What Causes Subarachnoid Hemorrhage? Understanding the Most Common Triggers

Subarachnoid hemorrhage (SAH) is a serious and potentially life-threatening type of stroke characterized by bleeding into the space between the brain and the tissues that cover it. While any condition that leads to intracranial bleeding can contribute to SAH, certain underlying causes are far more prevalent than others. Understanding these primary triggers is crucial for early diagnosis, effective treatment, and long-term prevention strategies.

1. Intracranial Aneurysms: The Leading Cause

Intracranial aneurysms are responsible for 50% to 85% of all spontaneous subarachnoid hemorrhage cases, making them the most common cause. These aneurysms typically form at branching points along major arteries at the base of the brain, particularly within the circle of Willis—a critical arterial network supplying blood to the brain. The anterior portion of this vascular ring is especially prone to aneurysm development. When such an aneurysm ruptures, it releases blood directly into the subarachnoid space, leading to sudden and severe neurological symptoms including thunderclap headaches, nausea, and loss of consciousness.

Risk Factors for Aneurysmal Rupture

Several factors increase the likelihood of an aneurysm rupturing, including uncontrolled hypertension, smoking, excessive alcohol consumption, and a family history of cerebral aneurysms. Certain genetic conditions such as polycystic kidney disease and connective tissue disorders like Ehlers-Danlos syndrome also elevate risk. Early detection through imaging techniques like CT angiography or MR angiography can help identify unruptured aneurysms before they become life-threatening.

2. Cerebrovascular Malformations: A Common Cause in Younger Patients

Arteriovenous malformations (AVMs) account for approximately 2% of subarachnoid hemorrhage cases and are more frequently diagnosed in adolescents and young adults. These congenital abnormalities involve a tangle of abnormal blood vessels connecting arteries and veins, bypassing the capillary system. Most AVMs are located in the cerebral hemispheres, particularly within the territory supplied by the middle cerebral artery. Although many remain asymptomatic for years, rupture can lead to sudden bleeding into the subarachnoid space, causing acute neurological deficits.

Diagnosis and Management of AVMs

Modern neuroimaging tools such as cerebral angiography, MRI, and perfusion studies play a vital role in diagnosing and planning treatment for AVMs. Treatment options include surgical resection, endovascular embolization, and stereotactic radiosurgery—often used in combination depending on the size, location, and complexity of the malformation.

3. Moyamoya Disease: A Rare but Significant Contributor

Also known as stenosis of the intracranial internal carotid arteries with abnormal collateral vessel formation, Moyamoya disease accounts for about 1% of all SAH cases. This progressive cerebrovascular disorder is characterized by the narrowing or occlusion of the terminal portions of the internal carotid arteries, followed by the development of a fragile network of tiny collateral vessels that resemble a "puff of smoke" on angiograms—hence the name "Moyamoya," which means exactly that in Japanese. These delicate vessels are prone to rupture, leading to hemorrhagic strokes, including subarachnoid bleeding.

Demographics and Clinical Presentation

Moyamoya primarily affects children and young adults, with peak incidence in pediatric populations presenting with ischemic symptoms and adults more likely to experience hemorrhagic events. Early recognition and surgical intervention—such as indirect or direct bypass procedures—are essential to restore cerebral blood flow and reduce the risk of future bleeding episodes.

4. Other Medical Conditions That Can Trigger SAH

Beyond aneurysms and vascular malformations, several other medical conditions may precipitate subarachnoid hemorrhage:

Vascular dissections: Trauma or spontaneous tearing of arterial walls, particularly in the carotid or vertebral arteries, can lead to bleeding.
Vasculitis: Inflammatory diseases affecting blood vessels, such as CNS vasculitis, weaken vessel integrity and increase hemorrhage risk.
Cerebral venous sinus thrombosis: Clotting in the brain's venous system can raise intracranial pressure and indirectly cause SAH.
Coagulopathies and anticoagulant therapy: Disorders affecting blood clotting—including hemophilia, thrombocytopenia, or complications from warfarin or newer oral anticoagulants—can predispose individuals to bleeding.
Metastatic tumors and primary brain cancers: Tumors invading blood vessels may erode arterial walls and trigger hemorrhage.
Connective tissue diseases: Conditions like Marfan syndrome or lupus can compromise vascular strength.

5. Idiopathic and Perimesencephalic Hemorrhages

A small subset of patients presents with non-aneurysmal perimesencephalic subarachnoid hemorrhage, where bleeding is localized around the midbrain without evidence of a structural lesion. These cases make up a portion of idiopathic SAH and generally carry a better prognosis compared to aneurysmal bleeds. The exact source of bleeding remains unclear, though possible origins include small venous structures or capillary-level vessels. Importantly, these patients typically do not require invasive interventions and have lower recurrence rates.

Importance of Comprehensive Evaluation

Even when initial imaging fails to identify a clear cause, thorough follow-up with advanced vascular imaging is recommended to rule out hidden aneurysms or vascular anomalies. Prompt and accurate diagnosis ensures appropriate management and reduces the risk of secondary complications such as vasospasm, hydrocephalus, or rebleeding.

Conclusion: Early Detection Saves Lives

Recognizing the diverse etiologies behind subarachnoid hemorrhage—from aneurysms and AVMs to rare conditions like Moyamoya disease—is essential for timely intervention. Public awareness, improved screening methods, and advances in neurocritical care continue to enhance outcomes for patients facing this medical emergency. If you or someone you know experiences a sudden, severe headache described as "the worst ever," seek immediate medical attention—early action can mean the difference between recovery and permanent disability.