Headache red flags and secondary headache — KFP-style written assessment

KFP 1 (10 marks)

A 43-year-old woman presents to the emergency department with a sudden-onset severe headache that peaked within one minute of onset. She describes it as the worst headache of her life. She is neurologically intact. CT head performed 3 hours after onset is normal. ^{[1] [2]}

1. What is the differential diagnosis for this presentation? (3) ^[1]
2. What does the evidence say about CT sensitivity at this time point? (4) ^[2]
3. If CT is negative and suspicion remains, what further investigation is needed and when should it be performed? (3)

Model answers

1. Subarachnoid haemorrhage is the primary concern (thunderclap onset). Other differentials include cerebral venous sinus thrombosis, reversible cerebral vasoconstriction syndrome, cervical artery dissection, and pituitary apoplexy. ^[1]
2. In the Christchurch study of 347 SAH patients imaged with modern MSCT, sensitivity for aneurysmal SAH was 99.6% (95% CI 97.6-100) at 48 hours and 100% (95% CI 98.3-100) at 24 hours after headache onset. This suggests modern CT may extend the rule-out window beyond the traditional 6-hour threshold. ^[2]
3. If CT is negative but clinical suspicion persists (especially beyond 24-48 hours), perform lumbar puncture for xanthochromia. Xanthochromia is detectable from approximately 12 hours to 2 weeks after SAH. Before LP, ensure CT has excluded mass effect causing coning risk. ^[2]