Neuromyelitis Optica (NMO)
Neuromyelitis optica (NMO) affects the nerves in the eyes and spinal cord, causing patches of myelin (the substance that covers most nerve fibers) and the nerve fibers under them to be damaged or destroyed.
(See also Overview of Demyelinating Disorders.)
Neuromyelitis optica is a demyelinating disorder. It causes symptoms similar to those of multiple sclerosis (MS) and used to be considered a variant of multiple sclerosis. However, neuromyelitis optica typically affects only the eyes and the spinal cord, and multiple sclerosis also affects the brain. The risk of becoming disabled is greater with neuromyelitis optica than with multiple sclerosis. Thus, people with symptoms suggesting neuromyelitis optica should see a doctor promptly.
Neuromyelitis optica is an autoimmune disorder. Autoimmune disorders occur when the immune system malfunctions and attacks the body’s own tissues. In neuromyelitis optica, the target of the autoimmune attack is a protein called aquaporin 4, which is present on the surface of support cells (called astrocytes) in the brain, spinal cord, and optic nerves. Damage to astrocytes is thought to lead to demyelination.
Neuromyelitis optica causes inflammation of the optic nerve (optic neuritis). One or both eyes may be affected. The disorder causes episodes of eye pain and dim, blurred, or lost vision.
Days to weeks (sometimes years) later, the limbs are affected. People may temporarily lose sensation. They may have painful muscle spasms, and the arms and legs may become weak and sometimes paralyzed. People may be unable to control bladder and bowel function.
In some people, the part of the spinal cord that controls breathing is inflamed, leading to difficulty breathing, which is life threatening.
The disorder progresses differently in each person. As the disorder progresses, people may have brief, frequent, painful muscle spasms. Eventually, blindness, loss of sensation and muscle weakness in the limbs, and bladder and bowel dysfunction may become permanent.
To diagnose neuromyelitis optica, doctors evaluate the nervous system (neurologic examination) during a physical examination. The optic nerve is examined with an ophthalmoscope (see What Is an Ophthalmoscope?).
Usually, tests include magnetic resonance imaging (MRI) of the brain to rule out multiple sclerosis, MRI of the spinal cord and evoked responses are done to confirm the diagnosis of neuromyelitis optica. When the evoked responses test is used to diagnose neuromyelitis optica, stimuli for sight (such as a flashing light) are used to activate certain areas of the brain. Then, electroencephalography is used to detect the response to the stimuli. Based on these responses, doctors can tell how well the structures of the eye are working.
A blood test to detect specific antibodies to aquaporin 4 is done to distinguish neuromyelitis optica from multiple sclerosis.
There is no cure for neuromyelitis optica. However, treatments can stop episodes, control symptoms, and prevent episodes from recurring.
A corticosteroid (such as methylprednisolone) and a drug that suppresses the immune system (an immunosuppressant, such as azathioprine) are often used to stop and prevent episodes. Rituximab (a monoclonal antibody used as an immunosuppressant and a chemotherapy drug) may be used to reduce the number of abnormal antibodies and to control the disorder.
Plasma exchange may help people who do not respond to corticosteroids. For this treatment, blood is removed, then abnormal antibodies are removed, and the blood is returned to the person (see Controlling Diseases by Purifying the Blood).
Treatment of symptoms is similar to that for multiple sclerosis. Baclofen or tizanidine may relieve muscle spasms.