Tuberculous meningitis is also called "TB meningitis".

Tuberculous meningitis is Mycobacterium tuberculosis infection of the meninges. It is the most common form of CNS tuberculosis.

Clinical features

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Fever and headache are the cardinal features. Confusion is a late feature and coma bears a poor prognosis. Meningism is absent in a fifth of patients with TB meningitis. Patients may also have focal neurological deficits.

Pathology

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Mycobacterium tuberculosis of the meninges is the cardinal feature and the inflammation is concentrated towards the base of the brain. Infection begins in the lungs and may spread to the meninges by a variety of routes.

Blood-borne spread certainly occures and 25% of patients with miliary TB have TB meningitis; but a proportion of patients may get TB meningitis from rupture of a cortical focus (a so-called Rich focus); an even smaller proportion get it from rupture of a bony focus in the spine. It is rare and unusual for TB of the spine to cause TB of the central nervous system, but isolated cases have been described.

Diagnosis

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Diagnosis of TB meningitis is made by analysing CSF collected by lumbar puncture. When collecting CSF for suspected TB meningitis, a minimum of 1ml of fluid should be taken (preferably 5 to 10ml).

The CSF usually has a high protein, low glucose and a raised number of lymphocytes. Acid-fast bacilli are sometimes seen on a CSF smear, but more commonly, M. tuberculosis is grown in culture. PCR may be used to demonstrate the presence of TB in the CSF but is less sensitive than culture. A spiderweb clot in the collected CSF is characteristic of TB meningitis, but is a rare finding.

More than half of cases of TB meningitis cannot be confirmed microbiologically, and these patients are treated on the basis of clinical suspicion only. The culture of TB from CSF takes a minimum of two weeks, and therefore the majority of patients with TB meningitis are started on treatment before the diagnosis is confirmed.

Imaging studies such as CT or MRI may show features strongly suggestive of TB meningitis, but cannot diagnose it.

Treatment

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See: tuberculosis treatment

The treatment of TB meningitis is isoniazid, rifampicin, pyrazinamide and ethambutol for two months, followed by isoniazid and rifampicin alone for a further ten months. Steroids are always used in the first six weeks of treatment (and sometimes for longer). A few patients may require immunomodulatory agents such as thalidomide.

Treatment must be started as soon as there is a reasonable suspicion of the diagnosis. Treatment must not be delayed while waiting for confirmation of the diagnosis.

Hydrocephalus occurs as a complication in about a third of patients with TB meningitis and will require a ventricular shunt.

References

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Tuberculosis