Headache Red Flags

NeuroWiki

Headache Red Flags & When to Image

Most headaches presenting to a neurologist are primary headache disorders. The challenge is identifying the minority that signal a secondary — and potentially dangerous — cause. A structured approach to red flags guides when to investigate, what imaging to order, and how urgently to act.

Bottom Line

Thunderclap headache (peak intensity in <1 minute) is a neurovascular emergency until proven otherwise — CT head immediately, then LP if CT is negative
New headache >50 years: Always consider GCA (ESR, CRP, temporal artery assessment) and intracranial pathology
Normal neurologic exam does not exclude secondary headache — imaging decisions are driven by the history
MRI with contrast is the study of choice for most non-emergent headache workups; CT without contrast for emergencies (SAH, acute bleed)
Routine imaging for typical migraine with normal exam is not indicated (AAN Practice Guideline)
Use SNNOOP10 as a systematic checklist — any positive element warrants investigation

SNNOOP10 Red Flag Framework

The SNNOOP10 mnemonic (Do et al., Neurology 2019) is the current evidence-based framework for identifying secondary headache warning signs. It expands on earlier versions to include 10 distinct red flag categories.

Letter	Red Flag	Concern	Action
S	Systemic symptoms (fever, weight loss, fatigue) or systemic disease (malignancy, HIV, immunosuppression)	Infection (meningitis, abscess), metastasis, opportunistic infection	MRI with contrast; consider LP; CBC, metabolic panel
N	Neurologic signs (focal deficits, papilledema, altered consciousness, seizure, cognitive change)	Mass lesion, stroke, venous thrombosis, raised ICP, encephalitis	Urgent MRI (or CT if hyperacute); MRV if papilledema
N	Neoplasm history	Brain metastasis, leptomeningeal carcinomatosis	MRI with contrast; consider LP for CSF cytology
O	Onset sudden (thunderclap — peak in <1 min)	SAH, RCVS, CVT, cervical dissection, pituitary apoplexy	CT head STAT → LP if CT negative → CTA/MRA
O	Older age (new onset >50 years)	GCA, intracranial neoplasm, subdural hematoma	ESR/CRP; MRI with contrast; temporal artery evaluation
P	Pattern change (progressive worsening, new type, change from prior headaches)	Mass lesion, medication overuse masking pathology, CSF pressure disorder	MRI with and without contrast; reassess if MOH present
1	Positional (worse lying down or standing)	Raised ICP (worse supine), intracranial hypotension (worse upright)	MRI with contrast; MRV if raised ICP; LP for opening pressure
0	Precipitated by Valsalva, cough, exertion, or sexual activity	Chiari malformation, posterior fossa lesion, SAH, CSF leak	MRI with craniocervical junction; if first/worst → rule out SAH
P	Papilledema	IIH, CVT, mass lesion with raised ICP	MRI + MRV; then LP for opening pressure (after excluding mass)
P	Progressive headache or atypical presentation	Chronic subdural, slow-growing mass, chronic meningitis	MRI with contrast; consider LP if infectious/inflammatory cause suspected
P	Pregnancy or postpartum	CVT, RCVS, preeclampsia/PRES, pituitary apoplexy	MRI without gadolinium + MRV; BP monitoring; preeclampsia labs
P	Painful eye with autonomic features	Acute glaucoma, cavernous sinus pathology, Tolosa-Hunt, cluster headache	Ophthalmology eval (IOP); MRI orbits/cavernous sinus with contrast
P	Post-traumatic onset	Subdural/epidural hematoma, CSF leak, dissection, DAI	CT head (acute); MRI if subacute/chronic or symptoms persist
P	Pathology of immune system (HIV, immunosuppression)	Toxoplasmosis, PML, cryptococcal meningitis, CNS lymphoma	MRI with contrast; LP with CSF analysis (cryptococcal Ag, cytology)
P	Painkiller overuse or new medication	MOH masking secondary cause; drug-induced headache (nitrates, PDE5i)	Detailed medication history; withdraw overused meds; re-image if no improvement

🔴 Medication Overuse Can Mask Secondary Headache

Patients with chronic daily headache attributed to MOH who fail to improve after medication withdrawal should be re-evaluated for secondary causes
Progressive headache despite preventive therapy warrants repeat imaging
A "new" pattern emerging within chronic headache always requires fresh investigation

Thunderclap Headache

A headache reaching maximum intensity within <1 minute. This is a neurologic emergency — the differential includes multiple life-threatening conditions.

