that the antibodies themselves may play an important part in the pathogenesis of the disorders. Antibody levels may correlate with disease activity; procedures to reduce immune response may result in clinical improvement; and in some cases, animal models have been developed mimicking the effects of antigen depletion. The genesis of these disorders is generally conceptualized as beginning with the development of an immune response to an autoantigen in the nervous system, either spontaneously due to exposure of the immune system to antigens displayed by a tumor, virus, or microbe or due to unmasking of otherwise privileged nervous system antigens due to brain injury or infection. The antigen provoking the response may be the actual neural antigen or a cross-reacting closely related molecule due to “molecular mimicry.” Once an immune response has occurred, there is immune-mediated attack on normal nervous system tissue. Such attack may occur after any inciting infection or injury has already resolved.
TABLE 71.1 Immune-mediated Meningoencephalitides
the distinguishing feature of the immune-mediated disorders is inflammation. Even if DWI signal abnormalities are seen, they are accompanied by more extensive FLAIR or T2-weighted imaging abnormalities not common in prion disease. And contrast-enhanced imaging may show areas of breakdown of the blood-brain barrier, definitely inconsistent with prion disease but common in inflammatory disease. But it is CSF and blood studies that best help define the immune-mediated disorders. CSF may show evidence of central nervous system inflammation such as the presence of pleocytosis and elevated protein. Cellular fluid is not characteristically seen in prion disease. Blood and CSF studies may identify marker antibodies that characterize distinct clinicopathologic entities with characteristic diagnostic, prognostic, and therapeutic features. Some of these immune syndromes are sufficiently recognizable disorders through their clinical phenotype, even though it may take some days to weeks to confirm an immune-mediated disorder with an antibody test. Other syndromes may not be as easily recognized but nonetheless may show a somewhat consistent epidemiology and clinical presentation.