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Cerebral Aneurysms
Counselor,

Saccular aneurysms are rounded, berrylike outpouchings that arise from arterial bifurcation points. These are true aneurysms, that is, they are dilatations of a vascular lumen caused by weakness of all vessel wall layers. A normal artery wall consists of 3 layers: the intima, which is the innermost endothelial layer; the media, which consists of smooth muscle; and the adventitia, the outermost layer, which consists of connective tissue. The aneurysmal sac itself is usually composed only of intima and adventitia. The intima is typically normal, although subintimal cellular proliferation is common. Atherosclerotic changes in the parent vessel are also common.

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Etiology:

Most saccular or intracranial berry aneurysms were thought to be congenital in origin, arising from focal defects in the media and gradually developing over a period of years as arterial pressure first weakens and subsequently balloons out the vessel wall. Recent studies, however, have found evidence for developmental weakness of the arterial wall. Although genetic conditions are associated with increased risk of aneurysm development, most intracranial aneurysms probably result from hemodynamically induced degenerative vascular injury. The occurrence of intracranial saccular aneurysms can often be explained by abnormal hemodynamic shear stresses on the walls of large cerebral arteries, particularly at bifurcation points.
 
 

Epidemiology:

About 75% of patients with multiple intracranial aneurysms have 2 aneurysms, 15% have 3, and 10% have more than 3. A strong female predilection is observed with multiple aneurysms. While the overall female-to-male ratio is 5:1, the ratio rises to 11:1 in patients with more than 3 aneurysms. Multiple aneurysms are also associated with vasculopathies such as FMD and other connective tissue disorders. Polycystic kidney disease has a 10% incidence of associated aneurysms; these aneurysms are also often multiple. Aneurysms typically become symptomatic in people aged 40-60 years. Intracranial aneurysms are uncommon in children, accounting for fewer than 2% of all cases.
 

Location:

Aneurysms commonly arise at the bifurcations of major arteries. Most saccular aneurysms arise on the circle of Willis or the middle cerebral artery (MCA) bifurcation. Anterior circulation aneurysms: Approximately 85% of all intracranial aneurysms arise on the anterior (carotid) circulation. Common locations are the anterior communicating artery (30-35%), the internal carotid artery (ICA) at the posterior communicating artery origin (30-35%), and the MCA bifurcation (20%). Posterior circulation aneurysms: About 15% of all intracranial aneurysms arise on the posterior (vertebrobasilar) circulation. Five percent arise from the basilar artery bifurcation, and the remaining 1-5% arise from other posterior fossa vessels.
 

Clinical Presentation:

Most aneurysms do not cause symptoms until they expand, leak and rupture; when they rupture, they are associated with significant morbidity and mortality. The bleeding causes a subarachnoid hemorrhage (SAH). 80-90%% of SAH is due to ruptured aneurysms; 5-15% are due to arterio-venous malformations (AVM). On presentation, patients typically report experiencing the worst headache of their lives, often associated with a stiff neck (meningele signs), confusion, nausea and vomiting, near syncope or syncope and even seizures.

The most widely used clinical method for grading the clinical severity of SAH is the Hunt and Hess scale which grades the patient on a scale of 1 to 5 depending upon the clinical presentation with 1 being a minimal headache and slight neck stiffness to 5 being a deep terminal coma.

 

Clinical Outcome:

Vasospasm is the leading cause of disability and death from aneurysm rupture. Of patients with SAH, 10-15% die, 50% of whom die within a month, and 50% of survivors have neurological deficits. Ruptured aneurysms have their highest rebleeding rate within the first day; if untreated, at least 50% rebleed during the 6 months after the initial hemorrhage. Ultra- early referral, the earliest possible surgery, and aggressive anti-ischemic treatment (ie, antivasospastic drugs, intravascular volume expansion, transcranial doppler monitoring) gradually are improving the outcome.
 

Natural History:

The risk of aneurysm rupture is difficult to determine precisely but is estimated at 1-2% per year, cumulative, for asymptomatic lesions that have not yet ruptured. With a combined operative mortality rate and major morbidity risk of about 3.5% for aneurysm surgery performed by a skilled physician, recent conclusions are that any patient with a life expectancy of more than 3 years would benefit from surgical obliteration of an unruptured asymptomatic aneurysm.

Ruptured aneurysms that are not operated on have a very high risk of rebleeding after the initial hemorrhage has occurred. The risk is estimated at 20-50% in the first 2 weeks, and such rebleeding carries a mortality rate of nearly 85%.