Differential Diagnosis

Subarachnoid hemorrhage — the primary concern; ~25% of thunderclap headaches with negative CT have SAH on LP
Reversible cerebral vasoconstriction syndrome (RCVS) — recurrent thunderclap headaches, often triggered by vasoactive substances; CTA/MRA may show segmental vasoconstriction
Cerebral venous thrombosis
Cervical artery dissection
Pituitary apoplexy
Spontaneous intracranial hypotension (with acute CSF leak)
Primary thunderclap headache — diagnosis of exclusion only

Workup Protocol

Thunderclap Headache: Step-by-Step

CT head without contrast — sensitivity for SAH: ~98-100% within 6 hours (modern scanners), ~93% at 24 hours, drops significantly after 3 days
If CT is negative: lumbar puncture — look for xanthochromia (spectrophotometry preferred) and elevated RBCs in tube 4. Ideally performed ≥6 hours after onset to allow xanthochromia development
If LP is also negative: CTA or MRA — evaluate for RCVS, dissection, aneurysm without rupture
Consider MRI with contrast if the above are negative and suspicion remains — evaluate for CVT, pituitary apoplexy, posterior fossa lesion
If RCVS suspected but initial CTA normal: Repeat vascular imaging in 1-2 weeks (vasoconstriction may not be present initially)

🔴 Ottawa SAH Rule: When CT Alone May Suffice

The Ottawa SAH Rule suggests CT alone within 6 hours by experienced radiologist may be sufficient to rule out SAH only if ALL of the following are met:
- Alert patient (GCS 15)
- No focal neurologic deficits
- CT performed on modern (≥64-slice) scanner
- Interpreted by experienced neuroradiologist
This remains debated — many centers still perform LP after negative CT
After 6 hours, CT sensitivity drops and LP becomes essential
CTA can identify an unruptured aneurysm even with negative CT and LP, but does not rule out RCVS (may need repeat imaging)

RCVS vs Primary Angiitis of CNS (PACNS)

Both conditions present with headache and multifocal vessel irregularity on angiography, but management differs dramatically. RCVS is self-limiting; PACNS requires aggressive immunosuppression.

Feature	RCVS	PACNS
Headache pattern	Recurrent thunderclap headaches (multiple over days-weeks)	Progressive, insidious headache over weeks-months
Triggers	Common: vasoactive drugs (cannabis, triptans, SSRIs, decongestants), postpartum, sexual activity	None typically
Age/Sex	Female predominance; mean age 40-45	Slight male predominance; mean age 50
CSF	Normal or near-normal (<10 WBC, protein <80)	Abnormal in 80-90% (elevated protein, lymphocytic pleocytosis)
MRI brain	Often normal; may show watershed infarcts, convexity SAH, PRES-like edema	Multiple infarcts in different vascular territories; often deep/subcortical
Angiography	Multifocal segmental vasoconstriction; "string of beads"	Similar appearance (cannot distinguish on imaging alone)
Vasoconstriction course	Resolves within 3 months (hallmark finding)	Persists or progresses
Brain biopsy	Normal or nonspecific	Granulomatous or lymphocytic vasculitis (but patchy — can be falsely negative)
Treatment	Remove triggers; supportive care; CCBs (nimodipine, verapamil) empirically; no steroids (may worsen)	High-dose corticosteroids + cyclophosphamide or other immunosuppression
Prognosis	Excellent; full resolution typical; recurrence rare (~5%)	Progressive without treatment; significant morbidity/mortality

