The prevalence of motor nerve diseases — including amyotrophic lateral sclerosis (ALS) — is on the rise worldwide, mostly because of an aging population. The disease burden is greatest in high-income countries, according to a global analysis of data from 1990 to 2016.
The study, “Global, regional, and national burden of motor neuron diseases 1990–2016: a systematic analysis for the Global Burden of Disease Study 2016,” was published Nov. 5 in The Lancet Neurology. The study was funded by the Bill & Melinda Gates Foundation.
ALS belongs to a group of rare and severe neurodegenerative disorders called motor nerve diseases. These also include spinal muscular atrophy, hereditary spastic paraplegia, primary lateral sclerosis, progressive muscular atrophy, and pseudobulbar palsy.
It is estimated that ALS has a worldwide median prevalence (proportion of a population with a disease) of 4.48 per 100,000 — 5.40 in Europe, 3.40 in the United States, and 2.34 in Asia.
While recent studies have provided additional insights into the frequency of motor nerve diseases, particularly ALS, in the U.S. and in Europe, obtaining enough data to generate global burden for all these diseases is challenging because of their rareness and a lack of population-based registries in some parts of the world.
By analyzing all available data between 1990 and 2016 within the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2016, the team provided the first report of the burden of motor nerve diseases for 195 countries and territories.
The GBD collects and analyzes data from more than 350 diseases and injuries in 195 countries, from 1990 to the present, allowing the quantification of disease burden and risk factors.
The team quantified the incidence (rate of new cases of the disease), prevalence, mortality, and disease burden associated with motor nerve diseases between 1990 and 2016 by age, gender, year, and geographical location.
They also analyzed the associations between the burden of these disorders and the countries’ socio-demographic index (SDI), a measure of social development based on education, income, and fertility.
The results showed that in 2016, 330,918 individuals worldwide had a motor nerve disease, with a higher proportion of patients (48.9%) living in countries with higher SDI levels and the lowest proportion (2.6%) in countries with lower SDI levels. The global prevalence was 4.5 per 100,000 people — 10 in Europe, 19.37 in the U.S., and 3.13 in Asia.
From 1990 to 2016, the number of people living with these diseases increased by 4.5%, largely due to an aging population. Also, the prevalence of these diseases over time was found to be consistently higher in men than in women across all ages, with the highest values at ages 85–89 for men and 80–84 for women.
Close to half of all people living with these diseases were related to three high-income regions: North America, western Europe, and Australasia. Accordingly the regions with high SDI levels also showed the highest disease burden.
“This finding is unsurprising because health services [in these countries] are well-developed and provide high standards of clinical care,” Orla Harding, an Irish consultant neurologist, wrote in an accompanying comment in the same journal issue. “With aging of the world population, the burden of motor neuron diseases on health services is likely to increase substantially in coming decades.”
While the results could suggest that the higher prevalence and incidence of these diseases were associated with high SDI levels, the lower incidence (and disease burden) in the high-income Asia Pacific region highlighted that factors other than sociodemographic development are responsible for this geographic variation.
The team also found that the geographic variation could not be explained by any of the 84 risk factors quantified in GBD, suggesting a role of unmeasured risk factors, such as ancestral origin and genetic background.
In agreement, “a 2017 study in which subcontinents were considered as surrogates of ancestries, a higher prevalence of amyotrophic lateral sclerosis was reported in Europe, the USA, and New Zealand than in east Asia,” the researchers said.
However, the team noted that the disease burden may have been underestimated in this study, because its assessment was based only on motor degeneration and did not include the potential burden of cognitive and behavioral deficits, which affect about half of ALS patients.
Also, the results were based on extrapolation for large areas of the globe due to lack of data. Thus, additional studies in these areas, including low- to middle-income and low-income countries — particularly in sub-Saharan Africa, Latin America, the Caribbean, and Asia — are required.
“This report … is an important first step in defining the societal impact of these conditions … and shows the substantial gaps in our knowledge, particularly relating to understudied populations of non-European or mixed ancestry,” Harding said.
“The estimates presented here, as well as future estimates based on data from a greater number of countries, will be important in the planning of services for people with motor neuron diseases worldwide,” the researchers concluded.