An intracranial aneurysm ("cerebral aneurysm," "brain aneurysm") is a protruding bubble or sac on a brain artery that balloons out over time. Aneurysms have thin, weak walls and have a tendency to rupture causing hemorrhage into and around vital brain structures.

The actual incidence is difficult to estimate since not all aneurysms present to medical attention. Autopsy studies indicate a prevalence in the general population of approximately 5 percent. Only a fraction those aneurysms rupture.

The etiology of aneurysm formation is debated but is thought to be due to a small area of weakness in the blood vessel wall near a branch point of the blood vessel. A number of factors may contribute to aneurysm formation. These include:

• Congenital predisposition
• Arterial hypertensive disease
• Cigarette smoking
• Complications of blood infections
• Craumatic injury

Intracranial aneurysms most frequently present to medical attention because they bleed. Unfortunately the majority of aneurysms that rupture do so without any preliminary signs or symptoms. Aneurysm rupture is called Subarachnoid Hemorrhage (SAH), and is accompanied by an extremely severe headache, typically described as "the worst headache of my life." Other manifestations of aneurysm rupture include nuchal rigidity (stiff neck) photophobia (intolerance of bright light), nausea, vomiting, seizures, paralysis and loss of consciousness.

As many as 20 percent of aneurysms that go on to rupture present with a "warning leak" several days prior to SAH. In these patients this warning headache may be much milder.

The diagnosis of aneurysmal SAH should be strongly suspected in any individual presenting to the emergency department with the sudden onset of a severe headache.

Aneurysms can also cause symptoms without enlarging causing unusual headaches when they enlarge prior to rupture, or by compressing adjacent brain structures such as cranial nerves.

Patients may also present with small strokes or seizures.

A large number of patients are found to harbor incidental or unruptured aneurysms.

The advent of safe noninvasive imaging techniques such as MRI and MRA has recently led to increasing numbers of unruptured or incidental intracranial aneurysms coming to medical attention. Typically the scans are performed to evaluate unrelated conditions such as mild headaches, vertigo, sinusitis, mild head trauma, etc.

In contrast to the management of a recently ruptured aneurysm, management of an unruptured intracranial aneurysm is complicated by a number of factors. These factors can be broadly divided into those affecting the "natural history" (the likelihood of rupture of the aneurysm) and those affecting the risks of treatment.

Both the natural history and the treatment are influenced by patient factors such as age, sex, and coexisting medical conditions, aneurysm characteristics such as size, location, shape, symptoms and other factors such as the experience of the surgical team and the treating hospital.

Patients with ruptured aneurysms producing SAH are usually diagnosed by CT scan. However, no test is 100 percent sensitive, and CT scans may not always detect SAH, particularly when it is mild, or if it occurred more than 24 hours before the scan. Therefore, whenever a diagnosis of SAH is being entertained, if the CT scan is negative, a lumbar puncture (spinal tap) must be performed, analyzing the spinal fluid for blood or its byproducts ("xanthochromia").

All patients with a diagnosis of SAH, or in whom an aneurysm is suspected require 4-vessel cerebral angiography. Cerebral angiography is currently the only test sensitive enough to definitively confirm the presence of an aneurysm, and also provides critical information regarding the size, shape, and location of the aneurysm, as well as the presence of vasospasm.

For patients without recent SAH, the initial diagnostic test is usually an MRI (Magnetic Resonance Image), an MRA (Magnetic Resonance Angiogram), or a CTA (Computed Tomographic Angiogram. These noninvasive tests have become increasingly sensitive in detecting intracranial aneurysms, and are generally sufficient to decide if 4 vessel angiography is warranted. In certain situation noninvasive imaging can be used to make treatment decisions as well.

Aneurysms that have ruptured require treatment to prevent another rupture (re-bleeding). Following the initial rupture of an aneurysm, re-bleeding is very common (especially within the first two weeks after rupture), and is usually more severe than the initial rupture. Therefore, ruptured aneurysms need to be treated immediately to prevent this risk.

The two primary treatment methods are surgical clipping and endovascular coiling. The optimal treatment choice depends upon the patient's history, physical examination, age, risk factors, and the anatomical characteristics of the aneurysm.

Optimal treatment of patients with intracranial aneurysm requires a highly experienced center that is capable of both coiling and clipping, and one in which a cooperative environment permits joint evaluation of every patient.

Each patient's aneurysm should be reviewed prospectively by an experienced cerebrovascular surgeon and an experienced interventional neuroradiologist, such that the best treatment (coiling vs. clipping) is offered appropriately.