Clinical Pearl: Distinguishing RCVS from PACNS

Thunderclap headache strongly favors RCVS — PACNS almost never presents this way
Normal CSF strongly favors RCVS — abnormal CSF should raise concern for PACNS
Resolution of vasoconstriction at 12 weeks confirms RCVS — repeat CTA/MRA is essential
When uncertain, avoid immunosuppression initially; follow clinically and with repeat imaging
Brain biopsy is rarely needed but may be considered when PACNS is strongly suspected and CSF is abnormal

Giant Cell Arteritis (GCA)

GCA is the most important consideration in new headache >50 years. Delayed diagnosis risks permanent vision loss. A high index of suspicion and low threshold for empiric treatment are essential.

GCA: Clinical Features and Diagnostic Approach

Classic features: New headache (often temporal), scalp tenderness, jaw claudication, visual symptoms (transient or permanent vision loss, diplopia), polymyalgia rheumatica symptoms (shoulder/hip girdle pain and stiffness)
Constitutional symptoms: Fever, weight loss, malaise, anorexia are common
Exam findings: Tender, thickened, or pulseless temporal artery; fundoscopy may show pale/swollen optic disc (anterior ischemic optic neuropathy)

Laboratory Workup

ESR: Typically >50 mm/hr (often >100); age-adjusted upper limit = age/2 (men) or (age+10)/2 (women)
CRP: Usually elevated; may be more sensitive than ESR
CBC: Normocytic anemia, thrombocytosis common
Note: ~5% of biopsy-proven GCA have normal ESR — do not exclude GCA based on normal inflammatory markers alone

Diagnostic Confirmation

Temporal artery biopsy: Gold standard; obtain ≥1 cm sample; sensitivity ~85-90% (skip lesions can cause false negatives); bilateral biopsy increases yield
Temporal artery ultrasound: "Halo sign" (dark hypoechoic ring around artery lumen) is highly specific (~100%) when present; sensitivity 54-81%; increasingly used as first-line in experienced centers
2022 ACR/EULAR criteria: Age ≥50 + new temporal headache or scalp tenderness + temporal artery abnormality or elevated ESR/CRP; classification criteria (not diagnostic criteria) but useful framework

Treatment Urgency

Do not delay treatment for biopsy — start prednisone 1 mg/kg/day (or 40-60 mg/day) immediately if clinical suspicion is moderate-high
Biopsy remains positive for 2-4 weeks after starting steroids
If visual symptoms present: IV methylprednisolone 1g daily x 3 days, then oral prednisone
Add low-dose aspirin (81-100 mg) for ischemic complication prevention

🔴 GCA Visual Emergency

Transient visual loss (amaurosis fugax) in suspected GCA is an emergency — permanent vision loss can occur within hours to days
Once vision is lost, it is rarely recoverable; treatment prevents contralateral eye involvement
Admit for IV steroids and urgent ophthalmology consultation

Spontaneous Intracranial Hypotension (SIH)

SIH results from CSF leak, most commonly at the thoracic or cervicothoracic spine. The classic presentation is orthostatic headache (worse upright, better supine), though atypical presentations are common.

Clinical Features

Orthostatic headache: Worsens within 15 minutes of standing; improves within 15-30 minutes of lying down. May become less positional over time.
Associated symptoms: Neck stiffness, nausea, photophobia, hearing changes (muffled hearing, tinnitus), diplopia (CN VI palsy from brain sagging)
Atypical presentations: "Second-half-of-the-day headache," thunderclap headache, frontotemporal dementia-like syndrome (behavioral changes from bilateral subdural hygromas), coma

MRI Findings: The SEEPS Mnemonic

SEEPS: MRI Signs of Intracranial Hypotension

S — Subdural collections (hygromas or hematomas) — bilateral, often thin
E — Enhancement of pachymeninges (diffuse, smooth, non-nodular dural enhancement on post-contrast T1)
E — Engorgement of venous structures (distended dural venous sinuses, prominent epidural venous plexus)
P — Pituitary enlargement (hyperemia; may mimic pituitary adenoma)
S — Sagging of brain (downward displacement of cerebellar tonsils, flattening of pons against clivus, crowding of posterior fossa)

Note: MRI can be normal in ~20% of SIH cases, especially early. If clinical suspicion is high, proceed with further workup.

