Thunderclap headache with a negative CT — the lumbar puncture and the CSF interpretation

(a) Differential diagnosis and the next investigation (2 marks).

• The leading diagnosis is a subarachnoid haemorrhage — the sudden-onset, maximum-intensity-within-seconds thunderclap headache with exertional onset and neck stiffness is the classic presentation.
• The next investigation is a lumbar puncture with CSF spectrophotometry — because CT can miss some SAH patients, spectrophotometry follows a negative or equivocal scan, detecting both oxyhaemoglobin and bilirubin since either or both may cause xanthochromia.^[3][13]

(b) The rules that shape the LP decision (3 marks).

• The Ottawa SAH rule carries 100 percent sensitivity (CI 94.6-100) with 13.6 percent specificity and can rule out SAH in alert, neurologically intact headache; the 6-hour CT rule guides when to forgo the LP — and implementing both rules meaningfully decreases testing and admissions.
• This patient is alert and intact with a normal 2-hour CT, so the rules point toward forgoing the LP — but with a classic thunderclap at maximum intensity in seconds, shared decision-making may still favour spectrophotometry since CT can miss SAH.^[3][4][13]

(c) The lumbar puncture, step by step (6 marks).

• Localise with ultrasound — 90.0 versus 81.4 percent success, fewer traumatic taps; consider before all LPs especially difficult anatomy.
• Use the atraumatic pencil-point needle in all patients — traumatic needles carry risk ratio 2.14 for headache; atraumatic is no less likely to work.
• Measure the opening pressure — the tap accesses the subarachnoid space for pressure measurement and sampling.
• Interpret carefully — CSF abnormalities vary with cause, age, immune status and prior treatment; history and examination alone sometimes cannot confirm or exclude meningitis.^[2][8][9]

(d) The post-dural puncture headache management (4 marks).

• The presentation is the classic post-dural puncture headache — worse within 15 minutes upright, relieved within 15 minutes flat.
• Caffeine treats the established headache — reduced PDPH at one to two hours versus placebo with less need for supplementary measures.
• Respect the broad differential — other dural-puncture complications and the underlying disease both mimic PDPH; the atypical pattern is re-evaluated, not labelled.^[7][14]