What is NMO?

What is NMO?

Neuromyelitis optica spectrum disorder (NMO) is a rare inflammatory disease of the central nervous system (eye nerves, spinal cord, and brain). It is sometimes also called neuromyelitis optica or NMO.1

NMO is an autoimmune disease. In autoimmune diseases, the immune system cannot tell the difference between healthy cells and invaders like viruses, fungi, or bacteria. Because it cannot tell the difference, the body begins to attack and damage healthy cells.

NMO is also chronic. This means that it lasts for a long time or never goes away completely. There is no cure for NMO, but there is treatment.1

The history

Neuromyelitis optica (formerly known as Devic disease) was first described 125 years ago. In 2006, the phrase “spectrum disorder” was added to describe the range of brain involvement and symptoms linked to the disease. In 2015, the International Panel for NMO Diagnosis (IPND) changed the diagnosis guidelines for NMO.2,3

NMO and multiple sclerosis

NMO was once thought of as a type of multiple sclerosis (MS). We now understand that it is its own condition, but it may share features with MS. Because it is often misdiagnosed as MS, the number of people who have NMO may be much higher than reported.1

Who gets NMO?

NMO affects 1 to 10 per 100,000 people worldwide. Doctors do not know the exact number of people living with NMO. There are between 4,000 and 15,000 people in the United States living with NMO, according to different sources. About 250,000 people have NMO worldwide.1,4

Gender, age, and ethnicity

NMO occurs in all parts of the world. Women are 4 to 5 times more likely to be diagnosed with the relapsing form of NMO, especially those with the antibody that targets aquaporin-4 (AQP4). Most people with NMO have no other family member with the condition. The average age of onset for NMO symptoms is between 40 and 50, but NMO may occur at any age. NMO occurs more commonly in those of Asian and African descent.1,4

Fewer than 5 percent of those with NMO are under the age of 18. Children are more likely to have optic neuritis (inflammation of the nerve bundles in the eye) at the onset of the disease. Children are more likely to have a single NMO attack than adults are.1,5

Cause

Doctors do not know the exact cause of NMO. In NMO, the layer that divides the blood and brain can weaken. This layer is known as the blood-brain barrier.1,6

The blood-brain barrier has several jobs:6

  • Protect the brain from substances in the blood that could injure it (“foreign” substances)
  • Allow some healthy substances to reach the brain
  • Maintain a constant environment for the brain

Antibodies are proteins the immune system makes to kill germs. In autoimmune disease, antibodies can also be harmful. A weakened blood-brain barrier allows AQP4 antibodies (AQP4-IgG) to cause inflammation in the brain.7

AQP4 is found in brain cells called astrocytes and helps control water balance in the brain. The brain, spinal cord, and eye nerve (optic nerve) have AQP4. AQP4 antibodies (AQP4-IgG) damage the protective coating (myelin) of the brain cells and produce other proteins that lead to inflammation.7,8

While many with NMO test positive for AQP4 antibodies (AQP4-IgG), about 1 in 4 do not. This adds to the mystery behind the cause of the disease.8

Antibodies targeting another protein called myelin oligodendrocyte glycoprotein (MOG) can also be present in NMO. They are believed to cause similar optic nerve or spinal cord attacks in MOG antibody (MOG-IgG) NMO. However, symptoms of this condition develop differently and need to be treated differently from those of AQP4 antibody (AQP4-IgG) NMO. Men and women are equally likely to get MOG antibody (MOG-IgG) NMO. Children have this disease more often than they have AQP4 antibody (AQP4-IgG) NMO.7-10

It is possible that MOG antibody (MOG-IgG) NMO and AQP4 antibody (AQP4-IgG) NMO will eventually be considered separate diseases, though this is debated. For now, they are considered subtypes of NMO.9,10

Patterns

There are 2 patterns of NMO. Doctors treat the disease the same, regardless of pattern. Doctors assume every person has relapsing NMO, even after a single attack.8,11

Monophasic

  • More common in children5
  • About 5 to 10 percent of cases11
  • A single attack8
  • Affects men and women equally8

Relapsing

  • 90 to 95 percent of cases11
  • Much more common in women than in men11
  • Periodic attacks with some time in-between11

Progression

People with NMO have “attacks” of the disease. Most of these attacks worsen over days and may last weeks to months. These attacks may include vision loss and paralysis, which may or may not improve over time.8

Untreated relapsing NMO relapses with an unpredictable course. Those who are on treatment may continue to relapse. The amount of time between relapses may be longer in those who are treated. However, this is also highly variable.11

Prognosis and life expectancy

New studies have shown that life expectancy for NMO may be longer in recent years because of advances in treatment. NMO is not a disease that is in itself fatal. Instead, the disease may lead to complications that can cause death. Death from NMO varies widely from person to person and is based on factors such as:11

  • Severity of symptoms
  • Delayed or misdiagnosis
  • Other diseases or conditions
  • Choice of treatment

Deaths from NMO usually occur because of complications from the disease such as respiratory failure.11

The 5-year survival rate for monophasic NMO is 90 percent, while the survival rate for the relapsing form is nearly 70 percent. This changes as treatment improves.12

As of 2021, 3 drugs approved by the U.S. Food and Drug Administration (FDA) show promise in preventing the damaging attacks of NMO.1,8

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Written by: Katie Murphy │ Last reviewed: September 2021