# Acute Headache Imaging

Urgent neuroimaging is indicated when acute headache has thunderclap onset, focal deficits, altered consciousness, fever, papilledema, pregnancy or puerperium, cancer, immunosuppression, coagulopathy, trauma, or a progressive new pattern. Test selection should follow the suspected vascular, infectious, pressure-related, or sellar emergency.

**Clinical question:** Which adults with acute headache need urgent imaging, which test should be ordered first, and what should follow a negative scan?

Updated: 2026-09-15T21:16:35.377669+00:00

## What matters in practice
- Obtain emergent neuroimaging for thunderclap headache and for acute headache accompanied by focal neurologic deficit, impaired consciousness, fever, progressive symptoms, papilledema, visual symptoms, pregnancy or puerperium, cancer, coagulopathy, or immunosuppression. [8][10]
- For suspected subarachnoid hemorrhage, noncontrast head CT is the initial study; ACEP addresses whether a normal noncontrast CT obtained within 6 hours of onset can end further subarachnoid hemorrhage workup in selected ED patients. [5][6]
- A negative noncontrast CT does not exclude several important secondary causes, including meningitis, intracranial-pressure disorders, and subarachnoid hemorrhage presenting more than 6 hours after onset; use lumbar puncture or MRI when the clinical branch requires it. [8][9]
- Recurrent thunderclap headaches over days to weeks warrant CTA or MRA for reversible cerebral vasoconstriction syndrome; diagnostic confirmation requires demonstration that vasoconstriction reverses within 12 weeks. [9][15]
- Abrupt severe headache with visual loss, ophthalmoplegia, altered consciousness, or hemodynamic instability should trigger urgent MRI for pituitary apoplexy and immediate high-dose corticosteroid treatment while arranging multidisciplinary assessment. [12]

## Who needs emergent imaging for acute headache?

Image for a secondary-headache signal, not headache intensity alone.

Obtain urgent cranial imaging when headache is abrupt and severe, new or progressive, or associated with fever, impaired consciousness, focal neurologic deficit, seizure, meningeal signs, visual disturbance, papilledema, exertional provocation, pregnancy or puerperium, cancer, coagulopathy, immunosuppression, or trauma. These findings redirect evaluation from a primary-headache pathway toward hemorrhagic, infectious, vascular, mass-effect, or pressure-related disease. [8][10]

Before imaging, document onset-to-peak time, time of onset, prior pattern, trauma, vasoactive or toxic exposure, anticoagulation or coagulopathy, pregnancy/postpartum state, cancer and immunosuppression, fever, neck stiffness, and positional symptoms. Perform vital signs, a complete neurologic examination, fundoscopy when feasible, and targeted ocular examination for red eye, pupillary abnormality, peri-orbital pain, or visual complaints. [10]

Do not routinely image a patient with an established migraine pattern, normal neurologic examination, and no new symptoms or red flags. New fever, seizure, trauma, or substantial change in a persistent headache pattern is an exception and should reopen secondary-cause evaluation. [4]
- Thunderclap headache: presume subarachnoid hemorrhage until excluded with urgent CT, with lumbar puncture when indicated by the diagnostic pathway. [9]
- Fever with altered mental status, meningeal signs, focal deficits, HIV, or another immunocompromised state: add lumbar puncture and CSF testing when infection is suspected. [11]
- New persistent headache with progressive focal or nonfocal symptoms plus an abnormal neurologic examination: evaluate for raised intracranial pressure and structural disease rather than treating as uncomplicated primary headache. [9]
- Orthostatic headache: consider low-pressure headache; the positional component can fade over time, so a previously positional history remains diagnostically relevant. [9]

*Clinical patterns that change the initial imaging pathway. [8][9][10][12][15]*

| Presentation pattern | Immediate diagnostic concern | Next test or escalation |
| --- | --- | --- |
| Abrupt severe headache peaking within minutes | Subarachnoid hemorrhage, other vascular catastrophe | Urgent noncontrast head CT; pursue additional vascular imaging or lumbar puncture according to timing and residual suspicion. [5][6][9] |
| Recurrent thunderclap headaches over days to weeks | Reversible cerebral vasoconstriction syndrome | CTA or MRA for multifocal arterial narrowing; plan follow-up evidence of reversibility within 12 weeks. [9][15] |
| Headache with fever, altered mental status, meningismus, focal deficit, or immunocompromise | CNS infection | CT when clinically needed before LP, then lumbar puncture with CSF studies without delay when meningitis is suspected. [9][11] |
| Headache with papilledema or progressive abnormal neurologic findings | Raised intracranial pressure, mass lesion, hydrocephalus, or venous disease | Urgent cranial imaging; use MRI or venographic imaging when the suspected process is not resolved by noncontrast CT. [8][9] |
| Abrupt headache with visual field loss, ophthalmoplegia, vomiting, altered consciousness, or hemodynamic instability | Pituitary apoplexy with acute adrenal insufficiency | Urgent MRI and immediate high-dose corticosteroids; obtain neurosurgical, endocrine, and ophthalmologic assessment. [12] |

