Thunderclap Headache
Severe headache that peaks within 1 minute. Treat as possible SAH until excluded.
All Pathways
NeurologyDiagnostic
Thunderclap Headache Evaluation
Thunderclap Headache Evaluation: Thunderclap Headache → Assess: Onset Time and Red Flags → High Risk: No Clinical Rule-Out → Ottawa SAH Rule: Positive i...
Pathway Overview
26 steps
Algorithm Steps
26 total
01Start 02Action Assess: Onset Time and Red Flags
Record the exact time of onset; it decides the 6-hour CT rule
- Time of onset and time to peak intensity
- Reduced GCS, seizure, new focal deficit, loss of consciousness
- Neck pain or stiffness, vomiting, photophobia
- Anticoagulant or antiplatelet use; pregnancy or postpartum
- Onset with exertion or sex; similar recent headache
- Recent vasoactive or illicit drug use (RCVS trigger)
- Give effective analgesia, opioid if needed; note it before neuro checks
03Warning High Risk: No Clinical Rule-Out
These patients need a different approach
- GCS below 15, new deficit or seizure: resuscitate, urgent CT, call neurosurgery early
- Anticoagulant, low platelets or coagulopathy: no LP (use CTA); reverse at once if bleeding
- Pregnant or postpartum: also consider eclampsia, PRES, CVT, RCVS, pituitary apoplexy
04Action Ottawa SAH Rule: Positive in Every Thunderclap Patient
Thunderclap onset is one of the rule items, so this rule never rules out SAH here
- For alert patients aged 16 or over with new severe non-traumatic headache peaking within 1 h
- Investigate if ANY: age 40 or over, neck pain or stiffness, witnessed LOC, onset with exertion, thunderclap onset, limited neck flexion
- Not valid if: GCS below 15, new deficit, papilloedema, prior aneurysm, SAH, brain tumour, shunt or hydrocephalus
- Not valid if: 3 or more similar headaches over more than 6 months, head injury in last 7 days, onset over 14 days ago
05Action Urgent Non-Contrast CT Head
Record the time from headache onset to scan
- Within 6 h of onset: sensitivity about 99% (modern CT, reported by a radiologist)
- Sensitivity falls after 6 h
- Look for subarachnoid blood, ICH, hydrocephalus, mass
- Get a radiologist report; emergency clinician reads can miss SAH
06Decision Blood on CT?
Subarachnoid blood on non-contrast CT
- If Yes
- Blood on CT
07Warning Blood on CT: SAH Diagnosed
Call neurosurgery now
- On an anticoagulant: reverse at once
- CTA head without delay to find the source
- Transfer early to a centre with aSAH expertise
08Action Confirmed SAH: Immediate Medical Care
Start once SAH is confirmed (blood on CT or xanthochromia).
- Anticoagulant: reverse at once (life-threatening bleed protocol)
- Hydrocephalus with reduced GCS: urgent CSF drainage (EVD)
- BP: avoid hypotension (MAP below 65 mm Hg) and large swings; lower gradually if SBP above 180-200 mm Hg
- No routine seizure prophylaxis; avoid phenytoin; treat seizures for 7 days
- No routine tranexamic acid
- Keep euvolaemia; no prophylactic hypervolaemia
09Action Nimodipine 60 mg Orally Every 4 h (Adult)
Start as soon as possible, within 4 days. Caution if SBP below 100 mm Hg. Liver impairment: 30 mg every 4 h. Not with rifampicin, phenytoin, carbamazepine or phenobarbital. CYP3A4 inhibitors (e.g. erythromycin, ritonavir, azole antifungals) or other BP drugs: monitor BP; dose may need reduction.
- By mouth or NG tube; never inject capsule contents
- Duration as neurosurgery advises (AU PI oral course 10-14 days; US label 21 days)
- If BP falls: treat hypotension; pause only if BP is unstable
- IV nimodipine: specialist setting only
10Decision CTA or DSA: Aneurysm Found?
CTA head without delay. DSA is the reference standard.
