Bleeding into the subarachnoid space; most spontaneous (nontraumatic) cases are 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. Trauma is the most common cause overall; ~85% of spontaneous (nontraumatic) cases are due to rupture of a saccular ('berry') aneurysm, and the rest are perimesencephalic 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
Real questions from the FirstPassPA bank, with the full explanation. Pick an answer — no signup, no email.
Question 1NeurologyMedium
A 46-year-old woman comes to the emergency department after a headache began abruptly at 7:15 AM and reached maximum intensity within seconds; she vomited twice on the way in. She has never had a headache like this and takes no medications. On arrival at 8:00 AM the temperature is 36.9 C, blood pressure is 152/94 mm Hg, and pulse is 88/min. She is awake and fully oriented, the neck is supple, and the neurologic examination, including funduscopy, is normal. A noncontrast head CT completed at 9:20 AM on the department multidetector scanner is read by the neuroradiologist on duty as showing no subarachnoid, intraparenchymal, or intraventricular blood. Which of the following is the most appropriate next step in management?
AIntravenous labetalol for blood pressure control
BAdmission for nimodipine and close observation
CDischarge with return precautions and follow-up
DLumbar puncture for cerebrospinal fluid studies
ERepeat the noncontrast head CT in 6 to 12 hours
Reveal answer & full explanation
Correct answer: C — Discharge with return precautions and follow-up
AIntravenous labetalol for blood pressure control
BAdmission for nimodipine and close observation
CDischarge with return precautions and follow-up✓
DLumbar puncture for cerebrospinal fluid studies
ERepeat the noncontrast head CT in 6 to 12 hours
Why Discharge with return precautions and follow-up is correct
A headache that reaches maximum intensity within seconds is aneurysmal subarachnoid hemorrhage (SAH) until proven otherwise, and the first test is a noncontrast head CT.
Work out the interval before deciding what the scan means: onset was 7:15 AM and the CT was completed at 9:20 AM, so it was obtained about 2 hours after onset, inside the 6-hour window, on a multidetector scanner read by a neuroradiologist, in a woman who is awake, fully oriented, and neurologically normal. That is the population in which the rule was derived and validated.
In exactly that setting a normal noncontrast CT rules out nontraumatic SAH: ACEP 2019 gives this a Level B recommendation, and the pooled data behind it show sensitivity 98.7%, a negative likelihood ratio of 0.010, and missed SAH in 1.46 per 1,000 scans; the 2023 AHA/ASA aneurysmal SAH guideline agrees (Class 2a) for the patient who presents within 6 hours of onset, as she did.
Nothing in the stem points toward the other dangerous causes, and an unenhanced CT does not formally exclude them, which is why the disposition carries explicit precautions rather than bare reassurance: she is afebrile with a supple neck, funduscopy and the neurologic examination are normal, and this is a single first-ever attack in a woman taking no medications. The blood pressure of 152/94 mm Hg is the expected pain response to a severe headache rather than a hypertensive emergency, which takes far higher pressures with acute end-organ damage, so it does not redirect the workup. The next step is therefore symptomatic treatment, a scheduled follow-up visit, and explicit return precautions for a second thunderclap headache, fever, neck stiffness, or any new neurologic symptom.
Why the others are wrong
Lumbar puncture for cerebrospinal fluid studies - CT followed by LP is the pathway for the patient who presents outside the 6-hour window or with a new neurologic deficit, where the 2023 AHA/ASA guideline gives it a Class 1 recommendation; a qualifying early scan has already answered the SAH question here, and a headache that peaks within seconds is not the tempo of meningitis, which builds over hours to days. Tapping her now carries a real rate of traumatic taps and post-LP headache for a vanishingly small yield, and xanthochromia is not reliably present until roughly 12 hours after onset, so a negative xanthochromia this early would add nothing the qualifying CT has not already settled (the trap is running the CT-then-LP reflex on a CT that has already answered the question).
Repeat the noncontrast head CT in 6 to 12 hours - subarachnoid blood grows less dense and less conspicuous as it is cleared, so a second unenhanced scan is less sensitive than the first; a later CT rescues nothing, adds radiation, and only delays disposition (the trap is treating an early scan as premature when it is in fact the most sensitive one).
Admission for nimodipine and close observation - nimodipine reduces delayed cerebral ischemia and improves neurologic outcome after a confirmed aneurysmal SAH, where the 2023 AHA/ASA guideline makes it a Class 1 recommendation, and it is used off label in reversible cerebral vasoconstriction syndrome (RCVS), but neither diagnosis is established: the qualifying early CT excludes SAH, and RCVS is defined by reversible segmental narrowing of the cerebral arteries, which takes vascular imaging she has not had and which most often declares itself through repeated thunderclap attacks over days to weeks rather than one episode (the trap is treating a diagnosis that has not been established).
