What causes ALS

ALS happens when nerve cells in the brain and spinal cord that control muscles gradually break down and die. Doctors do not yet know exactly why this happens in most cases. The disease damages motor neurons — the nerve cells that send signals telling your muscles to move. As these neurons die, your muscles weaken, stop responding to commands, and eventually waste away.

About 90 percent of ALS cases are sporadic, meaning they appear randomly with no family history. The other 10 percent are familial ALS (fALS), which runs in families and is caused by inherited genetic mutations. Even in familial cases, inheriting the mutation does not may provide you will develop the disease — penetrance varies, meaning some people carry the gene but never get sick.

Scientists have identified more than 25 genes linked to ALS, but having one of these mutations is not the same as having a cause. Most likely, ALS results from a combination of genetic factors, environmental exposures, and random cellular events that happen over time.

Key Takeaways

  • ALS develops when motor neurons in the brain and spinal cord break down, but doctors do not yet know why this happens in most people.
  • About 9 in 10 ALS cases are sporadic with no family history, while 1 in 10 are familial and inherited from a parent who carries a genetic mutation.
  • Carrying a gene linked to ALS does not mean you will definitely develop the disease, and some people with the mutation never become ill.
  • Researchers believe ALS likely results from a mix of genetic predisposition, environmental factors, and chance events rather than a single cause.

Genetic mutations and familial ALS

If ALS runs in your family, there is a chance you inherited a mutation that increases your risk. The most common familial ALS genes are SOD1, C9orf72, and FUS. Each mutation works differently — some are dominant, meaning you need only one copy from one parent to carry the risk, while others are recessive, requiring copies from both parents.

Having a familial ALS mutation does not mean you will get the disease. Some people with the mutation live their whole lives without symptoms, while others develop ALS in their 30s or 80s. This unpredictability is one reason genetic testing is complicated — a positive result tells you about risk, not destiny.

If you have a family history of ALS, a genetic counselor can explain your personal risk and discuss whether genetic testing makes sense for you. They can also help you understand what a positive or negative result would mean for your health decisions and your family.

Environmental and lifestyle factors under investigation

Researchers have studied whether environmental exposures trigger ALS, but no single environmental cause has been proven. Some studies suggest possible links to heavy metals, pesticides, military service, and intense physical activity, but the evidence remains mixed and incomplete.

Military veterans develop ALS at higher rates than the general population, and the VA recognizes ALS as a service-connected condition. The reason is unclear — it may involve exposure to toxins, physical stress, infections, or a combination. Similarly, some athletes, particularly those in contact sports, show higher ALS rates, though whether this reflects head injury, intense exertion, or selection bias is still debated.

The bottom line is that no lifestyle choice or exposure has been definitively shown to cause ALS in most people. This is why ALS is so difficult to prevent — there is no known way to avoid it based on current evidence.

Age and other risk factors

ALS most commonly appears between ages 55 and 75, though it can develop at any age. The average age at diagnosis is around 63. Younger-onset ALS (before age 50) is less common but does occur, and familial cases sometimes appear earlier than sporadic ones.

Men are slightly more likely to develop ALS than women, though the difference is small. Some research suggests that estrogen may offer some protection, but this remains an area of active study. Race and ethnicity do not appear to be major risk factors, though some populations have been studied less thoroughly than others.

How doctors rule out other conditions

Because ALS symptoms — weakness, muscle twitching, stiffness — can look like other neurological diseases, doctors must do careful testing to confirm the diagnosis. They use electromyography (EMG), a test that records electrical activity in muscles, and nerve conduction studies to see how well nerves send signals. These tests show a pattern typical of ALS: muscle damage without nerve damage.

MRI scans of the brain and spinal cord rule out tumors, strokes, and other structural problems that could cause similar symptoms. Blood tests and sometimes spinal fluid tests check for infections or other conditions. This process of elimination is how neurologists arrive at an ALS diagnosis — there is no single test that proves you have ALS.

What happens after diagnosis

Once a neurologist diagnoses ALS, the next step is usually to see a specialist who focuses on ALS care. An ALS clinic or multidisciplinary team typically includes a neurologist, respiratory therapist, physical therapist, speech therapist, social worker, and nutritionist. This team approach helps manage the different ways ALS affects the body as the disease progresses.

Your doctor will discuss treatment options, including the medications riluzole and edaravone, which may slow disease progression slightly. They will also talk about planning ahead — advance directives, breathing support options, feeding tubes, and other decisions that become important as the disease advances. Genetic testing may be offered if your case could be familial, both for your own understanding and to help your family members make informed decisions.

Frequently Asked Questions

Can you catch ALS from someone else?

No. ALS is not contagious and cannot spread from person to person. It is a disease of the nervous system that develops inside the body, either from inherited genes or from sporadic changes that occur randomly.

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

Not necessarily. If your parent has familial ALS caused by a dominant mutation, you have a 50 percent chance of inheriting the gene. But inheriting the gene does not may provide you will develop symptoms — some people with the mutation never get sick. A genetic counselor can discuss your specific situation based on your family's genetic pattern.

Can stress or trauma cause ALS?

There is no scientific evidence that emotional stress or physical trauma causes ALS. Some people notice symptoms appearing after a stressful event, but this is likely coincidence rather than cause. ALS develops over years as neurons gradually break down, not suddenly in response to life events.

Is there a test that can predict whether I will get ALS?

If you carry a familial ALS gene mutation, genetic testing can confirm that. However, a positive genetic test does not predict whether or when you will develop symptoms. For sporadic ALS, there is currently no predictive test. A genetic counselor can explain what genetic testing can and cannot tell you about your future health.

Does diet or exercise prevent ALS?

There is no proven way to prevent ALS through diet, exercise, or lifestyle changes. While staying healthy is always worthwhile, no specific prevention strategy has been shown to reduce ALS risk. If you have a family history of ALS, talk with your doctor about what you should know and monitor.