Encephalitis is defined as inflammation of the parenchyma of the brain. Strictly speaking, it is a pathologic diagnosis, in which the presence of inflammation, edema, and neuronophagia (neuronal cell death) is demonstrated by histopathology. This is generally associated with an infiltration of inflammatory cells and perivascular cuffing.
Myelitis, by corollary, is defined as inflammation of the parenchyma of the spinal cord. An inflammatory process involving both the brain and the spinal cord may be referred to as “encephalomyelitis”. Demonstration of this inflammation, regardless of the underlying cause, is considered confirmatory in the diagnosis of encephalitis/myelitis. A clinical diagnosis of encephalitis depends on the demonstration of brain dysfunction in the presence of evidence of an inflammatory process; this dysfunction may be cortical, subcortical (e.g., deep gray nuclei, brainstem), or both. Cerebral or cortical/subcortical dysfunction is generally manifested as (a) diffuse or global cortical dysfunction resulting in alteration of consciousness, impairment of mentation, or acute alteration of a person’s normal behavior, (b) focal cortical/subcortical dysfunction, resulting in so-called focal neurologic deficits, or (c) manifestations of both global dysfunction and focal deficits.
For the background rates we will differentiate between viral and other encephalitides.