New Scientist Live: Why do some people get MND?

Increasing age is one of the only confirmed risk factors for MND. The older someone is, the greater their chance of developing MND. Most people diagnosed with MND are aged between 50 and 70 years old. Research has shown that normal ageing can affect the structure and function of motor neurons. These age-related effects could also play a vital role in development of MND alongside other lifestyle, environmental and genetic factors.

MND can be caused by changes in genes that are passed down in families. Around 1 in 10 people with MND have a family history of the disease, and this is called inherited MND. When there is no evidence of a family history of MND, it is known as sporadic MND. Genes can still be involved in sporadic MND – some changes can increase the risk of someone developing MND, but other environmental and lifestyle factors also play a role. 

Male gender is another confirmed risk factor for MND, with men more likely to be diagnosed with MND than women. Research is still ongoing into why this is the case.

MND sometimes appears in clusters, where there are more cases in a particular area than would be expected. Several things are thought to contribute to these ‘hotspots’ including age (if there is a larger proportion of older adults the risk increases), genetic variability (if the community is remote and tends not to have a large gene pool then genetic changes that are related to MND can be passed between the community), and environmental chemical levels. However, we can’t say for sure why these clusters appear, or why MND occurs more in some areas than others. 

Different types of pollution have been researched as potential risk factors for MND, with some studies finding a link between pollutants in the atmosphere and the risk of developing MND. One potential theory is that fine particles (for example, from exhaust fumes) are inhaled and disrupt the barrier between the brain and the blood. Heavy metals present in air or water pollution may also increase the risk of developing MND by affecting how motor neurons work. The results of these studies have not been consistent, so we can’t say for certain that these links exist without further research. 

Pesticides are an unconfirmed risk factor for MND. Some studies have found that people who have MND are more likely to have been previously exposed to pesticides. Research is ongoing into the ways pesticides could potentially contribute to the risk of developing MND. 

There have been some studies in the United States which suggested people who served in the military have a higher risk of being diagnosed with MND. A study in the UK also identified an increased risk for veterans compared to non-veterans. However, the researchers were keen to highlight that the overall risk was extremely low. The studies found the increased risk was seen across different conflicts and deployments, potentially ruling out any specific environmental exposure from a particular deployment. The length of service in the military was also found to not increase the risk. Researchers have also mentioned that other lifestyle factors cannot be ruled out as the potential reason for this increased risk. This includes smoking, as there are higher rates of smoking in military personnel, and trauma.  

A small number of studies have hinted that smoking may increase the risk of developing MND. In one study, no overall change in risk was associated with smoking, but when participants were grouped by their sex, women who had smoked showed a potential greater risk. Theories around how smoking may influence MND risk include motor neuron damage from chemicals found in tobacco.

Several studies have investigated a potential link between strenuous exercise and MND. However, each of these studies had a different definition of strenuous, with some classing it as playing professional rugby or football, and others classing it as frequent and intense bursts of activity. Some studies found that really high levels of intense exercise may be linked, while others did not. The studies which did suggest a link had limitations, such as only looking at a small number of people. Many researchers investigating whether exercise is linked to MND still state that the benefits of exercise, such as reducing the risk of cardiovascular disease, far outweigh any potential risk. Research is ongoing into the links between physical activity and MND. 

Many people who play contact sports, such as football and rugby, go on to develop MND. One potential theory as to why this might be is because they are more prone to head injuries. Research has suggested that traumatic brain injuries are a risk factor for a number of neurodegenerative diseases, including dementia. Current research investigating head injury and MND is conflicting, with some studies suggesting an increased risk, while others show no links between head injuries and MND.