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.
Try two board-style Subarachnoid Hemorrhage questions
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Question 1NeurologyMedium
A 72-year-old woman presents with the sudden onset of the "worst headache of her life," which was followed by a brief loss of consciousness. She is now awake with a nonfocal neurologic examination. A noncontrast CT of the head obtained 8 hours after symptom onset is negative. Which of the following is the most important next step in management?
AMRI brain with contrast
BLumbar puncture
CAspirin 325 mg
DCT angiography of the head
Reveal answer & full explanation
Correct answer: B — Lumbar puncture
AMRI brain with contrast
BLumbar puncture✓
CAspirin 325 mg
DCT angiography of the head
Why lumbar puncture is correct
A thunderclap "worst headache of life" with a transient loss of consciousness is subarachnoid hemorrhage until proven otherwise.
Noncontrast CT misses roughly 5-10% of subarachnoid hemorrhages, with sensitivity falling further beyond 6-12 hours after onset, so a negative CT does not exclude it.
Lumbar puncture detects xanthochromia or an elevated red blood cell count that does not clear across sequential tubes, confirming the diagnosis.
Why the others are wrong
MRI brain with contrast — not the standard next step after a negative CT for suspected acute subarachnoid hemorrhage; it is less available and not validated for excluding a hyperacute bleed (right-idea-wrong-test).
Aspirin 325 mg — appropriate for acute ischemic stroke, but giving an antiplatelet agent before a hemorrhage is excluded can be harmful (anchoring on "stroke = aspirin").
CT angiography of the head — helps localize an aneurysm once subarachnoid hemorrhage is confirmed, but it does not establish the diagnosis and is not the immediate step before lumbar puncture (right-diagnosis-wrong-step).
Additional high-yield points
Do not discharge a patient with thunderclap headache on the basis of a negative CT alone.
Question 2NeurologyMedium
A 75-year-old female has sudden onset of the worst headache of her life, neck stiffness, and photophobia. CT head is negative. Lumbar puncture (LP) shows 4200 RBCs in tube 1 and 4100 in tube 4 (no xanthochromia yet — LP performed 45 minutes after headache onset). In general, which of the following findings most reliably distinguishes a traumatic tap from subarachnoid hemorrhage?
ACT angiography findings
BDecreasing RBC count from tube 1 to tube 4
CXanthochromia of CSF
DElevated CSF protein above 100 mg/dL
Reveal answer & full explanation
Correct answer: C — Xanthochromia of CSF
ACT angiography findings
BDecreasing RBC count from tube 1 to tube 4
CXanthochromia of CSF✓
DElevated CSF protein above 100 mg/dL
Why Xanthochromia of CSF is correct
Xanthochromia (yellow CSF from RBC lysis) is the most reliable marker for subarachnoid hemorrhage (SAH).
Requires a minimum of 2 hours after bleed onset to develop; sensitivity is 93–100% at 12 hours.
In this case the LP was performed only 45 minutes after headache onset, so absence of xanthochromia does not yet rule out SAH.
Why the others are wrong
B) Decreasing RBC count from tube 1 to tube 4 — slightly favors traumatic tap, but this finding is unreliable: SAH RBCs should remain equal across tubes, yet traumatic taps can also show thousands of RBCs and may not clear consistently.
A) CT angiography findings — CT angiography has sensitivity of approximately 98–99% for detecting aneurysms above 3 mm; useful as a next step, but it identifies the aneurysm rather than directly distinguishing traumatic tap from hemorrhage.
D) Elevated CSF protein above 100 mg/dL — not a standardized or reliable distinguishing criterion between traumatic tap and SAH.
Additional high-yield points
D-dimers in CSF may help distinguish traumatic tap from SAH but are not yet standardized.
Clinical approach when LP is performed less than 2 hours after headache onset with no xanthochromia and high clinical suspicion: repeat LP at 12 hours OR proceed with CT angiography.
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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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