Intravenous labetalol for blood pressure control - the 152/94 mm Hg reading invites treatment because blood pressure is lowered in aneurysmal SAH until the aneurysm is secured, but the qualifying early CT has excluded hemorrhage, and a modest rise without acute end-organ damage is the expected pain response rather than a hypertensive emergency, so intravenous antihypertensive therapy has no indication here (the trap is treating a number instead of the diagnosis).
Additional high-yield points
The 6-hour rule is conditional, and each document sets its own conditions: ACEP 2019 (Level B) asks for a normal noncontrast head CT obtained within 6 hours of symptom onset on a minimum third-generation scanner in a patient with a normal neurologic examination, while the 2023 AHA/ASA guideline (Class 2a) sets its window by presentation within 6 hours of onset and asks for a high-quality scan interpreted by a board-certified neuroradiologist. If the scan falls outside the window, the equipment or the reader does not meet that standard, or the patient has a focal deficit, papilledema, or depressed consciousness, do not stop at the CT.
Anchor the window to symptom onset rather than to arrival or to the time the study was ordered; a patient who waited at home can be outside the window on a scan that feels early in the visit.
When concern for SAH survives a nondiagnostic noncontrast CT, which is usually the beyond-6-hour situation, the workup continues rather than stopping: the 2023 AHA/ASA guideline gives noncontrast CT followed by LP a Class 1 recommendation beyond 6 hours or with a new neurologic deficit, and ACEP (Level C) lets lumbar puncture or CT angiography of the head be chosen by shared decision making in any patient still considered to be at risk after a negative scan.
A normal early CT excludes SAH but not every dangerous cause of thunderclap headache. Cerebral venous sinus thrombosis is the classic miss, since the unenhanced scan is usually normal and the study that finds it is CT or MR venography; cervical artery dissection is suggested by neck pain or a Horner syndrome, RCVS by repeated attacks or exposure to a vasoactive drug, pituitary apoplexy by visual field loss with hypopituitarism, and meningitis by fever and meningismus. That residual list, together with the cerebrospinal fluid diagnoses a lumbar puncture would have caught, is why return precautions and a scheduled visit rather than bare reassurance end this encounter.
ACEP 2019 also gives a Level A recommendation to preferentially use nonopioid rather than opioid medications in the treatment of acute primary headache in the emergency department, a recommendation that applies once a secondary cause like this one has been excluded.
The Ottawa SAH Rule (age 40 years or older, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap onset, limited neck flexion on examination) is highly sensitive but poorly specific, so it decides who gets imaged rather than who has bled.
Question 2NeurologyMedium
A 75-year-old woman comes to the emergency department 13 hours after the sudden onset of the worst headache of her life, which reached maximal intensity within seconds; she stayed home overnight hoping it would pass. She reports photophobia and has neck stiffness on examination; the neurologic examination is otherwise normal. A noncontrast head CT obtained on arrival shows no hemorrhage. Lumbar puncture is performed 14 hours after headache onset; it requires several needle passes, and the first tube of cerebrospinal fluid is blood-tinged. Which of the following findings would most reliably distinguish a traumatic tap from subarachnoid hemorrhage in this patient?
ACrenated red blood cells in the CSF
BXanthochromia of CSF
CVisible clotting of the CSF
DDecreasing RBC count from tube 1 to tube 4
EElevated CSF protein above 100 mg/dL
Reveal answer & full explanation
Correct answer: B — Xanthochromia of CSF
ACrenated red blood cells in the CSF
BXanthochromia of CSF✓
CVisible clotting of the CSF
DDecreasing RBC count from tube 1 to tube 4
EElevated CSF protein above 100 mg/dL
Why Xanthochromia of CSF is correct
Xanthochromia is yellow CSF supernatant from bilirubin, which forms in vivo over hours from the oxyhemoglobin that lysed RBCs release; blood introduced by the needle cannot form it before the sample is processed, so xanthochromia is the most specific CSF finding for telling true subarachnoid hemorrhage (SAH) from a traumatic tap.
It is reliably detectable from about 12 hours after onset (UK NEQAS 2008 advises lumbar puncture (LP) only >12 hours after onset) and persists for at least 2 weeks, so this LP at 14 hours can be interpreted: if she bled, bilirubin should already be present.
Spectrophotometry is the most sensitive method; visual inspection, common in North American labs, is highly specific but less sensitive (pooled sensitivity 71%; 47% in Perry 2015). ACEP 2019 notes that no method for separating the two, xanthochromia included, is agreed to be reliable: it is the best discriminator, not an infallible one.
Why the others are wrong
Visible clotting of the CSF — the classic bedside sign of a traumatic tap because the needle introduces peripheral fibrinogen, but clotting needs a heavily bloody sample, so a lightly blood-tinged traumatic tap usually does not clot and a missing clot cannot exclude one; ACEP 2019 lists clot formation among the methods not agreed to be reliable.
