Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder that causes muscle weakness. SMA symptoms vary greatly in their impact on physical function, quality of life, and life expectancy. Type of SMA and age of diagnosis are two of the most important variables that determine SMA symptom severity.

Depending on how severe the symptoms are, a person with SMA may need help with daily tasks like bathing, dressing, and eating.

Muscle Weakness and SMA

Muscle weakness, also referred to as hypotonia or lack of muscle tone, is the primary symptom of all types of SMA. Muscle weakness in SMA is caused by the loss of nerve cells that transmit signals from the brain and spinal cord to the muscles. A mutation (change) on the survival motor neuron 1 (SMN1) gene is responsible for SMA types 0 to 4, which account for nearly all cases of the condition. The SMN2 gene determines which type of SMA a person develops.

In most types of SMA, muscle weakness affects the proximal muscles (those closest to the center of the body) and lower extremities. Weakness is usually on both sides of the body. Muscle weakness can impact the development of motor skills, mobility functions like walking, and actions like swallowing and breathing. Muscle weakness generally worsens over time, meaning people may lose physical abilities as SMA progresses.

Muscle weakness is most profound in type 0 and type 1 (Werdnig-Hoffmann disease), the most severe forms of SMA. Babies with types 0 and 1 have no head control and do not reach developmental milestones. Severe muscle weakness impacts their breathing ability and their life expectancy.

In some types of SMA, muscle weakness comes with hyporeflexia, or severely reduced muscle reflexes. Babies and children with types 0 and 1 show symptoms of hyporeflexia. Lack of muscle reflexes is also common in type 2.

Mobility Limitations and SMA

Depending on the type and severity of SMA, a child or adult may have mobility limitations ranging from the inability to lift their head to slight difficulty walking.

Sucking, Chewing, and Swallowing

Problems with sucking and swallowing are caused by weak muscles in the throat and mouth. These issues are commonly seen in babies with type 1 SMA. Poor ability to suck and swallow can prevent infants from receiving adequate nutrition and can also cause problems with inhaling food or liquid into the lungs. Older babies and toddlers with SMA type 2 may struggle with chewing, in addition to sucking and swallowing. Infants and children with SMA may need a feeding tube to receive enough nutrition.

Sitting Up

Infants and children with SMA types 0 and 1 are never able to sit up unsupported. Babies and toddlers with SMA type 2 will develop the muscle strength to sit up without help. However, because of the degenerative nature of SMA, individuals with type 2 often lose the ability to sit up by themselves by their mid-teen years.

Standing

Children with SMA type 2 rarely develop the ability to stand, whereas those with type 3 (Kugelberg-Welander disease) will be able to stand. Children with type 3a, diagnosed before age 3, may take longer to learn how to stand on their own. In types 3a and 3b, standing ability may slowly decline over time. Sometimes, people with SMA type 3 will have problems moving from a sitting to a standing position.

Walking

Babies and children with types 0 to 2 will never develop the ability to walk. Depending on the age at diagnosis, young children with type 3a generally learn to walk but may start experiencing falls or difficulty with stairs around 2 to 3 years old. People with type 3b may also have problems with falling and stairs as SMA progresses and muscles weaken. They may develop foot deformities or abnormal walking patterns (gait) that can make walking more challenging. Adults with type 4 may also develop walking problems as they age and SMA progresses.

Among the rarest forms of SMA, SMA with lower extremity predominance (SMA-LED) also causes walking difficulties due to muscle weakness in the legs, especially the thighs.

Respiratory Function and SMA

Problems with respiratory function can occur because of weakness in the muscles that control breathing. Weakness in the respiratory muscles can be the most dangerous aspect of SMA. Complications related to breathing can be responsible for reduced life expectancy, especially in the more severe types of SMA.

Breathing

A person without SMA breathes using their intercostal muscles, the muscles that cause the chest to expand and contract when breathing, and the diaphragm, the muscle that moves the rib cage up and down. SMA can cause weakness in the intercostal muscles — mostly in types 0 to 2 and rarely in type 3. The diaphragm is forced to compensate for ineffective intercostal muscles. Weakness in the intercostal muscles and overcompensation from the diaphragm leads to abnormal breathing patterns and underdeveloped lungs.