- If Yes
- Aneurysm found
11Action Aneurysm: Secure Early, Preferably Within 24 h
Coiling or clipping, chosen by neurosurgery and neurointervention
- Rebleeding risk is highest in the first 24 h
- Aim for complete occlusion
- Start or continue SAH medical care and nimodipine as neurosurgery advises
12Outcome SAH: Neurosurgical or Neurocritical Care
Monitor for rebleeding, hydrocephalus and delayed cerebral ischaemia
- Neuro observations by trained nurses
- Watch sodium and volume status; keep euvolaemia
- Continue nimodipine as advised
If No- No aneurysm
13Decision No Aneurysm Found: Was SAH Confirmed?
Confirmed = blood on CT or xanthochromia on LP
- If Yes
- SAH confirmed
14Action Confirmed SAH, No Aneurysm: Stay Under Neurosurgery
Angiogram-negative SAH still needs SAH care
- Diffuse SAH: DSA even if CTA is normal
- Repeat DSA, CTA or MRI as neurosurgery advises
- Continue SAH medical care as advised
- Path rejoins step 12Shared downstream outcome
If No- SAH not confirmed
15Action SAH Excluded: Look for Other Causes
Primary thunderclap headache is a last-resort diagnosis. Pregnant or postpartum: also eclampsia, PRES, CVT, RCVS.
- RCVS, artery dissection, cerebral venous thrombosis, pituitary apoplexy
- ICH, unruptured aneurysm, meningitis, colloid cyst, spontaneous intracranial hypotension
- Brain vessel imaging (CTA or MRA) and venous imaging (CTV or MRV) are needed before a primary diagnosis
- Image before discharge if: recurrent thunderclap, neck pain, deficit, seizure, pregnancy or postpartum, vasoactive drugs
16Decision Other Cause Found?
Based on history, exam, CSF and imaging
- If Yes
- Cause found
17Outcome Other Cause Found: Treat and Refer
Manage the specific cause with the right team
- RCVS: stop vasoactive drugs; neurology review
- Dissection or CVT: stroke or neurology team
- Pituitary apoplexy: urgent endocrine and neurosurgical review
If No- No cause found
18Outcome Discharge When Serious Causes Excluded
Return at once if new or worse headache, neuro symptoms, seizure or fever. Until RCVS is excluded, avoid triptans and other vasoactive drugs.
- Until RCVS is excluded: avoid triptans, ergots, decongestants and illicit stimulants
- GP follow-up; neurology referral if thunderclap recurs
- Make sure any planned vessel imaging is booked
If No- No blood
19Decision No Blood: CT Within 6 h of Onset, Alert, Normal Neuro Exam?
Yes only if ALL: age 16 or over, CT within 6 h of onset, alert, normal neuro exam, peak within 1 h, modern CT reported by a radiologist, and not mainly neck pain, syncope or seizure
- CT done within 6 h of headache ONSET (not arrival)
- Alert, normal neuro exam, headache peaked within 1 h
- Modern CT reported by a radiologist
- Not for atypical presentations: neck pain only, syncope, seizure, focal deficit; not validated under 16
- If Yes
- All conditions met
20Action All Conditions Met: SAH Excluded
No routine LP. Senior review; seek specialist advice if in doubt.
- Suspicion still high: LP or CTA anyway
- Still look for other causes (next step)
- Path rejoins step 15Shared downstream outcome
If No- Not met or over 6 h
21Action Conditions Not Met or CT After 6 h: LP (or CTA)
LP at least 12 h after onset. No LP if anticoagulated, low platelets or coagulopathy: use CTA.
- Record opening pressure
- Cell count in first and last tube, protein, glucose
- Xanthochromia by spectrophotometry
- CTA instead of LP is acceptable; an aneurysm with no blood needs neurosurgical advice
22Decision LP or CTA Positive or Indeterminate?
Yes: xanthochromia, final-tube RBC 2000 x10^6/L or more, or aneurysm on CTA. No: no xanthochromia and final-tube RBC below 2000 x10^6/L, or normal CTA.
- Do not rely on RBCs falling between tubes alone
- Normal CTA can miss aneurysms under 3 mm and non-aneurysmal SAH: LP if suspicion stays high
- If Yes
- Positive or indeterminate
23Warning LP or CTA Positive or Indeterminate: Call Neurosurgery
Treat as possible SAH until neurosurgery reviews
- Call neurosurgery
- CTA head now if not done
- Xanthochromia = SAH confirmed
24Decision Xanthochromia on LP?
