A 35-year-old woman presents with acute-onset severe headache, neck stiffness, and fever. Clinical suspicion for meningitis is high. Which neuroimaging modality is the most commonly used investigation of choice to rule out contraindications to lumbar puncture before performing CSF analysis?

Explanation

## Neuroimaging in Suspected Meningitis: Pre-LP Assessment **Key Point:** Non-contrast CT head is the most commonly used, fastest, and most practical imaging modality to exclude contraindications to lumbar puncture (LP) in suspected meningitis. ### Rationale for Non-Contrast CT as First-Line 1. **Speed of acquisition** — Critical in acute meningitis; imaging should not delay antibiotic therapy 2. **Detects mass effect** — Identifies space-occupying lesions, midline shift, or hydrocephalus that would contraindicate LP 3. **Identifies hemorrhage** — Rules out subarachnoid hemorrhage mimicking meningitis 4. **Widely available** — Present in all emergency departments; no need for contrast or specialized sequences 5. **No contraindications** — Non-contrast imaging avoids delays from contrast administration ### When Is Imaging Needed Before LP? | Clinical Feature | Imaging Indicated? | Rationale | |------------------|-------------------|----------| | **Immunocompromised state** | Yes | Risk of opportunistic infections with mass effect (toxoplasmosis, cryptococcoma) | | **Papilledema on fundoscopy** | Yes | Suggests raised ICP; risk of herniation with LP | | **Focal neurological signs** | Yes | Suggests focal lesion; mass effect must be excluded | | **Altered consciousness** | Yes | May indicate increased ICP or mass effect | | **Immunocompetent, no focal signs** | No | LP can proceed without imaging if clinical suspicion is high | **High-Yield:** In immunocompetent patients with typical meningitis presentation and no focal neurological signs, LP can proceed without imaging. However, if imaging is performed, non-contrast CT is the modality of choice. ### Why Other Modalities Are Not First-Line | Modality | Advantage | Why Not First-Line | |----------|-----------|-------------------| | **Contrast-enhanced MRI with DWI** | Excellent for detecting meningeal enhancement, infarction, abscess | Time-consuming (30–45 min); delays antibiotics; not practical in acute setting | | **CT perfusion** | Assesses cerebral blood flow; detects vasculitis | Unnecessary for pre-LP assessment; adds radiation and time | | **Transcranial Doppler** | Non-invasive assessment of cerebral hemodynamics | Operator-dependent; does not detect mass effect or hemorrhage | **Clinical Pearl:** Current guidelines recommend that if imaging is required before LP in suspected meningitis, non-contrast CT is preferred. However, imaging should not delay antibiotics in immunocompetent patients with typical presentation. The decision to image should be made within minutes, not hours. ### Clinical Algorithm ```mermaid flowchart TD A[Suspected Meningitis]:::outcome --> B{Immunocompromised or focal signs?}:::decision B -->|No| C[Proceed to LP without imaging]:::action B -->|Yes| D[Non-contrast CT head]:::action D --> E{Mass effect or hemorrhage?}:::decision E -->|No| F[Proceed to LP]:::action E -->|Yes| G[Defer LP; treat empirically]:::urgent C --> H[CSF analysis + antibiotics]:::action F --> H ``` **Mnemonic:** **FAST imaging for meningitis** — **F**ast (non-contrast CT), **A**cute setting, **S**pace-occupying lesion exclusion, **T**herapy (antibiotics) should not be delayed. [cite:Harrison 21e Ch 381; Robbins 10e Ch 28]