Thunderclap Headache With Negative Noncontrast CT
54-year-old woman presents with sudden severe occipital headache that peaked within seconds while lifting a box. She has nausea and photophobia. Noncontrast head CT performed at an outside ED 7 hours after onset is reported as negative. No lumbar puncture has been done. She has autosomal dominant polycystic kidney disease, hypertension, and a 20-pack-year smoking history. Examination shows mild nuchal rigidity, no focal neurologic deficit, and GCS 15. Blood pressure is 168/96. She is on lisinopril; creatinine is 1.4 mg/dL. The accepting team wonders whether to discharge her with analgesics, proceed to lumbar puncture, or obtain CT angiography. The radiologist is asked to recommend the next step and to explain what to do if the study is negative.
1. What is the next diagnostic step? Explain the limitations of the negative CT and the role of lumbar puncture.
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Obtain CT angiography of the head and neck now. Although noncontrast CT is highly sensitive for subarachnoid hemorrhage, sensitivity is not 100%, and this patient has ADPKD, which increases the risk of intracranial aneurysm. A negative CT at 7 hours does not exclude a small aneurysm or a perimesencephalic bleed. If CTA is negative, perform lumbar puncture at least 12 hours after onset to look for xanthochromia; earlier LP can be falsely negative. Do not discharge her with analgesics. MRI is not the first test in this acute setting.
2. CTA shows a 5-mm anterior communicating artery aneurysm. How does this change management?
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A 5-mm anterior communicating artery aneurysm requires neurointervention or neurosurgical consultation and a catheter angiogram for treatment planning. If lumbar puncture shows xanthochromia, she has aneurysmal subarachnoid hemorrhage and should receive nimodipine, blood pressure control, and urgent aneurysm securing. If the LP is negative, the aneurysm may be incidental, but it still warrants specialist evaluation because of size and location. In either case, discharge without follow-up is unsafe.
3. If CTA and lumbar puncture are negative, what alternative diagnoses and imaging would you consider?
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If CTA and LP are negative, consider reversible cerebral vasoconstriction syndrome, cerebral venous thrombosis, pituitary apoplexy, arterial dissection, and posterior fossa pathology. MRI of the brain with MR venography is the next useful study; catheter angiography may be needed if RCVS is suspected and noninvasive studies are equivocal. The history of a thunderclap headache still matters, so arrange close follow-up and return precautions. A negative CT alone is not enough to close the case.