People with spinal muscular atrophy (SMA) may have a higher risk for developing certain other medical conditions and further complications. More severe types of SMA may lead to the development of more severe complications.
As new treatments for spinal muscular atrophy become available, the relationships between SMA and other conditions may change. It is unknown whether recently approved or future treatments for SMA may treat these comorbidities in addition to SMA, or perhaps prevent them from developing. More research and clinical trials are needed to understand the relationship between SMA and its common comorbidities.
Spinal muscular atrophy has its most profound effects on the muscles, and SMA symptoms like muscle weakness (hypotonia) can lead to serious complications in the musculoskeletal system.
Scoliosis is a complication of SMA so common that it is often considered a symptom. Scoliosis is most common in SMA type 1 (Werdnig-Hoffmann disease) and type 2 and also affects people with type 3 (Kugelberg-Welander disease). In people with SMA, the muscles are too weak to hold the spine in the correct alignment, and gravity slowly pulls the curve of the spine into a side-to-side conformation. About 90 percent of children with SMA develop scoliosis. In children with SMA, scoliosis develops on average between ages 6 and 8, but onset can be as early as 8 or 9 months of age. Age of onset is later in children who continue to be able to walk. Scoliosis may be treated with bracing, surgery, or an implanted vertical expandable prosthetic titanium rib (VEPTR) device. Scoliosis can cause or worsen respiratory dysfunction, affect balance while sitting, interfere with use of the arms, and change appearance.
Other disorders of spinal conformation can occur along with scoliosis. In kyphosis, the upper section of the spine curves back, forming a hunched shape. In lordosis, the spine curves forward or inward. The pelvis can also become tilted, a condition known as pelvic obliquity. About one-third of people with SMA have kyphosis, lordosis, or pelvic obliquity as well as scoliosis.
Pelvic obliquity can, in turn, lead to hip dislocation or subluxation (partial dislocation). Hip dislocation is less prevalent in people with SMA who continue to stand or walk. Some people with SMA who experience hip dislocation have hip surgery to reduce pain and pressure and improve balance.
If people with SMA lack the muscle tone to move their joints through the full range of motion, they may develop joint contractures – a situation where joints become too tight to move. Joint contracture can become permanent and limit movement. Splints, braces, and physical therapy or occupational therapy may help prevent joint contractures.
In some people with SMA type 2, problems with the jaw may develop. If range of motion is lost in the mandibular joints (hinges of the jaw), opening of the mouth may be limited.
In cases of SMA type 1 and SMA type 2, weak intercostal muscles (located between the ribs) and a relatively stronger diaphragm can lead to the development of a bell-shaped chest and pectus excavatum (sunken chest). Scoliosis can also contribute to distorting the shape of the chest. Changes to the shape of the chest compromise breathing and can lead to chronic respiratory failure, which can become life-threatening without treatment.
Recurrent respiratory infections are common complications of SMA due to compromised respiratory function and impaired coughing ability.
Apart from complications related to the muscles and skeleton, abnormalities of the cardiovascular system (heart and blood vessels) are some of the most frequently reported comorbidities in people with SMA. Some researchers suspect that the genetic mutation on the survival motor neuron (SMN1) gene that results in deficiency in SMN protein — the cause of SMA symptoms — may also lead to effects in the cardiovascular system. Cardiovascular disorders differ by spinal muscular atrophy type. People with type 1 are more likely to have structural abnormalities of the heart such as changes to heart valves or septa (walls dividing the heart chambers). Arrhythmias – disturbances to heart rhythm – are seen in SMA types 1, 2, and 3.
There are ways to manage most complications of spinal muscular atrophy. Treating complications and comorbidities can help people with SMA maintain function and quality of life. If there are signs that you or your child may be developing a complication of SMA, talk to your doctor as soon as possible.