm.
Cerebral aneurysms are classified both by size and shape. Small
aneurysms have a diameter of less than 15 mm. Larger aneurysms include
those classified as large (15 to 25 mm), giant (25 to 50 mm), and
super-giant (over 50 mm).
Saccular aneurysms
Saccular
aneurysms, also known as berry aneurysms, appear as a round outpouching
and are the most common form of cerebral aneurysm.
Fusiform aneurysms
Fusiform dolichoectatic
aneurysms represent a widening of a segment of an artery around the
entire blood vessel, rather than just arising from a side of an artery's
wall. They can rupture but usually do not.
Microaneurysms
Microaneurysms, also known as Charcot-Bouchard aneurysms, typically occur in small blood vessels (less than 300 micrometre diameter), most often the lenticulostriate vessels of the basal ganglia, and are associated with chronic hypertension.[3] Charcot–Bouchard aneurysms are a common cause of intracranial hemorrhage.
Signs and symptoms
A
small, unchanging aneurysm will produce few, if any, symptoms. Before a
larger aneurysm ruptures, the individual may experience such symptoms
as a sudden and unusually severe headache, nausea, vision impairment, vomiting, and loss of consciousness, or the individual may experience no symptoms at all.
Subarachnoid bleed
If an aneurysm ruptures, blood leaks into the space around the brain. This is called a subarachnoid hemorrhage. Onset is usually sudden without prodrome, classically presenting as a "thunderclap headache" worse than previous headaches. Symptoms of a subarachnoid hemorrhage differ depending on the site and size of the aneurysm. Symptoms of a ruptured aneurysm can include:
- a sudden severe headache that can last from several hours to days
- nausea and vomiting
- drowsiness, confusion and/or loss of consciousness
- visual abnormalities
- meningism
Almost all aneurysms rupture at their apex. This leads to hemorrhage
in the subarachnoid space and sometimes in brain parenchyma. Minor
leakage from aneurysm may precede rupture, causing warning headaches.
About 60% of patients die immediately after rupture. Larger aneurysms have a greater tendency to rupture, though most ruptured aneurysms are less than 10 mm in diameter.
The risk of a subarachnoid hemorrhage is greater with a saccular aneurysm than a fusiform aneurysm.
Microaneurysms
A ruptured microaneurysm may cause an intracerebral hemorrhage, presenting as a focal neurological deficit.
Rebleeding, hydrocephalus (the excessive accumulation of cerebrospinal fluid), vasospasm
(spasm, or narrowing, of the blood vessels), or multiple aneurysms may
also occur. The risk of rupture from a cerebral aneurysm varies
according to the size of an aneurysm, with the risk rising as the
aneurysm size increases.
Vasospasm
Vasospasm,
referring to blood vessel constriction, can occur secondary to
subarachnoid hemorrhage following a ruptured aneurysm. This is most
likely to occur within 21 days and is seen radiologically within 60% of
such patients. The vasospasm is thought to be secondary to the apoptosis
of inflammatory cells such as macrophages and neutrophils
that become trapped in the subarachnoid space. These cells initially
invade the subarachnoid space from the circulation in order to phagocytose
the hemorrhaged red blood cells. Following apoptosis, it is thought
there is a massive degranulation of vasoconstrictors, including endothelins and free radicals, that cause the vasospasm.
Risk factors
Intracranial aneurysms may result from diseases acquired during life, or from genetic conditions. Lifestyle diseases including hypertension, smoking, excessive alcoholism, and obesity are associated with the development of brain aneurysms. Cocaine use has also been associated with the development of intracranial aneurysms.
Other acquired associations with intracranial aneurysms include head trauma and infections.
Genetic associations
Coarctation of the aorta is also a known risk factor, as is arteriovenous malformation. Genetic conditions associated with connective tissue disease may also be associated with the development of aneurysms. This includes:
- autosomal dominant polycystic kidney disease,
- neurofibromatosis type I,
- Marfan syndrome,
- multiple endocrine neoplasia type I,
- pseudoxanthoma elasticum,
- hereditary hemorrhagic telangiectasia and
- Ehlers-Danlos syndrome types II and IV.
