General: Anaemia, leucocytosis. Hyponatremia (if inappropriate ADH secretion occurs).
Cerebrospinal fluid: A lymphocytic pleocytosis is usually present, though in acute cases polymorphonuclear cells may predominate - 500/mm3. (range 50-4000/mm3) The protein is elevated - 1-5 g/1.
The glucose level is usually less than two-thirds of simultaneously measured blood glucose.
Microscopy (Ziehl Neelsen stain) reveals acid-fast bacilli in 20% of patients. CSF culture (6 weeks in Lowenstein-Jensen medium) should confirm the diagnosis. Polymerase chain reaction (PCR) is increasingly available for the detection of bacterial DNA.
Chest X-ray: Reveals changes of old or recent tuberculosis in 50-70% of adults and 90% of children.
PPD skin test (tuberculin): Positive to intermediate strength in 60%. Patients developing TBM while on steroids or with recently acquired primary tuberculosis may give a negative response.
CT/MRI: Shows meningeal enhancement on basal views, ventricular enlargement, associated infarction and tuberculomas in 10% (see page 347).
Diagnosis is based on the clinical presentation with characteristic CSF findings. Even if Ziehl Neelsen staining is negative, in view of the progressive disease course do not await the results of cultures before starting treatment.
DIFFERENTIAL DIAGNOSIS of subacute/chronic meningitis (see pages 497, 498).
Viral meningoencephalitis (with normal CSF sugar). Carcinomatous meningitis (with high CSF protein, low CSF sugar). Partially treated bacterial meningitis. Fungal meningitis. Sarcoidosis.
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