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SMA and ALS: 5 Differences and 3 Similarities

Medically reviewed by Dennrik Abrahan, M.D.
Written by Emily Wagner, M.S.
Posted on September 22, 2023

If you or a loved one is living with spinal muscular atrophy (SMA), you may be researching to learn about related conditions. Amyotrophic lateral sclerosis (ALS) is another motor neuron disease that shares some similarities with SMA. These diseases both affect the nerve cells (neurons) responsible for controlling muscle movements.

However, some differences make these conditions stand apart from each other — including who develops them, how they’re diagnosed, and how they’re treated. In this article, we’ll cover five differences and three similarities between SMA and ALS.

Differences Between SMA and ALS

Here are five key differences between SMA and ALS.

1. SMA and ALS Typically Develop in Different Age Groups

One difference between SMA and ALS is the typical age when they’re diagnosed. There are five types of SMA — types 0, 1, 2, 3, and 4. The SMA type is determined by how severe it is and how early the symptoms start. The symptoms of SMA types 1, 2, and 3 affect infants and children at different ages:

  • Type 1 — 6 months to 1 year old
  • Type 2 — 6 to 18 months old
  • Type 3 — 18 months and older

Type 4 affects adults and accounts for only 5 percent of cases. Type 4 SMA usually has mild symptoms, and people with this type commonly reach all motor milestones and remain mobile throughout their lives.

ALS can develop at any age, but most people start experiencing symptoms as adults from ages 55 to 75 years. The ALS Association also notes that the average age of an ALS diagnosis is 55 years. People as young as 20 to 30 years old can also develop ALS.

2. SMA and ALS Have Different Causes

SMA is a neuromuscular disease caused by changes in the survival motor neuron 1 (SMN1) gene. The SMN1 gene is found on chromosome 5. (Most people have two copies of 23 chromosomes for a total of 46.) The SMN1 gene provides instructions for making the SMN protein.

This protein is responsible for healthy motor neuron function. These nerve cells are responsible for sending electrical signals from the brain and spinal cord to the skeletal muscles. People with SMA can’t make enough SMN proteins to send signals properly, leading to muscle weakness and wasting.

On the other hand, doctors and researchers still aren’t quite sure what causes ALS. They believe it may be due to a combination of gene mutations (changes) that affect motor neurons and environmental factors. In fact, most cases occur randomly and don’t appear linked to any family history or specific risk factors.

Researchers have found that military veterans may be at a higher risk of developing ALS compared to the general population. They’re not quite sure why, but they believe exposure to environmental toxins like pesticides or lead may be to blame.

3. SMA Is Inherited, While ALS Typically Isn’t

SMA is a genetic disease passed from parent to child. SMA is known as a recessive disease. This means a person must inherit two faulty copies of the SMN1 gene to develop SMA. A person with SMA inherits one copy of a gene from each parent, giving them two copies.

About 9 out of 10 ALS cases develop without any known cause. This means that the other 5 percent to 10 percent of cases are caused by genetic mutations passed down in families. Familial ALS is often the result of two gene mutations — C9ORF72 and SOD1 mutations.

4. SMA and ALS Are Diagnosed Using Different Tests

Because SMA is caused by mutations in the SMN1 gene, it’s most often diagnosed using genetic testing. DNA sequencing can look for changes in a blood sample to confirm a diagnosis. If the test results don’t show SMN1 mutations, your doctor can also use additional tests to check muscle function.

On the other hand, there’s no single test that can diagnose ALS. Instead, doctors use a combination of physical exam results, muscle function tests, and imaging techniques to make a diagnosis.

5. Treatments Come From Different Classes of Medication

There’s currently no complete cure for SMA and ALS — but new treatments are constantly being developed with the hopes of helping people live longer healthier lives.

Because SMA is caused by faulty SMN genes, doctors and researchers have developed gene therapy and medications to boost SMN protein production. The U.S. Food and Drug Administration (FDA) has approved three SMA drugs:

Current SMA treatments have been on the market for less than 10 years, so doctors and researchers don’t know how they’ll affect life expectancy and disease progression in the long term.

On the other hand, ALS treatments focus on slowing functional decline and helping people live longer. The FDA has approved the following medications for treating ALS:

  • Riluzole (sold under the brand names Exservan, Rilutek, and Tiglutik)
  • Sodium phenylbutyrate and taurursodiol (Relyvrio)
  • Edaravone (Radicava)
  • Tofersen (Qalsody)

Similarities Between SMA and ALS

Below are three key similarities between SMA and ALS.

1. Both Are Motor Neuron Diseases That Affect Mobility

Despite having different causes, both SMA and ALS are motor neuron diseases that affect muscle movement or motor function.

There are two types of motor neurons — upper and lower. Upper motor neurons are found in the brain and spinal cord. They send electrical signals to the lower motor neurons found in the brain stem. This is the bottom part of your brain that connects to the spinal cord. Lower motor neurons send the signals to the rest of the body, telling muscles to move.

SMA affects the lower motor neurons, while ALS can affect the upper, lower, or both sets of motor neurons. The types of neurons affected play a role in what symptoms you may have.

People with SMA and ALS have trouble moving the muscles in their:

  • Face, mouth, and throat (known as bulbar muscles)
  • Arms and legs
  • Chest

Motor neurons are also responsible for controlling the skeletal muscles involved in speaking, walking, swallowing, and breathing. People with SMA and ALS have motor neuron dysfunction that can lead to:

  • Muscle twitching
  • Muscle weakness and wasting
  • Shortness of breath or trouble breathing
  • Trouble standing and walking

2. Both Diseases Are Progressive

Another similarity between SMA and ALS is that they are both progressive diseases that eventually lead to disability. Progressive means that once the disease begins, it continues to get worse over time. The speed of the worsening varies and is different for each person.

3. Both Diseases Can Shorten the Life Span

Some types of SMA and all cases of ALS shorten a person’s life span. This is because they’re neurodegenerative diseases that cause motor neurons to die off. When these nerve cells die, they can’t grow back. They can no longer send signals to the muscles, which causes atrophy (muscle wasting away or shrinking).

All people with ALS and some with SMA eventually develop respiratory failure, meaning they can’t breathe on their own. Because treatments for SMA are very new, it’s unknown whether or by how much they might improve life expectancy.

Although SMA and ALS are different diseases that affect the nervous system, it’s crucial to understand their differences and similarities. These conditions may show up in different ways and progress at different speeds. Learning more about these conditions, whether it’s recognizing the common signs of nerve damage or understanding when they usually begin, can provide individuals and families with greater hope as they navigate these challenges.

Talk With Others Who Understand

On mySMAteam, the social network for people with SMA and their loved ones, more than 2,300 members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

Do you have more questions about the differences and similarities between spinal muscular atrophy and amyotrophic lateral sclerosis? Share them in the comments below, or start a conversation by posting on your Activities page.

    Posted on September 22, 2023
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    Dennrik Abrahan, M.D. received his medical degree from the University of Central Florida. Learn more about him here.
    Emily Wagner, M.S. holds a Master of Science in biomedical sciences with a focus in pharmacology. She is passionate about immunology, cancer biology, and molecular biology. Learn more about her here.

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