Understanding Spinal Muscular Atrophy (SMA)

What is SMA?

Spinal Muscular Atrophy, or SMA, is a genetic disorder that impacts the motor neurons in the spinal cord, which are crucial for muscle movement. The condition leads to the degeneration of these neurons, resulting in muscle weakness and paralysis.

Causes of SMA

SMA is an inherited condition that occurs in approximately one in every 6,000 to 10,000 live births and primarily presents during childhood. It ranks among the top three genetic causes of child mortality in North America, alongside Cystic Fibrosis and Tay-Sachs disease. The disorder arises from a mutation in the survival motor neuron 1 (SMN1) gene, which is essential for producing a protein that protects motor neurons. A mutation in this gene results in insufficient protein production, leading to motor neuron death. Approximately one in 50 individuals carry the SMA gene, and if both parents possess the gene, there is a 25% chance their child may develop the disorder.

Types of SMA

SMA is categorized into five types based on the age of onset and physical milestones achieved. This classification system is evolving as new insights into the condition and its treatments emerge.

Type 0: Prenatal Onset SMA

Type 0 SMA is an extremely rare form that begins in utero. Affected fetuses may exhibit reduced movement during late pregnancy. At birth, these infants typically display significant muscle weakness and respiratory difficulties.

Type 1: Werdnig-Hoffmann Disease

Type 1 SMA, also known as Werdnig-Hoffmann disease, accounts for about 80% of SMA cases. Symptoms usually appear before six months of age, with infants showing signs of muscle weakness, poor muscle tone, and challenges with breathing and swallowing. Without intervention, children with Type 1 SMA often do not survive past the age of two.

Type 2 SMA

Symptoms of Type 2 SMA generally manifest between 6 and 18 months. Children with this type can typically sit without support but experience muscle weakness in their legs. Breathing problems may develop over time, and most individuals with Type 2 SMA live into adolescence or adulthood.

Type 3: Kugelberg-Welander Disease

Type 3 SMA, referred to as Kugelberg-Welander disease, becomes apparent after 18 months of age. Affected individuals can walk as children but may struggle with strength-demanding activities such as running or climbing stairs. Some may eventually lose the ability to walk and require a wheelchair.

Type 4: Adult-Onset SMA

Type 4 SMA typically arises in adulthood, usually after the age of 20. The progression of symptoms is slower compared to other types, with individuals experiencing mild to moderate muscle weakness, primarily in the legs and hips. Most maintain their ability to walk throughout their lives and have an average life expectancy.

Symptoms of SMA

The symptoms of SMA vary based on the type and progression of the disease. Common symptoms include:

Muscle Weakness

Muscle weakness is the most significant symptom of SMA, affecting the legs, arms, chest, and face, which leads to difficulties in movement and physical activities.

Poor Muscle Tone

Individuals with SMA may experience low muscle tone, resulting in weak or loose limbs.

Breathing Problems

Weakness in respiratory muscles can cause breathing difficulties and increased vulnerability to lung infections.

Feeding and Swallowing Difficulties

Muscle weakness may complicate chewing and swallowing, leading to nutritional challenges, choking risks, and weight loss.

Unmet Motor Milestones

Infants and children with SMA might struggle to achieve essential motor milestones such as sitting, crawling, and walking.

Treatment Options for SMA

While there is currently no cure for SMA, recent advancements have significantly enhanced the prognosis for affected individuals.

Medications

Several medications have been developed to treat SMA. Early screening of newborns is crucial, as treatment is most effective when initiated before symptom onset. However, some medications approved for use in both children and adults may not be covered by insurance, which can limit access to treatment. The cost of these medications can reach hundreds of thousands of dollars annually in the United States.

Approved Treatments

– **Nusinersen (Spinraza)**: A gene therapy that promotes SMN protein production, helping to partially compensate for the defective SMN1 gene. It is administered through spinal injections and is approved for all age groups.
– **Onasemnogene abeparvovec (Zolgensma)**: This gene therapy provides a functional copy of the SMN1 gene to motor neurons via a single intravenous infusion, specifically for children under two years old.
– **Risdiplam (Evrysdi)**: An oral medication that functions similarly to Spinraza, increasing SMN protein production. It is suitable for patients of all ages and can be taken at home.

Supportive Therapies

Supportive therapies can help alleviate SMA symptoms and enhance the quality of life. These include:

– **Physical Therapy**: Maintains muscle strength and flexibility while preventing contractures and improving mobility.
– **Occupational Therapy**: Aids in daily activities and adapts home environments to meet individual needs.
– **Respiratory Care**: Utilizes techniques and devices to support breathing, such as cough assist machines and BiPAP machines, along with regular lung function monitoring.
– **Nutritional Support**: Ensures adequate nutrition, potentially involving dietary adjustments and feeding tubes.
– **Assistive Devices**: Various tools, including wheelchairs, braces, and standing frames, assist individuals with SMA in maintaining independence. Adaptive technology can also facilitate communication and engagement with the world.

Living with SMA

Living with SMA can present various challenges; however, with the right support and resources, individuals can lead fulfilling lives. Advocates are working to improve access to treatment and supportive care.

Acknowledgments

Special thanks to Karli Drew for her valuable contributions and edits.

References

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Emmady, P. D., & Bodle, J. (2023). Werdnig-Hoffmann Disease. In StatPearls. StatPearls Publishing. Accessed June 17, 2024 from https://pubmed.ncbi.nlm.nih.gov/32644359/#:~:text=Werdnig%2DHoffmann%20disease%2C%20also%20known,and%20lack%20of%20motor%20development.

Erdos, J., & Wild, C. (2022). Mid- and long-term (at least 12 months) follow-up of patients with spinal muscular atrophy (SMA) treated with nusinersen, onasemnogene abeparvovec, risdiplam or combination therapies: A systematic review of real-world study data. European journal of paediatric neurology : EJPN : official journal of the European Paediatric Neurology Society, 39, 1–10. https://doi.org/10.1016/j.ejpn.2022.04.006

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