Diagnostic Workup

MRI brain with and without contrast: First-line; look for SEEPS findings
LP (if done): Low opening pressure (<60 mm H₂O); but LP can worsen symptoms and may be normal in some cases
MRI spine with contrast: May show extradural CSF collection, epidural venous engorgement, or meningeal diverticula
CT myelography: Gold standard for leak localization; performed if blood patch fails or precise localization needed for surgical repair
Dynamic CT myelography or digital subtraction myelography: For fast-flow leaks (CSF-venous fistulas)

Treatment

Conservative: Bed rest, hydration, caffeine (300-500 mg/day), abdominal binder — effective in mild cases
Epidural blood patch (EBP): First-line interventional treatment; 15-30 mL autologous blood injected into epidural space. Success rate ~30-90% per patch; may require multiple attempts.
Targeted blood patch or fibrin glue: If leak site is identified on imaging
Surgical repair: For refractory cases with identified leak (especially meningeal diverticula or ventral dural tears)

Idiopathic Intracranial Hypertension (IIH)

IIH (formerly pseudotumor cerebri) presents with headache and papilledema due to elevated ICP without identifiable cause. It predominantly affects young women with obesity.

IIH: Diagnosis and Management

Typical patient: Obese woman of childbearing age with daily headache, transient visual obscurations, pulsatile tinnitus, papilledema
Diagnostic criteria (modified Dandy):
- Signs/symptoms of elevated ICP (headache, papilledema, visual symptoms)
- Elevated LP opening pressure (>250 mm H₂O in adults; >280 in children)
- Normal CSF composition
- No other cause identified (normal MRI, MRV)
Imaging: MRI + MRV to exclude CVT and mass; MRI may show empty sella, flattened posterior globes, optic nerve sheath distension, transverse sinus stenosis
Ophthalmology referral: Essential; monitor visual fields (automated perimetry) — vision loss is the major morbidity
Treatment:
- Weight loss (5-10% body weight can resolve IIH)
- Acetazolamide 500-1000 mg BID (titrate up as tolerated); IIHTT showed modest improvement in visual field function with acetazolamide + diet vs diet alone
- Topiramate (alternative — also promotes weight loss)
- Surgical: optic nerve sheath fenestration or CSF shunting for refractory cases with progressive visual loss

When to Image: Indications by Scenario

Imaging NOT Routinely Indicated

Typical migraine with or without aura meeting ICHD-3 criteria with normal neurologic exam (AAN Level B recommendation)
Chronic stable headache with unchanged pattern and normal exam
Tension-type headache with typical features

Imaging Indicated

Scenario	Study of Choice	Rationale
Thunderclap headache	CT without contrast (emergent) → LP → CTA	Rule out SAH, RCVS, dissection
New focal neurologic deficit	MRI with contrast	Mass, stroke, demyelination, abscess
Papilledema on exam	MRI + MRV	Rule out mass, CVT; then LP for opening pressure
New headache >50 years	MRI with contrast + ESR/CRP	Neoplasm, GCA, subdural
Progressive worsening over weeks	MRI with and without contrast	Mass lesion, chronic subdural, hydrocephalus
Positional headache (worse upright)	MRI brain + spine with contrast	Pachymeningeal enhancement, brain sagging (SIH)
Positional headache (worse supine)	MRI + MRV	Raised ICP; rule out mass and CVT
Cough/Valsalva-triggered headache	MRI with craniocervical junction	Chiari I malformation, posterior fossa lesion
Headache with fever + meningismus	CT without contrast (before LP if altered mental status or focal signs)	Rule out mass/shift before LP; LP for meningitis
Immunocompromised patient with new headache	MRI with contrast	Toxoplasmosis, lymphoma, PML, cryptococcal disease
Pregnancy/postpartum with new headache	MRI without gadolinium + MRV	CVT, RCVS, preeclampsia/PRES
Cancer patient with new headache	MRI with contrast	Brain metastasis, leptomeningeal carcinomatosis

