Causes of Subarachnoid Hemorrhage: Understanding the Key Risk Factors and Underlying Conditions

Subarachnoid hemorrhage (SAH) is a life-threatening type of stroke characterized by bleeding into the space surrounding the brain. Identifying the underlying causes is crucial for timely diagnosis, effective treatment, and long-term prevention. While some cases remain unexplained, several well-documented medical conditions significantly increase the risk of SAH.

Primary Causes of Subarachnoid Hemorrhage

Intracranial Aneurysms: The Leading Cause

Intracranial aneurysms are responsible for 50% to 80% of all subarachnoid hemorrhage cases, making them the most common cause. Among these, saccular aneurysms—often referred to as "berry aneurysms"—account for approximately 75% of aneurysm-related SAH. These balloon-like bulges in the blood vessel walls typically develop at arterial branching points in the brain's Circle of Willis. Chronic hypertension and atherosclerosis contribute to their formation and rupture. Less commonly, fusiform aneurysms—linked to high blood pressure and degenerative vascular changes—can also lead to bleeding.

Cerebral Arteriovenous Malformations (AVMs)

Arteriovenous malformations are congenital vascular abnormalities that affect the connections between arteries and veins in the brain. Representing about 10% of SAH cases, AVMs are more frequently diagnosed in younger adults and adolescents. These tangled clusters of blood vessels bypass normal capillary networks, creating high-pressure shunts that are prone to rupture. Early detection through imaging studies like MRI or cerebral angiography can help prevent catastrophic bleeding events.

Moyamoya Disease: A Major Factor in Pediatric Cases

Also known as moyamoya syndrome or basal intracranial vascular anomalies, this rare cerebrovascular disorder plays a significant role in childhood SAH, accounting for nearly 20% of pediatric cases. Characterized by progressive stenosis of the internal carotid arteries and the development of fragile collateral vessels, moyamoya increases the risk not only of subarachnoid hemorrhage but also of intracerebral hemorrhage and ischemic strokes. Children under the age of 10 who experience unexplained brain bleeds should be thoroughly evaluated for this condition using advanced neuroimaging techniques.

Other Contributing Factors and Rare Causes

Beyond the primary causes, a range of secondary factors can trigger subarachnoid hemorrhage:

• Brain tumors: Certain neoplasms, especially those with rich vascularity such as glioblastomas or metastatic tumors, may erode blood vessels and cause bleeding.
• Cerebral vasculitis: Inflammatory conditions affecting blood vessel walls can weaken arteries and increase rupture risk.
• Blood disorders: Conditions like hemophilia, thrombocytopenia, or leukemia impair normal clotting mechanisms.
• Coagulopathies and anticoagulant use: Patients on blood thinners (e.g., warfarin, heparin, or direct oral anticoagulants) face elevated bleeding risks, particularly if dosing is not carefully monitored.
• Cerebral venous sinus thrombosis: This uncommon form of stroke can secondarily lead to hemorrhagic complications, including SAH.

Emerging Risk: Substance Abuse and SAH

In recent years, substance misuse has emerged as a growing contributor to subarachnoid hemorrhage. Illicit drugs such as cocaine and amphetamines induce acute spikes in blood pressure and promote vasospasm, significantly increasing the likelihood of vessel rupture. Public health efforts must address drug-related neurological emergencies, particularly among younger populations.

Idiopathic Cases: When No Clear Cause Is Found

Despite comprehensive diagnostic evaluations, approximately 10% of SAH patients have no identifiable cause. Some of these cases involve perimesencephalic hemorrhage, a subtype where bleeding is localized around the midbrain. These often carry a better prognosis and lower risk of rebleeding compared to aneurysmal SAH. Ongoing research aims to uncover potential genetic, microvascular, or hemodynamic factors behind these cryptogenic cases.

Early recognition of symptoms—such as sudden "thunderclap" headache, nausea, vomiting, neck stiffness, and altered consciousness—is vital. Prompt neuroimaging, lumbar puncture, and vascular studies (CTA, MRA, or DSA) are essential for accurate diagnosis and intervention. Preventive strategies, including blood pressure control, lifestyle modifications, and screening for at-risk individuals, play a key role in reducing the global burden of subarachnoid hemorrhage.