Bleeding into the subarachnoid space, usually from a ruptured saccular aneurysm.
Also known as: SAH, aneurysmal subarachnoid hemorrhage, thunderclap headache
Overview
Acute bleeding into the subarachnoid space between the arachnoid and pia mater. Most cases (~85%) are due to rupture of a saccular ('berry') aneurysm; others are perimesencephalic, traumatic, or from AVM, vasculitis, or coagulopathy.
Epidemiology
Incidence ~9/100,000 person-years. Peak age 40-60. Female predominance (3:2). Mortality 25-50% overall; ~10-15% die before reaching hospital.
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Sympathomimetic drug use (cocaine, methamphetamine)
Family history of SAH (first-degree relative)
Connective tissue disorders: autosomal dominant polycystic kidney disease, Ehlers-Danlos type IV, Marfan, neurofibromatosis
Female sex, especially post-menopausal
Pathophysiology
Most saccular aneurysms arise at branch points of the circle of Willis (anterior communicating artery most common, then posterior communicating, then MCA bifurcation). Aneurysm rupture releases arterial blood into the subarachnoid cisterns, causing sudden severe headache, transient global cerebral ischemia, and elevated ICP. Subsequent complications (rebleeding, vasospasm, hydrocephalus, seizures) determine outcome.
Clinical presentation
Symptoms
Sudden severe headache ('thunderclap,' 'worst headache of life,' maximal within seconds)
Brief loss of consciousness in ~50%
Nausea, vomiting, photophobia
Meningismus develops over hours from blood irritating meninges
Sentinel headache in days-weeks before rupture in 10-40% (warning leak)
Seizures at onset in ~10%
Signs / physical exam
Meningismus (Kernig and Brudzinski signs)
Decreased level of consciousness
Focal deficits: CN III palsy (posterior communicating aneurysm — ipsilateral fixed dilated pupil), hemiparesis (vasospasm or hematoma)
Subhyaloid (preretinal) hemorrhages on funduscopy (Terson syndrome)
Elevated BP and pulse pressure
Classic findings
Thunderclap headache + meningismus + LOC. Hunt-Hess and World Federation of Neurosurgical Societies (WFNS) grading scales predict outcome.
Differential diagnosis
Primary thunderclap headache / RCVS (reversible cerebral vasoconstriction) — Recurrent thunderclap headaches over days-weeks; segmental vasoconstriction on angiography reverses by 12 weeks
Migraine — Usually slower onset, history of similar episodes; SAH must be excluded for first severe headache
Meningitis — Fever, photophobia, nuchal rigidity, gradual onset; CSF pleocytosis with no xanthochromia
Cervical artery dissection — Neck or facial pain, partial Horner syndrome (carotid) or posterior circulation symptoms (vertebral); CTA/MRA diagnostic
Pituitary apoplexy — Severe headache + visual field defect + cranial nerve palsies + endocrine dysfunction; sellar imaging
Non-contrast head CT within 6 h of symptom onset is ~100% sensitive (drops to ~85% at 24 h, ~50% at 1 week)
Lumbar puncture if CT negative but clinical suspicion remains — xanthochromia (yellow CSF from hemoglobin breakdown) confirms SAH; RBCs that don't clear between tubes 1 and 4
CT within 6 hours of headache onset, interpreted by an experienced reader, is essentially 100% sensitive — LP may be safely omitted in this setting (Perry rules).
Xanthochromia takes 6-12 hours to develop — LP done too early may miss SAH.
Posterior communicating artery aneurysm → ipsilateral CN III palsy with pupil involvement (compressive); diabetic CN III spares the pupil.
ISAT trial: coiling was superior to clipping for survival and functional independence in ruptured anterior circulation aneurysms.
Nimodipine improves outcomes but does NOT reliably prevent angiographic vasospasm — mechanism likely neuroprotective.
References
AHA/ASA 2023 — 2023 Guideline for the Management of Patients With Aneurysmal Subarachnoid Hemorrhage (Hoh et al., Stroke 2023)
ISAT — International Subarachnoid Aneurysm Trial: Coiling versus Clipping (Molyneux et al., Lancet 2002, 2005)
Perry Rules — Clinical Decision Rules to Rule Out Subarachnoid Hemorrhage for Acute Headache (Perry et al., JAMA 2013)
Hunt-Hess — Surgical Risk as Related to Time of Intervention in Repair of Intracranial Aneurysms (Hunt & Hess, J Neurosurg 1968)
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