Most people with ALS do not inherit it, but some forms run in families

About 9 out of 10 people diagnosed with ALS develop the disease without a family history of it. These cases are called sporadic ALS, and they happen when genetic changes occur randomly during a person's lifetime rather than being passed down from a parent. The remaining 1 out of 10 people have familial ALS (fALS), meaning at least one parent, sibling, or child also has or had the disease.

Whether your ALS is inherited or sporadic does not change how the disease progresses or how it is treated. The symptoms and course are the same. What matters for your family is whether you carry a genetic mutation that could be passed to your children.

Key Takeaways

  • Sporadic ALS (no family history) accounts for about 90 percent of cases and does not run in families.
  • Familial ALS (fALS) means at least one blood relative has the disease, and a genetic mutation may have been inherited from a parent.
  • The most common inherited form is caused by mutations in the C9orf72 gene, followed by SOD1 and FUS genes.
  • A genetic counselor can explain your specific mutation, what it means for your children, and whether other relatives should know about it.
  • Genetic testing is optional and does not change your treatment, but it can help your family make informed decisions.

How genetic mutations cause familial ALS

In familial ALS, a person inherits a mutation in one of several genes known to cause the disease. The most common is the C9orf72 gene, which accounts for about 40 percent of familial cases in people of European descent. Mutations in the SOD1 gene and FUS gene are the next most frequent causes. Dozens of other genes have been linked to ALS, but they are much rarer.

When you inherit a mutation in one of these genes, you have a 50 percent chance of passing it to each of your children. Not everyone who carries the mutation will develop ALS—some people inherit it but never get sick, a pattern called incomplete penetrance. This means your child could carry the mutation and remain healthy their whole life, or they could develop symptoms years later.

The age at which symptoms start varies widely, even within the same family. One parent might develop ALS at 45 and another at 70, or a sibling might never develop it at all despite carrying the same mutation.

How sporadic ALS develops without family history

In sporadic ALS, genetic changes happen randomly in the cells of the nervous system, usually during adulthood. These mutations are not inherited from a parent and cannot be passed to your children. Scientists do not yet fully understand why these random changes occur, but they appear to involve a combination of genetic factors and environmental exposures over time.

Some people with sporadic ALS may carry genetic variations that make them more vulnerable to the disease, but these are not the same as the strong inherited mutations seen in familial ALS. Even if you have sporadic ALS, your children do not inherit an increased risk of developing the disease.

When and how to get genetic testing

Genetic testing for ALS involves a blood test that looks for mutations in known ALS genes. Your neurologist can order the test, or you can ask for a referral to a genetic counselor who specializes in neuromuscular disease. Testing is optional and does not affect your medical care or treatment.

You might consider testing if you have a family history of ALS, if you want to know whether you can pass the mutation to your children, or if you are interested in participating in research studies. Some clinical trials focus on people with specific genetic mutations, and knowing your status could open those options to you.

If you test positive for a mutation, a genetic counselor will explain what it means, how it might affect your family members, and whether relatives should be informed. If you test negative, it confirms you have sporadic ALS and your children are not at genetic risk.

What to tell your family members

If you have familial ALS or carry a mutation, your blood relatives may want to know. This is a personal decision, and there is no single right answer. Some families choose to share the information so relatives can make informed choices about their own health and family planning. Others prefer not to discuss it.

A genetic counselor can help you think through how and when to tell family members, and what information they might need. Some relatives may want to pursue genetic testing themselves, while others may not. Your neurologist or the ALS Association can connect you with a counselor in your area.

How genetic information affects family planning

If you carry a mutation that causes familial ALS, you have a 50 percent chance of passing it to each child. Some people use this information to make decisions about having biological children, pursuing adoption, or using reproductive technologies like in vitro fertilization (IVF) with genetic screening.

A genetic counselor can discuss these options with you and your partner in detail. They can also explain what it means to be a carrier—someone who has the mutation but may never develop symptoms—and how that differs from having the disease itself.

Genetic research and clinical trials

Researchers are actively studying the genetic causes of ALS and developing treatments targeted at specific mutations. If you have a known genetic mutation, you may be may be able to access for clinical trials testing new therapies designed for your particular form of the disease.

The ALS Association and ClinicalTrials.gov maintain lists of ongoing studies. Your neurologist can also tell you about trials recruiting in your area. Participating in research is voluntary and does not replace your standard medical care.

Frequently Asked Questions

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

No. Even if you inherit the mutation, you may never develop ALS. Some people carry the mutation their whole life without getting sick. If you are concerned, talk to your doctor about genetic testing and counseling to understand your specific situation.

Can I pass sporadic ALS to my children?

No. Sporadic ALS is not inherited, so your children do not have an increased genetic risk of developing the disease. The random genetic changes that caused your ALS happened during your lifetime and are not passed down.

What does it mean if I carry the mutation but don't have symptoms?

You are a carrier of the mutation but have not yet developed ALS. Some carriers never develop symptoms, while others may develop them later in life. A genetic counselor can discuss what this means for your health and your family.

Should I tell my siblings if I have familial ALS?

That is your choice. Some families find it helpful to share genetic information so relatives can make informed decisions about their health and family planning. A genetic counselor can help you think through how and when to have that conversation.

Does knowing my genetic mutation change how I am treated?

Not yet for most mutations, but genetic testing may open access to clinical trials testing therapies designed for your specific form of ALS. Talk to your neurologist about whether any trials are recruiting for your mutation.