What’s the Difference Between Spinal Muscle Atrophy and Muscular Dystrophy?

Spinal muscle atrophy (SMA) is a condition in which the motor neurons in the brain stem and spinal cord are destroyed. Motor neurons are nerve cells that control the movement of your skeletal muscles.

Skeletal muscles are those that you can move voluntarily. They’re vital for body functions like walking, talking, and swallowing. When the motor neurons that control them are destroyed, they begin to weaken and shrink.

There are a few different types of the most common form of SMA. These are generally divided based on when the symptoms of the condition first appear. The table below shows the different types and when they typically come on.

Spinal muscular atrophy type comparison

Muscular dystrophy (MD) refers to a group of conditions that lead to muscle weakness and shrinking. It most commonly affects muscle fibers, which are groups of cells surrounded by a membrane that make up your skeletal muscles.

A group of proteins called the dystrophin glycoprotein complex is found on the membranes of muscle fibers. These proteins help to prevent damage from happening as your muscles contract and then relax.

MD often impacts proteins in the dystrophin glycoprotein complex. When these proteins don’t function as they should, they can damage muscle fibers, leading to muscle weakness and shrinking.

There are nine major groups of MD. These are broken down based on factors like the age at which the condition develops and the muscle groups that are typically impacted. The table below gives a short overview of each type of MD.

Muscular dystrophy type comparison

SMA and MD are related in that they can cause a similar set of symptoms, particularly gradual muscle weakness and shrinking.

As such, both conditions are associated with increasing problems with skeletal muscle function that can hamper movement.

They can both also cause serious or life threatening issues with other body functions that involve muscle activity. These include breathing and swallowing.

While similar, the symptoms of both conditions are mediated by different factors. In SMA, the motor neurons that serve the muscles that are located in the spinal cord are destroyed. In MD, the damage is often occurring to the muscle fibers themselves.

Both SMA and MD are genetic conditions. That means that they happen due to harmful changes to genes. Genes are stretches of DNA that often act as instructions for making proteins and other physical structures.

Causes of spinal muscular atrophy

SMA is inherited, which means that it’s passed down to you from your parents. About 95% of people with SMA have a deletion in the SMN1 gene, which encodes the survival motor neuron (SMN) protein.

SMN protein works to keep motor neurons healthy and functioning. When there’s not enough SMN protein around, motor neurons begin to die. This is what leads to the muscle weakness and shrinking associated with SMA.

Causes of muscular dystrophy

MD is also typically inherited but can also happen due to spontaneous genetic changes. For example, one-third of instances of DMD and BMD are caused by spontaneous genetic changes.

Many different genes and proteins can be affected in MD, but it’s most often caused by changes impacting proteins in the membranes of muscle fibers, such as those of the dystrophin glycoprotein complex.

Both SMA and MD can cause a variety of symptoms that depend on the specific muscle group that’s impacted. Some of the general musculoskeletal symptoms can include:

• muscle weakness
• muscle shrinking
• muscle twitching
• low muscle tone
• reduced reflexes
• contractures, which are when muscles, tendons, or ligaments harden and become shorted
• scoliosis
• changes in posture
• difficulty with movement and mobility
• trouble swallowing, breathing, or speaking

Early signs of these conditions can depend on when they come on. For example, infants may have poor head control or trouble feeding. Meanwhile, an older child may struggle to sit up or experience difficulty walking and frequent falls.

MD is also sometimes associated with problems in other parts of the body. These can include but aren’t limited to the heart, lungs, and eyes.

There isn’t a cure for SMA or MD. Instead, treatment options focus on reducing symptoms, slowing progression, and boosting quality of life.

Some of the treatments that may be recommended for both conditions include:

• physical therapy
• occupational therapy
• speech therapy
• assistive devices like wheelchairs, braces, or speech synthesizers
• assisted ventilation
• emotional support, such as through talk therapy or other types of therapy
• surgery to correct complications, such as severe contractures or scoliosis
• an optimal diet to help promote overall health

Medical treatments for spinal muscular atrophy

The Food and Drug Administration (FDA) has approved several treatments specifically for SMA. These are:

• Nusinersen (Spinraza): Given through injection, Spinraza helps to increase the production of SMN protein in adults and children with SMA.
• Onasemnogene abeparvovec-xioi (Zolgensma): A type of gene therapy for children that have type I SMA, Zolgensma uses a modified virus to deliver a healthy copy of the SMN gene to motor neurons.
• Risdiplam (Evrysdi): This oral drug helps to increase the amount of functional SMN protein in adults and children with SMA.

