Neuromyelitis optica spectrum disorder (NMOSD) is a rare inflammatory disease of the central nervous system (eye nerves, spinal cord, and brain). NMOSD is also sometimes called neuromyelitis optica or NMO.1
NMOSD 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.1
Research and understanding of NMOSD has changed over time. Understanding the history of NMOSD, patterns of disease, and the various types of NMOSD is helpful to understanding the disease as a whole.
History of NMOSD
The symptoms recognized as neuromyelitis optica spectrum disorder (formerly known as Devic disease) were first described 125 years ago. In 2006, “spectrum disorder” was added to the name 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 NMOSD.2,3
NMOSD was once thought to be a type of multiple sclerosis (MS). Though it is in some ways similar to MS, NMOSD is its own condition. Because NMOSD is often misdiagnosed as MS, the number of people who have this disease may be higher than reported.1
Spectrum disorder
Spectrum disorders like NMOSD have wide variations in the type and severity of symptoms. Not only does this cause diagnosis challenges, it also means that doctors do not always agree on how to describe these conditions. Some people may use “patterns” and “types” interchangeably, though many do not.1
Patterns of NMOSD
Patterns of disease in NMOSD relate to the number of attacks that can occur in the condition.
There are 2 patterns or courses of illness in NMOSD. Doctors treat the disease the same, regardless of the pattern. These patterns are monophasic and relapsing. Doctors assume every person has relapsing NMOSD, even after a single attack.4
Monophasic NMOSD
Only about 5 to 10 percent of cases are monophasic. Monophasic means having a single attack. A single attack usually lasts 1 to 2 months but varies with each person. Men and women are equally likely to get this pattern of NMOSD.3,4
To be diagnosed with this type of NMOSD, the IPND recommends certain criteria should be met:3,4
- The person should be attack-free for 5 or more years
- Those who test positive for AQP4 antibodies (AQP4-IgG) likely do not have this pattern of NMOSD
- Those who test positive for MOG antibodies (MOG-IgG) are more likely to have this pattern of disease
Relapsing NMOSD
Up to 95 percent of all NMOSD cases are relapsing, meaning they involve attacks that occur more than once with time in between. Women have this course of NMOSD up to 10 times more often than men do. Because this pattern represents the vast majority of cases, doctors first treat all cases of NMOSD as relapsing.4
Types of NMOSD
The most commonly accepted types of NMOSD include:3
- AQP4-IgG positive, which is when a person tests positive for AQP4 antibodies
- MOG-IgG positive, which is when a person tests positive for MOG antibodies
- Seronegative, which is when a person has negative blood tests for both AQP4 antibodies and MOG antibodies
AQP4 and MOG antibodies
Antibodies are proteins the immune system makes to kill germs. In some diseases, antibodies can also be harmful. One type of antibody that causes inflammation in the brain targets a protein called aquaporin-4 (AQP4).5
AQP4 is found on 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) attack these areas and damage the protective coating (myelin) of the brain cells, producing more proteins that lead to inflammation.5
While many with NMOSD test positive for AQP4 antibodies (AQP4-IgG), about 1 in 4 do not. This adds to the mystery behind the cause of the disease.4
Antibodies targeting another protein called myelin oligodendrocyte glycoprotein (MOG) can also be present in NMOSD. They are believed to cause similar optic nerve or spinal cord attacks in MOG antibody (MOG-IgG) NMOSD. However, symptoms of this condition develop differently and need to be treated differently from those of AQP4 antibody (AQP4-IgG) NMOSD. Men and women are equally likely to get MOG antibody (MOG-IgG) NMOSD. Children have this disease more often than they have AQP4 antibody (AQP4-IgG) NMOSD.4-7
It is possible that MOG antibody (MOG-IgG) NMOSD and AQP4 antibody (AQP4-IgG) NMOSD disease will eventually be considered separate diseases, though this is debated. For now, they are considered subtypes of NMOSD.6,7
Living with a rare, chronic disease like NMOSD can be unpredictable and confusing at times. NMOSD is a complex disease with different patterns and types. Knowing these can give you more understanding about the disease, its treatments, and outcomes.