Onset of symptoms in MG more likely in summer, study reports
But more research is needed to ID potential environmental triggers
Myasthenia gravis (MG) may be more likely to begin manifesting in the summer, with the onset of symptoms more common in hotter weather, according to a new study by researchers in Italy.
The team theorized that the increased incidence of MG may occur because temperatures at that time of the year are higher. But they said more research is needed to understand why such seasonal variations occur.
“Our findings suggest that MG onset may be more common in summer and at higher environmental temperatures,” the researchers wrote. “Identifying the environmental triggers underlying the association between seasonality and MG onset could provide opportunities for the development of new preventive and treatment strategies.”
The study, “Seasonal variation in myasthenia gravis incidence,” was published as a short communication in the Journal of Neuroimmunology.
Onset of MG symptoms in summer seen for over a third of patients
MG occurs when the body’s immune system produces self-reactive antibodies that mistakenly attack proteins needed for nerve-muscle communication, causing muscle weakness and other symptoms.
Environmental factors, such as the levels of vitamin D, may be linked to MG. Like vitamin D, which usually increases as daylight hours become longer during the spring and summer months, other factors also change with the seasons.
While seasonal patterns are seen in other autoimmune diseases, little is known about how MG changes with the time of the year. That prompted this team of researchers, from several institutions in Italy, to find out if MG develops more commonly in certain seasons during the year.
The study included 316 patients in the country — 214 from a hospital-based registry and 102 from a population registry — whose MG started to manifest sometime in 2010 to 2019. All tested positive for antibodies targeting the acetylcholine receptor (AChR), the most common type of self-reactive antibodies causing the disease.
The median age at disease onset was 66, with patient ages ranging from 8 to 92. More than two-thirds of the patients — 219 or 69.3% — had late-onset MG, meaning they were age 50 or older at the time disease symptoms started. A total of 39 (12.3%) had early-onset MG, meaning the disease had begun to manifest before the age of 50.
For 58 of individuals (18.4%), MG was associated with a thymoma, a tumor that forms in the thymus. The thymus gland, located in the chest between the lungs, is part of the immune system. It’s thought that abnormalities in the thymus may play a role in the production of the self-reactive antibodies that drive MG.
Each patient was assigned to one of the four seasons — spring, summer, autumn, or winter — based on the month that symptoms started.
Notably, in five of [10] years examined, the number of new MG cases in summer was higher than 10 cases.
Compared with other seasons, the number of new MG cases was significantly higher in the summer, with more than one-third (34.2%) occurring in the months of June, July, or August. The number of new cases per season was also highest in summer, with a median of 11. This was followed by winter, with 8.5 new cases, then spring, with seven new cases, and finally autumn, with six.
“Notably, in five of [10] years examined, the number of new MG cases in summer was higher than 10 cases,” the researchers wrote.
This increase during the summer months was more evident for late-onset MG. In addition, the team observed that this increase seemed to be linked to higher temperatures.
While the number of new MG cases increased with an increase in average temperature, there were some “fluctuations … across the temperature range, suggesting that certain temperature [ranges] may influence MG incidence more strongly than others,” the researchers wrote.
The team noted that, because this study involved only Italian patients, its findings may not apply to other countries — although similar seasonal patterns have been seen in the U.S.
“[Those] findings are consistent with our results,” the team wrote. They added that “further research on larger populations is needed to confirm our data and explore new hypotheses regarding environmental factors in MG development.”
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