Headache Red and Green Flags: A Practical Guide to Identifying Secondary Causes and Selecting Appropriate Neuroimaging

Headache disorders are among the most prevalent disorders worldwide and among the leading causes of years lived with disability.¹ Most headaches encountered in clinical practice are caused by primary headache disorders, for example, migraine, tension-type headache, and cluster headache. It is critical for clinicians to distinguish these benign primary headache conditions from rare or potentially life-threatening secondary causes of headache. Only about 1 percent of patients who present to outpatient neurology clinics with headaches have significant intracranial abnormalities.² This is even lower in primary care.² However, the consequences of misdiagnosing a secondary headache can be detrimental.

This article provides an overview of the SNNOOP10 red-flag criteria, describes headache green flags, and guides providers on when to order imaging and which type of neuroimaging is appropriate. 

The SNNOOP10 Red Flags: A Screening Tool

In 2003, the original SNOOP mnemonic tool was proposed. SNOOP stood for systemic symptoms and signs, neurologic symptoms, onset-sudden or after the age of 40 and pattern change.³ We now use the current SNNOOP10 list, which includes 15 red flag items.³ “Red flags” are clinical findings or symptoms that warrant further testing, while “orange flags” become alarming when combined with other flags.³

The SNNOOP10 categories and potential causes of secondary headache:^2,3

1. Systemic symptoms, including fever: Systemic or neurologic infection, vasculitis, rheumatic or other inflammatory disease. Isolated fever is an orange flag.
2. Neoplasm history: Metastatic disease to the brain, intracranial neoplasm, or leptomeningeal disease. 
3. Neurologic deficit or altered level of consciousness: CNS infectious process, vascular malformation, ischemic stroke, intracranial hemorrhage, abscess, or mass. 
4. Onset that is sudden or abrupt (i.e., thunderclap headache): Associated with a high pretest probability of serious intracranial pathology, including subarachnoid hemorrhage, warranting prompt evaluation. Other secondary causes include stroke, posterior fossa mass lesion, cerebral venous or sinus thrombosis, intracranial hypotension, reversible cerebral vasoconstriction syndrome (RCVS).
5. Older age (onset after age 50): Infection, giant cell arteritis, cerebrovascular disease, or neoplasm. 
6. Pattern change or recent onset of a new headache: Mass, new intracranial abnormality, cerebral venous thrombosis, or medication overuse. 
7. Positional Headache: Intracranial hypertension or hypotension (due to a CSF leak). 
8. Precipitated by sneezing, coughing, or exercise: Chiari malformation type 1 or other posterior fossa structural lesions (e.g., tumors, cysts); with sudden or thunderclap onset, consider subarachnoid hemorrhage or reversible cerebral vasoconstriction syndrome (RCVS). 
9. Papilledema: Intracranial hypertension, intracranial tumor, space-occupying lesion, or non-vascular intracranial disorder. 
10. Progressive headache and atypical presentations: Space-occupying lesion, cerebral venous thrombosis, or other intracranial disorders. 
11. Pregnancy and Postpartum: Cerebral venous sinus thrombosis, preeclampsia, diabetes, anemia, hypothyroidism, post-dural puncture headache, pituitary disorders, or cranial or cervical vascular disorders. 
12. Painful eye with autonomic features: Primary ophthalmic disorders, Tolosa-Hunt Syndrome, glaucoma, corneal disorders, posterior fossa, pituitary, or cavernous sinus pathology. 
13. Posttraumatic onset: Acute or chronic posttraumatic headache or subdural hematoma. If the duration of headache is chronic, it is an orange flag.
14. Pathology of the immune system (i.e., HIV): CNS lesions or opportunistic infections.
15. Painkiller overuse or new drug at onset: Medication-overuse headache or a medication-related secondary headache. 

This is not a comprehensive list of potential secondary headaches.

A few SNNOOP10 findings require emergency evaluation and workup, including acute thunderclap headache; fever with nuchal rigidity or other signs of meningeal irritation; papilledema with or without an associated focal neurologic deficit or change in level of consciousness; and concern for acute angle-closure glaucoma.²

It is important to acknowledge that even with this framework and screening for red flags, most neuroimaging studies yield no clinically relevant findings. A study found that 14.5% of newly presenting patients at the Birmingham Headache Service in the UK underwent imaging based on clinical assessment, including history, neurological examination, and red and orange/yellow flags, and 2.1% of those imaged had a clinically significant abnormality.⁴ These findings support a selective approach to neuroimaging rather than routinely imaging all patients with headache.

Headache Green Flags: Identifying Primary Headache Disorders

While red flags raise suspicion for secondary causes, green flags are clinical features that support a primary headache disorder. They may provide additional reassurance when no red flags are present, but they should not be used to rule out a secondary headache or eliminate the need for further evaluation when clinically indicated. Unlike established red flag tools such as SNNOOP10, green flags lack a formally validated assessment tool. The Green Flags described below are based on clinical evidence and expert consensus but have not been prospectively validated as a diagnostic tool.

