Characteristic CSF Picture in Tubercular Meningitis
The characteristic cerebrospinal fluid (CSF) picture in tubercular meningitis includes raised opening pressure, clear or cloudy appearance, lymphocytic pleocytosis (typically 5-500 cells/μL), markedly elevated protein, very low glucose, and very low CSF/plasma glucose ratio. 1
CSF Parameters in Tubercular Meningitis
Opening Pressure and Appearance
- Opening pressure is typically raised in tubercular meningitis 1
- CSF appearance is usually clear or cloudy, unlike the turbid or purulent appearance commonly seen in bacterial meningitis 1
Cell Count and Differential
- CSF white cell count (WCC) is typically elevated, ranging from 5-500 cells/μL 1
- Lymphocytes predominate in the cell differential, though neutrophils may predominate early in the disease course 1
- In some cases, CSF WCC may be normal, especially in immunodeficient patients 1
- Plasmocytes (plasma cells) may appear from the third week of disease in approximately 30% of cases 2
Protein and Glucose
- CSF protein is markedly raised, typically >1 g/L, which has a sensitivity of 78% and specificity of 94% for diagnosing tubercular meningitis 1, 3
- CSF glucose is very low, with values typically <2.2 mmol/L (sensitivity 68%, specificity 96%) 1, 3
- CSF/plasma glucose ratio is very low, typically <0.5 (sensitivity 90%) 1, 3
Diagnostic Considerations
Temporal Evolution of CSF Findings
- In the first 10 days of disease, neutrophils may predominate (60-80%) 2
- As the disease progresses, mononuclear cells become predominant, including lymphocytes, lymphoid cells, monocytoid cells, and macrophages 2
- CSF abnormalities may persist for weeks to months, with pleocytosis sometimes lasting up to two years in atypical cases 2
Diagnostic Challenges
- CSF acid-fast smear and culture have relatively low sensitivity but yield increases with multiple, large-volume samples 4
- Nucleic acid amplification tests (PCR) on CSF are highly specific but have suboptimal sensitivity 4
- In some cases, diagnosis may be delayed until the 37th day or later, as reported in case studies 5
Comparison with Other Types of Meningitis
| Parameter | Tuberculous | Bacterial | Viral | Fungal |
|---|---|---|---|---|
| Opening Pressure | Raised | Raised | Normal/mildly raised | Raised |
| Appearance | Clear or cloudy | Turbid, cloudy | Clear | Clear or cloudy |
| WCC (cells/μL) | 5-500 | >100 | 5-1000 | 5-500 |
| Cell type | Lymphocytes* | Neutrophils | Lymphocytes | Lymphocytes |
| Protein (g/L) | Markedly raised | Raised | Mildly raised | Raised |
| Glucose | Very low | Very low | Normal/slightly low | Low |
| CSF/plasma glucose ratio | Very low | Very low | Normal/slightly low | Low |
*May be neutrophils early in the disease 1
Clinical Pearls and Pitfalls
- CSF findings alone cannot provide an absolute diagnosis; they must be interpreted in the clinical context 1
- Early empiric treatment should be initiated as soon as clinical suspicion is supported by initial CSF studies 4
- Adjunctive corticosteroids have been shown to improve mortality in tubercular meningitis 4, 6
- A positive TB contact, Mantoux skin test, chest radiograph, CT/MRI brain scan, and demonstration of TB bacilli from extraneural sources are invaluable in supporting a diagnosis of tubercular meningitis 6
- CSF to serum glucose ratio is more informative than absolute glucose value alone 3