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Brain aneurysms are abnormal dilatations or outpunching of cerebral vessels which can cause a number of different symptoms in patients resulting from pressure effects on nerves or surrounding tissue and can also place patients at risk for hemorrhage in the brain with serious consequences.

Types of Aneurysms
Signs and Symptoms & Discovery on Imaging Studies
Treatment of Brain Aneurysms
Subarachnoid Hemorrhage from Aneurysms

© 2011 Swedish Neuroscience Institute

Types of Aneurysms

There are a number of types of aneurysms which affect the brain and may behave quite differently depending on the type, size, location, shape of each aneurysm and whether they have caused symptoms or not. Most aneurysms result from the disruption of the normal structure of the cerebral blood vessels which can occur from various inherited factors, or from factors which effect patients during their life and cause damage to the vessels resulting in aneurysm formation.

The major types of brain aneurysms include:

  • Saccular or Berry aneurysms
  • Fusiform aneurysms
  • Traumatic aneurysms
  • Infectious or Mycotic aneurysms

Saccular, or berry aneurysms are the most common types of aneurysms, and usually occur at predictable locations along the cerebral vasculature. They resemble a small berry or sac, and project off of the side of cerebral vessels, usually at branch points.

© 2011 Swedish Neuroscience Institute

These aneurysms may also be referred to in the medical or healthcare literature as "congenital aneurysms" or "congenital berry aneurysms", however these terms are based on information that is now known not to be accurate when it was believed that patients were born with aneurysms and they only ruptured later in life. In the past, before modern imaging techniques including magnetic resonance angiography (MRA) or computerized tomography angiography (CTA), aneurysms were only diagnosed with cerebral angiography, generally after they ruptured or caused symptoms and therefore were often not detected before rupture.

It is rare to find aneurysms in infants, children or adolescents, however they do occur in these age groups on occasion on a very infrequent basis. Saccular aneurysms are thought to arise from focal defects in the blood vessel wall combined with sheer stresses caused by the pulsation jets and turbulence which results from the blood flow within the vessels as it reaches branch points and significant angles and direction shifts.

Fusiform brain aneurysms are dilatations of the cerebral vessels that result in the loss of the normal tubular structure of the vessel and the appearance of significant irregular sac-like widening and sometimes elongation of the vessels. Fusiform aneurysm also are believed to be caused by structural defects in the blood vessel wall which can be caused by dissection, or tearing of the internal lining of the vessel wall resulting in the blood flow entering the wall of the blood vessel resulting in and enlargement of the space where the blood flow is contained.

Fusiform aneurysms can arise in patients with a normal vasculature, who may have environmentally induced stressful insults to the vessels, and also can arise with patients with collagen (fibrous material that holds body tissue together) vascular disease, who have genetic abnormalities which prevents them from forming and maintaining the normal structural elements of the blood vessel. Weakening of the structure of the blood vessels from a variety of causes can result in the blood pressure inside the vessel forcing the vessel wall apart to form fusiform aneurysms which resemble sac or sausage like dilatations of the vessels.

Traumatic aneurysms are rare and usually occur only after a vessel is directly injured by trauma. Head trauma is often classified as either closed head injury, where the covering of the brain is not violated, or as penetrating injury, where an object has penetrated through the covering of the brain and can directly injure the brain tissues and also the brain blood vessels.

Examples of penetrating injury include gunshot wounds to the head, stab wounds to the head, or compound depressed skull fractures where bone fragments can be driven into the brain and cause direct injury. Traumatic aneurysms are almost exclusively seen after penetrating injury and rarely occur following closed head injury. It is believed that direct injury to the vessel wall results in out pouching of the inner lumen or inside tubular structure of the vessel to cause the formation of traumatic aneurysms.

Infectious or mycotic aneurysms are most commonly seen after a blood stream infection where microorganisms are growing inside the vessel walls. This situation can occur with intravascular infections from a variety of causes, however the most common setting in which mycotic brain aneurysms are seen is in subacute bacterial endocarditis.

In this condition bacteria begin to grow and form vegetations, or small outgrowths of tissue on the leaflets of the heart valves. These vegetations or outgrowths can break off into the bloodstream and travel up into the cerebral circulation where they can lodge in the more peripherally located brain vessels. The bacteria can weaken the vessel wall and the vessel dilates and becomes a mycotic aneurysm.

Signs and Symptoms & Discovery on Imaging Studies

Signs and symptoms of brain aneurysms fall into several categories. The most common signs and symptoms are either from direct pressure effects from enlarging aneurysms on either the cranial nerves or other brain structures, or preparing to rupture resulting in a so called "warning leak". Occasionally larger aneurysms can undergo blood clot formation within them and cause transient ischemic attacks (warning signs of stroke), or actual strokes. In rare circumstances larger aneurysms can cause seizures by direct pressure effect on the brain.

