By Jai S. Perumal, MD

Assistant Professor of Neurology, Weill Cornell Medical College

Dr. Perumal reports she receives grant/research support from Genzyme Corp., and is on the speakers bureau for Biogen Idec, Genzyme Corp., Acorda Therapeutics, and Teva Pharmaceuticals.

SYNOPSIS: Based on an epidemiological study in two ethnically and geographically distinct populations of patients diagnosed with central nervous system inflammatory demyelinating diseases, the authors report a higher prevalence among Afro-Caribbean patients in Martinique in the eastern Caribbean sea vs. a predominantly Caucasian population in Olmstead County in Minnesota. The study demonstrates a propensity for neuromyelitis optica to affect blacks more than Caucasians.

SOURCE: Flanagan EP, Cabre P, Weinshenker BG, et al. Epidemiology of aquaporin-4 autoimmunity and neuromyelitis optica spectrum. Ann Neurol 2016;79:775-783.

Neuromyelitis optica (NMO) is an inflammatory disease of the central nervous system that preferentially affects the optic nerves and spinal cord. Classic NMO or Devic's disease is characterized by concurrent episodes of optic neuritis (ON) and transverse myelitis (TM). NMO spectrum disorder (NMOSD) is diagnosed in patients with isolated ON or TM who have the NMO IgG (aquaporin-4) antibody, which is potentially pathogenic and has high specificity for this group of diseases. NMO was accepted as a distinct entity from multiple sclerosis (MS)after the discovery of the aquaporin-4 antibody in 2004. The aquaporin-4 test is > 80% sensitive and > 99% specific for NMOSD. Proper identification of NMOSD is imperative as the disease course and treatment options for this disease are different from that of MS.

A few studies from Europe that have reported the prevalence of NMO/NMOSD describe rates between 0.72-4.4/100,000. Large epidemiologic studies have not been reported from the United States, and though it appears that blacks may have a higher incidence when compared to Caucasians, there have not been large studies examining differences between the ethnicities. Given these limitations, the authors undertook a population-based study of the seroprevalence and seroincidence of aquaporin-4 antibody and NMO/NMOSD among patients with inflammatory demyelinating diseases in Martinique, French West Indies, and Olmstead County, Minnesota, respectively.

In Olmstead County, patients were identified from Rochester Epidemiological Project medical records linkage system, which is a database that includes patients seen by all medical practitioners in Olmstead County. Patients with a diagnosis of inflammatory demyelinating diseases (IDD), including MS, NMO, optic neuritis, transverse myelitis, clinically isolated syndrome, ADEMS, or other IDDs, between January 1, 1985, and Dec. 31, 2011, were included. Identified patients were contacted and invited to participate in the study and provide a blood sample for aquaporin-4 antibody assay between 2007 and 2015. Blood samples were collected from 363 of the 434 (84%) prevalent and 104 of 130 (80%) of incident cases of IDD. In Martinique, an IDD population registry has been maintained from 1992, which captures all cases of IDD from hospital-based and clinic-based neurology services and ophthalmology and rehabilitation services, health insurance data, and MS patient associations. One hundred ninety-three of 237 (81%) prevalent and 111 of 122 (91%) incident cases were included in the study. All blood samples collected as a part of this registry were sent to the Mayo Clinic for AQP4-IgG assay. The Olmstead population was 82% Caucasian and the Martinique population 90% black.

The overall age- and sex-adjusted prevalence of NMO/NMOSD on Dec 31, 2011, was greater in Martinique (10 of 100,000) than in Olmstead County (3.9 of 100,000; P = 0.01). Correspondingly, the age- and sex-adjusted incidence rate of NMO/NMOSD from 2003-2011 was also higher in Martinique (7.3 of 100,000) than Olmstead County (0.7 of 100,000; P < 0.001). The ratio of MS to NMO/NMOSD was lower in Martinique than in Olmstead County (3.5:1 vs. 54:1). Due to the small numbers of Asians and Hispanics in the Olmstead study population, data for these groups are limited. The prevalence in Olmstead County (3.9/100,000) is similar to that reported from a Dutch Caucasian study (4.4/100,000) and higher than studies from the United Kingdom (0.72-1.96/100,000), India (2.6/100,000), and Japan ( 0.9/100,000). The median age of onset (35-37 years) and the female-to-male ratio (5-9:1) was similar in both Olmstead and Martinique population.

COMMENTARY

In this one-of-a-kind epidemiological study of the incidence and prevalence of NMO/NMOSD in two distinct populations — a predominantly black population in Martinique, West Indies, and a predominantly Caucasian population in Olmstead County, Minnesota — the authors report a higher prevalence and incidence in Martinique. The authors also report a higher proportion of NMO/NMOSD among inflammatory demyelinating diseases in this population. This is in accordance with earlier studies which suggested that, in addition to having a higher incidence of NMO/NMOSD among blacks, there might be a relatively lower incidence of MS in this population when compared to Caucasians. This study again highlights the genetic and environmental influences in the manifestations of IDDs and adds valuable data on the epidemiology of NMO/NMOSD.