What we know about what triggers ALS

ALS does not have a single trigger. Most people who develop it have no family history of the disease, and researchers have not found one cause that explains all cases. Instead, ALS appears to result from a combination of genetic and environmental factors that damage motor neurons over time. In roughly 10 percent of cases, a person inherits a genetic mutation that makes ALS more likely. In the other 90 percent—called sporadic ALS—the cause remains unclear, though scientists have identified several factors that appear to increase risk.

The disease itself is well understood: motor neurons in the brain and spinal cord gradually die, and the muscles they control weaken and stop working. What is not yet clear is why this happens in some people and not others, or what combination of events sets it in motion. This uncertainty is one reason ALS is difficult to prevent and why treatment focuses on slowing progression rather than stopping it at the source.

Key Takeaways

  • ALS results from motor neuron death, but no single trigger causes this in most people—it appears to involve both genes and environment.
  • About 10 percent of ALS cases run in families and are linked to inherited genetic mutations; the other 90 percent are sporadic with no clear cause.
  • Researchers have identified risk factors associated with ALS, including age, sex, military service, and exposure to certain chemicals, but none of these may provide the disease will develop.
  • Genetic testing can identify some inherited forms of ALS, but a negative test does not rule out the disease or mean you will not develop it.

Genetic mutations that cause inherited ALS

When ALS runs in a family, it is usually caused by a mutation in one of several genes. The most common is a mutation in the C9orf72 gene, which accounts for roughly 40 percent of familial ALS cases in North America. Other genes linked to inherited ALS include SOD1, FUS, and TARDBP. Each of these genes normally helps control how motor neurons work, and a mutation in any of them can cause the cells to malfunction and die.

If one of your parents carries a mutation in one of these genes, you have a 50 percent chance of inheriting it. However, inheriting the mutation does not may provide you will develop ALS. Some people with the mutation never show symptoms, or symptoms appear very late in life. This unpredictability—called incomplete penetrance—means genetic testing can tell you whether you carry a mutation, but not whether or when you will become ill.

Genetic counseling before or after testing can help you understand what a result means for your health and your family. A genetic counselor can explain the inheritance pattern, discuss the limitations of testing, and help you decide whether testing makes sense for your situation.

Environmental and lifestyle factors associated with ALS

Researchers have found that certain exposures and life circumstances appear more common in people with ALS, though none of them directly cause the disease. Military service is one of the strongest associations: veterans have a higher rate of ALS than the general population, possibly because of exposure to pesticides, heavy metals, or other hazards during deployment. Smoking and heavy alcohol use have also been linked to higher ALS risk in some studies, though the relationship is not fully understood.

Occupational exposure to heavy metals like lead and mercury, and to pesticides used in agriculture, has been investigated as a possible risk factor. Some studies suggest a link, but the evidence is not strong enough to say these exposures directly cause ALS. Similarly, physical trauma and intense physical activity have been studied as possible triggers, with mixed results. A few studies have suggested that people who played professional sports or had significant head injuries may have higher ALS risk, but this remains controversial and is not proven.

Age is the strongest demographic risk factor: ALS typically appears between ages 55 and 75, though it can occur earlier or later. Men are diagnosed more often than women, though the reason is not known.

Why some people develop ALS and others do not

The fact that most ALS cases are sporadic—with no family history and no obvious trigger—suggests that the disease requires multiple hits to the nervous system. A person might carry a genetic variation that makes motor neurons slightly more vulnerable, then encounter an environmental exposure that damages them further, and then experience an infection or other stress that pushes them over the edge. This multi-step model explains why ALS is rare even though many people are exposed to the same environmental hazards.

It also explains why researchers have struggled to find a single cause. A risk factor that appears in many ALS patients may be necessary but not sufficient—meaning it increases risk but does not may provide disease. This is why a person can smoke for decades without developing ALS, while another person who never smoked develops it anyway.

What genetic testing can and cannot tell you

If ALS runs in your family, genetic testing can identify whether you carry a mutation in one of the known ALS genes. The test uses a blood sample and looks for changes in genes like C9orf72, SOD1, FUS, and others. A positive result means you carry a mutation; a negative result means you do not carry mutations in the genes that were tested.

However, a negative test does not rule out inherited ALS. Researchers continue to discover new genes linked to the disease, so a negative result today may change if new genes are identified. A negative test also does not mean you will not develop sporadic ALS. And a positive result does not mean you will definitely become ill—some people with mutations never develop symptoms.

Genetic testing is most useful for people with a strong family history of ALS, because it can identify who carries a mutation and may benefit from closer monitoring or from clinical trials of preventive treatments. For people without a family history, genetic testing is less informative because most sporadic ALS is not caused by the known inherited mutations.

Current research into ALS causes

Scientists are pursuing several lines of investigation to understand what triggers ALS. Some researchers are studying how misfolded proteins accumulate in motor neurons and damage them. Others are examining whether infections—particularly viral infections—might trigger the disease in genetically vulnerable people. Still others are investigating whether the immune system plays a role, attacking motor neurons in some cases.

Research into the C9orf72 mutation has revealed that it may cause problems with how cells clear out waste, leading to toxic buildup inside neurons. This finding has opened new avenues for treatment, with several drugs now in clinical trials that aim to restore the cell's cleanup process. Similar research into other genes is revealing different mechanisms of damage, suggesting that ALS may actually be several different diseases that look similar from the outside.

This research is still in early stages, and no treatment yet targets the root cause of ALS in most patients. Current medications slow progression in some people, but they do not stop the disease or reverse damage that has already occurred.

Frequently Asked Questions

Can stress or trauma cause ALS?

Stress and trauma have been studied as possible ALS triggers, but evidence for a direct causal link is weak. Some people develop ALS after a stressful event or injury, but many others experience similar events without developing the disease. Stress may worsen symptoms in people who already have ALS, but it is not considered a proven cause.

If my parent has ALS, will I definitely get it?

Not necessarily. If your parent has inherited ALS caused by a genetic mutation, you have a 50 percent chance of inheriting that mutation. But inheriting the mutation does not may provide you will develop symptoms—some people with the mutation never become ill. Genetic testing and counseling can help you understand your specific risk.

Does diet or exercise prevent ALS?

No diet or exercise regimen has been proven to prevent ALS. Some research suggests that overall good health may slow progression once the disease appears, but nothing prevents the disease from starting. People at genetic risk should discuss preventive options with their doctor, as some clinical trials are testing whether early treatment can delay symptom onset.

Can environmental exposure alone cause ALS?

Environmental exposures like pesticides or heavy metals appear more common in people with ALS, but exposure alone does not cause the disease. Most people exposed to these hazards do not develop ALS, suggesting that genetic factors also play a role. The disease likely requires both genetic vulnerability and environmental exposure.

What should I do if ALS runs in my family?

Talk with your doctor about genetic testing and counseling. A genetic counselor can explain your risk, discuss what testing can and cannot tell you, and help you decide whether testing makes sense. Even without testing, you can discuss with your doctor what symptoms to watch for and whether monitoring is recommended.