What is Neuromyelitis Optica Spectrum Disorder (NMOSD)?

Neuromyelitis optica spectrum disorder (NMOSD) is a progressive autoimmune disease that affects the nervous system. It primarily leads to inflammation of the optic nerve — the nerve that sends and receives signals from the eye — and the spinal cord. This leads to issues with vision and the control of muscles. Episodes of inflammation typically reoccur periodically, separated by periods of remission.

It is a rare disease, and estimates vary but suggest that approximately 15,000 people in the U.S. have it.

What causes NMOSD?

NMOSD is an autoimmune disease in which the body’s own immune system mistakenly attacks cells of the central nervous system, comprised of the brain and spinal cord. However, scientists do not know the exact underlying cause of NMOSD.

About 70% of patients with NMOSD have elevated levels of an antibody called anti-aquaporin-4 (AQP4-IgG). These antibodies target proteins that carry water across the membranes of astrocytes. Astrocytes are cells of the nervous system that support nerve cells.

Another antibody that is present in some patients, anti-MOG, targets myelin oligodendrocyte glycoprotein (MOG). MOG is found on the outside of special types of cells in the nervous system called oligodendrocytes. These cells serve as support cells for nerve cells by covering them in an insulating material called myelin.

The immune attack on astrocytes or oligodendrocytes causes inflammation that leads to damage to the surrounding nerve cells and myelin.

What are the symptoms?

When the disease affects the optic nerve (optic neuritis), it can result in pain inside the eye and possibly blurry vision or vision loss. When it affects the spinal cord, it often results in a condition known as longitudinally extensive transverse myelitis, in which large sections of the spinal cord are affected at one time. The immune attacks on the spinal cord can lead to a wide range of symptoms. These include weakness or paralysis of the arms and legs, loss of sensation, pain in the spine or limbs, muscle spasticity, trouble breathing, and inability to control the bladder and bowel.

In rare cases, NMOSD also can affect the brainstem, leading to symptoms such as intractable (unstoppable) hiccups or vomiting, slurred speech, double vision, lack of coordination, and sleep attacks.

Finally, the disease can lead to psychological issues such as depression and impaired cognition (thinking).

How do doctors diagnose NMOSD?

Early on in the disease, it may be hard to differentiate NMOSD from multiple sclerosis (MS), because optic neuritis and myelitis can appear in both conditions. This can often lead to misdiagnosis. However, several tests are available to help differentiate between the two diseases.

Doctors will usually start with a review of a patient’s medical history and symptoms, followed by physical and neurological exams to assess muscle strength and coordination, vision, speech, memory, and ability to think. In addition, clinicians will look for any signs of numbness. Based on the findings of the medical history review and examinations, the physician may recommend further testing.

Magnetic resonance imaging (MRI) can look for evidence of damage and determine whether the disease mostly affects the spinal cord or the brain. NMOSD usually affects the spinal cord more than the brain, which can help differentiate the condition. When it does affect the brain, this is usually a different region than in MS.

Doctors also can use MRI or evoked responses to assess the status of the optic nerve. In evoked responses, they shine a light in the patient’s eyes and observe the response in the brain using electroencephalography (EEG).

Antibody tests from the blood and cerebrospinal fluid (the fluid that surrounds the brain and spinal cord) also can help diagnose NMOSD. Here, doctors look for AQP4-IgG and anti-MOG antibodies.