AFM Can Mimic MOG Antibody Myelitis

Spinal cord inflammation associated with an antibody to myelin oligodendrocyte glycoprotein (MOG) can mimic acute flaccid myelitis (AFM), a small retrospective study showed.

Of 54 patients with myelin oligodendrocyte glycoprotein autoantibody (MOG-IgG) myelitis, 19% were initially misdiagnosed with AFM, reported Eoin Flanagan, MB, BCh, of the Mayo Clinic in Rochester, Minnesota, and colleagues in JAMA Neurology.

The study focused on transverse myelitis, for which MOG is a biomarker, and the mimicry was an unexpected finding, Flanagan said. “When we were looking at cases with transverse myelitis, we recognized that some of those cases had been diagnosed with AFM,” he told MedPage Today.

Patients with MOG antibodies may respond to steroids and plasma exchange, but patients with AFM do not. “If you have a patient who presents with acute flaccid myelitis, you should consider MOG testing,” Flanagan noted. “It would have both treatment implications and prognostic implications because patients with MOG antibodies tend to improve much more.”

Like AFM, MOG-IgG myelitis can affect children and young adults. “As clinicians, we need to consider both disorders when we’re evaluating kids who present acutely with weakness,” observed Matthew Elrick, MD, of Johns Hopkins University in Baltimore, who was not involved with the research.

Part of the confusion may arise from the fact that AFM has an epidemiologic case definition that’s intended to be broad, Elrick added. “There isn’t really a good, agreed-upon clinical definition of what AFM is,” Elrick told MedPage Today. “As a result, patients with other disorders are included and this is an example of that scenario.”

Indeed, Elrick had published a paper last month making exactly that point, and offering a definition that he and colleagues said could be more useful for clinical diagnosis and investigations into the disease’s pathophysiology.

In the new paper, Flanagan and his group contrasted differences between MOG-IgG myelitis and other inflammatory causes of myelitis, namely myelitis with aquaporin-4–IgG (AQP4-IgG) and multiple sclerosis (MS). This was probably the first and largest study to focus on MOG, said co-author Divyanshu Dubey, MBBS, also of the Mayo Clinic.

“It highlights that in patients presenting with myelitis, MOG is something people should consider,” Dubey told MedPage Today. “If you look at the data, nearly one-third of the patients become wheelchair-bound at the disease nadir, so it can be pretty devastating.”

The paper emphasized clinical and MRI features to help select patients for MOG-IgG testing, which is crucial because testing in low-probability situations substantially increases the risk of false-positives. It also highlighted that follow-up is important; patients who persistently tested positive for MOG over time had higher relapse rates. “If you check for MOG initially and it’s positive, consider testing the patient again in 6 months to a year,” Dubey said.

The study had several limitations: it was retrospective with variable follow-up and researchers could not assess long-term effects in all patients. The MOG-IgG group had a higher proportion of children than other groups and serial samples of MOG-IgG were available only in a few patients.