Aquaporin 4 IgG Serostatus and Outcome in Recurrent Longitudinally Extensive Transverse Myelitis

Neuromyelitis Optica is an autoimmune disorder that affects both adults and children commonly causing severe recurrent bouts of optic neuritis and transverse myelitis. Patients often present with either optic neuritis or transverse myelitis only to have future relapses confirming the diagnosis of NMO. In 2004 an antibody was identified in a large number of NMO patients. This antibody recognized the AQP4 protein in astrocytes (a specialized cell in the central nervous system). Patients with this anti-AQP4 antibody have been shown to develop NMO when followed over time, but initially many patients tested negative for this antibody. In this scenario, scientists often question, do patients who test negative lack the antibody or is the test not sensitive enough to identify them?

Dr. Pittock’s group from Mayo Clinic recently published their original work focused on recurrent longitudinally extensive transverse myelitis (rLETM). The goal of the study was to study AQP4-IgG, a known clinical biomarker of NMO spectrum disorders, from patients with rLETM using a recombinant human AQP4 based assay who were initially classified as negative for this antibody based on an assay known as IIF. The authors did a search of the Mayo Clinic database based on diagnosis from Oct 2005 to Nov 2011 and identified 48 patients with rLETM, 75% of whom were positive for the NMO signature of AQP4 IgG using the IIF method with serial serum specimens. On retesting those that were negative using the recombinant human AQP4-based assays, the overall AQP4-IgG seropositivity increased from 75% to 89%.

The authors also set out to define the clinical characteristics and motor disability outcomes in those who were positive for AQP4 IgG. This study showed that 36% of rLETM patients who were seropositive will likely need a cane to walk within 5 years after onset, and the median time from onset to first optic neuritis attack was 54 months in those who went on to be diagnosed as NMO. The median number of acute attacks was 3 in those rLETM patients who were seropositive (ranged from 2-22 attacks). Immunosuppression reduced the relapse rate in both seropositive and seronegative rLETM patients.

The authors concluded that recombinant antigen based assays can better detect AQP4-IgG in rLETM patients and generally adults with rLETM are seropositive, and are likely to go on and be diagnosed as having NMO. For rLETM patients who are at higher risk to convert to NMO, testing for this antibody AQP4 using more sensitive assays is the recommendation of the authors if less sensitive tests show negative results.