Cerebral aneurysms: an overview

Many asymptomatic aneurysms can be safely left alone, but there are some cases when intervention should be considered, writes Dr Jason Wenderoth

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A cerebral aneurysm is a bulge or a sac that develops in a brain artery because of a weakened blood vessel wall. It is estimated around 4% to 6% of the adult population has a brain aneurysm.

In the majority of cases it is not known what causes cerebral aneurysms. However, cigarette smoking is a recognised risk factor, for both development and rupture of aneurysms, and in a small percentage of patients, there is a strong family history of the condition suggesting a genetic predisposition.

Diseases such as Marfan’s syndrome or autosomal dominant polycystic kidney disease are also known to be associated with an increased risk of developing these aneurysms.

Aneurysms occurring elsewhere in the body, such as the aorta, are unrelated to brain aneurysms, and do not confer any increased risk of their development.

Intracranial aneurysms may present in one of three ways.

If they bleed they are likely to result in a subarachnoid haemorrhage. The patient will usually experience a sudden, severe “thunderclap” headache, loss of consciousness, and/or nausea and vomiting.

Occasionally, an aneurysm may cause neurological symptoms as a result of compression of a nerve or tissue. Such symptoms may include pain, weakness, numbness, or paralysis of one side of the face or body.

However, in most cases, cerebral aneurysms are discovered by accident – an incidental finding when imaging the brain for other reasons, or as part of a screening test in individuals with a strong family history of aneurysms.

Many asymptomatic aneurysms can be safely left alone and monitored. However, if the aneurysm is large, or growing, there is a risk of rupture, and intervention should be considered.

Assessment and treatment planning for brain aneurysms requires imaging (with MRI, CT or angiography) and specialist review.

The two common options available for the treatment of intracranial aneurysms are cranial surgery with aneurysm clipping or minimally invasive endovascular repair.

IS IT URGENT?

Aneurysms that have not ruptured very rarely pose an immediate risk to life.

This means that in almost all cases, patients do not need to go straight to hospital, or even to see a specialist urgently.

Upon diagnosis of an asymptomatic brain aneurysm, it is recommended that expert advice be sought to advise on the relative risk of conservative management versus intervention.

However, a ruptured aneurysm should be considered an emergency. In almost all cases of aneurysm rupture, the patient experiences a sudden onset of a thunderclap headache.

This is often associated with drowsiness or loss of consciousness, nausea and vomiting, neck stiffness and photophobia – general features of meningism. Patients with these symptoms require urgent evaluation at a hospital emergency department.

IMAGING AND SCREENING

While up to 6% of the adult population have at least one intracranial aneurysm, very few people are at significant risk of having the aneurysm rupture.

The incidence of subarachnoid hemorrhage caused by the rupture of a cerebral aneurysm is only eight per 100,000 adult population.

Patients need to be made aware that a positive finding of an aneurysm detected incidentally or on screening may not lead to active treatment.

In terms of screening, it is very important to determine, prior to any investigation, which patients are at increased risk of bleeding from intracranial aneurysms, and in whom the finding of an aneurysm may have serious consequences with regard to anxiety.

In general, there are six groups of patients who should be encouraged to undergo screening for intracranial aneurysms:

•   Patients with isolated thunderclap headache (but no other suspicious features e.g. meningism)
• Patients with acute onset third nerve palsy (especially if the pupil is involved)
• Patients who have had a previous aneurysm rupture and/or repair
• Patients with two or more first-degree relatives (parent, sibling, child) with intracranial aneurysms
• Patients with genetic conditions know to predispose to intracranial aneurysm (e.g. autosomal dominant polycystic kidney disease, mitral valve prolapse, Marfan’s syndrome)
• Patients who operate vehicles or equipment with the potential to harm others (eg. operators of public transport, commercial pilots)

In patients with a thunderclap headache, or a recent-onset third nerve palsy, imaging should be conducted relatively urgently, that is, within 24 to 48 hours.

The imaging test of choice for initial aneurysm screening in high-risk patients (those with a family history, suspicious symptoms or signs or known genetic associations) is a non-contrast CT of the brain followed by CT angiography of the Circle of Willis.

This should be done, wherever possible, at a location where expert diagnostic and/or interventional neuroradiologists are available to review the scans.

There are several reasons CT is the preferred modality for initial evaluation of intracranial aneurysms in high-risk patients.

Firstly, for most patients, CT is more widely available and accessible than MRI.

Secondly, CT is more sensitive and specific for the presence of subarachnoid haemorrhage than MRI in the acute phase (98% up to 48 hours).

Thirdly, CT angiography is more sensitive and specific for detection of small aneurysms than MR angiography (99% vs 93-95% in the hands of trained neuroradiologists).

Finally, there are fewer contraindications to CT than for MRI (pacemakers, claustrophobia, metallic implants et cetera).

MRI with MR angiography is usually satisfactory for initial screening of patients with relatively low risk.

Any patient in whom an aneurysm is identified should have a consultation with an interventional neuroradiologist or vascular neurosurgeon.

ANEURYSM TREATMENT

In deciding to treat an aneurysm there are a few key issues that must be considered.

These include:

• Whether the aneurysm has ruptured or is symptomatic
• The size, location and morphology of the aneurysm
• Whether the aneurysm is single or one of several
• The patient’s age, family history, smoking status and comorbidities
• The patient’s occupation – risk to the public

Ruptured, Symptomatic or Unruptured?

As a general rule, ruptured and symptomatic aneurysms should be treated as soon as possible. Ruptured aneurysms need to be treated within 24 hours of ictus. Symptomatic but unruptured aneurysms should be treated within three to seven days of symptom onset, since there is a high short-term risk of subsequent rupture.

