The burden on the nervous system worldwide
Based on the GBD database, this study conducted a decennial statistical analysis of the mortality rates caused by motor neuron diseases among males and females globally and in China from 1990 to 2020. As shown in Figure 1a, the global mortality rate of motor neuron diseases shows an annual upward trend, while the mortality rate of motor neuron diseases in China presents a fluctuating upward pattern. Specific data show that the global mortality rate of ALS/MNDs increased from approximately 0.29 per 100,000 people in 1990 to 0.35 in 2000, further increasing to 0.42 in 2010, and reaching 0.49 in 2020. In China, the mortality rate of motor neuron diseases was 0.13 around 1990 and rose to 0.17 around 2000; although it slightly dropped to 0.15 around 2010, it rose again to 0.23 in 2020. Overall, from 1990 to 2020, the mortality rate of motor neuron diseases, despite certain fluctuations, showed an overall upward trend. Consistent with the global annual upward trend of neurological disease mortality rates, changes in the mortality rate of motor neuron diseases are influenced by multiple factors. Unhealthy lifestyles, such as poor diet, lack of exercise, smoking, and excessive drinking, as well as environmental pollution and chemical exposure, are the main causes of the increased burden of nervous system diseases.18 Although medical progress has improved diagnostic accuracy, the uneven distribution of medical resources globally has made it difficult for patients in low-income countries to receive timely treatment, thereby increasing the mortality rate.
To further illustrate the global influence of ALS/MNDs and the universality of the increasing trend, this study used the G8 country blocs and China as representatives to conduct a statistical analysis of the number of deaths caused by ALS/MNDs. The results are shown in Figure 1b. It can be seen that not only China but also other countries witnessed an increasing trend in the number of deaths due to ALS/MNDs every decade from 1990 to 2020. Among them, the United States consistently had the highest number of ALS/MND deaths, and its rate of increase was also the MND pronounced. The number rose from 3,900 in 1990 to 8,200 in 2020, accounting for approximately one quarter of global ALS/MND deaths in 2020. As the country with the second-highest number of ALS/MND deaths, China can conduct relevant targeted research to make significant contributions to the development of the ALS/MND research field.
Figure 2 presents the relevant data on the number of patients globally and in China from 1990 to 2020. The X-axis of this chart denotes the time span from 1990 to 2020, with a 10-year interval between each data point. The Y-axis, scaled approximately from 0 to 30 (×104), quantifies the global number of patients. Different colors distinguish between regions, namely the global context and China, with the specific color-code correspondence provided in the legend beneath the chart—red signifies the global dataset, while blue represents China. In terms of the overall trend, the data points exhibit an upward trajectory over time. Specifically, the global prevalence of motor neuron diseases has risen significantly, from 16.1 in 1990 to 18.8 in 2000, further to 22.9 in 2010, and ultimately reaching 27.0 in 2020. This trend indicates a general increase in global prevalence over the past three decades. Similarly, the prevalence of motor neuron diseases in China has shown a year-on-year increase, from 2.5 in 1990 to 2.8 in 2000, 3.0 in 2010, and 3.3 in 2020. Nevertheless, due to a relatively smaller population base, the upward trend in China is less pronounced compared to the global prevalence.
Figure 3 shows data on the number of patients in the G8 country blocs and China from 1990 to 2020, further illustrating the global impact of motor neuron diseases. The X-axis represents the time span from 1990 to 2020, with a 10-year interval between each data point. The Y-axis, ranging approximately from 0 to 4.5 (×104), quantifies the number of patients, with the G8 country blocs and China each having fewer than 40,000 patients with motor neurological disorders. Different colors are used in the figure to distinguish regions, with the legend at the bottom indicating the specific color-code correspondence. From the overall trend, except for Russia, the number of patients with motor neuron diseases in the countries shown increased from 1990 to 2020. China had the largest number of patients in 1990 and 2000, while the United States had the largest number in 2010 and 2020. The country with the fastest growth is the United States, which may be related to factors such as the aging population, the level of medical diagnosis, and the rapid development of statistical tools. The trends in Germany, France, and Japan were similar, showing a steady upward trajectory. Combining the information in the figure, it is evident that the population affected by motor neuron diseases is increasing year by year, which is of great significance for the study of this disease.