Breathing problems can also be related to scoliosis (curvature of the spine), which is a very common complication of SMA types 1 to 3. The changed shape of the spine can reduce the capacity of the lungs. The resulting breathing problems are the leading cause of illness and reduced life span in babies and children with SMA. Nearly every baby or child with respiratory weakness will require respiratory care to support breathing.

Sleep problems are common in children with types 1 and 2 because of respiratory weakness. Weak respiratory muscles can cause breathing during sleep to be too slow or too shallow (hypoventilation). As a result, the person with SMA isn’t breathing in enough oxygen or breathing out enough carbon dioxide. A mechanical breathing machine like a bi-level positive airway pressure, or BiPAP, machine can be used to support breathing at night.

Headaches sometimes occur because of hypoventilation. A breathing machine can help ensure a person with SMA gets enough oxygen, which can prevent headaches.

Coughing

Weak respiratory muscles can make it difficult for people with SMA to effectively cough and clear secretions or mucus from the lungs. The inability to clear the lungs can make breathing more difficult and, in some cases, lead to complete or partial lung collapse. A weak cough also increases the risk of chest infections. A cough machine or manual cough assistance are options for clearing the lungs.

Infection

Respiratory weakness, aspiration of food or liquids, or an inability to effectively cough can lead to pneumonia and other chest infections. Chest infections like pneumonia are most common in SMA types 1 and 2 and SMA with respiratory distress. Respiratory infections can be managed with various methods that help to clear the lungs, including manual methods and mechanical cough machines.

Musculoskeletal Symptoms of SMA

Scoliosis is the curvature of the spine to the side. Instead of the spine being held by muscles in a normal curve, the spine creates an ‘S’ shape. Scoliosis impacts nearly all children with SMA types 1 and 2 and about half of people with type 3. The change in the shape of the spine occurs because of muscle weakness, and it can cause breathing problems and limited mobility. SMA-related scoliosis is often treated with surgery. People with SMA may also have kyphosis, a forward bend in the spine.

Joint contracture is a deformity caused by the shortening or tightening of muscles or tendons. Joint contractures in the lower limbs are common for people with SMA and occur more often than contractures in the upper limbs. The hips, knees, and ankles are common sites of contractures in the lower body, and the elbows, shoulders, and wrists are common sites in the upper body. Contractures can be treated with physical therapy or surgery, though surgery is more likely to be recommended for lower body contractures.

Hip dislocation and other problems with the hip joints are common in SMA types 1 to 3. Problems with the hip joints generally occur because of weakened muscles and lack of use. Surgery to correct hip problems is not usually recommended due to limited effectiveness and risks associated with operating.

Bone fractures occur frequently in SMA types 2 and 3. Fractures in the femur near the knee and the ankle are common in type 2, and fractures in the arms are common in type 3. In some cases, infants born with SMA may have congenital fractures (broken bones present at birth). Treatment for bone breaks will depend on a person’s level of mobility.

Other Symptoms of SMA

Tremors in the hands are often the first noticeable symptom of SMA type 4 and Kennedy disease (X-linked spinal and bulbar muscular atrophy). Tremors in the fingers can also occur in SMA type 2 and type 3a.

Tongue fasciculation, or tongue quivering, is common in severe types of SMA and can also occur in Kennedy disease. Tongue fasciculations occur because of the involvement of the bulbar motor neurons, which impact the muscles that control speech, swallowing, and chewing.

Mental Health and SMA

Depression and mood disorders are common for people with chronic health conditions, and they can affect people with SMA and their families. In a series of interviews with 19 young people with SMA, several reported experiencing depression or feelings of sadness related to loss of ability or concerns around the possibility of losing function. The researchers concluded that more effort should be made to integrate mental health care into overall health care for people with SMA. If you’re experiencing symptoms of depression, speak to your health care provider about treatment options, including therapy or medication.

Parents of babies and children with SMA can experience depression following diagnosis and as they face the challenges of caring for a child with a severe disability. Seeking support from a mental health professional can help caregivers manage their emotional well-being. In-person or online support groups like mySMAteam can also be beneficial.

Find Your Team

On mySMAteam, the social network for people with SMA and their family members, more than 2,600 members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

Have you experienced some of these symptoms of SMA? Do you have any tips for managing them? Share your experience in the comments below, or start a conversation by posting on your Activities page.