Key Green Flags Include:^2,3,5,6,7

• Fulfillment of ICHD-3 Diagnostic Criteria for a primary headache disorder. One emergency department study found that meeting ICHD-3 criteria was strongly associated with a primary headache diagnosis (OR 18.7, 95% CI 7.1–48.6), meaning patients meeting the criteria had about 19 times the odds of having a primary headache diagnosis.⁵
• Personal or family history of migraine: Patients with a history of migraine had about 3 times the odds of receiving a primary headache diagnosis (OR 2.9, 95% CI 2.1–3.9).⁵ A close family member with the same headache phenotype also supports a potentially inherited primary headache disorder, particularly migraine or cluster headache.^5,7
• History of similar episodes: Patients with a history of similar episodes had about 3 times the odds of receiving a primary headache diagnosis (OR 2.7, 95% CI 2.3–3.3).⁵
• The headache has been present since childhood: In adults, a headache present since childhood supports a primary headache disorder.⁷
• Headache-free days: The presence of headache-free days supports a primary headache disorder, reflecting the intermittent nature of many primary headaches.⁷ However, the absence of headache-free days does not by itself indicate a secondary headache, as many primary headache disorders can occur daily.⁷
• Normal neurologic examination: A systematic review and meta-analysis of 41 studies including 15,760 participants presenting with headache and a normal neurologic examination found that the combined prevalence of unexpected findings and normal variants was 17.5%. Vascular findings accounted for 6.6%, neoplastic findings for 1.4%, and non-neoplastic findings for 9.6%.⁶ The authors concluded that clinically significant vascular or neoplastic abnormalities are rare in patients presenting with headache who have a normal neurologic examination and are better detected with MRI.⁶
• Predictable timing or known triggers: such as menstrual attacks or headaches provoked by specific environmental factors.^2,7
• Headache occurred or resolved more than one week ago: Generally supports primary headache disorder, as dangerous secondary headaches typically present with concerning features within a shorter period, but it does not completely exclude a secondary headache.⁷
• Presence of typical migraine features: Unilateral location, pulsating quality, moderate to severe pain intensity, aggravation by routine physical activity, and associated nausea, photophobia, and phonophobia.²
• Typical aura characteristics: Gradual onset of positive and/or negative visual, sensory, or speech/language symptoms, each typically lasting 5–60 minutes and often followed by headache. This gradual evolution can help distinguish migraine aura from the more sudden onset typical of many cerebrovascular events.²

Overall, these features can increase the likelihood of a primary headache disorder, but they are not validated diagnostic criteria and should be interpreted alongside red flags and the overall clinical picture.

When Should Imaging Be Considered and What Should Be Ordered?

Neuroimaging decisions should be guided by clinical context, including a detailed history, the presence or absence of red and green flags, and a knowledge of the specific primary and secondary headache disorders being considered.

When Neuroimaging Is Not Indicated

The American College of Radiology (ACR) criteria and multiple guidelines agree that imaging is usually not appropriate for patients who present with a new primary migraine or tension-type headache who have a normal neurologic examination.^2,6,8 Likewise, imaging is not usually appropriate for patients with chronic headaches with no new features, locations, or neurologic deficits on examination.^2,6,8

When to Order a Noncontrast CT of the Brain

Computed tomography (CT) is the first-line emergent imaging modality for sudden, severe headache, including thunderclap headache.² CT is sensitive for detecting acute intracranial hemorrhage and mass effect.² If CT is negative, additional evaluation may be necessary to exclude subarachnoid hemorrhage.⁹ The American Heart Association/American Stroke Association recommends lumbar puncture after a negative non-contrast head CT in patients with acute onset of severe headache who present more than 6 hours after symptom onset or who have a new neurological deficit.⁹ When lumbar puncture is performed, cerebrospinal fluid is evaluated for evidence of subarachnoid hemorrhage, including xanthochromia. 

When to Order an MRI of the Brain

MRI is generally preferred over CT for non-emergent headache evaluation when concerning features are present.^2,6 Brain MRI is preferred over CT for identifying intracranial mass lesions, vascular pathology, posterior fossa abnormalities, and cervicomedullary lesions.² The use of contrast should be determined by the suspected underlying pathology.

The following are clinical scenarios in which MRI brain may be appropriate, with the specific protocol determined by the suspected underlying condition:^2,8

• Trigeminal autonomic cephalalgias:⁸ MRI with and without contrast to exclude secondary pathology, including pituitary or cavernous sinus lesions, particularly when the presentation is new or atypical.² These disorders typically present with severe unilateral head pain accompanied by ipsilateral autonomic symptoms such as nasal congestion, lacrimation, or forehead and facial sweating.
• Headache with features of intracranial hypertension: MRI with and without contrast (e.g., papilledema, pulsatile tinnitus, or visual symptoms that worsen with Valsalva).⁸
• Headache with features of intracranial hypotension: MRI with and without contrast (e.g., headache that is positional, worse when upright, and improves when lying down).⁸
• Headache with new onset or a new pattern during pregnancy or the peripartum period: Initial MRI without contrast.⁸
• Headache with one or more red flags: Increasing frequency or severity, fever, neurologic deficit, history of cancer or immunocompromise, new-onset headache after age 50, or posttraumatic onset.⁸
• Cough headache: MRI with and without contrast (to exclude Chiari malformation and other posterior fossa lesions).^2,3,10

