IM17.6-9 | Headache Diagnostic Testing and Lumbar Puncture — Summary & Reflection

KEY TAKEAWAYS

Imaging selection: CT head (acute red flag/thunderclap/haemorrhage); MRI with contrast (progressive headache, encephalitis, posterior fossa); MRI+MRV (suspected CVT); CTA (confirmed SAH to locate aneurysm).

LP indications: suspected SAH after negative CT (LP ≥12 hours); bacterial/viral/TBM/cryptococcal meningitis; IIH (opening pressure); carcinomatous meningitis; MS work-up. Absolute contraindications: raised ICP with herniation risk (papilloedema, focal deficit, effaced basal cisterns on CT), infection at puncture site, uncorrected coagulopathy.

LP technique: L3–L4 or L4–L5 (Tuffier's line = L4); lateral decubitus foetal position for pressure measurement; 22G Quincke needle bevel parallel to dural fibres; normal opening pressure 10–20 cmH₂O; 4 numbered collection tubes.

CSF patterns (memorise the glucose ratio):
- Bacterial meningitis: turbid, neutrophils 500–10,000, protein >100 mg/dL, glucose ratio <0.4, Gram stain +ve
- Viral meningitis: clear, lymphocytes 10–500, protein 50–200 mg/dL, glucose ratio ≥0.6 (NORMAL)
- TBM: clear/opalescent, lymphocytes 50–500, protein 100–500 mg/dL, glucose ratio <0.5, AFB smear 10–40%
- SAH: uniformly blood-stained all tubes, xanthochromic spun supernatant (≥12 hours), normal glucose

Traumatic tap vs SAH: blood clears tube 1→4 (traumatic); uniform blood + xanthochromia (SAH).

REFLECT

Arjun's question — 'why LP when CT is normal?' — now has a precise, evidence-based answer you can deliver with confidence. More importantly, you can explain it to a patient's family: the CT showed no blood, but a very small amount of blood in the fluid around the brain can only be detected by sampling that fluid directly, and the test cannot be done until at least 12 hours after the headache started because the blood needs time to produce a detectable colour change. That explanation, delivered calmly and accurately, converts a family from refusal to cooperation — and potentially saves a life. Diagnostic skill and communication skill are not separable: the best investigation is useless if the patient refuses it because no one explained why it was necessary.