An aneurysm is a balloon-like dilatation that originates from an intracranial vessel.  The sight of aneurysm formation is most frequently where its wall is weakest specificaly in curves or bifurcation.  These saccular dilatations tend to origine from blood vessels found at the base of the skull.  Such parent vessels course within the subarachnoid space, located between one of the membranes covering the brain, the arachnoid, and the brain and basal cisterns. 

Aneurysms are often viewed as an acquired pathology with high blood pressure and cigarette smoking being two common risk factors.  Other less frequent acquired etiologies include trauma (traumatic aneurysms) and infection (mycotic aneurysm).  Furthermore, some conditions are recognized to be associated with cerebral aneurysms such as autosomal dominant polycystic kidney disease, fibromuscular dysplasia, connective tissue disorders, etc.

When two or more members of second degree or closer harbor an imaged aneurysm, they may constitute a familial aneurysm syndrome.

Symptoms
An aneurysm may be discovered incidentally while performing brain imaging for an unrelated symptom. 

One of the potentially devastating clinical presentations is a hemorrhage from the aneurysm. Up to 60% of patients may recall having had a headache in the weeks preceding the hemorrhage (refered to as a sentinel hemorrhage).  When an aneurysm ruptures, blood acutely fills the surrounding subarachnoid spaces and may spill to distant spaces and be termed diffuse.  The hemorrhage may also extend or be solely located in the ventricles (intraventricular hemorrhage) or  in the brain parenchyma (intraparenchymal hematoma).  Following an aneurysmal rupture, the patient often reports a severe headache of sudden onset refered to the worst headache of my life.  Nausea, Vomiting, Nuchal rigidity, sensitivity to light and noise might accompany this inaugural headache.  Depending on the severity and extent of the hemorrhage, focal neurological deficits as well as altered consciousness (up to a comatous state) may be noted. 

Aneurysms may also manifest without bleeding.  These symptomatic unruptured aneurysms may present with various presentations depending on their location and size.  Seizures, transient ischemic attacks or strokes and mass effect on surrounding structures may be accounted by the aneurysm.  More specifically, an aneurysm may exert mass effect on the adjacent brain parenchyma, brainstem and cranial nerves such as the optic nerve (visual loss) and the third cranial nerve (dilated pupil and diplopia).

Diagnosis
The sequence of evalution differs if a subarachnoid hemorrhage is suspected or not.

If a hemorrhage is suspected, a CT scanner without contrast must be done initially to document the presence of blood in the subarachnoid spaces and potentially in other locations (ventricles and/or parenchyma).  If CT scan does not show any blood or intracranial mass and the history is hightly suggestive of a hemorrhage, a lumbar punture should be performed to evalute if blood products are present in the cerebrospinal fluid.  If this exam is also negative, then pursuing further investigation remains to the treating physicans discretion.

When subarachnoid hemorrhage is confirmed (CT or Lumbar puncture), aneurysm imaging should be sought.  To date, many centers are performing high quality CT scans to image the vessels of the skull base and document aneurysms.  In other centers, an angiography remains the gold standard. 

For patients with symptomatic unruptured aneurysms, preferred imaging modality varies between centers depending on the quality of the CT scanner and MRI scanner.  Sometimes an angiography may be needed to detail the morphology of the aneurysm, the artery from which it arises and the distal branches and perforating arteries.  

In the presence of a cerebral aneurysm, the first question to consider is if the aneurysm requires treatment.  If the aneurysm is symptomatic with a subachnoid hemorrhage, the aneurysm must be treated.  Unruptured aneurysms that  are clinically manifest are generally considered for treatment as some studies showed that they were more at risk for a futur hemorrhage.  Regarding asymptomatic unruptured cerebral aneurysms, the recommandation of treatment takes into consideration numerous factors related to the aneurysm and to the patient.  A discussion with the treating physician is imperative regarding the benefits and risks of treatment.

When a decision to treat an aneurysm is taken, two modalities may be considered including endovascular treatment (embolization) and surgical treatment.

Regarding surgery, an adapted approach is required depending on the aneurysm.

Surgery usually implies putting a clip accross the balloon-like dilatation to prevent blood from entering it and therefore prevent hemorrhage.  Various craniotomies and skull base approaches enable the neurovascular surgeon to access the aneurysm with no damage to the brain and nerve tissue.  In some circumstances, a cerebral aneurysm may be treated through minimally invasive approaches such as a brow-incision craniotomies and endoscopic approches.  (describe this teams experience with such approches).