Neuromyelitis optica (NMO) is an autoimmune disorder in which the immune system attacks the optic nerves and spinal cord, causing episodes of vision loss, weakness, and paralysis. Once thought to be a variant of multiple sclerosis, it is now recognized as a distinct condition with a different biological cause and a different treatment approach. The broader term used today is neuromyelitis optica spectrum disorder (NMOSD), which captures the full range of presentations.
What Happens in the Body
In most people with NMOSD, the immune system produces antibodies that target a protein called aquaporin-4, a water channel found on the surface of certain brain and spinal cord cells called astrocytes. Astrocytes perform critical housekeeping in the nervous system: they regulate water balance, deliver nutrients, and clear excess signaling chemicals between nerve cells.
When those antibodies lock onto astrocytes, they trigger a cascade of inflammation. The body’s complement system, a set of proteins that normally helps fight infections, activates and punches holes in astrocyte membranes. Immune cells then flood into the area, destroying astrocytes first and then the surrounding insulation (myelin) that protects nerve fibers. The result is damage to the optic nerves, spinal cord, and sometimes the brainstem. This is why NMOSD is now classified as an “autoimmune astrocytopathy,” a disease driven by astrocyte destruction, rather than a primary demyelinating disease like MS.
Symptoms of an Attack
NMOSD typically causes distinct attacks, or relapses, that affect specific parts of the nervous system. The two hallmark presentations are optic neuritis and transverse myelitis.
Optic neuritis involves inflammation of the optic nerve. It can cause blurred vision or complete vision loss in one or both eyes, loss of color vision, and eye pain that often worsens with eye movement.
Transverse myelitis involves inflammation across a section of the spinal cord. Symptoms include stiffness, weakness, or numbness in the legs and sometimes the arms, tingling or shooting pain in the neck, back, or abdomen, and difficulty with bladder or bowel control. These symptoms can develop over hours to days and range from mild to severe.
Some people also experience prolonged nausea, vomiting, or intractable hiccups. These brainstem symptoms, seen in roughly 40% of people with aquaporin-4 antibodies, occur when inflammation hits an area of the brainstem called the area postrema, which controls the vomiting reflex. Painful muscle spasms are another common feature, affecting over 40% of aquaporin-4 antibody-positive patients.
Who Gets NMOSD
NMOSD is rare, with a prevalence of roughly 0.5 to 4.4 per 100,000 people depending on the population studied. Like many autoimmune diseases, it disproportionately affects women, who make up 66 to 88% of patients. In the relapsing form, which is by far the most common, the female-to-male ratio can reach 7:1. The condition occurs across all ethnicities but appears more common in people of African, Asian, and Latin American descent compared to White populations.
How It Differs From Multiple Sclerosis
For decades, NMO was misdiagnosed as MS because both cause vision problems and spinal cord inflammation. The distinction matters because some MS medications can actually worsen NMOSD. Several key differences help separate the two.
On spinal cord MRI, NMOSD produces long, continuous lesions that stretch across three or more vertebral segments and sit centrally within the cord. MS spinal lesions are typically short (rarely more than one segment), patchy, and positioned near the periphery of the cord. Brain MRI in NMOSD may show lesions, but they tend to follow a pattern distinct from the scattered white-matter lesions typical of MS. Importantly, having brain lesions does not rule out NMOSD, as was once believed.
The most definitive difference is the blood test. About 70 to 80% of NMOSD patients carry aquaporin-4 antibodies, which are not found in MS.
Diagnosis
Diagnosis relies on a combination of clinical symptoms, MRI findings, and blood testing for aquaporin-4 antibodies. The 2015 international consensus criteria define NMOSD with aquaporin-4 antibodies as requiring at least one core clinical feature (optic neuritis, transverse myelitis, brainstem symptoms, or certain brain presentations) along with a positive antibody test. For people who test negative for the antibody, or when testing is unavailable, stricter clinical and imaging criteria are required.
The best available blood test is the cell-based assay, which detects aquaporin-4 antibodies with about 76% sensitivity and 99% specificity. That high specificity means a positive result is very reliable, but the 76% sensitivity means about one in four people with the disease may test negative, so a negative result doesn’t completely rule it out.
MOG Antibody Disease
A subset of people who were once grouped under NMOSD actually have antibodies against a different protein called MOG (myelin oligodendrocyte glycoprotein). This condition, now called MOG antibody-associated disease, looks similar on the surface but behaves differently. MOG-positive patients are more often male, more frequently have bilateral optic neuritis (affecting both eyes at once, in roughly 73% of attacks), and tend to recover better. Their median disability scores are significantly lower than those of aquaporin-4 antibody-positive patients. Spinal cord lesions in MOG disease tend to sit lower, in the thoracolumbar region, while aquaporin-4 disease favors the cervicothoracic cord. MOG-positive patients are also much less likely to experience the prolonged nausea, vomiting, or hiccups typical of aquaporin-4 NMOSD.
What Happens Without Treatment
About 90% of NMOSD patients have a relapsing course, meaning attacks recur over time. Half experience their first relapse within a year, and 90% relapse within five years. Each attack carries the risk of permanent damage, and unlike MS, recovery between attacks is often incomplete. Without treatment, 50% of patients with relapsing NMOSD become blind in one or both eyes or need assistance walking within five years.
Treating Acute Attacks
When an attack occurs, the immediate goal is to stop the inflammation and limit permanent damage. The first-line treatment is high-dose intravenous corticosteroids, which broadly suppress the immune response. If symptoms don’t improve, or if the attack is severe, plasma exchange (plasmapheresis) is used. This process filters the blood to physically remove the harmful antibodies. A typical course involves five to seven sessions over 10 to 14 days, done daily or every other day.
Preventing Future Attacks
Long-term treatment focuses on preventing relapses, and this is where treatment has been transformed in recent years. Four targeted biologic therapies are now FDA-approved specifically for aquaporin-4 antibody-positive NMOSD, each blocking a different step in the inflammatory chain.
Two of these therapies block complement protein C5, the same immune component that punches holes in astrocytes during an attack. In clinical trials, one reduced attacks by 94% compared to placebo, and the other, a longer-acting version, eliminated relapses entirely over a median follow-up of about 18 months, representing a 98.6% reduction in relapse risk.
A third option depletes B cells, the immune cells that produce the harmful antibodies, and reduced relapse risk by 77% overall and 79% in aquaporin-4 antibody-positive patients. The fourth blocks a signaling molecule called IL-6 that drives inflammation, reducing relapses by 74 to 79% in aquaporin-4 antibody-positive patients depending on whether it was used alone or alongside other immunosuppressants.
These therapies represent a major shift. Before their approval, doctors relied on general immunosuppressants borrowed from other conditions. The targeted drugs are more effective and, for many patients, have made the disease manageable in a way that was not possible a decade ago. All four are specifically approved for people who test positive for aquaporin-4 antibodies, and treatment decisions for antibody-negative patients remain more complex.