Spinal muscular atrophy vs. muscular dystrophy

Both spinal muscular atrophy (SMA) and muscular dystrophy (MD) are genetic conditions characterized by muscle weakness that can lead to mobility and breathing issues. However, the two diseases have different underlying causes and treatment strategies, so it is important to get the correct diagnosis and care.

There are several types of both SMA and muscular dystrophy, which can vary widely in inheritance patterns, symptoms, treatment, and prognosis. Both conditions are considered rare, with SMA estimated to affect about 1 of every 10,000 newborns, while muscular dystrophy affects 1 of every 5,000 to 10,000.

What is the difference between SMA and MD?

SMA and MD are both rare diseases that lead to muscle weakness, but how this occurs is a key difference between them. While nerve cells are the source of the problem in SMA, muscular dystrophy is primarily a muscle issue.

More specifically, in SMA, the specialized nerve cells that control muscles, called motor neurons, are progressively lost. Without the right input from motor neurons, muscles don’t contract as they should, and without being used properly, they eventually waste away, or (atrophy).

In muscular dystrophy, problems with proteins that support muscle health cause muscles to progressively weaken and degenerate.

What causes SMA and muscular dystrophy?

Both SMA and MD are caused by genetic mutations:

• SMA causes: In the most common types of SMA (types 0-4), motor neuron death is caused by a deficiency in survival motor neuron (SMN) protein due to mutations in the SMN1 gene. Rarer SMA types involve mutations in other genes that affect motor neuron health in different ways.
• Muscular dystrophy causes: There are more than 30 types of MD, each caused by mutations in genes that affect different proteins needed for muscle health. The most common type that affects children, Duchenne muscular dystrophy (DMD), is caused by mutations in the DMD gene that result in a lack of the muscle-protecting protein dystrophin.

For both SMA and MD, inheritance patterns vary by disease type. SMA types 0-4 follow an autosomal recessive inheritance pattern, meaning that someone needs to inherit two mutated copies of SMN1  — one from each parent — to develop the condition. It affects males and females similarly.

DMD and some other muscular dystrophies are X-linked recessive — the disease-causing mutation is on the X chromosome, one of the two sex-determining chromosomes in the body. With this inheritance pattern, males are more likely than females to develop the disease.

Other subtypes of either disease can have different inheritance patterns, including autosomal recessive, X-linked, or autosomal dominant, meaning one mutated gene copy can cause the disease.

How do the symptoms compare?

Several possible symptoms of SMA and MD overlap, including:

• muscle weakness
• bone problems, including abnormal side-to-side curvature of the spine (scoliosis)
• breathing problems
• fatigue
• speech and swallowing problems
• nutrition problems

SMA symptoms can emerge at different age ranges depending on the disease type, from birth in SMA type 0 to adulthood in SMA type 4. Symptom severity and progression also vary. In the most common type, SMA type 1, symptoms usually start in the first six months of life. Other common symptoms in SMA include gastrointestinal problems, tremors, and involuntary tongue movements.

Muscular dystrophy symptoms also vary by disease type, as each can affect different muscle groups. For example, DMD primarily affects the muscles near the body’s core, so symptoms often include mobility challenges. Other types may primarily cause problems in the face or upper limbs. Symptom severity and age at onset also vary by subtype. MD may also cause heart, vision, or cognitive problems.

How are SMA and muscular dystrophy diagnosed?

The diagnosis of both SMA and MD often involves a series of tests to determine the origin of muscle weakness and rule out other conditions. These may include a physical examination, thorough medical history, and any of the following:

• genetic testing, to identify disease-causing mutations
• electromyography (EMG), a technique for recording muscle activity
• nerve conduction studies, which measure how efficiently nerves deliver signals
• muscle biopsy, a small sample of muscle tissue that is examined under a microscope

Results from EMG and nerve conduction studies help doctors determine whether the main problem lies in the motor neurons or the muscles. For example, nerve conduction is often normal in MD but abnormal in SMA.

A muscular dystrophy diagnosis may also involve muscle MRI imaging and blood tests to identify markers of muscle damage.

Newborn screening programs can enable an SMA diagnosis shortly after birth, even in the absence of symptoms. In some places, this may also be true for DMD.

How does treatment differ?

SMA and MD have different disease-modifying treatments designed to slow disease progression.

• SMA treatment: All approved disease-modifying therapies for SMA are for forms of the disease caused by SMN1 mutations. They generally work in different ways to boost SMN production.
• Muscular dystrophy treatment: Approved disease-modifying therapies for muscular dystrophy are generally for DMD. They work in various ways to either boost the production of functional dystrophin or protect muscles in other ways.

Corticosteroids, a class of broad-acting immunosuppressant medications, are routinely used for certain types of MD to help reduce inflammation that contributes to muscle damage. This is not a routine treatment for SMA.

Medications for symptom management can vary. For example, SMA care may also include treatment for drooling and muscle spasms, while MD care may include heart medications.

Other supportive care approaches to ease symptoms and improve quality of life often overlap between SMA and MD. They may include:

• physical therapy
• occupational therapy
• respiratory support
• back or neck braces for scoliosis
• nutritional support
• speech therapy
• mental health care
• pain management
• assistive devices and home adaptations

How can mobility and daily life be affected?

Degrees of mobility and independence vary widely by SMA or MD type, but both diseases can make walking and other daily activities difficult. Many cases of SMA and MD are progressive, meaning muscle weakness and related symptoms tend to get worse over time, but the rate at which this happens varies widely. With either disease, some people may retain their ability to walk, while others will become wheelchair-reliant.

Life expectancy estimates also vary by disease type, although severe disease can affect survival for both SMA and muscular dystrophy.

Why an accurate diagnosis matters

Getting an accurate diagnosis is vital to ensuring that the proper treatment and care plan are in place as early as possible. Appropriate treatment and proactive monitoring can help preserve function and quality of life with either condition.

Knowing the right diagnosis also helps individuals and families understand their long-term outlook and plan for the future. Understanding the inheritance pattern of a specific disease type also informs family planning and genetic testing decisions.

During the testing process, individuals may want to ask for a referral to a neuromuscular disease specialist who has the expertise to accurately distinguish SMA vs. MD and guide an appropriate care plan.

Questions to ask your healthcare team

Patients and families who are undergoing testing for a neuromuscular condition may have questions about the differences between SMA and MD. Depending on the situation, they may want to ask:

• how doctors intend to narrow in on a diagnosis and exclude other possibilities
• how a specific diagnosis affects treatment options and long-term outlook
• if there are specific warning signs or symptoms tthey should keep track of to help with the diagnosis
• for a referral to a genetic counselor for support throughout the process

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