Causes and Risk Factors of Subarachnoid Hemorrhage: A Comprehensive Overview
Subarachnoid hemorrhage (SAH) is a serious neurological condition characterized by bleeding into the subarachnoid space—the area between the brain and the surrounding membrane. This type of hemorrhage can lead to life-threatening complications if not promptly diagnosed and treated. Understanding its causes is crucial for early detection, effective management, and long-term prevention.
Primary Types of Subarachnoid Hemorrhage
There are two main categories of subarachnoid hemorrhage: spontaneous and secondary. Spontaneous SAH occurs without external trauma and is typically caused by underlying vascular abnormalities in the brain. In contrast, secondary SAH results from head injuries due to accidents, falls, or physical assaults.
Spontaneous Subarachnoid Hemorrhage: Common Causes
The most frequent cause of spontaneous SAH is a ruptured intracranial aneurysm, which accounts for approximately 80% to 85% of all cases. An aneurysm is a weakened, bulging spot on a blood vessel wall that can burst under pressure, leading to sudden bleeding in the subarachnoid space. These aneurysms are more commonly found in adults and older individuals, with prevalence increasing with age.
Another significant contributor is cerebrovascular malformations, particularly arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs). These abnormal connections between arteries and veins disrupt normal blood flow and increase the risk of rupture. Unlike aneurysms, AVMs are often present from birth and are more frequently diagnosed in children and younger patients.
Less Common but Important Causes
Beyond aneurysms and AVMs, several other medical conditions can lead to spontaneous SAH. These include Moyamoya disease, a rare cerebrovascular disorder involving the narrowing of arteries at the base of the brain and the development of a fragile network of collateral vessels. Additionally, vasculitis—inflammation of blood vessel walls—can weaken vessels and predispose them to rupture.
Hematological disorders such as hemophilia or thrombocytopenia may impair clotting mechanisms, making even minor bleeds more dangerous. Furthermore, certain brain tumors, especially those with rich vascularity, can erode blood vessels and trigger hemorrhage into the subarachnoid space.
Secondary Subarachnoid Hemorrhage: Trauma-Related Causes
Traumatic brain injury is the leading cause of secondary SAH. Events such as motor vehicle accidents, falls from height, or blunt force impacts can result in skull fractures or direct damage to cerebral vessels. Even seemingly mild head injuries should be taken seriously, as they may conceal internal bleeding that becomes apparent only through imaging studies.
Diagnosis and Advanced Imaging Techniques
Many patients remain unaware of their underlying vascular issues until symptoms appear—often dramatically. Sudden, severe headaches described as "the worst headache of my life," neck stiffness, photophobia, and altered consciousness are classic signs of SAH due to meningeal irritation. In some cases, intracranial hematoma formation may also occur, requiring urgent intervention.
Initial diagnosis typically begins with a non-contrast head CT scan, which can detect blood in the subarachnoid space with high accuracy. For more detailed evaluation, a CT angiography (CTA) is highly effective in identifying aneurysms, AVMs, and other vascular anomalies. When further precision is needed, a digital subtraction angiography (DSA)—also known as a cerebral angiogram—remains the gold standard for visualizing the brain's vascular architecture in real time.
Age and Individual Risk Factors Matter
Recent clinical data suggest that the distribution of SAH causes varies significantly based on patient demographics. While AVMs are more prevalent in pediatric populations, aneurysmal ruptures dominate in middle-aged and elderly adults. Lifestyle factors such as hypertension, smoking, excessive alcohol consumption, and drug use (especially cocaine) also play a critical role in increasing the risk of vessel rupture.
Early screening and awareness are essential, particularly for individuals with a family history of aneurysms or other cerebrovascular diseases. With advances in neuroimaging and minimally invasive treatment options like endovascular coiling or surgical clipping, outcomes for SAH patients have improved considerably over the past decade.