## How should thunderclap headache be imaged?

The first objective is exclusion of subarachnoid hemorrhage.

Order noncontrast head CT immediately for thunderclap headache because it is the imaging modality of choice for acute subarachnoid hemorrhage and is superior to MRI for detecting acute subarachnoid blood. If CT shows subarachnoid or parenchymal hemorrhage, proceed to vascular evaluation for aneurysm or vascular malformation. [5]

For an adult ED patient, ACEP specifically evaluates whether a normal noncontrast CT performed within 6 hours of headache onset can preclude further diagnostic workup for subarachnoid hemorrhage. The timing must be established accurately; a normal CT obtained later, or a clinical picture with persisting high concern, should not be treated as equivalent. [6][8]

When CT is normal but subarachnoid hemorrhage remains clinically suspected, perform lumbar puncture with opening pressure, red and white blood cell counts, protein, glucose, and spectrophotometry for xanthochromia. In the absence of another indication such as meningitis, delay LP for at least 6 hours and preferably 12 hours after headache onset to permit xanthochromia detection. [9]
- Use the Ottawa SAH Rule as a clinical risk-stratification aid in patients at risk for subarachnoid hemorrhage; it is designed to avoid missed subarachnoid hemorrhage, not to establish the cause of headache independently. [8][5]
- Do not stop at a negative CT when red flags persist: infectious disease, increased or decreased intracranial pressure, and subarachnoid hemorrhage after 6 hours may not be apparent on CT. [8]
- If hemorrhage is identified, add vascular imaging to investigate aneurysm or arteriovenous malformation. [5]

### When CT and LP do not explain thunderclap headache

Choose further vascular or parenchymal imaging from the phenotype. CTA or MRA is appropriate for recurrent thunderclap attacks suggesting RCVS; MRI can identify cortical subarachnoid hemorrhage, posterior reversible encephalopathy syndrome, infarction, or hematoma in this setting. [9]
- Consider cerebral venous thrombosis when the clinical context supports venous disease; obtain CT venography or MR venography rather than relying on unenhanced CT alone. [8][14]
- Consider cervical artery dissection, ischemic stroke, hypertensive crisis, and RCVS among nonaneurysmal vascular causes of thunderclap headache. [15]
- With headache-associated blood-pressure surges, obtain urine catecholamines to evaluate for pheochromocytoma. [9]

*Sequential testing after thunderclap headache. [5][6][8][9][15]*

| Result or clinical condition | Interpretation | Next action |
| --- | --- | --- |
| Noncontrast CT shows subarachnoid or parenchymal hemorrhage | Hemorrhagic emergency | Obtain vascular evaluation for aneurysm or vascular malformation. [5] |
| Normal CT within 6 hours of onset | Use the ACEP timing-specific subarachnoid hemorrhage pathway; reassess whether the patient meets the clinical circumstances addressed by that policy. [6] | Do not extend this result to later presentations or alternate secondary-headache syndromes. [8] |
| Normal CT with residual concern for subarachnoid hemorrhage | CT alone may miss subarachnoid hemorrhage, particularly after 6 hours. [8][11] | LP with opening pressure and CSF cell count, protein, glucose, and xanthochromia assessment; absent suspected meningitis, perform at least 6 hours and preferably 12 hours after onset. [9] |
| Repeated thunderclap headaches; CT/MRI initially normal | RCVS remains possible | CTA, MRA, or angiography for multifocal vasoconstriction; establish reversibility by 12 weeks. [13][15] |

## When should MRI, CTA/MRA, or venography replace a CT-only approach?

Select the modality that tests the leading dangerous mechanism.

