According to a study published in Neurology, multiple sclerosis incidence in women has doubled in a 60-year period, though the increase for men was modest. The introduction of new diagnostic criteria and lifestyle changes over the decades, including fewer childbirths and a higher occurrence of obesity, may indicate an epidemiological change contributing to an increase in late-onset multiple sclerosis in women.
This study analyzed whether exposure to risk factors happened early in life or closer to onset, trends in incidence according to the presenting symptoms at onset (optic, brainstem, or sensory), and whether perinatal circumstances or seasonality of birth are considered risk factors. The study population included 19,536 patients (12,467 women and 7069 men). From the 1950-1959 period to the 2000-2009 period, the incidence in women rose 114%, whereas the incidence in men rose 30%. The increase in incidence, however, was not consistent across age groups or clinical subgroups.
Study results could not identify a specific pattern in onset symptoms over the decades, but indicated that in the last 30 years, sensory/brainstem symptoms were significantly more common in women than men. The age-at-onset analysis determined that, although all age groups and sexes experienced a positive local drift, women aged 50-64 had the highest relative increase in incidence between the decades. The study suggests that a reason for an increase in incidence is the introduction of new diagnostic criteria and tools, including magnetic resonance imaging.
The effect of birth month did not deviate significantly from the background population and, although smoking is an established risk factor for multiple sclerosis, the proportion of women and men who smoke decreased by more than half between 1970-2005. Obesity, another known risk factor, saw an increase of prevalence in female incidence starting in the 1970s. Women who had given birth traditionally had a higher risk of developing multiple sclerosis; delayed pregnancies and fewer childbirths, along with a decrease in the exposure to hormonal substitution therapy, may have influenced changes in the disease epidemiology.
The study concluded that the incidence of late-onset multiple sclerosis more than doubled among the female population, but only showed a modest increase among men. The contribution of established risk factors is still quantitatively unknown, and delayed pregnancies along with a decrease in exposure to hormone therapies should be further studied as late onset multiple sclerosis rises in women over 50.
Koch-Henriksen N, Thygesen LC, Stenager E, Laursen B, Magyari M. Incidence of MS has increased markedly over six decades in Denmark particularly with late onset and in women [published online May 2, 2018]. Neurology. doi: 10.1212/WNL.0000000000005612