This line graph illustrates changes in motor neuron disease data among men and women of different age groups worldwide from 1990 to 2020 (Fig. 4a). The age groups are categorized as follows: individuals under 20 years form one group; those aged 21 to 69 are divided into subgroups at five-year intervals; and people over 70 constitute a separate group. The horizontal axis depicts age, partitioning the data across all age groups, while the vertical axis denotes the number of deaths, spanning from 0 to 20,000. Multiple lines of distinct colors in the graph correspond to data from the 1990s, 2000s, 2010s, and 2020s, with specific labeling provided in the table beneath the chart. Specifically, the red line represents data on “global common other neurological diseases in both men and women” from 1990, while the yellow, blue, and green lines illustrate variations across different age groups in 2000, 2010, and 2020, respectively.
It is evident that from 2000 to 2020, the data across all age groups generally exhibited an upward trend. In 1990, the number of deaths in the 65–69 age group decreased, whereas the number of deaths among individuals over 70 increased, resulting in an overall upward trend. This suggests that motor neuron diseases primarily affect the elderly population, particularly those over 50 years of age. Additionally, from a temporal perspective, the incidence of motor neuron diseases has increased year by year.
ALS, also known as “Frozen Limbs Syndrome”, is a heterogeneous neurodegenerative disease. Along with cancer, AIDS, leukemia, and rheumatoid arthritis, it is listed by the World Health Organization as one of the “top five global difficulties and complex diseases”. The incidence of ALS is approximately one to 2.6 cases per 100,000 people per year, and the prevalence is about six cases per 100,000 people. Ninety percent of ALS patients are sporadic, while a small portion are familial. The average age of onset is 58–60 years, and the average survival period after diagnosis is about three to four years. The core pathological manifestation of ALS is the progressive death of upper and lower motor neurons, which involves complex pathological mechanisms. Currently, treatment options for ALS are extremely limited, and no effective means exist to directly cure ALS at the source of neuronal damage.
Figure 4b illustrates changes in motor neuron disease data among men and women of different age groups in China from 1990 to 2020. The age groups are categorized consistently with the previous figure. In the chart, lines of distinct colors correspond to data from 1990, 2000, 2010, and 2020, respectively, with specific data labels detailed in the table beneath the chart. Specifically, the red line denotes the 1990 data for “other neurological diseases common to both men and women in China”, while the yellow, blue, and green lines represent the variations of this data across different age groups in 2020, 2010, and 2000, respectively.
It is evident that from 1990 to 2020, the overall trend across all age groups first declined and then increased. Among individuals under 20 years of age, the mortality rate due to motor neurological diseases was relatively high. Subsequently, the trend shifted upward. The number of deaths in 2020 was higher, whereas the data from 2010 and 2000 were comparable, and the number of deaths in 1990 was relatively lower. This suggests that in China, in addition to the elderly population, there is a significant incidence of motor neurological diseases among adolescents and children under 20, which may be attributed to factors such as genetics and heredity. Furthermore, the upward trend over the years highlights the importance of research on motor neurological diseases in China.
Figure 5a summarizes the mortality rates of motor neuron diseases for both men and women worldwide in different years. Figure 5b shows the same summary for China. It is clear that, both globally and in China, mortality rates of motor neuron diseases for men and women have been continuously increasing over the years. For example, globally, the rate rose from 29% in 1990 to 49% in 2020.
The reasons for the increase in mortality rates are as follows. Firstly, population aging plays a role. Motor neuron diseases are more common in the elderly. As the global aging population grows, the proportion of elderly individuals increases, and the absolute number of people with motor neuron diseases also rises, leading to higher mortality rates. Secondly, environmental factors are closely related to this disease. In modern society, people are exposed to more environmental toxins. Long-term exposure to heavy metals (such as lead and mercury), organic solvents (such as benzene and formaldehyde), or pesticides may accelerate motor neuron death through oxidative stress. Thirdly, genetic factors also have an influence. Although most cases of motor neuron diseases are sporadic, 5–10% are related to genetic defects and inheritance. With the development of genetic testing, more people with genetic susceptibility have been identified. This group has a higher risk of developing the disease and may impact overall mortality rates. An important factor is the improvement in diagnostic capabilities. With the advancement of medical technology, diagnostic standards have become clearer, and diagnostic methods have improved, enabling more patients to be accurately diagnosed, which may increase statistical mortality rates. At the same time, the accuracy of death certificate collection has improved, reducing underreporting and misreporting, which also affects mortality statistics.
