A 52-year-old man undergoes spinal anesthesia for lower limb orthopedic surgery. Six hours postoperatively, he develops severe headache, neck stiffness, photophobia, and fever (38.5°C). On examination, Kernig's sign is positive. Which is the investigation of choice to confirm the suspected diagnosis?

Explanation

## Clinical Presentation Analysis The patient presents with classic meningitis signs postoperatively: headache, neck stiffness, photophobia, fever, and positive Kernig's sign. The temporal relationship to spinal anesthesia raises concern for **post-dural puncture meningitis (PDPM)**, a rare but serious complication. ## Investigation of Choice **Key Point:** Cerebrospinal fluid (CSF) analysis with culture and sensitivity is the gold standard and investigation of choice for diagnosing meningitis of any etiology, including post-dural puncture meningitis. ### Why CSF Analysis is Diagnostic | Finding | Bacterial Meningitis | Aseptic Meningitis | |---------|---------------------|-------------------| | **Cell count** | 100–10,000 (predominantly PMN) | 50–500 (lymphocytic) | | **Protein** | 100–500 mg/dL (elevated) | 40–100 mg/dL (normal to mildly elevated) | | **Glucose** | <40 mg/dL (low CSF:serum ratio <0.4) | Normal (>40 mg/dL) | | **Culture** | Positive in 70–80% | Negative | | **Gram stain** | Positive in 60–90% | Negative | **High-Yield:** CSF culture remains the gold standard for organism identification and guides antibiotic therapy. Gram stain provides rapid presumptive diagnosis within 5–10 minutes. ## Timing and Technique **Clinical Pearl:** Lumbar puncture for CSF analysis should be performed urgently (within 1 hour of clinical suspicion) in suspected meningitis. Opening pressure measurement also aids diagnosis—elevated pressure (>25 cm H₂O) is typical in bacterial meningitis. **Warning:** Do NOT delay LP for imaging if meningitis is clinically suspected. Imaging (CT/MRI) is indicated only if papilledema, focal neurological signs, or contraindications to LP are present—neither is present here. ## Post-Dural Puncture Meningitis Context **Key Point:** PDPM typically occurs 24–48 hours after spinal anesthesia (this patient at 6 hours is within the early window). Causative organisms are usually skin commensals (Staphylococcus epidermidis, Propionibacterium acnes) or oral flora if the anesthetist's mask slipped. CSF analysis will show: - Pleocytosis (lymphocytic or mixed) - Elevated protein - Normal or low glucose (depending on organism virulence) - Culture positive in 50–70% of cases