Neuromyelitis optica (NMO) is an uncommon disease syndrome of the central nervous system (CNS) that affects the optic nerves and spinal cord.
The relapsing form of NMO primarily affects women. The female to male ratio is greater than 4:1. Another form of NMO, in which an individual only has a single, severe attack extending over a month or two, is most likely a distinct disease that affects men and women with equal frequency. The onset of NMO varies from childhood to adulthood, with two peaks, one in childhood and the other in adults in their 40s.
Individuals with NMO develop optic neuritis, which causes pain in the eye and vision loss, and transverse myelitis, which causes weakness, numbness, and sometimes paralysis of the arms and legs, along with sensory disturbances and loss of bladder and bowel control. NMO leads to loss of myelin, which is a fatty substance that surrounds nerve fibers and helps nerve signals move from cell to cell. The syndrome can also damage nerve fibers and leave areas of broken-down tissue. In the disease process of NMO, for reasons that aren’t yet clear, immune system cells and antibodies attack and destroy myelin cells in the optic nerves and the spinal cord.
Historically, NMO was diagnosed in patients who experienced a rapid onset of blindness in one or both eyes, followed within days or weeks by varying degrees of paralysis in the arms and legs. In most cases, however, the interval between optic neuritis and transverse myelitis is significantly longer, sometimes as long as several years. After the initial attack, NMO follows an unpredictable course.
Disability is cumulative, the result of each attack damaging new areas of myelin. Some individuals are severely affected by NMO and can lose vision in both eyes and the use of their arms and legs. Most individuals experience a moderate degree of permanent limb weakness from myelitis. Muscle weakness can cause breathing difficulties and may require the use of artificial ventilation. The death of an individual with NMO is most often caused by respiratory complications from myelitis attacks.
In the past, NMO was considered to be a severe variant of multiple sclerosis (MS) because both can cause attacks of optic neuritis and myelitis. Recent discoveries, however, suggest it is a separate disease. NMO is different from MS in the severity of its attacks and its tendency to solely strike the optic nerves and spinal cord at the beginning of the disease. Symptoms outside of the optic nerves and spinal cord are rare, although certain symptoms, including uncontrollable vomiting and hiccups, are now recognized as relatively specific symptoms of NMO that are due to brainstem involvement.
The recent discovery of an antibody in the blood of individuals with NMO gives doctors a reliable biomarker to distinguish NMO from MS. The antibody, known as NMO-IgG, seems to be present in about 70 percent of those with NMO and is not found in people with MS or other similar conditions.
There is no cure for NMO, but there are therapies to treat an attack while it is happening, to reduce symptoms, and to prevent relapses. Doctors usually treat an initial attack of NMO with a combination of a corticosteroid drug (methylprednisolone) to stop the attack, and an immunosuppressive drug (azathioprine) for prevention of subsequent attacks. If frequent relapses occur, some individuals may need to continue a low dose of steroids for longer periods. Plasma exchange (plasmapheresis) is a technique that separates antibodies out of the blood stream and is used with people who are unresponsive to corticosteroid therapy. Pain, stiffness, muscle spasms, and bladder and bowel control problems can be managed with the appropriate medications and therapies.
Individuals with major disability will require the combined efforts of occupational therapists, physiotherapists, and social services professionals to address their complex rehabilitation needs.