Spinal muscular atrophy (SMA) is a genetic disorder characterized by progressive muscle wasting and weakness due to the loss of motor neurons in the spinal cord. This affects voluntary muscles used for activities such as crawling, walking, sitting up, and controlling head movement. Over time, individuals with SMA find it increasingly difficult to carry out these basic motor functions, affecting their daily lives.
The progressive nature of SMA means the condition worsens over time, leading to increased physical limitations and making everyday tasks more arduous. Individuals with SMA often require various forms of support and assistance to manage their condition, including physical therapy, mobility aids, and adaptive devices to maintain their independence and quality of life.
Managing SMA involves a multidisciplinary approach with healthcare professionals from various fields. Regular monitoring and assessments help tailor treatments and interventions to meet each person’s specific needs, ensuring they receive the best possible care. Addressing the emotional well-being of those affected by SMA through counseling, support groups, and mental health services is essential.
Advances in medical research have led to new treatments and therapies that offer hope for individuals with SMA. Although there is no cure, these treatments represent a significant step forward in managing the condition.
Here are five things to know about SMA:
1. SMA is classified into four main types, each varying in age of onset, severity, and approach to treatment.
SMA type 1 (Werdnig-Hoffmann disease): This is the most severe form, manifesting in infants within the first 6 months of life. It is characterized by significant muscle weakness and hypotonia, with infants having difficulty with basic motor functions such as sucking, swallowing, and breathing. Without intervention, they may not achieve the ability to sit independently. Respiratory complications are common, and life expectancy is often reduced, not extending beyond early childhood.
SMA type 2 (Dubowitz disease): Presenting between age 6 and 18 months, children with this type typically can sit without support but may never stand or walk unaided. The progression of muscle weakness is slower than in type 1, but it still leads to significant physical limitations. Respiratory and skeletal complications can arise, necessitating ongoing medical care. Despite these challenges, individuals often have a normal or near-normal life expectancy with appropriate interventions.
SMA type 3 (Kugelberg-Welander disease): This type appears after age 18 months, ranging from early childhood to early adulthood. Individuals with type 3 experience milder symptoms and can walk and stand independently for some time. The progression of muscle weakness is slower, and respiratory issues are less common. Many people lead relatively normal lives with proper management, although they may require mobility aids as they age.
SMA type 4: This adult-onset form appears in the second or third decade of life, characterized by mild symptoms and slow progression. Individuals typically experience muscle weakness primarily in the proximal muscles. Although physical limitations can occur, the effect on daily life is generally less severe and most individuals maintain a good quality of life with minimal intervention.
2. SMA is characterized by a range of signs and symptoms that primarily affect the motor function of individuals.
Although the severity and progression can vary, the hallmark feature of SMA is muscle weakness and atrophy, significantly affecting daily activities and overall quality of life. One of the earliest signs is hypotonia observed in infants and young children. This manifests as floppy limbs, poor head control, and a lack of resistance when the muscles are manipulated. Infants with SMA may struggle to support their own weight, hold their head up, or maintain an upright posture. As they grow, these children often find it challenging to sit, crawl, and walk.
Muscle weakness in SMA typically follows a proximal-distal pattern, affecting muscles in the shoulders, hips, and back more so than those more distant. This leads to difficulties with tasks such as lifting objects, climbing stairs, and raising the arms. Over time, weakness may progress to the distal muscles, including those in the hands and feet, limiting fine motor skills and dexterity.
Respiratory issues are common, particularly in the more severe types of SMA. The muscles involved in breathing, such as the diaphragm and intercostal muscles, can weaken, leading to compromised respiratory function. This can result in frequent respiratory infections, difficulty coughing, and inadequate ventilation, especially during sleep.
SMA can also affect swallowing and speech muscles. Dysphagia is a significant concern, leading to feeding difficulties, poor nutrition, and an increased risk for aspiration pneumonia. Speech problems may arise owing to weakness in the face, mouth, and throat muscles, making articulation challenging.
Skeletal deformities, such as scoliosis, are common as a result of muscle strength imbalances around the spine. This can worsen over time, leading to discomfort, pain, and further respiratory compromise. Contractures, or the permanent tightening of muscles and tendons, can also develop, limiting joint mobility and causing discomfort.
