Global Prevalence of SPMS Estimated, But Seen to Vary Widely
Finding of 22 in every 100,000 affected by country's latitude, health system
Worldwide, about 22 in every 100,000 people live with secondary progressive multiple sclerosis (SPMS), a review study analyzing data across various countries reported.
Its researchers found substantial variability in SPMS rates country-by-country, with countries closer to the poles generally reporting higher prevalence than those near the equator.
Rates also were probably influenced by differences in how healthcare systems across countries report this type of multiple sclerosis (MS). “It may be important to consider this context in the design of future epidemiological studies of SPMS,” the scientists wrote.
The study, “Systematic literature review and meta-analysis of the prevalence of secondary progressive multiple sclerosis in the USA, Europe, Canada, Australia, and Brazil,” was published in BMC Neurology.
People with MS are often initially diagnosed with relapsing-remitting disease, characterized by episodes of suddenly worsening symptoms (relapses) interspersed with periods of stable symptoms (remission). Given time, most of these patients will eventually progress to SPMS, where MS symptoms continually worsen over time regardless of relapse activity.
SPMS cases rising since 1990s, but more slowly than MS in general
Even though studies suggest that SPMS imposes a higher clinical and economic burden on patients than other MS types, little is known about the prevalence of SPMS. Here, researchers conducted a review of the scientific literature aiming to estimate the prevalence of SPMS in the U.S., Europe, Canada, Australia, and Brazil.
“These countries were selected based on the availability and quality of MS prevalence data,” the investigators noted.
A total of 86 studies with at least moderate quality in terms of patient sampling, sample size, outcome measurements, and interpretation of results were included in the review.
Most (76 studies) were conducted in Europe, with over a dozen studies each in Spain and Italy. No epidemiological studies in the U.S. had reported specifically on SPMS prevalence.
The researchers conducted a meta-analysis where they pooled data from all of these studies together in order to estimate the overall prevalence of SPMS worldwide. The overall combined prevalence of SPMS was 22.42 out of every 100,000 people.
However, substantial variability in SPMS prevalence was evident from country to country. Brazil had the lowest estimated prevalence, at 1.68 out of every 100,000 people. The prevalence in Canada was estimated to be 55.02 out of 100,000, while in Australia it was 10.32 out of 100,000.
In Europe overall, the prevalence was estimated to be 24.74 out of 100,000, but there was marked variation country-by-country. The highest prevalence in any country was reported in Sweden — 75 out of 100,000 — while Portugal had the lowest rate, at 7.88 per 100,000 people.
Some of this geographical variability is likely explained by latitude, the researchers said, since the risk of MS is known to be higher for populations living further from the equator.
“The analysis from this review found that Brazil reported a seven times lower pooled prevalence of SPMS than Australia, a 19 times lower pooled prevalence of SPMS than Europe, and a 42 times lower pooled prevalence of SPMS than Canada,” the scientists wrote.
However, they stressed that these differences cannot account for all of the variation seen here, since some data “directly conflict with any interpretation of the results based on latitude or longitude.” For instance, Spain and Italy are at similar distances from the equator, but the estimated prevalence was nearly two times higher in Italy: 22.49 vs. 11.89 out of every 100,000 people.
Other potential clinical factors also generally failed to account for the wide range of SPMS prevalence reported in different countries.
“Our review found high variability in the estimated SPMS prevalence and the quality of the studies conducted with no obvious explanation for variability based on what is known of the SPMS disease physiology. … These variations may therefore be due to the differences across healthcare systems in the reporting of SPMS and audit of treatments,” the researchers wrote.
Overall, SPMS prevalence has been increasing since the 1990s, correlating with a general increase in MS cases over the same time period, the team noted. However, the proportional increase was lesser for SPMS than for MS in general, most likely due to the introduction of disease-modifying therapies (DMTs), particularly oral DMTs starting in 2010, helping to delay the transition from RRMS.
“The prevalence of SPMS statistically correlated with that of MS. However, the extent of increase in the SPMS prevalence did not correlate with that of MS,” the scientists concluded. “The introduction of oral DMTs significantly influenced the overall pooled prevalence of SPMS.”