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Secondary headache

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Secondary headache is a broad topic. Here's the landscape before we go deeper.

What "secondary headache" covers

  1. Definition & classification - headache caused by an identifiable underlying disorder (vs. primary headache, which is the disorder itself), per the International Headache Society (ICHD-3) system.
  2. Red flags ("SNOOP" criteria) - features that should trigger urgent workup: sudden/thunderclap onset, new onset after age 50, papilledema, altered consciousness, focal neurologic deficits, worsening pattern, fever/cancer/immunosuppression, headache triggered by exertion/Valsalva/sex, pregnancy or postpartum.
  3. Vascular causes - subarachnoid hemorrhage, cerebral venous thrombosis, cervical artery dissection (carotid/vertebral), giant cell (temporal) arteritis, reversible cerebral vasoconstriction syndrome, hypertensive emergency, stroke.
  4. Infectious causes - meningitis, encephalitis, brain abscess, sinusitis-related headache.
  5. Intracranial pressure disorders - idiopathic intracranial hypertension, mass lesions/brain tumor, intracranial hypotension (post-LP or spontaneous CSF leak), hydrocephalus.
  6. Head/neck trauma - post-traumatic headache, subdural/epidural hematoma.
  7. Medication and substance-related - medication-overuse headache, drug-induced headache, withdrawal headache.
  8. Metabolic/systemic causes - hypoxia, hypercapnia, severe hypertension, systemic infection.
  9. Other structural causes - cervicogenic headache, temporomandibular disorder, disorders of the eyes/ears/nose/teeth, glaucoma.
  10. Clinical approach - history/exam clues distinguishing primary vs secondary, indications for neuroimaging (CT/MRI), lumbar puncture, and other targeted tests.

Quick overview

Headache is classified as primary (the headache itself is the disease, e.g., migraine, tension-type, cluster) or secondary (headache is a symptom of an underlying disorder). Secondary headaches are far less common than primary ones overall (in general practice roughly 1-10% of headaches are secondary), but they matter disproportionately because a small subset represents life-threatening disease - subarachnoid hemorrhage, meningitis, venous thrombosis, arterial dissection, mass lesions, or raised/lowered intracranial pressure.
The clinical priority in any patient with headache is to screen for red-flag features (sudden "thunderclap" onset, new headache after age 50, abnormal neuro exam, papilledema, fever, immunosuppression, cancer history, pregnancy, or headache provoked by exertion/Valsalva). Presence of any red flag lowers the threshold for neuroimaging and further workup (LP, CT/MRV, ESR/CRP for suspected giant cell arteritis, etc.). Absence of red flags with a pattern consistent with migraine or tension-type headache usually allows a clinical diagnosis without imaging.
Sources: Harrison's Principles of Internal Medicine 22E, Ch. 17; Rosen's Emergency Medicine, Ch. 89 (Secondary Headache Disorders); Frameworks for Internal Medicine.
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