Complications of Spinal Muscular Atrophy

Muscle weakness is progressive in the major types of spinal muscular atrophy (SMA). This means that over time, muscles become weaker. As important muscles weaken, complications can occur. For example, progressive chest muscle weakness can lead to respiratory failure. Progressive mouth and throat muscle weakness can lead to problems swallowing. Progressive leg and arm weakness can cause a decline in motor function.

In the more severe types of SMA, this progression is faster. People with more severe types of SMA are more likely to experience severe complications. However, people with the same type of SMA can have different experiences. Especially with new treatments, it is impossible to predict which complications someone may experience and their severity.

Scoliosis and other spine deformities

Scoliosis is a curvature of the spine caused by weakness in the back muscles. It usually refers to a sideways curve that forms an “S” shape. Other types of spine deformities are also possible, such as an inward bending (lordosis) and an outward bending (kyphosis). People with type 1 or 2 SMA have a high chance of developing scoliosis.^1-3

Signs of scoliosis include uneven shoulders, shoulder blades, or hips. Doctors usually diagnose scoliosis using visual exam and X-rays of the spine. Scoliosis reduces mobility and causes joints to stiffen or displace. Much of physical therapy for people with SMA is focused on preventing or delaying scoliosis.^3,4

Fractures

People with SMA often have poor bone health. Muscle weakness, immobility, and poor nutrition can lead to low bone density. Low bone density makes the bones of some people with SMA easily fracture during everyday activities. These fractures are most common in the thigh bone (femur). Nutritional support focuses on improving bone health. Physical therapy can also help by building muscle and helping those with SMA stay mobile.^5-8

Respiratory problems

People with SMA have progressive weakness in the muscles supporting the chest wall (intercostal muscles). This often leaves the diaphragm as the only breathing muscle, leading to breathing difficulties. This can cause oxygen deficiencies (hypoventilation), especially during sleep. Fatigue, daytime sleepiness, and headaches are common symptoms of nighttime breathing difficulties. It can also cause people with SMA to have a weak cough and be more vulnerable to infections.^9,10

Respiratory failure is the most common cause of death for children with type 1 or type 2 SMA. Combined with scoliosis, respiratory muscle weakness can also cause restrictive lung disease. This happens when the lung cannot expand to hold enough air. Most children with severe types of SMA require breathing support, such as airway clearance and non-invasive ventilation techniques.^1,2

Joint contractures

Muscle weakness in SMA can lead to joint contractures, especially in people with type 2 SMA. This happens when flexible muscles are replaced by stiffer tissues. Joint contractures cause a significant decrease in range of motion. Joints most affected in SMA include shoulders, knees, wrists, elbows, hips, and ankles. Contractures can also be very painful. Physical therapy and supportive braces to prevent contractures are priorities in the management of SMA.^11-14

Malnutrition and aspiration pneumonia

Weakness in the tongue, mouth, and throat muscles often leads to swallowing problems (dysphagia). Difficulty eating contributes to a risk of malnutrition for people with SMA. People with SMA typically have lower than normal body weight. Difficulty eating by mouth can require nutritional support in the form of personalized diets, supplements, or feeding tubes.^15,16

Swallowing problems can also cause aspiration. This is the inhalation of material into the lungs. Children with SMA also often develop acid reflux, which happens when material in the stomach travels into the throat and mouth. Children with swallowing difficulties may inhale this stomach material into the lungs. Aspiration of food or stomach contents into the lungs can lead to a lung infection. This is called aspiration pneumonia.¹⁰

Pain

People with SMA usually do not experience severe pain. However, children with SMA experience more pain than the general population. People with SMA feel pain most commonly in the neck, back, and legs. Joint contractures and other bone deformities often cause this pain.^17,18

Other possible complications

Children with type 1 or 2 SMA have a higher risk for urinary incontinence. This is involuntary urination, either during the day or at night. We do not yet understand why children with SMA have the risk of involuntary urination, but it may involve pelvic muscle weakness.¹⁹

One complication associated with a rare type of SMA is progressive myoclonic epilepsy (PME). This only happens in a very rare form of SMA. This form, called SMA-PME, is caused by mutations in the ASAH1 gene, not the SMN1 gene. People with SMA-PME show typical symptoms and complications of progressive muscle weakness. They also experience myoclonic seizures, usually starting between 3 and 12 years old. These are brief muscle jerks that do not cause a loss of consciousness.²⁰