Subarachnoid Hemorrhage

Subarachnoid hemorrhage is bleeding into the subarachnoid space, the cerebrospinal-fluid-filled compartment between the arachnoid and pia mater. Most non-traumatic cases result from a ruptured intracranial aneurysm and present with a sudden, severe 'thunderclap' headache. It is a high-mortality form of stroke whose course is shaped by the initial bleed, rebleeding, and delayed cerebral ischemia.

Definition

Subarachnoid hemorrhage is extravasation of blood into the subarachnoid space; in its most common non-traumatic form it results from rupture of an intracranial aneurysm and is associated with high early mortality and characteristic delayed complications.

Scope

This entry defines subarachnoid hemorrhage, focuses on the aneurysmal form, and introduces the clinical grading scales, the complication of delayed cerebral ischemia, and the principle of securing the ruptured aneurysm. It is a reference and educational overview and does not provide diagnostic or treatment recommendations for any individual.

Core questions

• What is the subarachnoid space, and what does bleeding into it cause?
• Why is most spontaneous subarachnoid hemorrhage attributed to ruptured aneurysms?
• How do clinical grading scales such as Hunt and Hess describe severity?
• What is delayed cerebral ischemia, and why does it matter for outcome?

Key concepts

• Subarachnoid space
• Aneurysmal versus non-aneurysmal causes
• Thunderclap headache
• Hunt and Hess and WFNS grading
• Rebleeding before the aneurysm is secured
• Cerebral vasospasm
• Delayed cerebral ischemia
• Hydrocephalus

Mechanisms

When an intracranial aneurysm ruptures, arterial blood enters the subarachnoid space, raising intracranial pressure and producing the abrupt, severe headache that characterizes the event. Until the ruptured aneurysm is secured, there is a risk of rebleeding, which worsens outcome; treatment therefore aims to exclude the aneurysm by clipping or coiling. In the days after the bleed, patients may develop cerebral vasospasm and delayed cerebral ischemia, a leading cause of secondary injury, the definition of which has been standardized for research (Vergouwen et al., 2010). Hydrocephalus may arise when blood impairs cerebrospinal-fluid circulation. Clinical grading scales such as Hunt and Hess (Hunt & Hess, 1968) summarize neurological severity at presentation and relate to prognosis.

Clinical relevance

Understanding the mechanisms, grading, and complications of subarachnoid hemorrhage supports critical reading of the neurovascular and neurocritical-care literature. This entry describes how the condition is characterized and studied; it is not a basis for diagnosis or treatment of an individual, which depends on clinical assessment and current guidelines and rests with the treating team.

Epidemiology

Aneurysmal subarachnoid hemorrhage is an uncommon form of stroke but carries high early mortality and substantial morbidity among survivors, often affecting people younger than those with other stroke subtypes. A minority of spontaneous cases are non-aneurysmal, including perimesencephalic hemorrhage, which generally has a more benign course.

Evidence & guidelines

Clinical grading derives from scales such as Hunt and Hess (Hunt & Hess, 1968). The International Subarachnoid Aneurysm Trial informed how ruptured aneurysms are secured (Molyneux et al., 2005). Delayed cerebral ischemia has a standardized research definition (Vergouwen et al., 2010), and contemporary management is summarized in AHA/ASA guidance for aneurysmal subarachnoid hemorrhage (Hoh et al., 2023).

History

The clinical grading of subarachnoid hemorrhage was formalized with the Hunt and Hess scale (1968), relating presenting severity to surgical risk and timing. The International Subarachnoid Aneurysm Trial (Molyneux et al., 2005) then reshaped how ruptured aneurysms are secured. Recognition of delayed cerebral ischemia as a distinct, definable complication, standardized for research by Vergouwen and colleagues (2010), refined how outcomes after the initial bleed are studied.

Debates

How best to define and study delayed cerebral ischemia

Because vasospasm, delayed cerebral ischemia, and infarction had been used inconsistently across studies, a multidisciplinary group proposed standardized definitions so that outcomes could be compared across trials; harmonizing these endpoints remains important for research.

Key figures

• William E. Hunt
• Andrew J. Molyneux
• Gabriel J. E. Rinkel
• Jan van Gijn

Seminal works

• hunt-hess-1968
• molyneux-2005
• vergouwen-2010

Frequently asked questions

What most commonly causes spontaneous subarachnoid hemorrhage?

Rupture of an intracranial aneurysm is the most common cause of spontaneous (non-traumatic) subarachnoid hemorrhage; a minority of cases are non-aneurysmal, such as perimesencephalic hemorrhage.

What is delayed cerebral ischemia?

It is a secondary deterioration that can occur in the days after the initial bleed, associated with cerebral vasospasm, and is an important cause of additional brain injury after aneurysmal subarachnoid hemorrhage. A standardized definition exists for research purposes.