What Causes Motor Neurone Disease and Who Is at Risk

Motor neurone disease (MND) does not have a single cause. Instead, researchers now believe it develops through a series of steps, perhaps six on average, involving a mix of inherited genetic susceptibility, environmental exposures, and cellular damage that accumulates over a lifetime. About 5 to 10 percent of cases run in families, with identifiable gene mutations, while the rest are classified as sporadic, meaning no clear family link exists. Even in those sporadic cases, though, genetics still plays a role alongside factors like pesticide exposure, heavy metals, strenuous physical activity, and head injury. The people most at risk tend to be men over 60, but the full picture of who develops MND and why remains one of the harder puzzles in neurology.

A Disease Built in Steps

One of the more useful ways to think about what causes MND comes from a model borrowed from cancer research. In cancer, tumors often arise after a cell accumulates several independent hits, whether mutations, exposures, or metabolic insults. Researchers applied the same logic to MND and found that the pattern of new cases across different ages fits a process requiring roughly six steps before the disease appears. A population-based study confirmed a strong linear relationship between the log of disease incidence and the log of age, consistent with this six-step model.¹

The practical meaning is straightforward: no single cause is likely to be sufficient on its own. A person might carry a genetic risk variant, get exposed to a toxin, suffer a head injury, and experience age-related cellular decline, and it is the combination that eventually tips the motor neurons over the edge. Importantly, carrying a strong genetic mutation can reduce the number of required steps. People with the most common MND gene mutation, called C9orf72, appear to need only about three steps, while those with SOD1 mutations fit a two-step model.² That helps explain why familial cases tend to show up earlier in life: fewer steps means fewer years needed to accumulate the necessary damage.

The Genetic Side

Around thirty genes have been linked to MND so far, but two stand out as especially important. The C9orf72 gene, discovered in 2011, contains a short segment of DNA that can repeat far more times than normal. When this repeat expansion occurs, it is the most common genetic cause of both MND and a related condition called frontotemporal dementia.³ It accounts for roughly a third of familial MND cases and about 6 percent of sporadic ones.⁴

The other major gene, SOD1, was the first to be linked to MND back in the 1990s and has been intensely studied since. The SOD1 protein normally helps protect cells from oxidative damage, but the mutations that cause MND do not simply remove that protection. Instead, the mutant protein appears to gain a toxic new property. Research in mouse models showed that animals expressing the mutant form developed motor neurone disease even when their overall SOD1 levels were high, proving that it is the altered protein itself, not a loss of the normal enzyme, that drives the damage.⁵ Later work suggested the toxicity involves both a gain of harmful function and a loss of normal protective function, particularly in the cell’s nucleus.⁶

Even when no known mutation is present, genetics still matters. Genome-wide studies have identified dozens of common gene variants that each raise MND risk by a small amount. A person who inherits an unlucky combination of these low-risk variants may need fewer environmental hits for the disease to emerge. The genetics of MND, in other words, is rarely an on-off switch. For most people it is more like a dimmer, setting a baseline level of vulnerability that other factors push higher or lower.

What Goes Wrong Inside Motor Neurons

Regardless of the trigger, MND converges on a few shared problems inside the nerve cells that control movement. One of the most consistent findings involves a protein called TDP-43. In the majority of sporadic MND cases, TDP-43 is found misplaced: instead of sitting in the nucleus where it belongs, it clumps into insoluble deposits in the surrounding cytoplasm.⁷ This creates a double problem. The cell loses a protein it needs for normal DNA repair and gene regulation in the nucleus, and the clumped protein in the cytoplasm may itself be toxic.⁸ Researchers are now investigating whether restoring TDP-43 to its proper location could slow the disease, and early work in cell and animal models has shown promise.⁹

Mitochondria, the structures that produce a cell’s energy, also malfunction early in MND. Studies in animal models suggest these mitochondrial problems begin before any symptoms appear, hinting that they may help drive the disease rather than being a late consequence of it.¹⁰ Damaged mitochondria make motor neurons more sensitive to glutamate, the brain’s main excitatory chemical signal. Motor neurons are already unusually vulnerable to glutamate overstimulation because they have low levels of the calcium-buffering proteins that protect other nerve cells.¹¹ This is one reason riluzole, the first drug approved for MND, works by dampening glutamate signaling.