Use MRI when noncontrast CT is unrevealing but the phenotype suggests a lesion CT may miss, including intracranial-pressure disorders, infection, ischemia, posterior reversible encephalopathy syndrome, or pituitary apoplexy. In patients with acute headache and red flags, lumbar puncture or MRI is warranted when the initial CT does not resolve the diagnostic concern. [8][12]

Use CTA or MRA when arterial pathology is suspected. Recurrent thunderclap headache, especially over days to weeks, should prompt CTA or MRA for the multifocal “string of beads” pattern of RCVS. Early angiography may be negative, so an initially normal vascular study does not definitively eliminate RCVS when the clinical syndrome remains compelling. [9][13]

Use CT venography or MR venography when cerebral venous thrombosis is a plausible cause, particularly in pregnancy or puerperium or when headache occurs with other red flags. Conventional CT may be normal in conditions requiring venographic imaging. [8][10][14]
- PRES: headache with encephalopathy or seizures, especially with hypertension, renal failure, immunosuppressive medication exposure, or autoimmune disease, should prompt brain imaging for subcortical vasogenic edema. [7]
- RCVS versus PRES: MRI evidence of parietal, frontal, or occipital vasogenic edema supports PRES; multifocal cerebral arterial vasoconstriction supports RCVS. [15]
- Pituitary apoplexy: MRI is the diagnostic imaging study when abrupt headache occurs with visual field deficit, ophthalmoplegia, or altered consciousness. [12]

*Imaging selection after the initial examination. [7][8][9][12][15]*

| Suspected process | Preferred imaging direction | Finding that changes management |
| --- | --- | --- |
| Aneurysmal subarachnoid hemorrhage or acute intracranial bleeding | Noncontrast head CT first | Subarachnoid or parenchymal blood requires vascular evaluation for aneurysm or vascular malformation. [5] |
| RCVS | CTA or MRA; angiography when necessary | Multifocal arterial narrowing supports RCVS; demonstrate reversal within 12 weeks for confirmation. [13][15] |
| Cerebral venous thrombosis | CT venography or MR venography | Venous imaging is required when plain CT does not answer the venous-thrombosis question. [8][14] |
| PRES | Brain MRI | Posterior-predominant vasogenic edema supports PRES and directs blood-pressure control and removal of precipitants. [7][15] |
| Pituitary apoplexy | Urgent MRI | Hemorrhagic or ischemic pituitary changes support emergency corticosteroid treatment and decompression assessment. [12] |

## When does acute headache require lumbar puncture?

LP answers hemorrhage, infection, and pressure questions that CT may not resolve.

Perform lumbar puncture promptly for suspected meningitis; do not delay CSF testing when the presentation includes fever, altered mental status, meningeal signs, focal neurologic deficits, HIV, or other immunocompromise and the diagnostic pathway supports CNS infection. [9][11]

After normal CT in suspected subarachnoid hemorrhage, LP should include opening pressure, red and white cells, protein, glucose, and spectrophotometry for xanthochromia. For the isolated subarachnoid hemorrhage question, obtain CSF no sooner than 6 hours and preferably 12 hours from headache onset unless another diagnostic indication requires earlier LP. [9]

Measure opening pressure whenever LP is used to investigate a pressure syndrome. Elevated opening pressure is diagnostically important when idiopathic intracranial hypertension is suspected, while an orthostatic phenotype should raise concern for low-pressure headache even if the positional feature has attenuated. [11][9]
- Interpret a negative CT cautiously: CT may be negative in up to 5% of subarachnoid hemorrhage cases, so CSF red blood cells or xanthochromia remain useful when clinical suspicion is high. [11]
- Use LP as an adjunct to—not a substitute for—appropriate vascular imaging when recurrent thunderclap headache, dissection, venous thrombosis, or RCVS remains under consideration. [9][15]
- A normal CT does not exclude infectious disease or pressure disorders; match CSF testing and MRI to the suspected branch. [8]

*Lumbar puncture components and the diagnostic question they address. [9][11]*

| LP component | Clinical question | Actionable interpretation |
| --- | --- | --- |
| Opening pressure | Raised intracranial pressure or idiopathic intracranial hypertension | Elevated opening pressure is diagnostically important in suspected idiopathic intracranial hypertension. [11] |
| Red blood cells and xanthochromia spectrophotometry | Subarachnoid hemorrhage after nondiagnostic CT | Support or reduce concern for subarachnoid hemorrhage; for isolated SAH evaluation, collect at least 6 hours and preferably 12 hours after onset. [9] |
| White cells, protein, and glucose | Meningitis or other CNS infection | Interpret with the clinical syndrome and pursue infection-directed management when CSF supports it. [9][11] |

## Which imaging phenotypes require immediate disease-directed action?

Imaging should trigger escalation, not terminate the evaluation.