Putting It All Together: A Practical Approach to Headache Evaluation

The systematic use of the SNNOOP10 red flag criteria provides a structured approach to identifying potential secondary headache disorders. Conversely, green flags, including meeting ICHD-3 criteria, a history of similar episodes, and a normal neurological examination, can support a primary headache diagnosis but should not be used to exclude a secondary cause when other concerning features are present. Neuroimaging should be guided by the clinical context, including red flags, changes in headache pattern, neurological findings, and other concerning features. Noncontrast CT is typically used as the initial imaging modality for emergent presentations such as suspected acute intracranial hemorrhage, while MRI is preferred for many non-emergent presentations with concerning features. This balanced approach promotes patient safety while reducing unnecessary testing and healthcare costs.

References

1. 1. GBD 2023 Headache Collaborators. Global, regional, and national burden of headache disorders, 1990-2023: a systematic analysis for the Global Burden of Disease Study 2023. The Lancet Neurology. 2025;24(12):1005-1015. doi:10.1016/S1474-4422(25)00402-8.
   2. Viera AJ, Antono B. Acute headache in adults: a diagnostic approach. American Family Physician. 2022;106(3):260-268.
   3. Do TP, Remmers A, Schytz HW, Schankin C, Nelson SE, Obermann M, Hansen JM, Sinclair AJ, Gantenbein AR, Schoonman GG. Red and orange flags for secondary headaches in clinical practice: SNNOOP10 list. Neurology. 2019;92(3):134-144. doi:10.1212/WNL.0000000000006697.
   4. Clarke CE, Edwards J, Nicholl DJ, Sivaguru A. Imaging results in a consecutive series of 530 new patients in the Birmingham Headache Service. Journal of Neurology. 2010;257(8):1274-1278. doi:10.1007/s00415-010-5506-7.
   5. Munoz-Ceron J, Marin-Careaga V, Peña L, Mutis J, Ortiz G. Headache at the emergency room: etiologies, diagnostic usefulness of the ICHD 3 criteria, red and green flags. PLoS One. 2019;14(1):e0208728. doi:10.1371/journal.pone.0208728.
   6. Kamtchum-Tatuene J, Kenteu B, Fogang YF, Zafack JG, Nyaga UF, Noubiap JJ. Neuroimaging findings in headache with normal neurologic examination: systematic review and meta-analysis. Journal of the Neurological Sciences. 2020;416:116997. doi:10.1016/j.jns.2020.116997.
   7. Pohl, H., Do, T.P., García-Azorín, D., Hansen, J.M., Kristoffersen, E.S., Nelson, S.E., Obermann, M., Sandor, P.S., Schankin, C.J., Schytz, H.W., Sinclair, A., Schoonman, G.G. and Gantenbein, A.R. (2021), Green Flags and headache: A concept study using the Delphi method. Headache: The Journal of Head and Face Pain, 61: 300-309. https://doi.org/10.1111/head.14054
   8. Expert Panel on Neurologic Imaging, Utukuri, P. S., Shih, R. Y., Ajam, A. A., Callahan, K. E., Chen, D., Dunkle, J. W., Hunt, C. H., Ivanidze, J., Ledbetter, L. N., Lee, R. K., Malu, O., Pannell, J. S., Pollock, J. M., Potrebic, S. X., Setzen, M., Shih, R. D., Soares, B. P., Staudt, M. D., . . . Burns, J. (2022). Headache [American College of Radiology Appropriateness Criteria]. American College of Radiology. acr.org
   9. Hoh BL, Ko NU, Amin-Hanjani S, Chou SH-Y, Cruz-Flores S, Dangayach NS, Derdeyn CP, Du R, Hänggi D, Hetts SW, Ifejika NL, Johnson R, Keigher KM, Leslie-Mazwi TM, Lucke-Wold B, Rabinstein AA, Robicsek SA, Stapleton CJ, Suarez JI, Tjoumakaris SI, Welch BG. 2023 Guideline for the Management of Patients With Aneurysmal Subarachnoid Hemorrhage: A Guideline From the American Heart Association/American Stroke Association. Stroke. 2023 Jul;54(7):e314-e370. doi: 10.1161/STR.0000000000000436. Epub 2023 May 22. Erratum in: Stroke. 2023 Dec;54(12):e516. doi: 10.1161/STR.0000000000000449. PMID: 37212182.
   10. Ahmed KA, Ahmed JA, Mohsin I, Ahmed MH. Primary Cough Headache: A Case Report. Cureus. 2023 Mar 31;15(3):e36971. doi: 10.7759/cureus.36971. PMID: 37025714; PMCID: PMC10072196.

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