On top of all this, the transport system that moves essential cargo along the long nerve fibres, called axonal transport, often breaks down. Motor neurons can stretch over a metre from the spinal cord to a foot muscle, and they depend on this internal delivery system more than almost any other cell type.¹² The immune cells of the brain, called microglia, add another layer: they initially try to protect motor neurons, but as the disease progresses they shift to a damaging inflammatory state that accelerates nerve cell death.¹³

Environmental Exposures That Raise the Risk

Pesticides are the environmental factor with the most consistent evidence. A systematic review and meta-analysis pooling multiple studies found that occupational exposure to pesticides raised the risk of MND by about 60 percent overall. Higher-level exposure pushed the risk even further, roughly tripling it compared to people with no exposure.¹⁴ A case-control study from New Zealand broke this down further, finding elevated risks for several specific classes including organophosphate insecticides, organochlorine insecticides, and fumigants. Exposure to the fumigant methyl bromide carried an especially pronounced risk.¹⁵ The same study found associations with petrol and diesel fuel exposure and with solvents.

Heavy metals are another recurring suspect. A Spanish geographic study found that areas where heavy metals like lead, zinc, and arsenic were released into rivers had around 15 to 21 percent higher MND death rates than unexposed areas.¹⁶ Autopsy research paints a more vivid picture: heavy metals were found inside the remaining spinal motor neurons of two-thirds of MND patients examined, and staining in a brain region called the locus ceruleus was significantly more common in MND patients than in control subjects.¹⁷ Blood-level studies are less clear-cut, however. One study found elevated plasma cadmium in MND patients but with a lot of overlap between patients and healthy controls, and no other metal reached a statistically meaningful difference.¹⁸ The disconnect between tissue findings and blood tests is a good reminder that the relationship between metal exposure and MND is real but complicated, and measuring it in living patients is difficult.

Military Service as a Risk Factor

Several studies across different countries have found that military veterans face a higher risk of MND. A Scottish study following veterans over decades found their risk was about 50 percent higher than non-veterans, a finding that held regardless of how long they served or which era they served in.¹⁹ A French study confirmed the pattern, finding MND incidence was 16 percent higher in male military personnel compared to the general population, though no increase was found in female military personnel.²⁰

The reasons are not entirely clear. Military service bundles together several known and suspected risk factors: intense physical exertion, exposure to chemicals and fuels, potential head trauma, and psychological stress. It is possible that no single aspect of military life is responsible and that the elevated risk reflects the cumulative burden of multiple exposures happening during the same period of a person’s life. This fits neatly with the multistep model, where different types of insults can all contribute to the same end point.

Strenuous Exercise and Head Injuries

The relationship between physical activity and MND is counterintuitive and sometimes alarming to people who value exercise. Being physically active is broadly protective against most diseases, but for MND the picture is different. A review of the evidence concluded that strenuous, anaerobic physical activity, the kind seen in competitive sport rather than casual recreation, is linked to a higher risk of developing MND. One large study found that people who engaged in high levels of vigorous exercise throughout adulthood had roughly 47 percent greater odds of developing the disease.²¹ This does not mean exercise causes MND in any straightforward way. The prevailing theory is that extreme motor neuron workload, sustained over years, could represent one of the “steps” in the multistep process, particularly in someone who already carries genetic or environmental vulnerabilities.

Head injury, specifically concussion, adds another risk layer. A New Zealand case-control study found that a history of concussion at least three years before diagnosis was associated with about a 50 percent increase in MND risk. The risk climbed steeply with the number of concussions: people with two reported head injuries had roughly four times the risk, and those with three or more had over double the risk compared to people with no concussions.²² The combination of intense physical activity and repeated head impacts may help explain why professional athletes, particularly in contact sports, appear to develop MND at higher rates than the general population.