Both in China and globally, the mortality rate of motor neuron diseases is higher in men than in women. Firstly, hormonal differences influence risk. Androgen levels in men are relatively higher, and androgens may adversely affect neurons, increasing the risk of motor neuron disease. Secondly, environmental exposure differs. In certain occupations, men may have greater exposure opportunities. For example, in metal processing and mechanical manufacturing, men are more likely to be exposed to toxic substances, heavy metals, and chemical solvents, which may damage nerve cells. Thirdly, unhealthy lifestyles affect health. Men may be more prone to smoking, excessive drinking, high-calorie diets, and lack of exercise, leading to metabolic disorders, increased oxidative stress, and nervous system impairment, thereby increasing disease risk. Fourthly, high-intensity physical activities may contribute. Some men engage in strenuous labor or vigorous exercise, which may cause chronic nerve cell damage and disrupt repair processes, making motor neurons more vulnerable and increasing disease risk.
In traditional Chinese medicine theory, men belong to the “yang gang body”, and yang qi is inherently more abundant. If they are chronically exhausted, have emotional disorders, or similar conditions, it can easily lead to excessive yang qi and fire, burning the yin fluid, and causing the tendons and meridians to lose nourishment. Moreover, men often face greater social role pressure, making them prone to liver stagnation, qi stagnation, and kidney essence depletion. Due to a higher proportion of physical labor, the risk of muscle and bone damage is also high. Multiple factors together lead to a higher incidence of motor neuron disease in men.
Figure 6 shows the ratios of DALYs, YLLs, and YLDs for the G8 country blocs and the Chinese population across different years.
DALY is a comprehensive indicator for measuring the loss of healthy life due to diseases, injuries, or risk factors. It combines the loss of life caused by premature death and the loss of healthy life due to disability (or incapacity) resulting from diseases, reflecting the overall disease burden. The higher the DALY value, the greater the harm of the disease to the population’s health, and it can be used to compare the burden of different diseases and evaluate the effectiveness of intervention measures. YLLs refer to the potential years of life lost due to premature death caused by a certain disease or injury, calculated as the sum of differences between the actual age at death and the expected lifespan (usually based on the average life expectancy of the region). YLDs refer to the lost healthy life years due to non-fatal health problems (such as disability or incapacity) caused by diseases or injuries, and must be calculated in combination with the duration of the disease and the severity of disability. They mainly reflect the “long-term disability” burden caused by diseases and are applicable for the assessment of chronic diseases (such as diabetes or mental disorders) or disabling diseases (such as spinal cord injury). The relationship among the three is as follows: DALYs = YLLs + YLDs. DALYs sum both YLLs and YLDs, covering the loss due to “early death” and the impact of “disability”, and thus more comprehensively reflect the overall harm of diseases to the population’s health. YLLs focus on life lost due to “death” and are suitable for measuring high-fatality diseases (such as AIDS or stroke), while YLDs focus on health loss “not related to death” and are suitable for measuring highly disabling diseases (such as Alzheimer’s disease or cerebral palsy). DALYs can also be used to compare disease burdens across diseases and regions (for example, comparing the overall health harm of depression and coronary heart disease).
From Figure 6, it can be seen that the values of DALYs, YLLs, and YLDs are all increasing in the G8 country blocs, globally, and in China, indicating that motor neuron diseases are increasing the overall burden. Population aging is an important factor in the changes of DALYs, YLLs, and YLDs values in China. As the degree of population aging deepens, the burden of motor neuron diseases has significantly increased, mainly because the elderly are the high-risk group for these diseases. Additionally, lifestyle and environmental changes also play a role. Economic development leads to lifestyle changes, such as increased sedentary time and higher intake of high-calorie diets, which alter the risk of some chronic diseases and cancers. Globally, unhealthy lifestyles, such as high-sugar and high-fat diets and lack of exercise, are becoming increasingly common, leading to higher incidences of obesity, diabetes, and related diseases, and thereby increasing DALYs, YLLs, and YLDs. At the same time, factors such as environmental pollution and occupational exposure may also contribute to an increased disease burden.