Yes = SAH confirmed. No = RBC-only LP, or CTA done instead of LP
- If Yes
- Xanthochromia
- Path rejoins step 08Shared downstream outcome
If No- No xanthochromia
25Action No Xanthochromia (RBC Only or CTA Route): Angiography Before SAH Care
RBC-only LP may be a traumatic tap; an aneurysm without blood may be incidental. Start SAH care and nimodipine only if neurosurgery confirms SAH.
- CTA now if not done; DSA as neurosurgery advises
- Aneurysm found: neurosurgery decides on SAH care and treatment
- No aneurysm: SAH not confirmed; look for other causes
- Path rejoins step 10Shared downstream outcome
If No- Negative
26Action LP Negative or Normal CTA: SAH Unlikely
No xanthochromia and final-tube RBC below 2000 x10^6/L, or normal CTA with low suspicion
- Check opening pressure, cells and protein for other causes
- Continue to look for other causes (next step)
- Path rejoins step 15Shared downstream outcome
Guideline Source
2023 AHA/ASA Guideline for the Management of Patients With Aneurysmal Subarachnoid Hemorrhage (Hoh et al, Stroke 2023)
Clinical Safety Information
Clinical Decision Support — Not a Substitute for Clinical Judgment
Individual patient factors may require deviation from these recommendations.
Known Limitations
- The 6-hour CT rule-out applies only to alert patients with a normal neuro exam, a clear onset time and a modern CT reported by a radiologist
- Excluding SAH does not exclude RCVS, dissection, CVT or pituitary apoplexy; vessel and venous imaging may still be needed
- CTA can miss small aneurysms; LP needs 12 h from onset and spectrophotometry
- SAH treatment steps are a summary; follow the neurosurgical unit protocol
Contraindicated Populations
Children under 16 (Ottawa rule and 6-hour CT rule not validated)GCS below 15, new focal deficit or seizure: no clinical rule-out; urgent CT and neurosurgeryPregnant or postpartum: also consider eclampsia, PRES, CVT, RCVS, pituitary apoplexyAnticoagulant, low platelets or coagulopathy: no LP; reverse anticoagulation if bleeding
Applicable Regions
USEUUKAUglobal
AU: Nimodipine 30 mg tablets (Nimotop) are TGA-registered; the AU PI oral course is 10-14 days. Check local access to CSF spectrophotometry.
EU: Similar approach; some centres use CTA first after a negative CT
UK: NICE NG228 (2022): no routine LP if CT within 6 h of onset is normal and radiologist-reported; LP at least 12 h after onset with spectrophotometry
US: AHA/ASA 2023 aSAH guideline; ACEP 2019 clinical policy on acute headache
Version 2Next review: 2027-09-30
Next steps
Finish the workflow by opening the most relevant calculator, then convert the session into a live account when you are ready.
Related Resources
Frequently Asked Questions
What is the Thunderclap Headache Evaluation?
The Thunderclap Headache Evaluation is a diagnostic clinical algorithm for Neurology. It provides a structured decision tree to guide clinical decision-making, based on 2023 AHA/ASA Guideline for the Management of Patients With Aneurysmal Subarachnoid Hemorrhage (Hoh et al, Stroke 2023).
What guideline is the Thunderclap Headache Evaluation based on?
This algorithm is based on 2023 AHA/ASA Guideline for the Management of Patients With Aneurysmal Subarachnoid Hemorrhage (Hoh et al, Stroke 2023) (DOI: 10.1161/STR.0000000000000436).
What are the limitations of the Thunderclap Headache Evaluation?
Known limitations include: The 6-hour CT rule-out applies only to alert patients with a normal neuro exam, a clear onset time and a modern CT reported by a radiologist; Excluding SAH does not exclude RCVS, dissection, CVT or pituitary apoplexy; vessel and venous imaging may still be needed; CTA can miss small aneurysms; LP needs 12 h from onset and spectrophotometry; SAH treatment steps are a summary; follow the neurosurgical unit protocol. Individual patient factors may require deviation from these recommendations.
Get AI-Powered Analysis Alongside This Algorithm
In AttendMe.ai, the Thunderclap Headache Evaluation appears automatically when your clinical question matches — alongside evidence from 3M+ peer-reviewed articles.
Try AttendMe Free