 

Diagnosis by Imaging:

CT scans and MRIs have excellent sensitivity in diagnosing ruptured aneursyms, but are not perfect, especially CT scans. When the diagnosis is suspected by clinical suspicions, and a CT scan is negative, the accepted standard of practice is to perform a lumbar puncture to assess for the presence of subarachnoid blood. The gold standard for delineating the location of the aneurysm is arteriography.
 

Treatment Considerations:

After an aneurysm is demonstrated arteriographically, the neurosurgeon must decide how and when to obliterate the aneurysm. In the earlier days of aneurysm treatment, surgery was delayed until the second or third week after the initial hemorrhage to avoid difficulty related to a swollen brain during surgery. Although this lowered surgical morbidity and mortality rates, management results were not always good because of a high incidence of rebleeding and difficulty in managing vasospasm. For this reason, most authorities advocate early surgical intervention within the first 48 hours after hemorrhage, especially in patients with good neurological grade. But before surgery is undertaken several other measures must first be taken:

All patients should be placed on calcium channel blockers (nimodipine for 21 d) on admission to prevent and treat vasospasm. All patients should be treated aggressively with HHH (hypertension, hypervolemia, hemodilution) therapy if vasospasm is suspected. This remains the most important aspect of the medical management of vasospasm, but, in refractory cases where medical management fails, endovascular methods with transluminal balloon angioplasty or intra-arterial papaverine should be used.

Lastly, blood in the subarachnoid space often obliterates the arachnoidal villi and can cause acute hydrocephalus, which can lead to neurologic worsening because of the raised intracranial pressure (ICP). In this situation, an immediate intraventricular catheter with CSF drainage should be placed. Not only can this be lifesaving, but also a patient's neurologic examination can improve dramatically after the hydrocephalus has been treated.

There are numerous surgical possibilities. The first is surgical clipping. The goal of this treatment is to place a clip across the neck of the aneurysm to exclude the aneurysm from the circulation without occluding normal vessels. When the aneurysm cannot be clipped because of the nature of the aneurysm or poor medical condition of the patient, other techniques can be used including wrapping, trapping, and proximal ligation. The goal at surgery is to obliterate the aneurysm from the normal circulation without compromising any of the adjacent vessels or small perforating branches of these vessels. The clips are manufactured in various types, shapes, sizes, and lengths and are usually MRI compatible. The operative mortality rate is less than 5% with an experienced physician. After surgery, good clip placement and obliteration of the aneursym should be confirmed by repeating the angiogram. With successful clipping, the rebleeding rate is negligibly low. However, after a successful surgery, the patient remains at significant risk for vasospasm and hydrocephalus for at least 7-10 days. Vasospasm is narrowing that can lead to delayed ischemia characterized by a sudden or gradual deterioration in mental status or the presence of focal neurologic deficits. If suspected, an angiogram should be done to confirm the diagnosis. Once diagnosed it should be aggressively managed because it can cause permanent disability or death.

The second surgical intervention is broadly called "endovascular surgery" and has come into vogue within the past decade. This involves direct obliteration of the aneurysmal lumen by microcoils. The most common coils used in endovascular procedures are platinum Guglielmi detachable coils (GDC). The purpose of the coil is to induce thrombosis at the site of deployment via electrothrombosis. Electrothrombosis occurs because white and red blood cells, platelets, and fibrinogen are negatively charged. If a positively charged electrode is placed in the bloodstream, it attracts these negatively charged blood components, promoting clot formation. The procedure involves using a microcatheter with 2 radiopaque markers which is advanced into the aneurysm cavity. Coils of decreasing sizes are delivered into the aneurysm cavity and electrolytically detached. Angiograms are obtained before detaching each coil to ensure preservation of the parent vessel. This process is continued until maximal angiographic obliteration of the aneurysm cavity is achieved.

 

Choice of Surgical Technique:

Whether to obliterate an aneurysm surgically through a craniotomy and clipping or to use endovascular methods is a decision made by the neurosurgeon and the endovascular radiologists as a team based on which approach best suits each patient's aneurysm. The general consensus today is that treatment depends on the age of the patient and the location of the aneurysm. Younger patients tend to undergo surgical clipping because coiling has a high recurrence rate. Posterior fossa aneurysms (especially the basilar artery tip) tend to be treated using the coil procedure. In most major aneurysm centers, most cases are still obliterated by surgical clipping, but coiling is being used more frequently.
 
 

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AMFS, Inc.
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