Investigations

The electrolytes should be tested to look for low sodium caused by SIADH which would favour a diagnosis of tuberculous meningitis.

A full blood count may help distinguish AIDS from leukaemia/lymphoma.

The brain should be imaged before a lumbar puncture is performed.

The lumbar puncture is often diagnostic in cases of fungal meningitis:

• pleocytosis is variable:
  • usually less than 15 cell/mm^3
  • initially excess polymorphonuclear cells but this changes to lymphocytes with time
  • patients with AIDS may not have a pleocytosis
    
• protein is raised
• glucose is usually reduced
• India ink staining is positive
• cryptococcal antigen tests are sensitive

Reference

1. Williamson PR et al. Cryptococcal meningitis: epidemiology, immunology, diagnosis and therapy. Nat Rev Neurol. 2017 Jan;13(1):13-24