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MI6.1-3 | Central Nervous System Infections — Case Study
CLINICAL SCENARIO
You are a microbiology resident on call. The ward nurse brings you four CSF reports from patients admitted to the neurology ward overnight. Each report includes clinical context and laboratory findings. Your task is to interpret each CSF panel, identify the most likely aetiology, justify your reasoning using the laboratory parameters, and recommend the single most important next diagnostic step for each patient.
Instructions
Instructions
Read the four CSF case summaries below carefully. For each case, write a structured response with three components:
- Interpretation: Describe what the CSF parameters tell you (appearance, opening pressure, cell count and differential, glucose ratio, protein level).
- Diagnosis: State the most likely aetiological diagnosis and justify it by integrating ALL the provided laboratory values.
- Next step: Identify the single most important diagnostic test or action to confirm the diagnosis or guide immediate management.
General guidance:
- Use the CSF:blood glucose ratio (normal ≥0.6) to assess glucose consumption.
- Correlate cell differential (neutrophils vs lymphocytes) with the clinical timeline.
- A cobweb clot and xanthochromia are clinically significant findings.
- State which microbiological test you would prioritise and why.
## Case 1
Clinical context: 55-year-old male, 2 days of fever (39°C), severe headache, photophobia, neck stiffness. No recent travel, no immunosuppression.
| Parameter | Result |
|---|---|
| Appearance | Turbid |
| Opening pressure | 320 mm H₂O |
| WBC | 2,400/mm³ (91% neutrophils) |
| RBC | 10/mm³ |
| Protein | 290 mg/dL |
| CSF glucose | 18 mg/dL |
| Blood glucose | 88 mg/dL |
| Gram stain | Gram-positive lancet-shaped diplococci, extracellular |
## Case 2
Clinical context: 28-year-old female, 3 weeks of subacute headache, low-grade fever (37.8°C), night sweats, weight loss. No HIV. Previous contact with a TB patient 1 year ago.
| Parameter | Result |
|---|---|
| Appearance | Xanthochromic, forms cobweb clot on standing |
| Opening pressure | 210 mm H₂O |
| WBC | 195/mm³ (78% lymphocytes, 22% monocytes) |
| RBC | 5/mm³ |
| Protein | 198 mg/dL |
| CSF glucose | 28 mg/dL |
| Blood glucose | 92 mg/dL |
| ZN stain | Negative |
| ADA | 22 IU/L |
## Case 3
Clinical context: 40-year-old HIV-positive man (CD4 count 32 cells/μL), 10 days of headache, blurred vision, vomiting. CT head shows no mass lesion.
| Parameter | Result |
|---|---|
| Appearance | Clear |
| Opening pressure | 410 mm H₂O |
| WBC | 12/mm³ (all lymphocytes) |
| RBC | 2/mm³ |
| Protein | 108 mg/dL |
| CSF glucose | 32 mg/dL |
| Blood glucose | 85 mg/dL |
| India ink | Round budding yeast with clear capsular halo |
## Case 4
Clinical context: 19-year-old male, 4 days of fever, headache, photophobia. No neck stiffness. No altered consciousness. Onset August, lives in a hostel.
| Parameter | Result |
|---|---|
| Appearance | Clear |
| Opening pressure | 165 mm H₂O |
| WBC | 310/mm³ (94% lymphocytes) |
| RBC | 3/mm³ |
| Protein | 68 mg/dL |
| CSF glucose | 58 mg/dL |
| Blood glucose | 90 mg/dL |
| Gram stain | No organisms seen |
| Culture | Pending |
Length: 600-800 words
What to Submit
Section 1: Case 1 — Bacterial Meningitis
Interpret the CSF findings in Case 1. Identify the most likely pathogen based on the Gram stain morphology and all laboratory parameters. What is the single most critical management action before awaiting full culture sensitivities?
Guidance: Focus on: turbid CSF + neutrophilic pleocytosis + very low CSF:blood glucose ratio (calculate it) + gram-positive lancet diplococci. Link Gram stain morphology to S. pneumoniae. State empiric antibiotic choice.
