Spinal Muscular Atrophy (SMA) is a hereditary disease that causes muscles to progressively weaken and waste away by affecting the nerves and muscles. It primarily affects infants and children, but can also develop in adults. The symptoms and prognosis of SMA vary depending on the type.
What is SMA (Spinal Muscular Atrophy)?
Spinal Muscular Atrophy (SMA) is a genetic (hereditary) neuromuscular disease that causes muscle weakness. Individuals with SMA lose a specific type of nerve cell called motor neurons, which control muscle movement in the spinal cord. Without these motor neurons, muscles do not receive the nerve signals necessary for movement. Atrophy is a medical term that refers to the shrinking and weakening of muscles due to disuse.
What are the Types of SMA?
There are four main types of SMA:
• Type 1 (severe): Approximately 60% of individuals with SMA have type 1, also known as Werdnig-Hoffman disease. Symptoms appear at birth or within the first six months of life. Babies with type 1 SMA have difficulty with swallowing and sucking. They struggle to hold their heads upright and sit without support. As muscles continue to weaken, children become more susceptible to respiratory infections and the collapse of the lungs (pneumothorax).
• Type 2 (intermediate): Symptoms of type 2 SMA manifest when a child is between six months and 18 months old. This type tends to affect the lower extremities. Children with type 2 SMA can sit but cannot walk. Most individuals with type 2 SMA survive into adolescence or young adulthood.
• Type 3 (mild): Symptoms of type 3 SMA appear after the first 18 months of a child’s life. Some individuals with type 3 SMA may not show signs of the disease until early adulthood. Symptoms include mild muscle weakness, difficulty walking, and frequent respiratory infections. Over time, the symptoms can affect walking or standing ability. Type 3 SMA does not significantly shorten life expectancy.
• Type 4 (adult onset): The rare adult-onset form of SMA typically does not manifest until the mid-30s. Symptoms of muscle weakness progress slowly, so most individuals with type 4 remain mobile and lead fulfilling lives.
What Causes SMA?
Individuals with SMA either have a missing portion or a mutated form of the SMN1 gene. A healthy SMN1 gene produces the SMN protein, which is necessary for motor neurons to survive and function properly. People with SMA do not produce enough SMN protein, leading to the shrinking and death of motor neurons. As a result, the brain cannot control voluntary movements, particularly in the head, neck, arms, and legs. Additionally, individuals have SMN2 genes that produce a small amount of SMN protein. A person can have up to eight copies of the SMN2 gene. Having multiple copies of the SMN2 gene typically results in milder SMA symptoms because the extra genes produce the missing SMN1 protein. In rare cases, mutations in non-SMN genes (other than chromosome 5) can cause SMA.
What are the Symptoms of SMA?
The symptoms of SMA vary depending on the type. Generally, individuals with SMA experience progressive loss of muscle control, movement, and strength. Muscle loss worsens with age. The disease tends to severely affect muscles closest to the trunk and neck. Some individuals with SMA may never walk, sit, or stand, while others gradually lose the ability to perform these actions.
How is SMA Diagnosed?
Diagnosis typically begins when parents or caregivers notice SMA symptoms in a child. Your healthcare provider will conduct a physical examination and take a medical history to determine the cause of the symptoms. They may also request one or more of the following tests to diagnose SMA:
• Blood tests
• Muscle biopsy
• Genetic tests
• Electromyography (EMG)
How is SMA Treated?
Currently, there is no cure for Spinal Muscular Atrophy (SMA), but research is ongoing to find new treatments. Treatment and support are available to manage symptoms and help individuals with SMA achieve the best possible quality of life. The treatments aimed at reducing the symptoms of SMA depend on the type and symptoms. Many individuals with SMA benefit from physical and occupational therapy and may use assistive devices such as orthopedic braces, crutches, walkers, and wheelchairs.
SMA is a genetic neuromuscular disease that significantly affects quality of life and life expectancy. It is a progressive disease that worsens over time. New disease-modifying and gene-replacement therapies hold promise. It is possible to carry the gene that causes SMA without being aware of it. If there is a history of SMA in your family, discuss ways to reduce the chances of your future baby having SMA with your doctor.