Who Gets MND and When

MND is not common, but it is not as rare as many people assume. An English study using three linked datasets found an incidence of about 5.7 per 100,000 person-years, with the median age at diagnosis being 72.²³ Men are consistently more affected than women. A UK study found the rate in men was 54 percent higher than in women, with incidence for both sexes peaking around age 75 to 79.²⁴

The male excess is not constant across the lifespan. A French nationwide study with an accompanying meta-analysis showed that the gap between men and women is widest in younger adults, where men outnumber women by more than two to one, then narrows in middle and older age before widening again after 85.²⁵ The narrowing in midlife has led some researchers to speculate that hormonal changes around menopause may remove a protective factor in women, but this remains an open question. Whatever the explanation, the overall pattern is clear: being male and being older are the two strongest demographic risk factors.

The Overlap with Frontotemporal Dementia

MND and frontotemporal dementia (FTD) were once considered completely separate conditions, one affecting movement and the other affecting behaviour and language. That boundary has blurred considerably. Up to half of people with MND show some degree of cognitive or behavioural change, and a proportion meet the full diagnostic criteria for FTD. Conversely, some FTD patients go on to develop motor symptoms. The C9orf72 gene mutation can cause either condition, and different members of the same family carrying the same mutation may develop MND, FTD, or both.²⁶

The discovery that TDP-43 clumps are a hallmark of both conditions provided a biological explanation for the overlap. TDP-43 pathology is now understood to be a shared feature linking sporadic and familial MND with FTD.²⁷ Most researchers today view MND and FTD as two ends of a spectrum rather than distinct diseases. For families and patients, this has a practical implication: a family history of either FTD or MND should be considered relevant when assessing someone’s genetic risk for MND.

What Affects How the Disease Progresses

Not everyone with MND follows the same course. Where symptoms first appear matters a lot. When MND begins with speech and swallowing difficulties, a presentation known as bulbar onset, the median survival from symptom onset is about 27 months.²⁸ Bulbar-onset patients face higher mortality once the respiratory system becomes involved, partly because they are more vulnerable to aspiration and chest infections and partly because they tolerate breathing support less well.²⁹ Limb-onset MND, where weakness begins in the arms or legs, generally has a somewhat longer survival.

Nutritional status also plays a role in how the disease progresses. People with a higher body mass index before their diagnosis tend to fare better, and maintaining weight and nutrition after diagnosis is associated with improved outcomes.³⁰ This is one of the few modifiable factors in MND, and it is why many specialist clinics place strong emphasis on early and aggressive nutritional support, including feeding tubes when needed.

Emerging Research on the Gut and Ageing

One unexpected avenue of research involves the gut. The idea that gut bacteria could influence a brain disease might sound unlikely, but a small study of MND patients found that all five patients examined had an altered gut microbiome with low bacterial diversity, along with signs of intestinal inflammation.³¹ Mouse models of MND showed similar gut changes before symptoms appeared. The research is still preliminary, limited to very small patient numbers, and no one is claiming that gut bacteria cause MND. But if chronic low-grade gut inflammation contributes even one of the several “steps” needed for disease onset, it might eventually become a target for prevention or early intervention. This line of investigation sits alongside broader interest in how systemic inflammation and the immune system interact with neurodegenerative diseases, and it is worth watching even if it has not yet changed clinical practice.

Separately, researchers have begun to look at biological ageing, as measured through chemical markers on DNA, rather than just calendar age. A 2024 paper explored whether accelerated biological ageing might serve as a marker of MND severity, though its full findings are still being digested by the field. The broader point is that ageing itself is not just the passage of time but an accumulation of cellular wear, and people whose cells age faster may be more susceptible to neurodegenerative disease. How much this concept will add to our understanding of MND risk beyond what chronological age already tells us remains to be seen.