In many cases, patients may present to medical attention with neurologic symptoms which are of uncertain significance, or symptoms such as light headedness or dizziness which may have nothing to do with an aneurysm, and an aneurysm may be discovered on either an MRI or CT which is ordered to evaluate the symptoms. If an aneurysm is found on an MRI or CT then more sensitive test called MRA, or magnetic resonance angiography, or CTA, computerized tomography angiography can be performed.

The gold standard test for detecting and defining the critical features of cerebral aneurysms is cerebral angiography where contrast material is directly injected in the proximal cerebral arteries through a trans femoral approach through the leg. X-rays are taken of the head with contrast material in the brain blood vessels, and 3D spin angiography is used to complement the x-ray images by performing computerized reconstructions of images obtained by filming the vascular structures as the x-ray tube is spun around in an arc around the patient.

© 2011 Swedish Neuroscience Institute

Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) refers to bleeding within the brain in the subarachnoid space, which is the space where the cerebrospinal fluid (CSF) circulates, over the surface of the brain but under the arachnoid membrane which is a cellophane like semi clear membrane that covers the brain and cerebral blood vessels. Cerebral aneurysms are the most frequent cause of spontaneous SAH, with arteriovenous malformations accounting for approximately 6%, Other causes of SAH include trauma, tumors, and vasculitis (inflammation of the blood vessels).

Many retrospective studies, which record symptoms in groups of patients that have aneurysms rupture, suggest that up to 40% of patients have “warning leaks” though some investigators would argue the true incidence is lower. Most occur 1–8 weeks before a major SAH and may not necessarily be seen on CT or lumbar puncture. Headache is usually milder but of similar nature and sudden-onset, lasting several days, and can be associated with nausea and vomiting, although stiff neck and light sensitivity are rare.

Patients who have experienced an SAH usually complain of an acute onset terrible headache described as a thunderclap headache, or the “worst headache of my life.” The headache is usually persistent, and is often accompanied by stiff neck and light sensitivity. SAH is often preceded in time (usually days or weeks) by a less severe headache, also of sudden onset, that clears within a 24 hour period and is referred to as a “warning headache” or “sentinel hemorrhage.” This event is thought to represent a small microscopic leak, or leak within the vessel wall from the aneurysm and a precursor to the full blown SAH.

A non-contrast head CT scan is the diagnostic test of choice to confirm the diagnosis of SAH. If performed within 24 hours of the SAH, the CT should be able to detect 90-95% of all SAH. The blood from SAH is usually located in the basilar cisterns, extending into the sylvian and/or interhemispheric fissure of the brain and can be accompanied by with variable amounts of intraventricular or intracerebral blood.

Lumbar puncture is reserved for patients who have a normal head CT despite a story suggestive of an SAH. Common reasons for a CT scan to be negative following SAH are, very minor hemorrhage, prolonged interval of over several days between the headache and the CT scan which may allow blood to diffuse away from the bleeding site and not be readily detectable on CT. Appropriate interpretation of the results of the cerebrospinal fluid obtained from lumbar puncture can make the diagnosis of SAH.

Treatment of Brain Aneurysms

There are four main options for treating patients with brain aneurysms: observation, craniotomy with clip ligation (“clipping” ) and endovascular occlusion using detachable coils (“coiling” ), or parent vessel ligation where the vessel leading up to an aneurysm is ligated or occluded. This can be done with either a clip surgically, or using an endovascular device.

All ruptured brain aneurysms are treated in patients with a reasonable chance of surviving, whereas incidentally discovered unruptured aneurysms may be either treated of observed depending on aneurysm and patient factors including aneurysm size and location and patient age and general health.  Observation includes routine periodic imaging and physician visits where these studies are reviewed.

Clipping of aneurysms requires open surgery using craniotomies performed by neurosurgeons under general anesthesia. The operating microscope is used to expose the aneurysm and allow safe clip placement. Permanent clips made from metals that are MRI-compatible are placed across the neck of the aneurysm, excluding it from the circulation.

© 2011 Swedish Nueroscience Institute

Endovascular occlusion or coiling is performed by either an interventional neuroradiologist, or a neurosurgeon or neurologist with interventional training. During coiling, the patient is most often placed under general anesthesia, though this is not always the case. Angiography is performed following which a microcatheter is navigated into the aneurysm and detachable coils of varying sizes and shapes are deployed within the aneurysm to decrease or eliminate filling of the aneurysm with blood. More recently intracranial stents have been utilized as an adjunct to coiling to prevent the coils from prolapsing into the parent vessel in aneurysms which have a wide neck.

© 2011 Swedish Neuroscience Institute