3. Diagnosing SMA and assessing motor function involve a comprehensive approach to accurately identify the condition and its effects on the individual.
Diagnosing SMA begins with a detailed physical examination by a neurologist or specialist, assessing muscle strength, bulk, tone, and reflexes. Observations of muscle weakness, especially in proximal muscles, and signs of hypotonia provide important clues.
A thorough neurologic assessment evaluates motor skills, coordination, and sensory function. Diminished or absent deep tendon reflexes are key indicators of SMA and help distinguish it from other neuromuscular conditions.
Genetic testing is the definitive diagnostic tool for SMA. A blood sample is analyzed to identify mutations or deletions in the survival motor neuron 1 (SMN1) gene, responsible for producing the SMN protein. This confirms the diagnosis and determines the specific type of SMA by evaluating the number of SMN2 gene copies, which can influence disease severity.
Emerging biomarkers aid in diagnosing and monitoring SMA. These biological molecules reflect motor neuron damage and disease progression, complementing genetic testing and clinical assessments.
Standardized motor function scales, such as the Hammersmith Functional Motor Scale and the Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders, quantify motor impairment and track disease progression. These scales measure muscle strength, coordination, and functional abilities, helping monitor intervention effectiveness and adjust care plans.
4. SMA is a genetic disorder primarily caused by mutations in the SMN1 gene.
The SMN1 gene is responsible for producing the SMN protein, crucial for the maintenance and function of motor neurons in the spinal cord. Motor neurons are essential for muscle movement; their degeneration leads to muscle weakness and atrophy characteristic of SMA.
In a healthy individual, the SMN1 gene provides instructions for adequate production of the SMN protein. However, in individuals with SMA, mutations or deletions in the SMN1 gene result in insufficient protein production. This deficiency leads to the progressive loss of motor neurons, impairing muscle function.
Humans have a second gene, SMN2, almost identical to SMN1. Owing to a single nucleotide difference, SMN2 predominantly produces a truncated, less functional SMN protein. The number of SMN2 gene copies varies among individuals and influences the severity of SMA. More copies of SMN2 can partially compensate for the defective SMN1 gene by producing some functional SMN protein, leading to milder forms of SMA.
5. Treating SMA involves a combination of medical, surgical, and supportive therapies managed by a multidisciplinary care team.
Medical treatments include the use of antisense oligonucleotides such as nusinersen, which promote the production of full-length SMN protein necessary for motor neuron survival. Administered via intrathecal injection, nusinersen has shown significant improvements in motor function and reduced ventilator dependency. Recombinant gene therapy, such as onasemnogene abeparvovec, introduces a functional copy of the SMN1 gene into the patient’s cells through a one-time intravenous infusion, halting disease progression and enabling SMN protein production. Additionally, SMN2 mRNA splicing modifiers such as risdiplam increase the production of functional SMN protein, improving motor function and survival rates in patients of all ages.
Although surgical interventions are not primary treatments for SMA, they may be necessary to address complications such as scoliosis or other orthopedic issues. Physical therapy plays a crucial role in managing SMA by maintaining muscle strength, flexibility, and mobility through personalized exercise programs focusing on stretching, strengthening, and motor skills training.
A comprehensive care team is essential for effectively managing SMA. This team typically includes neurologists, who specialize in diagnosing and treating SMA, as well as monitoring disease progression. Pulmonologists focus on respiratory health, given the effects of SMA on breathing muscles. Speech and language pathologists assist with communication and swallowing difficulties, and orthopedists address skeletal issues such as scoliosis, joint problems, and joint deformities. Physical and occupational therapists help maintain physical function and independence in daily activities. Gastroenterologists manage nutritional and digestive issues, with the support of dietitians who provide dietary guidance to ensure proper nutrition. Social workers offer support to patients and families, helping them navigate healthcare systems and access resources. This comprehensive and multidisciplinary approach ensures that patients with SMA receive holistic care addressing their medical, physical, and emotional needs.
Editor’s Note: This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.
SMA ranges from mild with slow progression to severe with loss of life in childhood. These five things to know are crucial to understanding the types, causes, diagnosis, and latest treatments.
