Multiple sclerosis (MS) is a disease in which the body’s immune system attacks the outside layer of nerve cells, called myelin.

Neuromyelitis optica (NMO), sometimes just called “neuromyelitis” or Devic's disease, is also an immune system attack. However, the attack is focused only on the optic nerve in neuromyelitis.

Neuromyelitis optica is a rare disease that damages the optic nerve and the spinal cord. Its cause is unknown. Neuromyelitis leads to optic neuritis, which causes pain in the eyes and a loss of vision. Other symptoms can include weakness, numbness, and bladder control problems. Doctors use magnetic resonance imaging (MRI) and check spinal fluid to diagnose neuromyelitis.

Unlike neuromyelitis, MS attacks the entire central nervous system. It can affect the brain, optic nerve, and spinal cord. The results include numbness, paralysis, vision loss, and other problems, and severity varies greatly from person to person.

While there’s currently no cure for MS, medications and treatments can help manage some symptoms. MS usually doesn’t affect life expectancy.

Because neuromyelitis is so similar to MS, scientists previously believed it might be a form of MS. However, scientific consensus now distinguishes neuromyelitis optica from MS and groups it with related syndromes under the unifying term neuromyelitis optica spectrum disorder (NMOSD).

The Cleveland Clinic reports that neuromyelitis attacks do more harm than MS does to certain parts of the body. The Clinic also points out that neuromyelitis doesn’t respond to some of the drugs that help relieve MS symptoms.

MS and neuromyelitis differ in the impact that episodes have on the body. Symptoms of MS episodes are often mild, especially in the early stages of the disease. The cumulative effects of episodes can become very serious. However, they also may have a limited impact on a patient’s ability to function.

Neuromyelitis attacks, on the other hand, can be severe and lead to health problems that can’t be reversed. Early and aggressive treatment is important in reducing the harm caused by neuromyelitis.

The course of both diseases can be quite similar. Some people with MS experience remitting episodes, ones in which symptoms come and go. The more common form of neuromyelitis also occurs in attacks that return on a regular basis.

But the two conditions can also be quite different. Neuromyelitis may strike once and last for a month or two. Some types of MS don’t have periods in which symptoms go into remission. In these cases, symptoms simply get progressively worse over time.

Multiple sclerosis is more common than neuromyelitis. More than 2 million people have MS, according to the National Multiple Sclerosis Society. And these people tend to be concentrated in areas with mild weather.

Neuromyelitis can be found in any climate, with about 500,000 cases around the world, including about 4,000 in the United States, according to the National Multiple Sclerosis Society.

Both MS and neuromyelitis are incurable. There is also no way to predict who will develop one or both of the diseases. However, medications can help treat symptoms.

Because neuromyelitis is likely to return after the first episode, patients are usually prescribed drugs to repress the body’s immune system. Randomized clinical trials are beginning to assess the efficacy and safety of a variety of immunotherapies in treating neuromyelitis. New MS medications are meant to reduce symptom flare-ups and treat the underlying causes of the disease.