Neuromyelitis optica (NMO) and NMO Spectrum Disorder (NMOSD), also known as Devic's disease, is an autoimmune disorder in which white blood cells and antibodies primarily attack the optic nerves and the spinal cord, but may also attack the brain. The damage to the optic nerves produces swelling and inflammation that cause pain and loss of vision; the damage to the spinal cord causes weakness or paralysis in the legs or arms, loss of sensation, and problems with bladder and bowel function.
NMO is a relapsing-remitting disease. During a relapse, new damage to the optic nerves and/or spinal cord can lead to accumulating disability. Unlike MS, there is no progressive phase of this disease. Therefore, preventing attacks is critical to a good long-term outcome.
The cause of NMO in the majority of cases is due to a specific attack on the aquaporin-4 (AQP4) water channel located within the optic nerves and spinal cord. Aquaporins (AQPs) are proteins that transport water across cell membranes. More than 70% of NMO and NMOSD patients test positive for an antibody biomarker in the blood called the NMO-IgG or anti-AQP4 antibody. In people with NMO who test negative for anti-AQP4 antibodies, up to a third may be positive for auto-antibodies directed against a component of myelin called myelin oligodendrocyte glycoprotein (MOG). People with anti-MOG related NMO similarly have episodes of transverse myelitis and optic neuritis, but recovery after attacks appears to be better than anti-AQP4 related disease.
Who gets NMO, and when?
There are an estimated 4,000 people with NMO in the United States and a quarter-million people worldwide.
NMO is more common in women (greater than 80 percent) than men;
NMO occurs in all parts of the world and may be the most common form of demyelinating disease in certain populations such as Africans, Asians and Native Americans.
NMO can occur at any age - in children as young as 3 and adults as old as 90 - but appears most often between ages 40 and 50.
How is NMO different from MS?
NMO is considered to be an autoimmune disease (where the immune system reacts against healthy tissue as if it was a threat). In the majority of cases of NMO, the immune system recognizes the AQP4 water channel as foreign and develops antibodies (NMO-IgG or anti-AQP4 antibody) to attack AQP4 on the surface of astrocytes, which in turn damages the astrocytes. Astrocytes are supportive cells in the brain, spinal cord and optic nerves, and damage to astrocytes is believed to lead to demyelination. In cases associated with anti-MOG antibodies, it is considered that anti-MOG antibodies may trigger an attack on the myelin sheath resulting in demyelination.
Symptoms are generally more severe after an NMO attack than an MS attack.
NMO rarely has a secondary progressive stage as in MS; disabilities accumulate from repeated acute attacks.
NMO is present across the world, especially among non-Caucasians; MS has a higher incidence in temperate climates and Caucasians.
Fatigue in NMO is usually an indirect result of living with the disease and its symptoms (secondary fatigue); in MS, fatigue is both primary (caused by the disease process itself) and secondary.
NMO usually affects only the optic nerve and spinal cord at the beginning of the disease, although there may be lesions present in other specific areas of the brain. MS typically affects the brain as well as the spinal cord and optic nerve.
NMO-IgG (anti-AQP4 antibody) is not found in people with MS but is found in 70 percent of those with NMO.
In MS, individual episodes are usually mild; their cumulative effect over time may or not not cause progressive disability. In NMO, the opposite is true and therefore early diagnosis is critical; acute episodes are usually severe and - if untreated - can have devastating, irreversible effects on function.
Some people with NMO also have other autoimmune diseases such as Sjogren’s Syndrome or Systemic Lupus Erythematosus (SLE)