What Is Subarachnoid Hemorrhage? Understanding Causes, Types, and Risks

Subarachnoid hemorrhage (SAH) is a life-threatening type of stroke that occurs when blood leaks into the space between the brain and the surrounding membrane known as the arachnoid layer. This condition typically results from the rupture of a blood vessel in the brain, leading to rapid accumulation of blood in the subarachnoid space. SAH is a medical emergency requiring immediate intervention due to its high mortality and long-term disability rates.

Types of Subarachnoid Hemorrhage

Primary Subarachnoid Hemorrhage

Primary SAH arises when blood vessels at the base or on the surface of the brain rupture spontaneously. The most common underlying causes include congenital cerebral aneurysms, arteriovenous malformations (AVMs), and microaneurysms caused by chronic hypertension and atherosclerosis. These vascular abnormalities weaken the arterial walls, making them prone to bursting under pressure. When the vessel ruptures, blood flows directly into the subarachnoid space, triggering a cascade of neurological symptoms.

This form accounts for approximately 10% of all acute strokes. Epidemiological studies show an annual incidence rate ranging from 1.9 to 5.4 cases per 100,000 people. In practical terms, this means between 6 and 20 individuals per 100,000 may experience a primary SAH each year, with higher prevalence among adults aged 40 to 65.

Secondary Subarachnoid Hemorrhage

Secondary SAH occurs when bleeding originates within the brain tissue—such as from an intracerebral hematoma—and then extends outward, breaking through brain structures and spilling into the subarachnoid space. Unlike primary SAH, which begins in the subarachnoid area, secondary cases are complications of other forms of intracranial bleeding. Traumatic brain injury, tumors, or hemorrhagic transformation of ischemic strokes can also lead to this type of bleed.

Symptoms and Early Warning Signs

One of the hallmark signs of subarachnoid hemorrhage is a sudden, severe headache often described by patients as "the worst headache of my life" or feeling like a "thunderclap" in the head. This explosive onset of pain typically peaks within seconds to minutes and may be accompanied by nausea, vomiting, neck stiffness, photophobia, confusion, or even loss of consciousness.

Because these symptoms can mimic migraines or meningitis, prompt diagnosis is critical. Delayed treatment significantly increases the risk of rebleeding, cerebral vasospasm, hydrocephalus, and permanent neurological damage.

Risks and Mortality Rates

Subarachnoid hemorrhage carries a devastating prognosis if not treated swiftly. The initial rupture has a fatality rate of around 30%, meaning nearly one-third of patients do not survive the first event. For those who do survive, the risk of a second hemorrhage dramatically increases—up to 70% mortality if rebleeding occurs. Even among survivors, many face long-term cognitive impairments, physical disabilities, or emotional challenges.

Early detection through imaging techniques such as non-contrast CT scans or lumbar puncture greatly improves outcomes. Advances in neurosurgical interventions—including endovascular coiling and surgical clipping—have enhanced survival rates, but prevention and awareness remain key.

Prevention and Awareness

Managing risk factors such as uncontrolled hypertension, smoking, excessive alcohol consumption, and drug use (especially cocaine) can reduce the likelihood of developing an aneurysm or experiencing a rupture. Regular screening for individuals with a family history of brain aneurysms or genetic conditions like polycystic kidney disease is also recommended.

Public education about the sudden nature of SAH and its warning signs plays a vital role in improving early diagnosis and saving lives. Recognizing the thunderclap headache as a red flag could make the difference between life and death.