Medical treatments for muscular dystrophy

There are also FDA-approved treatments for people with DMD that have certain gene changes. These include

• golodirsen (Vyondys 53)
• viltolarsen (Viltepso)
• casimersen (Amondys 45)

Other medical treatments that may be used for MD include corticosteroids or immunosuppressive drugs to help slow muscle damage. Other drugs, such as phenytoin or mexiletine, can help with muscle spasms or muscle weakness.

SMA and MD are inherited. As such, the main risk factor for them is a family history of either condition.

People assigned male or female at birth are generally at an equal risk of developing SMA. While this is true for many types of MD as well, some types are more common in certain sexes. For example, DMD mostly impacts people with XY chromosomes (assigned male at birth) and is very rare in people with XX chromosomes (assigned female at birth).

Both SMA and MD are progressive conditions. That means that they continue to get worse as time passes. However, it’s possible that treatment can help to improve quality of life and slow progression.

The outlook for people with SMA can vary based on which type a person has. Some types of SMA, such as type I, can be fatal if treatment isn’t given quickly. Generally speaking, the outlook is worse the younger a person is when SMA symptoms begin.

Similarly, the outlook for people with MD also depends on the type that you have. However, many people with MD have a shortened life expectancy compared with those without MD.

Improvements in treatment have improved the outlook for people with MD. For example, a 2021 review notes that people with DMD can now live into their 40s. In these individuals, death typically occurs due to complications related to the lungs or heart.

Below, we’ll cover some additional questions that you may have about SMA or MD.

How is spinal muscular atrophy inherited?

Most of the time, SMA is inherited in an autosomal recessive manner. This means that in order to have SMA, a person needs to receive a copy of a faulty gene from each parent, meaning they have two copies of the faulty gene.

How is muscular dystrophy inherited?

Depending on the type, MD can be inherited in several different ways:

• Autosomal recessive: Two copies of a faulty gene need to be present to have the condition.
• Autosomal dominant: Only one copy of a faulty gene needs to have been inherited to have the condition.
• X-linked: The faulty gene is carried on the X sex chromosome, meaning that people assigned male at birth, who have only one X chromosome, are more likely to be affected.

Genetic causes for muscular dystrophy

How common are spinal muscle atrophy and muscular dystrophy?

SMA is estimated to impact about 1 in 10,000 live births. Additionally, about 1 in 40 people in the general population are believed the carry a copy of the faulty SMN1 gene.

DMD and BMD are some of the most common types of MD. The Centers for Disease Control and Prevention (CDC) estimates they affect 1 in every 7,250 males between the ages of 5 and 24. Other types of MD are rarer.

How are spinal muscle atrophy and muscular dystrophy diagnosed?

The diagnosis of SMA and MD starts with a physical exam and a thorough review of your personal and family medical history. Additional tests that can be used to confirm the diagnosis include:

• Genetic testing: Genetic testing looks for specific genetic changes associated with either condition.
• Electromyography: Electromyography looks at the electrical activity in your muscles.
• Nerve conduction velocity tests: Nerve conduction velocity tests assess how fast electrical signals travel down a nerve.
• Muscle biopsy: A muscle biopsy involves collecting a sample of muscle tissue that can be examined under a microscope.
• Imaging tests: These tests, such as MRI, give a doctor a look at the condition of your muscles.
• Additional blood and urine tests: These tests can evaluate your overall health and look for signs of muscle damage.

Can spinal muscle atrophy and muscular dystrophy be prevented?

There’s no way to prevent SMA or MD at this time. If one of these conditions runs in your family, a doctor may suggest genetic testing and counseling to learn about the risk of future children inheriting it.

SMA and MD are conditions that lead to muscle weakness and shrinking. This can result not only in problems with movement but also with other body functions like swallowing and breathing.

SMA happens due to the destruction of motor neurons. MD typically results from damage to muscle fibers.

Both of the conditions are genetic, happening due to harmful changes in genes. They’re also typically inherited, meaning that they can be passed down from parent to child.

There’s no cure for SMA or MD, and treatments focus on easing symptoms, slowing progression, and boosting quality of life. There are different types of both SMA and MD, and the outlook depends on the specific type that you have.