Section 2: Case 2 — Tuberculous Meningitis
Interpret the CSF findings in Case 2. Explain why the cobweb clot and xanthochromia are significant. Given a negative ZN stain, what is the most important next diagnostic test, and what is the significance of the ADA level?
Guidance: Link subacute onset + lymphocytic pleocytosis + very low glucose + high protein + cobweb clot to TBM. Explain why ZN stain is often negative in TBM (low AFB load). Identify GeneXpert Ultra (CBNAAT) as the next test. Interpret ADA >10 IU/L as supportive.
Section 3: Case 3 — Cryptococcal Meningitis
Interpret the CSF findings in Case 3. Explain why the WBC count is paradoxically low despite active infection. What does the very high opening pressure indicate, and which confirmatory test should be ordered alongside India ink?
Guidance: Low WBC reflects profound immunosuppression (CD4 32). Very high opening pressure (410 mm H₂O) is from polysaccharide capsule blocking arachnoid granulations. India ink shows encapsulated yeast = Cryptococcus. Order CrAg by latex agglutination. Serial therapeutic LPs for ICP.
Section 4: Case 4 — Viral (Aseptic) Meningitis
Interpret the CSF findings in Case 4. How does the glucose ratio and cell differential help you classify this as viral meningitis rather than bacterial or TB? In the absence of an identified organism, what is the expected clinical course, and which specific virological test, if any, should be considered?
Guidance: Clear CSF + lymphocytic pleocytosis + near-normal glucose (CSF:blood = 0.64) + mildly elevated protein = viral aseptic meningitis. Most common cause in August (monsoon) in young adult = enterovirus. Enterovirus PCR on CSF is the specific test. Expected course: self-limiting in 7–10 days with supportive care.
Grading Rubric — CSF Interpretation Case Study Rubric (30 points)
| Criterion | Points | Full-marks descriptor |
|---|---|---|
| Case 1 — Bacterial Meningitis: Correct interpretation of turbid neutrophilic CSF, Gram stain identification of lancet-shaped diplococci as S. pneumoniae, and appropriate empiric management | 8 pts | Correctly calculates CSF:blood glucose ratio (<0.2), identifies S. pneumoniae from Gram stain morphology, names empiric antibiotics (ceftriaxone ± vancomycin ± dexamethasone), and states must not delay treatment. |
| Case 2 — Tuberculous Meningitis: Recognition of TBM pattern, significance of cobweb clot and ADA, and selection of GeneXpert Ultra as next test | 8 pts | Correctly identifies TBM based on subacute onset + lymphocytic pleocytosis + very low glucose + cobweb clot + xanthochromia. Explains ADA >10 IU/L significance. Names GeneXpert Ultra (CBNAAT) as the next test and explains its advantages over ZN stain. |
| Case 3 — Cryptococcal Meningitis: Explains paradoxically low WBC, mechanism of raised ICP, and confirmatory test | 7 pts | Correctly attributes low WBC to profound CD4 immunosuppression. Identifies polysaccharide capsule obstruction of arachnoid granulations as the ICP mechanism. Orders CrAg by latex agglutination as confirmatory test. Mentions serial therapeutic LPs for ICP. |
| Case 4 — Viral Aseptic Meningitis: Correct classification using glucose ratio, appropriate virological test, and expected prognosis | 7 pts | Calculates CSF:blood glucose ratio (0.64 — normal), correctly classifies as viral aseptic meningitis, identifies enterovirus PCR as the appropriate virological test, and correctly states the expected self-limiting course with supportive care. |
PEER REVIEW
Review your classmate's response by checking: (1) Did they calculate CSF:blood glucose ratios for Cases 1, 2, and 4? (2) Did they correctly identify the Gram stain morphology in Case 1? (3) Did they explain the mechanism of raised ICP in Case 3? (4) Did they select GeneXpert Ultra for Case 2 and justify it over ZN stain? Provide constructive written feedback of 100–150 words, identifying one strength and one area for improvement.