Wednesday, 31 August 2016

TB & parainfectious myelitis

Why are absent sensory nerve action potentials such an important finding in cases of myelitis? #TeachNeuro

I saw a patient last week on the ward with necrotic myelitis with lower motor neurone signs in the lower limbs and mixed upper and lower motor neurone signs in the upper limb. He had a sensory level at C8/T1 and a motor level at C7/C8. The presumptive diagnosis has been TB myelitis based on low CSF glucose level compared to the peripheral blood and raised total protein. CSF cultures have been negative and a peripheral screen has not shown systemic TB, including a whole body FDG-PET study. When I trained in South Africa I saw several cases of so called TB-related parainfectious myelitis. These patients all had active TB and developed their myelitis shortly before, or after, starting their anti-tuberculous drugs (see article by Silber et al. 1990). The hypothesis underlying these cases is that the myelitis is a monophasic autoimmune disease. Mycobacteria are potent adjuvants and are well known to trigger, or boost, autoimmune responses in animals. In fact, complete Freund's adjuvant (CFA or FCA) is composed of inactivated and dried mycobacteria (usually M. tuberculosis) in an oil emulsion. CFA is often referred to as the immunologist's dirty little secret and is what is used to trigger EAE (experimental allergic encephalomyelitis).

The one red flag in relation to this patient is that his nerve conduction studies show absent SNAPs (sensory nerve action potentials) in the lower limbs and a whole body work-up has not shown peripheral TB. Why is finding absent SNAPs such an important clue in this case?

Essential Reads

Silber et al. Neuromyelitis optica (Devic's syndrome) and pulmonary tuberculosis. Neurology. 1990 Jun;40(6):934-8.

Neuromyelitis optica and acute necrotic myelopathy occur in association with pulmonary tuberculosis. We studied 8 patients with either neuromyelitis optica (6), acute myelopathy (1), or acute optic neuropathy (1) in close temporal association with pulmonary tuberculosis, but with no evidence for CNS tuberculosis. Neurologic symptoms preceded the use of antituberculosis medication in 5 patients. Different patients showed similar clinical features, suggesting a consistent disease pattern. Autopsy examination (1 patient) revealed extensive spinal cord and optic nerve demyelination. We identified only 5 additional patients seen over the same period with idiopathic neuromyelitis optica, thus suggesting that the close temporal relationship to pulmonary tuberculosis is not coincidental. The syndrome is most likely due to an immune reaction to tuberculosisrather than the use of antituberculosis medication.

Hughes & Mair. Acute necrotic myelopathy with pulmonary tuberculosis. Brain. 1977 Jun;100(2):223-38.

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