CT vs MRI: Which to Order

Use CT	Use MRI
Acute SAH (<6 hours — highest sensitivity)	Non-emergent headache workup (superior soft tissue detail)
Trauma with suspected acute hemorrhage	Suspected mass, infection, or demyelination
Before emergent LP (rule out mass effect)	Suspected intracranial hypotension (pachymeningeal enhancement)
Acute altered mental status or focal signs in ED	Posterior fossa pathology (CT has beam-hardening artifact)
Patient cannot undergo MRI (pacemaker, severe claustrophobia)	Pituitary pathology, Chiari malformation, CVT (with MRV)

Additional Workup Beyond Imaging

Test	When to Order
ESR + CRP	New headache >50 years (GCA screening); consider if any systemic symptoms present
Lumbar puncture	Thunderclap with negative CT; suspected meningitis; papilledema (after imaging); suspected IIH or SIH (opening pressure)
MRA or CTA (head and neck)	Thunderclap headache workup; suspected dissection; suspected RCVS (may need repeat at 2-4 weeks)
MRV	Papilledema; suspected CVT; postpartum headache with red flags; IIH workup
Temporal artery ultrasound	Suspected GCA (look for halo sign); increasingly first-line in experienced centers
Temporal artery biopsy	Suspected GCA for definitive diagnosis; obtain within 2 weeks of starting steroids
CT myelography	SIH with failed blood patch or need for precise leak localization
CBC, metabolic panel	Systemic symptoms, infection concern, altered consciousness

Common Pitfalls

🔴 Avoid These Mistakes

Attributing a new headache pattern to migraine without workup — migraine is a diagnosis based on established pattern; a first-ever severe headache requires investigation
Accepting negative CT as ruling out SAH beyond 6 hours — sensitivity drops significantly; LP is needed
Missing CVT — CT is often normal; headache may mimic migraine or IIH; always consider in young women on OCP, peripartum, or with risk factors for thrombosis
Ordering CT when MRI is the appropriate study — for non-emergent workup, CT misses posterior fossa pathology, small tumors, intracranial hypotension, and pituitary lesions
Not checking opening pressure when doing LP for headache — always measure; it may diagnose IIH or confirm low-pressure headache
Forgetting to ask about pregnancy in women of childbearing age — changes both the differential (CVT, preeclampsia, RCVS) and imaging approach (avoid gadolinium)
Confusing RCVS with PACNS — both show vessel irregularity but treatment is opposite; thunderclap onset and normal CSF favor RCVS
Delaying steroids in suspected GCA while awaiting biopsy — vision loss is irreversible; treat empirically if suspicion is moderate-high
Assuming MOH is the only diagnosis — patients with medication overuse who don't improve with withdrawal need re-evaluation

References

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Ducros A, Wolff V. The typical thunderclap headache of reversible cerebral vasoconstriction syndrome and its various triggers. Headache. 2016;56(4):657-673.
Calabrese LH, et al. Primary angiitis of the central nervous system: diagnostic criteria and clinical approach. Cleve Clin J Med. 2015;82(4):216-226.
Dejaco C, et al. Giant cell arteritis and polymyalgia rheumatica: 2022 update. Lancet. 2022;400(10365):1771-1785.
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Friedman DI, et al. Diagnostic criteria for idiopathic intracranial hypertension. Neurology. 2013;81(13):1159-1165.
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