Neuromyelitis optica spectrum disorder (NMO) is a rare inflammatory disease of the central nervous system (CNS): eye nerves, spinal cord, and brain. NMO is also sometimes called neuromyelitis optica or NMOSD.1
Doctors do not know the exact cause of NMO. However, doctors do know some things that may cause it.
Nerve damage
Nerve fibers in your brain, spinal cord, and eyes have a protective coating called myelin. Damage to this layer is called demyelination. Conditions like NMO are demyelinating because they damage this protective layer.2
When this layer is damaged, nerve signals can slow or stop. This leads to brain and nerve problems.2
NMO and your immune system
NMO is an autoimmune disease. Autoimmune means that 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. In NMO, the nerve fibers of the eye, spine, and brain may be affected.3
Blood-brain barrier
With NMO, the area between the blood and the brain can weaken. This area is known as the blood-brain barrier. The blood-brain barrier has several jobs:4
- 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
AQP4 and MOG antibodies
Antibodies are chemicals the immune system makes to kill germs. In autoimmune diseases, antibodies can also be harmful. A weakened blood-brain barrier allows a type of antibody to cause inflammation in the brain. This antibody targets a protein called aquaporin-4 (AQP4).4,5
AQP4 is found on a type of 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 myelin of the brain cells and make more proteins that lead to inflammation.4,5
While many with NMO test positive for AQP4 antibodies (AQP4-IgG), about 1 in 4 do not. Some test positive for an antibody that targets a protein called myelin oligodendrocyte glycoprotein (MOG). Some test negative for MOG (MOG-IgG) and AQP4 (AQP4-IgG) antibodies. This adds to the mystery behind the cause of the disease.6
MOG (MOG-IgG) antibodies are believed to cause similar optic nerve or spinal cord attacks to those caused by AQP4 antibodies (AQP4-IgG) 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.5-8
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.7,8
Is NMO inherited?
Most people with NMO do not have a relative who has also been diagnosed with the disease. This means that it is not considered to be passed down from family.6
Some people with NMO may have a history of autoimmune disorders in the family or have another autoimmune condition themselves. Changes to the structure or job of some genes may be a reason why NMO develops. These changes may be there from birth or could develop over time. Doctors are looking at how genes factor into the development of NMO.6,9