Age of onset of RRMS increased in Norway over past century: Study

An increasing number of people in western Norway have been diagnosed with relapsing-remitting multiple sclerosis (RRMS) after age 50 over the past century, according to a new population study.

The rise was primarily driven by people being diagnosed at older ages, with the average age of onset rising from 28.6 years in the decades before 1970 to 36.2 years after 2010.

This was accompanied by a marked decrease in the time from the onset of symptoms to diagnosis, indicating that patients are now diagnosed faster after having their first symptoms.

Changes in environmental factors, diagnostic criteria, older age at first childbirth, or greater attention to later onset RRMS may explain these findings, the researchers said in “Increasing age of multiple sclerosis onset from 1920 to 2022: a population-based study,” which was published in the Journal of Neurology.

MS is an inflammatory disease that damages healthy parts of the brain and spinal cord, often leading to worsening physical disability over time. The disorder primarily affects young adults, with symptoms typically emerging between ages 20-40.

Recent evidence suggests the incidence of MS, or the number of new cases, is increasing over time among people older than 40, and there’s a particular increase in late-onset MS cases, or those developing after age 50.

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Increases in age of onset

Western Norway has one of the highest reported prevalence rates of MS, meaning the total number of cases per population, leading a research team led by scientists at Haukeland University Hospital, Norway to explore whether the age at MS onset has changed over the past century in the country.

The researchers collected data on 3,364 people in three counties in western Norway (Hordaland, Møre, and Romsdal) who developed RRMS, the most common type of MS, between 1920 and 2022. Among them, 67.1% were female and the age at the onset of symptoms ranged from 4 to 76 (mean, 33.5). The average time between symptom onset and diagnosis was 4.5 years, with a range between 0-59 years.

Data showed a significant increase in the mean age at RRMS onset between 1920 and 2022, which was mainly driven by more diagnoses above age 40 rather than a reduction in diagnoses in younger age groups.

In fact, the proportion of patients diagnosed after age 50 increased from 2.6% in the decades before 1970 to 11.9% between 2010-2022. When patients with primary progressive MS were included in the analysis, the results remained the same.

Diagnostic delay declines over time

In contrast, the mean time between symptom onset and diagnosis, often called the diagnostic delay, markedly decreased over the years, from 11.7 years before 1970 to one year between 2010 and 2022. This represented a “profound decrease in diagnostic delay during the study period,” the researchers said.

Twice as many women as men were diagnosed with RRMS during most of the study period. After the age of 45, even more women were diagnosed than men.

The likelihood of RRMS onset over age 40 increased during 2010-2022 compared to 1970-1979. Moreover, from 2010 to 2022, there was a diagnostic peak of around 30 years of age and a second peak between 40-45 years. This second peak was most prominent in women and happened at around age 45.

Two peaks of RRMS diagnosis between 2010 and 2022 “could indicate different susceptibility periods of MS or changes in exposure to risk factors during the observation period,” wrote the researchers, who said the increased age of RRMS onset over the past century may reflect changes in environmental factors, such as Epstein–Barr virus infections, vitamin D levels, smoking habits, or hormonal changes. Other factors cited included older age at first childbirth, changes in diagnostic criteria, or greater attention to late-onset RRMS.

“We found an increased age at onset of Norwegian [people with MS] over the past century, despite a significantly decreased interval from disease onset to diagnosis during the same period,” the researchers wrote. “This was mainly caused by an increasing number of [people with MS], predominantly female, experiencing onset after 40 years of age.”

More research on changes in environmental factors over time may lead to studies on the effects of age on the efficacy and side effects of disease-modifying therapies, the researchers said.