Lyme neuroborreliosis: known knowns, known unknowns.

Lyme borreliosis affects the nervous system in three principal ways-mononuclear cell meningitis, cranial neuropathies and radiculoneuropathies-the last a broad term encompassing painful radiculopathy, unifocal and multifocal peripheral nerve involvement. Diagnostic tools have been significantly refined-including improved peripheral blood and CSF serodiagnostics-and much has been learned about the interactions between the causative pathogen and the nervous system. Despite these advances in our understanding of this disease, a broad range of other disorders continue to be misattributed to nervous system Lyme borreliosis, supported by, at best, limited evidence. These misattributions often reflect limited understanding not only of Lyme neuroborreliosis but also of what constitutes nervous system disease generally. Fortunately, a large body of evidence now exists to clarify many of these issues, establishing a clear basis for diagnosing nervous system involvement in this infection and, based on well performed studies, clarifying which clinical disorders are associated with Lyme neuroborreliosis, which with non-neurologic Lyme borreliosis, and which with neither.