Opsoclonus-myoclonus syndrome is a putative autoimmune disorder, a "friendly fire" attack of the immune system on the brain (Pranzatelli 2000). On-coneural antigens have not been identified. Tumors from children with OMS are more highly infiltrated with lymphocytes than those from non-OMS counterparts (Martin and Beckwitz 1968); both B cells and T cells congregate in immune nodules (Cooper et al. 2001) and are recruited to the CNS (Pranzatelli 2000). B-cell and T-cell expansion correlates with neurological severity. Histopathologically, inflammatory cells, cerebellar vermian atrophy (Pranzatelli et al. 2002a), and cerebellar cell loss (Hayward et al. 2001) are sometimes identified (Clerico et al. 1993). Non-motor functions of the cerebellum, particularly language acquisition, are well accepted (Lieberman 2002); howev-er,the anatomic substrate of opsoclonus appears to be the brain stem, with mesencephalic and pontine ocular gaze centers containing the burst and omnipause cells that control saccadic eye movements (Fuchs et al. 1985). Also, the seat of myoclonus can be wide-ranging within the CNS, the nucleus gigantocellularis reticularis in the caudal medulla, is the closest to a "myoclonus center" (Pranzatelli 1992). A diffuse neural network originating in the cerebellum with brain stem and frontal connections may become dysfunctional. Purkinje neurons, the main cerebellar outflow to deep cerebellar nuclei, may play a crucial role.

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