Aneurysm size, location and morphology

The ISUIA and PHASES collaborations introduced some statistical rigour around which unruptured aneurysms may be safely monitored and which should be considered for treatment.

In general, in the absence of a family or personal history of aneurysm rupture, small aneurysms (<7mm) in the anterior circulation (except the posterior communicating artery origin) may be monitored without treatment, since their rupture risk is low (around 0% to 1% every five years).

Posterior communicating artery aneurysms and posterior circulation aneurysms should all be considered for treatment because of their higher risk of bleeding when compared with anterior circulation aneurysms.

Larger aneurysms (>7mm), and those with irregular morphology (multiple loculi, daughter aneurysms) are known to present a higher risk of rupture in the medium term, and should be considered for treatment, with the exception of extradural aneurysms (those outside the subrarachnoid space (e.g. In the cavernous sinus), which represent a very low risk to the patient.

Aneurysm number

If the patient has more than one aneurysm, even if these are individually considered low risk,  the patient is at a higher risk of morbidity from a subarachnoid haemorrhage.

The cumulative risk of rupture from multiple aneurysms means that treatment of one or more of the aneurysms may be advisable to reduce the overall risk of rupture.

Patient age, family history, smoking status and comorbidities

As with all invasive procedures, the decision to treat an intracranial aneurysm is largely determined by the patient’s life expectancy weighed against the risk of morbidity or mortality from their aneurysm.

In most cases, there is little to be gained in terms of life expectancy in treating unruptured aneurysms in patients over 75 years of age; for this reason, it is not advisable to initiate screening in patients or to conduct imaging follow-up of treated patients over 75.

Patients who are actively smoking have at least a two-fold risk of haemorrhage from their aneurysm if it is untreated, as well as a significantly greater risk of developing new aneurysms. These patients should be encouraged to cease smoking. In addition, smokers are at a higher risk from  general anaesthesia and perioperative complications.

Impact on the community

In some patients (e.g. operators of heavy machinery, firearm users, operators of public transport, pilots), the presence of an intracranial aneurysm may place the public at risk. While not mandatory, it may be considered worthwhile to screen such individuals for a brain aneurysm and, in those with a positive finding, to seek a risk assessment from an expert in the field.

A lower threshold for intervention may be considered for these individuals because of the increased risk of harm to the public should the aneurysm rupture in the course of their work duties. Identification and treatment of this subset of patients has the potential to reduce the risk of harm to the public and to mitigate corporate risk.

ENDOVASCULAR REPAIR

Today, the majority of brain aneurysms are treated via endovascular techniques (insertion of platinum coils into the aneurysm or reconstruction of the artery containing the aneurysm), with robust peer-reviewed evidence in the literature supporting these approaches.

Access to the cerebral arteries is usually gained through a 2mm to 3mm incision in the groin. A catheter is passed through the femoral artery into the relevant cervical vessel (carotid or vertebral artery) and angiography performed to outline the intracranial arterial anatomy and to delineate and characterise the aneurysm.

A microcatheter of around 1mm diameter is then advanced to the aneurysm through the first catheter. One or more soft platinum coils are passed through the catheter, positioned inside the aneurysm and detached so that they pack the aneurysm and block the blood flow into it. The coils initiate aneurysm thrombosis.

Simple coil occlusion may not be possible for aneurysms of certain sizes, shapes and locations.

In other cases, a stent can be placed permanently in the vessel across the aneurysm opening before or during coiling in order to hold the coils in place and to reconstruct the vessel. For these aneurysms, additional devices can be used to reconstruct the artery at the opening of the aneurysm in order to make coil placement possible.

In some cases, a balloon can be placed temporarily in the vessel next to the aneurysm and inflated and deflated to hold the coils inside the aneurysm.

Occasionally, a special type of stent, called a “flow diverting stent” can be inserted in the vessel to bypass the aneurysm without placing coils in the aneurysm at all.

While endovascular treatment is highly efficacious in the treatment of aneurysms, it is associated with certain risks. These risks and complications are usually less than those associated with craniotomy and clipping (open surgery), and are minimised by having the procedure performed by an experienced endovascular doctor.

A register of certified endovascular aneurysm doctors is available at: www.ccinr.org.au/register 

Some of the risks of endovascular aneurysm repair include:

• Thrombosis can occur inside the vessels that can embolise leading to stroke
• Rupture of the aneurysm when inserting devices
• Narrowing of the artery or vasospasm that may cause stroke
• Migration of the coils. Some coils may extend into the artery reducing the arterial diameter
• Incomplete filling of coils in the aneurysm allowing regrowth of aneurysm

OPEN SURGICAL REPAIR

Although the majority of cerebral aneurysms can be treated via endovascular techniques, some may require open repair.

In an open repair procedure, conducted under general anaesthetic, the surgeon accesses the brain through a small hole in the skull, gently displaces the brain near the aneurysm, and places one or more small clips across the opening to the aneurysm, ligating it from the rest of the circulation.

Risks associated with surgical aneurysm treatment include new or worsened neurologic deficits caused by brain retraction, temporary arterial occlusion, and intraoperative haemorrhage. Intraprocedural aneurysmal rupture occurred in 19% of 711 patients treated with surgical clipping in the CARAT study and was associated with periprocedural death and disability.

Procedure-related surgical complications occurred in 29 (20%) of 143 patients in a retrospective series, although functional outcome was good in 22 (76%) of those patients.

Dr Jason Wenderoth is an interventional neuroradiologist at Spectrum Medical Imaging where he works closely with a group of highly skilled endovascular aneurysm doctors at Sydney Neurointerventional Specialists (www.snis.com.au)