Specific genes have also had reported association with the development of intracranial aneurysms, including perlecan, elastin, collagen type 1 A2, endothelial nitric oxide synthase, endothelin receptor A and cyclin dependent kinase inhibitor. Mutations in interleukin 6 may be protective. Recently, several genetic loci
have been identified as relevant to the development of intracranial
aneurysms. These include 1p34-36, 2p14-15, 7q11, 11q25, and
19q13.1-13.3.
Pathophysiology
Aneurysm means an outpouching of a blood vessel
wall that is filled with blood. Aneurysms occur at a point of weakness
in the vessel wall. This can be because of acquired disease or
hereditary factors. The repeated trauma of blood flow against the vessel
wall presses against the point of weakness and causes the aneurysm to
enlarge. As described by the Law of Young-Laplace, the increasing area increases tension against the aneurysmal walls, leading to enlargement.
Both high and low wall shear stress
of flowing blood can cause aneurysm and rupture. However, the mechanism
of action is still unknown. It is speculated that low shear stress
causes growth and rupture of large aneurysms through inflammatory
response while high shear stress causes growth and rupture of small
aneurysm through mural response (response from the blood vessel wall).
Other risk factors that contributes to the formation of aneurysm are:
cigarette smoking, hypertension, female gender, family history of
cerebral aneurysm, infection, and trauma. Damage to structural integrity
of the arterial wall by shear stress causes an inflammatory response
with the recruitment of T cells, macrophages, and mast cells. The inflammatory mediators are: Interleukin 1 beta, Interleukin 6, Tumor necrosis factor alpha (TNF alpha), MMP1, MMP2, MMP9, prostaglandin E2, complement system, reactive oxygen species (ROS), and angiotensin II. On the other hand, smooth muscle cells from the tunica media layer of the artery moved into the tunica intima,
where the function of the smooth muscle cells changed from contractile
function into pro-inflammatory function. This causes the fibrosis of the
arterial wall, with reduction of number of smooth muscle cells,
abnormal collagen synthesis, resulting in thinning of arterial wall and
formation of aneurysm and rupture. On the other hand, no specific gene
loci has been identified to be associated with cerebral aneurysm.
Generally, aneurysms larger than 7 mm in diameter should be
treated because they are prone for rupture. Meanwhile, aneurysms less
than 7 mm arises from anterior and posterior communicating artery are more easily ruptured when compared to aneurysms arising from other locations.
Saccular aneurysms
The most common sites of intracranial saccular aneurysms.
Saccular aneurysms are almost always the result of hereditary
weakness in blood vessels and typically occur within the arteries of the
Circle of Willis, in order of frequency affecting the following arteries:
- Anterior communicating artery
- Posterior communicating artery
- Middle cerebral artery
- Internal carotid artery
- Tip of basilar artery
Saccular aneurysms tend to have a lack of tunica media
and elastic lamina around its dilated location (congenital), with wall
of sac made up of thickened hyalinized intima and adventitia. In addition, some parts of the brain vasculature are inherently weak—particularly areas along the Circle of Willis,
where small communicating vessels link the main cerebral vessels. These
areas are particularly susceptible to saccular aneurysms. Approximately 25% of patients have multiple aneurysms, predominantly when there is familial pattern.
Diagnosis
CT angiography showing aneurysm measuring 2.6 mm in diameter at the ACOM (anterior communicating artery).
Once suspected, intracranial aneurysms can be diagnosed radiologically using magnetic resonance or CT angiography.
