Spinal muscular atrophy (SMA) is a rare genetic neuromuscular disorder that causes low muscle tone (hypotonia) and progressive muscle weakness and wasting (atrophy). Depending on SMA type, severity, and age of onset, SMA can affect motor skills like walking, as well as eating and breathing.
SMA type 4, known as adult-onset SMA, is the mildest type of SMA. It accounts for fewer than 5 percent of SMA cases, according to the Muscular Dystrophy Association (MDA). SMA type 4 typically begins in adulthood and leads to mild motor impairment.
There are five main types of spinal muscular atrophy. In SMA types 0-2, symptoms typically appear in the womb or in infants and young children. People with SMA types 0-2 can have difficulty sucking, swallowing, and breathing, as well as major motor impairment. In utero, this can be noted by limited movement and increased amniotic fluid. Newborns with SMA might also have limb contractures at birth.
SMA type 3 can begin in childhood through early adulthood. People with SMA type 3 (Kugelberg-Welander disease) experience mobility impairment, but rarely have respiratory difficulties.
SMA type 4 generally occurs after age 35 — although symptoms can appear as early as the late teens, according to Cure SMA. Adult-onset SMA symptoms tend to be milder, and may include hand and finger tremors, achy or heavy muscles, numbness, and cramping. SMA type 4 rarely involves the swallowing and breathing dysfunction characteristic of SMA type 0 or 1.
While other types of SMA can severely limit mobility, people with SMA type 4 usually remain mobile into their 60s. In later years, some may need a wheelchair or motorized device to get around. Those with SMA type 4 tend to have a life expectancy similar to that of the general population.
Spinal muscular atrophy is caused by a genetic mutation on both copies of the survival motor neuron 1, or SMN1 gene. That mutation inhibits the production of SMN proteins, which maintain normal functioning of motor neurons. Motor neurons are nerve cells that enable messaging between the brain, spinal cord, and muscles. Without functioning motor neurons, the brain cannot send signals to the muscles, which leads to muscle wasting, or atrophy.
A second gene, called the SMN2 gene, also produces a small amount of SMN protein. The number of copies of SMN2 a person has — and the amount of SMN protein they produce — can influence the type and severity of SMA.
SMA type 4 typically starts in the leg and hip muscles, progressing steadily and slowly toward the shoulders and arms. As with all forms of SMA, weakness increases over time. Although the level of disability can vary, studies have shown that people with SMA type 4 generally do not develop severe disabilities.
Diagnosing SMA type 4 can take some time. That’s because symptoms are typically mild and may require additional tests to rule out other neuromuscular conditions.
A neurologist will take a medical history, conduct a physical exam, and order a DNA blood test to confirm SMA. Genetic testing looks for the absence of the SMN1 gene, or mutations on that gene. In approximately 95 percent of SMA cases, the SMN1 gene is missing entirely.
Other diagnostic tests may include:
Once a diagnosis of SMA type 4 has been confirmed, a neurologist may prescribe medication or physical therapy, or recommend nutritional or lifestyle changes. The goal of treatment is to keep a person healthy and mobile throughout their lives.
Spinraza (nusinersen) is approved by the U.S. Food and Drug Administration (FDA) for SMA in children and adults. Administered by injection into the fluid surrounding the spinal cord, Spinraza is believed to work by increasing the body’s production of SMN protein. Studies have shown the drug improves motor function in all types of SMA, including later-onset SMA. Spinraza has not been studied specifically in people with SMA type 4.
Clinical trials currently investigating new SMA treatments for adults include studies of the following drugs:
Physical therapy (also called physiotherapy or PT) is designed to help people manage movement dysfunction, improve functional ability, and prevent future disability. People with SMA type 4 are advised to get as much physical activity as is possible and comfortable, to maintain mobility and independence throughout their lives.
A physical therapist can design an exercise plan to help people with SMA type 4 manage symptoms, like muscle weakness and contractures, or learn practices for safer, more efficient walking.
Aquatic therapy, also known as hydrotherapy or water therapy, is among the therapies recommended by the International Standard of Care Committee for Spinal Muscular Atrophy to help maintain muscle strength and support stamina. It can be done in a bathtub, swimming pool, or hydrotherapy pool. Water buoyancy makes it possible to perform certain exercises that may not be possible on land.
Range of motion exercises and regular stretching can help increase flexibility and prevent painful contractures (the permanent shortening and tightening of joints). Flexibility and range of motion are important for everyone with SMA, regardless of mobility levels. If necessary, a physical therapist will recommend splints, braces, and standing devices.
Some people with SMA unconsciously adapt their gait to compensate for muscle weakness. This puts strain on other parts of the body, impacting balance and long-term walking ability. Although people with SMA type 4 rarely develop walking problems, gait training can help them maintain mobility longer and walk more safely.
Occupational therapy can help people with SMA 4 who’ve lost muscle strength to perform daily activities, such as dressing, using the computer, or completing home tasks. This might include learning alternative ways to put on clothing or learning to use assistive devices.
Although SMA type 4 does not normally cause breathing or swallowing problems, learning breathing exercises can boost general health and well-being. Respiratory function can also be improved by swimming or exercising in a hydrotherapy pool.
Reduced mobility and lack of exercise can cause weight gain in people with all types of SMA. Extra weight can put stress on muscles, bones, and joints, making it harder to exercise. Although SMA type 4 rarely causes difficulty with chewing and swallowing, a dietitian can customize a nutrition plan to help you maintain a healthy weight.
Individuals with SMA type 4 sometimes experience pain and fatigue when certain muscles are overused to compensate for weaker ones. A hot-water bottle or heating pad can help relax muscles, while a cold pack may be used for pain relief. Preventing your body from becoming overly tired can help manage fatigue. Meditation and mindfulness practices may also be helpful in managing pain and fatigue.
Most people with SMA type 4 remain mobile throughout their lives, but some people may eventually need assistive devices if walking becomes difficult. A wheelchair can improve quality of life by reducing fatigue and fear of falling.