Headache

1 · First minutes

1. Thunderclap, worst-ever, or any red flag → image.
2. A high-quality noncontrast CT within 6 h can exclude SAH in a neurologically intact adult under the ACEP pathway; persistent concern after a negative CT needs LP or CTA through shared decision-making.
3. Do not delay antibiotics (and dexamethasone if pneumococcal likely) for CT when meningitis is possible.

3 · Immediate workup

• Glucose, vitals including temperature, fundoscopy

4 · Don’t miss

• Subarachnoid hemorrhage
  Sudden worst-ever headache. High-quality noncontrast CT within 6 h with a normal neurologic exam can exclude SAH under the ACEP pathway. If residual risk remains, use LP or CTA with shared decision-making. Ottawa SAH applies only to alert, neurologically intact patients aged ≥16 with new nontraumatic severe headache peaking within 1 h; it does not exclude other secondary causes.
• Intracranial hemorrhage
  Focal deficits, anticoagulation, hypertension — non-contrast CT first.
• Meningitis / encephalitis
  Fever + meningismus ± altered mentation — antibiotics (and dexamethasone if pneumococcal likely) must not wait on CT; LP when safe.
• Cerebral venous sinus thrombosis
  Progressive headache, papilledema, hypercoagulable state, postpartum — CTV/MRV.
• Arterial dissection (carotid/vertebral)
  Head/neck pain + Horner syndrome or posterior-circulation symptoms after trauma or neck manipulation.