But these methods have limited sensitivity for diagnosis of small
aneurysms, and often cannot be used to specifically distinguish them
from infundibular dilations without performing a formal angiogram. The determination of whether an aneurysm is ruptured is critical to diagnosis. Lumbar puncture (LP) is the gold standard technique for determining aneurysm rupture (subarachnoid hemorrhage). Once an LP is performed, the CSF is evaluated for RBC count, and presence or absence of xanthochromia.
Treatment
Emergency treatment for individuals with a ruptured cerebral aneurysm generally includes restoring deteriorating respiration and reducing intracranial pressure. Currently there are two treatment options for securing intracranial aneurysms: surgical clipping or endovascular coiling.
If possible, either surgical clipping or endovascular coiling is
typically performed within the first 24 hours after bleeding to occlude
the ruptured aneurysm and reduce the risk of rebleeding.
While a large meta-analysis
found the outcomes and risks of surgical clipping and endovascular
coiling to be statistically similar, no consensus has been reached. In particular, the large randomised control trial International Subarachnoid Aneurysm Trial
appears to indicate a higher rate of recurrence when intracerebral
aneurysms are treated using endovascular coiling. Analysis of data from
this trial has indicated a 7% lower eight-year mortality rate with
coiling, a high rate of aneurysm recurrence in aneurysms treated with coiling—from 28.6-33.6% within a year, a 6.9 times greater rate of late retreatment for coiled aneurysms, and a rate of rebleeding 8 times higher than surgically-clipped aneurysms.
Surgical clipping
Aneurysms can be treated by clipping the base of the aneurysm with a
specially-designed clip. Whilst this is typically carried out by craniotomy, a new endoscopic endonasal approach is being trialled. Surgical clipping was introduced by Walter Dandy of the Johns Hopkins Hospital in 1937.
After clipping, a catheter angiogram or CTA can be performed to confirm complete clipping.
Endovascular coiling
Endovascular coiling refers to the insertion of platinum coils into the aneurysm. A catheter is inserted into a blood vessel, typically the femoral artery,
and passed through blood vessels into the cerebral circulation and the
aneurysm. Coils are pushed into the aneurysm, or released into the blood
stream ahead of the aneurysm. Upon depositing within the aneurysm, the
coils expand and initiate a thrombotic reaction within the aneurysm. If
successful, this prevents further bleeding from the aneurysm. In the case of broad-based aneurysms, a stent may be passed first into the parent artery to serve as a scaffold for the coils.
Cerebral bypass surgery
Cerebral bypass surgery was developed in the 1960s in Switzerland by Gazi Yasargil,
M.D. When a patient has an aneurysm involving a blood vessel or a tumor
at the base of the skull wrapping around a blood vessel, surgeons
eliminate the problem vessel by replacing it with an artery from another
part of the body.
Prognosis
Outcomes depend on the size of the aneurysm. Small aneurysms (less than 7 mm) have a low risk of rupture and increase in size slowly. The risk of rupture is less than a percent for aneurysms of this size.
The prognosis
for a ruptured cerebral aneurysm depends on the extent and location of
the aneurysm, the person's age, general health, and neurological
condition. Some individuals with a ruptured cerebral aneurysm die from
the initial bleeding. Other individuals with cerebral aneurysm recover
with little or no neurological deficit. The most significant factors in
determining outcome are the Hunt and Hess grade,
and age. Generally patients with Hunt and Hess grade I and II
hemorrhage on admission to the emergency room and patients who are
younger within the typical age range of vulnerability can anticipate a
good outcome, without death or permanent disability. Older patients and
those with poorer Hunt and Hess grades on admission have a poor
prognosis. Generally, about two-thirds of patients have a poor outcome,
death, or permanent disability.
Epidemiology
The prevalence of intracranial aneurysm is about 1-5% (10 million to 12 million persons in the United States) and the incidence
is 1 per 10,000 persons per year in the United States (approximately
27,000), with 30- to 60-year-olds being the age group most affected. Intracranial aneurysms occur more in women, by a ratio of 3 to 2, and are rarely seen in pediatric populations.