For pituitary apoplexy, treat acute adrenal insufficiency as the immediate life-threatening complication. In a patient with abrupt severe headache, visual field loss, ophthalmoplegia, nausea or vomiting, altered consciousness, or hemodynamic instability, obtain urgent MRI and administer high-dose corticosteroids while arranging neurosurgical, endocrine, and ophthalmologic evaluation; assess for surgical decompression when indicated. [12]

For PRES, image patients with headache plus encephalopathy or seizures in the setting of hypertension, renal failure, immunosuppressive medications, or autoimmune disease. Management includes removing precipitants and gradually reducing blood pressure by no more than 20% over 1 to 2 hours; use titratable agents to minimize blood-pressure variability. [7]

For RCVS, exclude competing hemorrhagic and vascular diagnoses before assigning the syndrome. CTA, MRA, or angiography demonstrating multifocal narrowing supports the diagnosis, but confirmation requires documented reversal within 12 weeks; differentiate from primary angiitis of the CNS, which more often has an insidious headache course. [13][15]
- If acute visual symptoms or ophthalmoplegia accompany severe headache, do not defer sellar imaging as a routine outpatient study; evaluate urgently for pituitary apoplexy. [12]
- If CT or MRI shows cortical subarachnoid hemorrhage, infarction, hematoma, or PRES-like edema during recurrent thunderclap attacks, pursue an RCVS-centered vascular evaluation while considering competing causes. [9][15]
- If clinical deterioration occurs despite initially negative CT, repeat the diagnostic branch with MRI, vascular imaging, and/or LP according to the evolving phenotype rather than labeling the event as primary headache. [8][10]

*Imaging-linked emergencies and immediate next steps. [7][9][12][15]*

| Emergency pattern | Diagnostic trigger | Immediate action |
| --- | --- | --- |
| Pituitary apoplexy | Abrupt headache with visual loss, ophthalmoplegia, altered consciousness, or MRI evidence of pituitary hemorrhage or infarction | Give high-dose corticosteroids and obtain urgent multidisciplinary assessment for decompression decisions. [12] |
| PRES | Headache, encephalopathy, or seizures with MRI vasogenic edema and a compatible hypertensive or systemic context | Remove precipitating factors and reduce blood pressure gradually by no more than 20% in 1 to 2 hours. [7] |
| RCVS | Recurrent thunderclap attacks with multifocal arterial narrowing | Use vascular imaging and document reversibility within 12 weeks; continue evaluation for hemorrhage, infarction, and PRES. [9][15] |

## References
1. Pituitary incidentaloma: a Pituitary Society international consensus guideline statement | Nature Reviews Endocrinology — www.nature.com — https://www.nature.com/articles/s41574-025-01134-8
2. ACR Appropriateness Criteria Headache — www.sciencedirect.com — https://www.sciencedirect.com/science/article/abs/pii/S154614401400163X
3. Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S0196064419305773
4. “Worst Headache of Life” in a Migraineur: Marginal Value of Emergency Department CT Scanning - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/abs/pii/S1546144018314613
5. Thunderclap headache: Diagnostic considerations and neuroimaging features — www.sciencedirect.com — https://www.sciencedirect.com/science/article/abs/pii/S0009926012005272
6. Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache — www.annemergmed.com — https://www.annemergmed.com/article/S0196-0644(19)30577-3/pdf
7. [PDF] Race Against Time Sticks and Stones - ACEP — www.acep.org — https://www.acep.org/siteassets/sites/acepanytime/media/documents/cdem-sampleissue.pdf
8. The Value of Cranial CT Imaging in Patients With Headache at the Emergency Department — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC8141591
9. Neuroimaging and other investigations in patients presenting with headache — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC3444223
10. Atypical Cause of Headache: A Potential Diagnostic Pitfall in Acute Medicine - PMC — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC13242071
11. Acute Headache - StatPearls - NCBI Bookshelf — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK554510
12. Pituitary Apoplexy - StatPearls - NCBI Bookshelf — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK559222
13. Reversible cerebral vasoconstriction syndrome associated with probable drug poisoning - PMC — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC9800248
14. Cerebral venous thrombosis presenting with subarachnoid hemorrhage: Case report and review - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/abs/pii/S0735675708000934
15. Reversible cerebral vasoconstriction syndrome: literature review — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC9833030

## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.