Urgency of treatment is something that many physicians may not fully appreciate when it comes to neuromyelitis optica (NMO), according to experts on this rare autoimmune demyelinating disorder. This may be partly due to its similar presentation to multiple sclerosis (MS), said Michael Levy, MD, PhD, associate professor, Harvard Medical School, research director, Division of Neuroimmunology & Neuroinfectious Disease, and director, Neuroimmunology Clinic and Research Laboratory, at Massachusetts General Hospital in Boston. But while the two conditions share many clinical characteristics, “immunologically, they are about as different as can be,” he warned.
The urgency of distinction is important because where MS is known to have a relatively gradual progression, NMO is now red-flagged to potentially cause rapid and irreversible damage. While the course of MS might be described as a slow burn, NMO should be treated like a wildfire.
“That message has gotten muddled, particularly because acute treatment in MS has never been shown to affect outcome,” said Jeffrey Bennett, MD, PhD, professor of neurology and ophthalmology at the University of Colorado School of Medicine, Aurora. In contrast, rapid diagnosis and treatment of NMO “means potentially preventing future devastating neurologic injury,” he said.
First described by Dr. Eugène Devic in 1894, and sometimes known as Devic’s disease, NMO is believed to have a prevalence that varies widely depending on ethnicity and gender. A recent report suggests a prevalence of approximately1/100,000 population among Whites with an annual incidence of less than 1/million in this population, while the prevalence is higher among East Asians (approximately 3.5/100,000), and may reach as high as 10/100,000 in Blacks.1 It has a high female-to-male ratio (up to 9:1) with a mean age of onset of about 40 years, although pediatric cases are described.
It has long been recognized that NMO lacks the “neurocerebritis” of MS, with inflammation predominant in the optic and spinal nerves, but it was not until 2004 that researchers at the Mayo Clinic identified serum aquaporin-4 immunoglobulin G antibodies (AQP4-IgG) that could reliably distinguish NMO from MS. In 2015, the international consensus diagnostic criteria for neuromyelitis optica2 cited core clinical characteristics required for patients with AQP4-IgG-positive NMO spectrum disorder (NMOSD) “including clinical syndromes or MRI findings related to optic nerve, spinal cord, area postrema, other brainstem, diencephalic, or cerebral presentations.” Rarely, NMO patients can be seronegative for AQP4-IgG, but are still considered to have NMOSD for which non-opticospinal clinical and MRI characteristics findings are described. MS patients testing negative for AQP4-IgG should also be tested for the related myelin oligodendrocyte glycoprotein antibody disease (MOGAD), which has a prevalence about four to five times greater than NMO, Dr. Bennett said.
Testing
Because both NMO and MOGAD can be identified by antibodies, they are less commonly misdiagnosed as MS compared to previously. But, prior to the identification of the AQP4-IgG antibody in 2004, the misdiagnosis rate of NMO was probably about 95% said Dr. Levy.
Dr. Michael Levy
“Of course, before we had the antibody test or clinical criteria, we couldn’t confirm a diagnosis of NMO, so basically everyone had a diagnosis of MS, and after the antibody test became commercially available in 2005/2006, we could confirm the diagnosis, with our study in 2012 showing a much lower misdiagnosis rate of 30%.”3 More recently, the misdiagnosis rates are even lower, he added. A recent study out of Argentina found a rate of only 12%.4
The specificity and sensitivity of cell binding assay serum AQP4-IgG testing is roughly 99% and 90%, respectively, better than ELIZA testing (which has a sensitivity in the 60-65% range), said Dr. Bennett. “That’s why we highly emphasize to physicians, that if you have a suspicion for NMOSD you go to a cell binding assay, and make sure that where you’re sending the serum, the lab can do that procedure.” Still, because of the risk of false positives, he urges restraint in testing for the disorder in the absence of a high suspicion for it. “If you test a lot of people indiscriminately for a rare disorder, you get a lot of false positives because the actual true positives are a very small fraction of that group. So, even with a specificity of around 99% that means 1% of the people you test are falsely positive. And if you’re testing a group of people indiscriminately, then your true positives are less than 1% by far, so then most of the people that you pick up are not truly with disease.”