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Anthrax Meningitis: Deadly, Easily Overlooked, and More Common Than Thought

By Eric Toner, M.D., May 9, 2005

While known to carry a very high mortality rate, meningitis has long been considered an infrequent manifestation of infection with Bacillus anthracis. However, results of primate studies using aerolsized spores as well as autopsy data from victims of an accidental release of B. anthracis in Sverdlosk (1979) indicate that meningitis actually occurs in 50% of patients with inhalational anthrax. Previous reviews, which were based mostly on cutaneous anthrax cases, estimated the frequency of meningitis at just 5%. This is one of the issues covered in a recent review of anthrax meningitis by Sejar, Tenover and Stephens of the CDC, which appears in this month’s Lancet Infectious Diseases. [The Lancet ID (vol5 May 2005)].

Anthrax meningitis is fulminant and rapidly fatal unless treated very aggressively. The mortality rate is 96%, and the majority of patients (75%) die within the first 24 hours, making early diagnosis and treatment critical. There is some correlation between survival and early treatment.

Anthrax meningitis occurs as a result of hematogenous or lymphatic spread from the site of primary infection across the blood brain barrier. The organism can be found in the brain parenchyma and meninges as well as in CSF. Hemorrhage, which is characteristic, often manifests as frankly bloody CSF on spinal tap or as subarachnoid or intra parenchymal hemorrhages on CT. Clinical presentation may be the same as that of other forms of bacterial meningitis, with headache and meningismus, or these symptoms may be absent initially, as was the case with the index patient from the 2001 attack. Since altered mental status is common in inhalational anthrax, meningitis may be overlooked unless specifically considered. Among victims of the 2001 anthrax attacks in the U.S., only one patient had clinically recognized meningitis. Several others had CNS symptoms but did not have spinal taps.

The diagnosis can be made readily with early lumbar puncture and gram stain of the CSF. Because of the presence of frank blood on LP or CT, however, the condition may be misdiagnosed as a spontaneous or traumatic hemorrhage. B. anthracis is an encapsulated gram positive rod with a characteristic box car appearance. The organism grows rapidly in routine culture media. Because some laboratories will dismiss the finding of Bacillus species as a contaminant, suspicion of anthrax should be relayed to the lab. PCR can also provide a rapid diagnosis but is generally available only through state labs or the CDC. Treatment with antibiotics prior to spinal tap may prevent culture growth but will not interfere with PCR.

Aggressive, broad spectrum antibiotic treatment of suspected anthrax meningitis must be initiated as early as possible. The first line drug of choice is a fluoroquinolone. Ciprofloxacin has excellent activity against B. anthracis and good CNS penetration. Because fluoroquinolones act synergistically with other antibiotics, multi-drug treatment with vancomycin, rifampin, or meropenem is recommended. Clindamycin, which is often recommended as a second drug for inhalational anthrax does not penetrate the blood brain barrier.

Given its frequency and lethality, meningitis should be considered in all patients with suspected inhalational anthrax.