Analysis of the correlation between mortality rates and different pathogenic bacteria in different years
Figures 9 show the death tolls caused by different pathogenic bacteria in different years globally and in China, respectively. From Figure 9a, it can be seen that the death tolls due to these bacterial infections have been decreasing in recent years. At the same time, we can also observe from Figure 9b that this pattern exists simultaneously at the national level and within China. We have collected four typical pathogenic bacteria, namely: Shiga's bacillus, Enteropathogenic Escherichia coli, Enterobacter spp., and Escherichia coli.
Shiga’s bacillus, belonging to the Enterobacteriaceae family, is the main pathogen causing bacillary dysentery in humans. It is mainly transmitted through the fecal-oral route, with contaminated food, water, or hand contact being common transmission methods. It adheres to the colonic mucosal epithelial cells through fimbriae, invades the cells to reproduce and release Shiga toxin, causing colonic mucosal inflammation and ulcers, and resulting in bloody diarrhea.
Enteropathogenic Escherichia coli is a pathogenic subtype of Escherichia coli and belongs to diarrheagenic Escherichia coli. It adheres to the epithelial cells of the small intestine through fimbriae (such as tufted fimbriae), damages the microvillus structure, causes dysfunction of mucosal absorption, and leads to secretory diarrhea.
Enterobacter spp. (the Enterobacter genus) belongs to the Enterobacteriaceae family and is an opportunistic pathogen. It is widely present in nature (in soil, water, and decomposed organic matter) and can also colonize human intestines and respiratory tracts. Usually, it is opportunistic, and when the body’s immunity is low (such as during long-term antibiotic use, hospitalization, or intensive care unit admission), it is prone to cause infections.
Escherichia coli belongs to the Enterobacteriaceae family and is normal flora in the intestines of humans and warm-blooded animals. It usually does not cause disease and can even synthesize vitamins B and K for the host.
In the GBD database, most diseases caused by bacteria are acute, such as cholera and dysentery. The analysis of intestinal microbiota has gradually deepened in recent years, and mechanisms such as the “intestinal-muscle axis” and “brain-intestinal axis” have been proposed. The metabolic products of intestinal microbiota are crucial for maintaining skeletal muscle function and metabolism. These metabolites can effectively target mitochondria and delay skeletal muscle aging. At the same time, the energy produced by mitochondria and moderate levels of reactive oxygen species can effectively promote this process. Mitochondria also promote immune responses, maintain the biodiversity of intestinal microbiota, and support intestinal barrier function.19 Additionally, different types of exercise can induce specific adaptive responses in mitochondria, which can be regulated by the intestinal microbiota or other systems crucial for maintaining internal balance and delaying skeletal muscle aging. For example, muscle atrophy occurs due to nutritional disorders in skeletal muscles, resulting in thinner muscle fibers and reduced muscle volume. The concept of “intestinal-muscle axis regulation” suggests that the state of the intestinal microbiota in patients with muscle atrophy can be modulated to improve muscle quality, providing new treatment ideas for clinical management of muscle atrophy-related diseases like ALS.
The direct links between Shigella bacteria, pathogenic Escherichia coli, and motor neuron diseases have not yet been confirmed. However, they may contribute by inducing intestinal inflammation, disrupting microbiota balance, and releasing pro-inflammatory substances. Our subsequent experiments have verified that some intestinal bacteria are directly associated with ALS onset. There is potential for exploration in this field. In the future, more high-quality clinical studies and mechanistic research are needed to clarify the relationship between microbiota and motor neuron diseases. For patients with motor neuron diseases, maintaining intestinal microbiota balance (through a reasonable diet and avoiding drug abuse) is a potential therapeutic direction.