What Is a Brain Aneurysm and How Does It Cause Subarachnoid Hemorrhage?

A brain aneurysm, also known as an intracranial aneurysm, is a localized weakening in the wall of a blood vessel within the skull that leads to an abnormal bulge or ballooning. Think of it like a tire with a weak spot—over time, constant pressure causes the weakened area to expand outward, forming a small blister-like protrusion. If this bulge ruptures, it can lead to life-threatening bleeding in the brain, particularly into the subarachnoid space—the area between the brain and the thin tissues that cover it. This condition, known as subarachnoid hemorrhage (SAH), is one of the most severe types of stroke and requires immediate medical attention.

Types and Locations of Intracranial Aneurysms

Brain aneurysms are classified based on their shape and location. The most common type is the saccular (or berry) aneurysm, which has a distinct sac-like appearance and accounts for the majority of cases. Other types include fusiform aneurysms, which involve a more uniform widening of the entire artery wall, and dissecting aneurysms, caused by a tear in the arterial lining leading to blood accumulation within the vessel wall.

These aneurysms tend to form at branching points of major arteries at the base of the brain. Common sites include:

• The junction of the internal carotid artery
• The anterior communicating artery complex
• The middle cerebral artery bifurcation
• The posterior communicating artery origin
• The vertebrobasilar system and basilar tip
• The posterior cerebral artery

Risk Factors and Demographics

While the exact cause of brain aneurysms remains unclear, several risk factors have been identified. They are most commonly diagnosed in individuals aged 60 to 79 years, with a higher prevalence in women than men—particularly after menopause, suggesting hormonal influences may play a role. Other contributing factors include:

• Smoking
• Hypertension (high blood pressure)
• Family history of aneurysms
• Certain genetic conditions such as polycystic kidney disease or connective tissue disorders
• Previous head trauma or infection affecting cerebral vessels

Lifestyle choices significantly impact aneurysm development and rupture risk, making prevention through healthy habits crucial.

Ruptured vs. Unruptured Aneurysms: Symptoms and Clinical Presentation

A key distinction in clinical management is whether the aneurysm has ruptured. A ruptured brain aneurysm often presents suddenly and dramatically. Patients typically describe the onset of an excruciating headache—"the worst headache of my life"—followed rapidly by nausea, vomiting, and neck stiffness due to irritation of the meninges from blood leakage.

Warning Signs of Rupture

Additional neurological symptoms may include:

• Photophobia (sensitivity to light)
• Altered mental status or confusion
• Focal deficits such as facial drooping, weakness in limbs, or speech difficulties
• Seizures
• In severe cases, loss of consciousness, hemiplegia (paralysis on one side), or even decerebrate posturing indicating critical brainstem involvement

In contrast, unruptured aneurysms may remain silent for years. Some are discovered incidentally during brain scans performed for unrelated reasons. However, if the aneurysm grows large enough, it can exert pressure on surrounding brain structures or cranial nerves, leading to symptoms such as double vision, eyelid drooping (ptosis), or facial numbness—especially when located near the optic nerve or oculomotor pathways.

Diagnosis: Advanced Imaging Techniques

Accurate diagnosis is essential for determining the appropriate course of action. Modern neuroimaging tools allow clinicians to detect aneurysms early and evaluate their size, shape, and risk of rupture. The primary diagnostic methods include:

Non-Invasive Screening Tools

Computed Tomography Angiography (CTA) combines detailed X-ray imaging with contrast dye to visualize blood flow and identify vascular abnormalities. It's fast, widely available, and highly effective in detecting acute bleeding and aneurysms.

Magnetic Resonance Angiography (MRA) uses magnetic fields and radio waves to generate high-resolution images without radiation exposure. It's ideal for routine screening in high-risk patients or monitoring known unruptured aneurysms over time.

The Gold Standard: Digital Subtraction Angiography (DSA)

Despite advances in non-invasive techniques, digital subtraction angiography (DSA) remains the gold standard for diagnosing intracranial aneurysms. This invasive procedure involves inserting a catheter into the femoral artery and guiding it to the brain while injecting contrast dye. DSA provides unparalleled detail of vascular anatomy, making it indispensable for planning complex treatments.

Treatment Options: Surgical Clipping vs. Endovascular Therapy

The choice of treatment depends on multiple factors including aneurysm size, location, patient age, overall health, and rupture status. Traditionally, open surgical clipping was the mainstay of treatment—where a neurosurgeon places a metal clip across the neck of the aneurysm to prevent blood flow into it.

However, in recent years, endovascular interventions have become the preferred approach for many patients. These minimally invasive procedures include:

• Coil embolization: Tiny platinum coils are inserted via a catheter to fill the aneurysm and promote clotting
• Flow diverter stents: Mesh-like devices placed in the parent artery to redirect blood flow away from the aneurysm
• Stent-assisted coiling: Combines stents and coils for wide-necked aneurysms

Why Aneurysms Lead to Subarachnoid Hemorrhage

When a brain aneurysm ruptures, blood is forcefully ejected under arterial pressure directly into the subarachnoid space. This sudden influx of blood increases intracranial pressure, disrupts cerebrospinal fluid circulation, and can trigger secondary complications such as vasospasm—a dangerous narrowing of brain arteries that occurs days after the initial bleed and may lead to delayed cerebral ischemia.

Without prompt treatment, subarachnoid hemorrhage carries a high mortality rate, with up to 50% of patients dying within the first month. Among survivors, long-term disability is common, underscoring the importance of early detection and preventive strategies.

Prevention and Long-Term Management

For individuals with unruptured aneurysms, regular monitoring and lifestyle modifications are key. Controlling hypertension, quitting smoking, avoiding excessive alcohol consumption, and managing stress can all help reduce the risk of rupture. In select cases, prophylactic treatment may be recommended based on aneurysm characteristics and individual risk profiles.

Ongoing research continues to improve our understanding of aneurysm formation and progression. Advances in genetic testing, predictive modeling, and next-generation endovascular devices offer hope for safer, more personalized care in the future.