In the US, brain aneurysms occur in 3 to 5% of people. Brain aneurysms can occur at any age but are most common among people aged 30 to 60 years. They are more common among women than in men.
Common contributing factors for aneurysms may include
Hereditary connective tissue disorders Introduction to Connective Tissue Disorders in Children There are over 200 disorders that involve connective tissue. Certain disorders are characterized by overactivity of the immune system with resulting inflammation and systemic damage to the tissues... read more (eg, Ehlers-Danlos syndrome, pseudoxanthoma elasticum, autosomal dominant polycystic kidney syndrome)
Family history of aneurysm (first-degree relative: parent, sibling, or child)
Cigarette smoking
Occasionally, septic emboli cause mycotic aneurysms.
Brain aneurysms are most often < 2.5 cm in diameter and saccular (noncircumferential); sometimes they have one or more small, thin-walled, outpouchings (berry aneurysm).
Most brain aneurysms occur along the middle or anterior cerebral arteries or the communicating branches of the circle of Willis, particularly at arterial bifurcations. Mycotic aneurysms usually develop distal to the first bifurcation of the arterial branches of the circle of Willis.
Symptoms and Signs of Brain Aneurysms
Many aneurysms are asymptomatic, but a few, usually large or growing aneurysms, cause symptoms by compressing adjacent structures. Ocular palsies, diplopia, squint, or orbital pain may indicate pressure on the 3rd, 4th, 5th, or 6th cranial nerves. Visual loss and a bitemporal field defect may indicate pressure on the optic chiasm.
Brain aneurysms may bleed into the subarachnoid space, causing subarachnoid hemorrhage Subarachnoid Hemorrhage (SAH) Subarachnoid hemorrhage is sudden bleeding into the subarachnoid space. The most common cause of spontaneous bleeding is a ruptured aneurysm. Symptoms include sudden, severe headache, usually... read more 
. Before rupture, aneurysms occasionally cause sentinel (warning) headaches due to painful expansion of the aneurysm or to blood leaking into the subarachnoid space. Actual rupture causes a sudden severe headache called a thunderclap headache. A ruptured aneurysm may also cause nausea, vomiting, a stiff neck, photosensitivity, loss of consciousness, and/or seizures.
Diagnosis of Brain Aneurysms
Neuroimaging
Neuroimaging may detect aneurysms incidentally.
Diagnosis of aneurysms requires angiography, CT angiography, or magnetic resonance angiography.
If a mycotic aneurysm is suspected, bacterial and fungal blood cultures should be done.
Treatment of Brain Aneurysms
For smaller, asymptomatic aneurysm, serial imaging
For large or symptomatic aneurysms, endovascular therapy
Treatment of unruptured aneurysms depends on
Type, size, and location of the aneurysm
Risk of rupture
Patient age and health
Personal and family medical history
Risks of treatment
Risk of rupture versus that of perioperative complications should be discussed frankly with the patient.
Control of atherosclerotic risk factors, especially smoking cessation and use of antihypertensive drugs as appropriate, is important.
If < 7 mm, asymptomatic aneurysms in the anterior circulation rarely rupture and do not warrant the risks of immediate treatment. They can be monitored with serial imaging.
If aneurysms are larger, are in the posterior circulation, or cause symptoms due to bleeding or due to compression of neural structures, endovascular therapy Treatment 
(eg, stents, coil embolization), if feasible, can be tried. Sometimes open surgery with placement of a microvascular clip is necessary.
Treatment of mycotic aneurysms is aggressive antibiotic therapy directed at the specific pathogen. Usually, mycotic aneurysms must also be surgically repaired.
Key Points
Before rupture, aneurysms occasionally cause sentinel (warning) headaches; actual rupture causes a sudden severe headache (thunderclap headache).
Diagnose using angiography, CT angiography, or magnetic resonance angiography
If the aneurysm is asymptomatic and < 7 mm, monitor with serial imaging; if the aneurysm is symptomatic, larger, and located in the posterior circulation, treat using endovascular therapy or sometimes open surgery.
