France leads Europe in incidence, prevalence of myasthenia gravis

Number of new and existing MG cases grew from 2008 to 2018

Patricia Inacio, PhD avatar

by Patricia Inacio, PhD |

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The incidence and prevalence of myasthenia gravis (MG) among the population in France are higher than reported for other European countries.

That’s according to a retrospective population-based study, which also found the incidence of MG tended to increase with age and was associated with a higher risk of cancer.

“Incidence and prevalence rates appear to be significantly higher than in the available literature data for other European countries, and suggest that MG should not be considered a rare disease,” the researchers wrote in “Epidemiology of myasthenia gravis in France: Incidence, prevalence, and comorbidities based on national healthcare insurance claims data,” which was published in Revue Neurologique.

MG is an autoimmune disease where self-reactive antibodies attack proteins involved in nerve-muscle communication, resulting in muscle weakness and fatigue.

Studies in Europe have reported variable incidence and prevalence rates for MG, but no epidemiological studies have reported on these parameters in France. Incidence refers to the number of people who develop a disease within a specific period of time, that is, new cases. Prevalence refers to the total number of people within a population who have a specific disorder within a time period, or new and existing cases.

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A challenge to previous studies

A team led by researchers at the Université Clermont Auvergne retrospectively analyzed data from the Échantillon généraliste de bénéficiares (EGB) from January 2008 to December 2018 to evaluate the incidence and prevalence of MG in France. The EGB is affiliated with the French national health insurance system and contains representative data on the French population.

During the 11 years analyzed, 331 new cases of MG were identified, 189 women and 142 men. This corresponded to an annual MG incidence of 50.3 cases per million people, 56.4 cases per million people for women and 44 cases per million people for men.

The incidence of MG gradually increased with age for women and men. Incidence started increasing for women at age 40 and for men from age 60.

The prevalence of MG also tended to increase over the study period. It ranged between 331 cases per million people in 2008 to 586 cases per million people in 2016. In the last five years analyzed, MG prevalence has consistently been above the 500 cases per million people.

“Our results appear to modify the epidemiological knowledge of MG, challenging previous studies conducted in Western countries, especially in Europe. In particular, we found an incidence of MG of over 50 per million person-years, far above the highest estimation in the literature of 30 per million person-years,” the researchers wrote. “Focusing on the last years of the study period, the prevalence was above 500 per million people … whereas the literature suggests a range of 15 to 320 per million.”

Analyzing presence of comorbidities

Researchers also analyzed the presence of other disorders, or comorbidities, particularly cancer, owing to reports that link both disorders.

Data from 296 patients were included in this analysis. A thymoma, or thymus tumor, and the surgical removal of the thymus gland (thymectomy) were much more frequent among MG patients (5.1% for thymoma, 4.7% for thymectomy) compared with the 703,261 people from the EGB database who served as controls (0.015% for thymoma, 0.03% for thymectomy).

Thyroid disorders and rheumatoid arthritis, two autoimmune disorders, also were more common in MG patients than controls (8.5% vs. 1.7% for thyroid disease; 1.4% vs. 0.5% for rheumatoid arthritis). Moreover, a higher proportion of MG patients received treatment for cancer (22%) during the study compared with controls (5.2%), “indicating a clear excess risk for MG patients to develop cancer.”

“The excess risk of cancer among MG patients, in particular the elderly, raises questions about the management of the disease, especially concerning the use of immunosuppressive drugs, which should be avoided for several years following cancer treatment.” wrote the researchers, who said the “results highlight the necessity of cancer screening for individuals with MG, if not at diagnosis, at least in patients whose disease is difficult to controlled.”