Decreasing RBC count from tube 1 to tube 4 — tube clearing is the widely taught bedside test, but counts show how much blood is in the tube, not how long it has been there, and a traumatic tap can occur on top of a true SAH, so a falling count is not a reliable rule-out unless the final tube is zero or near zero (ACEP 2019). In one retrospective series (Czuczman 2013) the simple initial-to-final-tube difference did not discriminate (area under the curve 0.45) and percent drop did better (0.84), but no cutoff has been validated, and Perry 2015 did not assess a safe drop.
Elevated CSF protein above 100 mg/dL — blood from any source raises CSF protein; in Perry 2015 the median was only modestly higher in SAH (56 vs 40 mg/dL, a gap the authors judged of doubtful clinical relevance), and both medians, like most SAH values, sit below this 100 mg/dL cutoff.
Crenated red blood cells in the CSF — once taught as a sign that blood had been in the CSF for hours, but red cells can crenate in either setting, including a traumatic tap, so crenation does not reliably separate the two, whereas xanthochromia reflects bilirubin formed in vivo.
Additional high-yield points
An LP is not needed after a normal noncontrast CT within 6 hours of onset in a patient with a normal neurologic exam, because that scan rules out nontraumatic SAH (ACEP 2019, Level B). A patient still at risk after a negative CT, as here, needs LP or CT angiography (CTA), chosen by shared decision making (ACEP 2019, Level C).
Perry 2015: no xanthochromia plus a final-tube RBC count below 2,000 x10^6/L excluded aneurysmal SAH (sensitivity 100%, 95% CI 74.7-100%; specificity 91.2%), but with only 15 SAH cases the interval is wide.
After a traumatic or uninterpretable tap, next tests include repeat LP at a different site, CTA, digital subtraction angiography, or MR angiography (ACEP 2019).
D-dimers in CSF may help distinguish traumatic tap from SAH but are not yet standardized.
That's 2. Get 7,700+ more.
Unlock the full FirstPassPA bank: 7,700+ board-style questions, flashcards, and an AI tutor that explains every answer. Your 7-day free trial includes everything, and there's no credit card required.
You've read your 2 free diagnosis previews. Create your free account to unlock the full Subarachnoid Hemorrhage (SAH) outline — plus all 514 diagnoses, 7,700+ board-style questions, flashcards, and an AI tutor. Your 7-day free trial includes everything, and there's no credit card required.
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 support SAH, but tube-to-tube clearing is unreliable, so xanthochromia is the most reliable way to distinguish SAH from a traumatic tap
Catheter angiography (DSA) — gold standard, used during coiling
Repeat MRI/MRA or DSA in 1-2 weeks if initial study negative but SAH confirmed
Diagnostic algorithm
Hunt-Hess Grade
Clinical Findings
Mortality
I
Asymptomatic or minimal headache, mild nuchal rigidity
~1%
II
Moderate-severe headache, nuchal rigidity, no deficit except cranial nerve palsy
~5%
III
Drowsy, confused, mild focal deficit
~19%
IV
Stupor, moderate-severe hemiparesis, early decerebrate posturing
~42%
V
Deep coma, decerebrate posturing, moribund
~77%
Hunt-Hess grading scale for aneurysmal subarachnoid hemorrhage.
Treatment
First-line
ABCs; intubate if GCS ≤8 or airway compromise
BP control: target SBP <160 (or MAP <110) until aneurysm secured — IV nicardipine, clevidipine, or labetalol
Pain control (acetaminophen, opioids; avoid antiplatelet/NSAID)
Antiemetics, stool softeners (avoid Valsalva)
Nimodipine 60 mg PO/NG q4h × 21 days — improves neurologic outcomes by reducing vasospasm-related ischemia (not by preventing vasospasm itself)
Definitive aneurysm treatment as early as feasible, preferably within 24 h of onset (AHA/ASA 2023): endovascular coiling (preferred when feasible — ISAT) or surgical clipping
Seizure prophylaxis (short-course levetiracetam) controversial; many centers use only with documented seizures
Second-line / adjunct
External ventricular drain for hydrocephalus or to monitor ICP
Transcranial doppler daily to monitor for vasospasm (peak days 4-14)
CT within 6 hours of headache onset, interpreted by an experienced reader, is essentially 100% sensitive in a patient with no new neurologic deficit — LP may be safely omitted in this setting (Perry et al., BMJ 2011; AHA/ASA 2023).
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 2011 (CT within 6 h) — Sensitivity of Computed Tomography Performed Within Six Hours of Onset of Headache for Diagnosis of Subarachnoid Haemorrhage (Perry et al., BMJ 2011;343:d4277)
Hunt-Hess — Surgical Risk as Related to Time of Intervention in Repair of Intracranial Aneurysms (Hunt & Hess, J Neurosurg 1968)
Practice Neurology questions on FirstPassPA
Turn this outline into retention. 7,700+ board-style questions with an AI tutor that explains every answer — free to start, no card required.
Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.