What Causes Subarachnoid Hemorrhage? Understanding Triggers, Risk Factors, and Recovery Outlook

Subarachnoid hemorrhage (SAH) is a serious medical condition characterized by bleeding into the subarachnoid space—the area between the brain and the thin tissues that cover it. This type of hemorrhage typically results from the rupture of abnormal blood vessels at the base or surface of the brain, leading to a sudden and severe neurological event. The most common symptoms include an explosive headache often described as "the worst headache of my life," nausea, vomiting, neck stiffness, and in severe cases, loss of consciousness or coma.

Primary Causes of Subarachnoid Hemorrhage

Understanding the underlying causes of SAH is critical for both prevention and effective treatment. While several factors can contribute to this condition, certain vascular abnormalities are far more prevalent than others.

Intracranial Aneurysms: The Leading Cause

Intracranial aneurysms account for over 75% of spontaneous subarachnoid hemorrhage cases. These are weak, bulging spots on the walls of brain arteries that can rupture under pressure. Most commonly found at branching points in the Circle of Willis—a network of arteries at the base of the brain—aneurysms may remain asymptomatic for years until they burst. Risk factors include hypertension, smoking, family history, and certain genetic conditions such as polycystic kidney disease or connective tissue disorders like Ehlers-Danlos syndrome.

Cerebrovascular Malformations

Another significant contributor is cerebral arteriovenous malformations (AVMs), which are tangles of abnormally formed blood vessels connecting arteries and veins. These defects disrupt normal blood flow and increase the risk of rupture. Although less common than aneurysms, AVMs can still lead to life-threatening bleeding in the subarachnoid space, especially in younger patients.

Moyamoya Disease: A Rare but Important Factor

Moyamoya disease is a rare, progressive cerebrovascular disorder marked by the narrowing of major arteries at the base of the brain and the development of a fragile network of tiny collateral vessels. These new vessels are prone to leakage or clotting, increasing the likelihood of hemorrhagic stroke, including subarachnoid hemorrhage. It's more frequently diagnosed in children and young adults, particularly in East Asian populations, though cases occur worldwide.

Less Common but Notable Causes

While aneurysms and vascular malformations dominate the landscape of SAH etiology, other medical conditions and complications must not be overlooked.

Vasculitis, or inflammation of blood vessel walls, can weaken cerebral arteries and predispose them to rupture. Autoimmune disorders such as systemic lupus erythematosus (SLE) and other connective tissue diseases may also elevate the risk. Additionally, hematologic conditions—including leukemia, thrombocytopenia, and coagulopathies—can impair the body's ability to form clots, making even minor bleeds more dangerous.

Brain tumors, although a rarer cause, can erode nearby vasculature or promote abnormal angiogenesis, potentially leading to hemorrhage. Furthermore, certain medical treatments—such as anticoagulant therapy or anti-inflammatory drugs used in managing chronic conditions—can inadvertently increase bleeding risks, especially if not closely monitored.

Symptoms and Prognosis After SAH

Unlike ischemic strokes, subarachnoid hemorrhages typically do not present with immediate limb weakness or paralysis. Instead, patients often experience a sudden, thunderclap headache accompanied by photophobia, dizziness, and altered mental status. Early recognition of these signs is crucial for timely intervention.

With prompt diagnosis and appropriate treatment—such as surgical clipping or endovascular coiling of aneurysms—many patients can achieve full recovery. If managed effectively during the acute phase and without major complications like rebleeding or vasospasm, individuals may return to normal neurological function without lasting deficits such as hemiparesis or aphasia.

However, SAH remains a medical emergency with high morbidity and mortality rates. Long-term outcomes depend heavily on early detection, access to specialized care, and aggressive management of secondary complications. Lifestyle modifications, including quitting smoking and controlling blood pressure, play a vital role in reducing recurrence risk.