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In 1894, French physician Eugène Devic described patients he had seen with a serious condition affecting their vision, movement, and bladder control. They had rapidly lost sight in one or both eyes and, soon afterward, began experiencing severe weakness and spasticity in their limbs and loss of bladder control. Devic linked his patients’ symptoms to inflammation of the optic nerves (Figure) and spinal cord.With future reports of similar cases, the disease came to be known as Devic’s disease, also called neuromyelitis optica (noor'-oh-myuh- lye'-tis op’-ti-kuh), or NMO.
The basic similarities between NMO and multiple sclerosis (MS) were unmistakable from the outset: both diseases act on the central nervous system, and they overlap on the key symptoms of eye, limb, and bladder dysfunction. Indeed, in the century after Devic’s report, many experts considered NMO to be a rare and severe form of MS. But a great deal has been learned about NMO in the recent past—chiefly, that it is a distinct disease from MS, with its own profile of symptoms, nerve targets, appearance on imaging scans, and, quite helpfully, immune markers found in the blood and spinal fluid (Table). This knowledge offers neurologists more certainty and speed in the diagnosis of NMO, factors that are crucial to prompt the use of treatments to intervene in NMO symptoms and prevent relapses.
Epidemiology of NMO: Who Is Affected?
The prevalence of NMO has been hard to establish because the disease is so often confused with MS; an estimated 3% to 8% of diagnoses of MS in some regions of the world may actually be NMO. In Japan, up to 30% of people identified as having “opticospinal MS” are believed to have NMO. Although NMO is clearly less prevalent than MS in Europe and North America, it is more prevalent than MS in other populations, including Japanese, indigenous Africans, Afro-Brazilians, West Indians, Hong Kong Chinese, Singaporeans, and Indians. NMO is up to 9 times more common in women than men, for no known reason. The median age of onset is 39 years, but the disease can appear in children and the elderly as well. Having an affected family member does not seem to increase risk for other members, although some familial cases of NMO are known to exist in Japan. There is a great deal yet to be learned about what makes individuals and groups susceptible to NMO.
The Role of Autoimmunity
NMO and MS are autoimmune diseases, which means they occur because the body’s immune system turns against normal tissues as if they were foreign (not from one’s own body) tissues. In both diseases, an inflammatory autoimmune response results in damage to myelin, a fatty substance in the central nervous system that surrounds nerve fibers and helps with cell-to-cell signal transmission. The damaged areas, called “lesions,” account for signal problems and disruption of bodily functions: vision, sensations, movement, bladder and bowel control, speech, and cognition, among others. In the case of NMO, lesions appear mainly in the optic nerves and spinal cord and affect the specific eye and muscle control functions managed by these nerves. NMO lesions can also be found in the brainstem but usually only in advanced stages of the disease. MS, by contrast, does not ordinarily spare the brain early on.
As with MS, no one knows what triggers the autoimmune response in NMO. Some experts suspect that it develops in relation to a coexisting condition such as a viral infection or another autoimmune disease. In 2005, the molecular target of autoimmunity in NMO was discovered by researchers in the United States. It is a protein, called aquaporin-4, found not on nerve cells but on astrocytes—star-shaped cells that support the blood-brain barrier, a buffer zone that keeps harmful blood-borne substances from passing into the central nervous system. Aquaporin-4 is responsible for maintaining a critical balance of water within and outside the astrocytes. Some scientists now theorize that the autoimmune assault on aquaporin-4 results in a breach in the blood-brain barrier that allows self-directed immune cells to enter and start inflaming myelin. This scenario would help explain the observation that there are weakened areas in the blood-brain barriers of people with NMO.
Symptoms of NMO
The main symptoms of NMO arise from optic neuritis and acute myelitis, the terms for inflammation of the optic nerves and spinal cord, respectively. These 2 conditions usually occur in sequence (one after the other), rather than together. Relapses of one or both are likely to happen, but these relapses can occur months, years, or even decades apart.
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