Recent insights into the pathology of multiple sclerosis and neuromyelitis optica.

Multiple sclerosis (MS) is the most common inflammatory demyelinating disease of the central nervous system. Traditionally, demyelinating lesions in the white matter have been regarded as the most important pathological feature in MS, but recent pathological and imaging studies confirmed substantial changes in grey matter and normal-appearing white matter. MS lesions are characterized by inflammation, demyelination, axonal damage and astrogliosis. During early MS lesion formation acute axonal injury is extensive and correlates with inflammation. In addition to focal lesions, diffuse wide-spread changes including neuroaxonal degeneration and compartmentalized inflammation are likely to contribute to increasing disability in progressive MS. Neuromyelitis optica (NMO) is classically characterized by severe transverse myelitis and optic neuritis, but brain lesions are also present in the majority of NMO patients. The discovery of the NMO-specific antibody demonstrated that NMO is a disease entity distinct from MS. This antibody binds to aquaporin-4 expressed in astrocytes and ependymal cells. NMO lesions are characterized by inflammation, demyelination, axonal damage and a marked loss of aquaporin-4. Early NMO lesions demonstrate a pronounced humoral inflammatory response and astrocytic cell death with loss of aquaporin-4, followed by inflammatory demyelination and axonal damage. These recent findings contribute to a better understanding of different mechanisms leading to inflammatory demyelination.