Neurological expression of genetic immunodeficiencies and of opportunistic infections.

Immunodeficiencies may result from genetic defects or may be acquired after viral infection or therapeutic immunosuppression. In either case, neurological symptoms are frequent. Hemophagocytic lymphohistiocytosis (HLH), previously designated as macrophage activation syndrome (MAS), results usually from genetic defects impairing the cytotoxicity of CD8 T lymphocytes and natural killer (NK) cells. Neurological symptoms may be the first and only manifestation of the disease. The neurological and neuroradiological aspects of MAS, if isolated, may closely resemble those of other central nervous system (CNS) diseases such as acute disseminated encephalomyelitis (ADEM) or encephalitis. An early treatment including allogeneic hematopoietic stem cell transplantation can prevent further brain lesions. The main infections of the CNS observed in conjunction with therapeutic immunosuppression are described. It is important to know which phase of immunosuppression the child is in at the time of suspected CNS infection and always to consider that a relapse of the primary tumor may mimic a CNS infection.