Symptoms of spinal muscular atrophy

Written by Lindsey Shapiro, PhD | Fact-checked by Patricia Silva, PhD | Updated Dec. 18, 2024

Medically reviewed by Natalie Katz, MD

Spinal muscular atrophy (SMA) is a rare genetic condition resulting in symptoms that most commonly include progressive muscle weakness and poor muscle tone. While motor function is typically most affected, patients also can experience breathing problems, fatigue, scoliosis, and nutritional deficiencies.

The disease is marked by a progressive loss of specialized nerve cells, called motor neurons, that control voluntary movements and lead to its motor and nonmotor symptoms.

How SMA type affects symptoms

SMA symptoms, as well as their severity and age of onset, vary by SMA type. The five main types of SMA — 0, 1, 2, 3, and 4 — are classified based on the age of symptom onset and the highest motor milestone achieved.

The most severe form of the disease is SMA type 0, with symptoms that appear before birth and are severe from the moment the child is born. In the other SMA forms, symptoms usually emerge in infancy or childhood. Symptoms in SMA type 1 typically manifest in newborns within the first six months, and start between the ages of 6 and 18 months in SMA type 2, both more severe disease types. In SMA type 3, a less severe form of the disease, signs of the disease are usually seen between 18 months of age and late adolescence. The mildest form, SMA type 4, is associated with symptom onset in adulthood.

More severe disease types are associated with a shorter SMA life expectancy, while patients with milder disease types have a normal life expectancy.

Muscle weakness

When muscles don’t receive enough input from motor neurons, they are prone to weakness, atrophy (wasting), and poor muscle tone — a lack of tension in the muscles, which can cause floppiness — due to their inactivity.

These are the hallmark symptoms of SMA, which and lead to impaired motor development and/or future mobility problems.

Muscle weakness usually affects the proximal muscles, or those closer to the center of the body — such as those of the shoulders, thighs, and hips — more than distal muscles, which include those in the hands and feet. In SMA, both sides of the body are typically similarly affected.

Given that proximal muscles enable activities such as crawling, head control, sitting, standing, and walking, SMA patients may never acquire these motor milestones or may gradually lose these functions.

Loss of motor milestones or a failure/delay in meeting them are typically the first signs leading to a suspicion of SMA. In types 0 and 1, there also may be an absence or reduction of fetal movements during pregnancy.

Besides impaired mobility and muscle control, patients’ posture often worsens over time as muscles shrink, resulting in balance problems and possible curvature of the spine, called scoliosis, that can affect breathing.

Breathing difficulties

While patients with the milder forms of SMA may never experience breathing problems, those with types 0, 1, or 2 typically have breathing difficulties due to severe weakness of the muscles supporting the chest wall. These muscles, called the intercostal muscles, allow the lungs to fill with air by the expansion of the chest. This leaves the diaphragm, a thin muscle which sits at the base of the chest and separates it from the abdomen, as the main breathing muscle.

As a consequence, patients may experience breathing complications that include:

• lung underdevelopment
• reduced lung function
• difficulty coughing and clearing respiratory secretions.

These symptoms, in turn, can lead to a risk of additional complications, including:

• hypoventilation, or shallow breathing, characterized by a lack of oxygen and excess carbon dioxide buildup
• lung infections, such as aspiration pneumonia — which develops when food or liquid is breathed into the airways instead of being swallowed.
• prolonged recovery from infection due to weakness of the respiratory muscles and cough.

Breathing problems and hypoventilation are usually worse at night, when patients are lying down. In this position, any abdominal contents are pushed against the diaphragm and gravity is reduced, making it harder to take deep breaths. This may lead to sleep problems, such as obstructive sleep apnea (when a person repeatedly stops and starts breathing during sleep), and to daytime sleepiness and headaches, which can affect a person’s ability to function during the day.

Patients with severe breathing difficulties may require temporary or assisted breathing support, including the use of a BiPAP machine — a bilevel positive airway pressure machine.

In children with SMA types 0, 1, and 2, breathing difficulties can be especially severe, and are usually one of the main causes of death among these patients.

Fatigue

Fatigue is a common symptom of SMA, although reports about its prevalence vary between studies. Patients describe the symptom as an overwhelming sense of tiredness and exhaustion during repetitive daily activities, for example when lifting an armwhile eating or walking short distances.

This symptom can decrease motivation and affect a patient’s everyday activities, work, and social life.

“Fatigue feels like my body weighs twice as much as it used to,” said Brianna Albers, a columnist at SMA News Today, who was diagnosed with SMA type 2 at 9 months old. “I struggle with everything from swallowing to operating my wheelchair. Everyday activities become next to impossible.”

Fatigue from SMA can be physiological muscle fatigue — defined as muscle weakness and exhaustion during daily activities — as well as perceived fatigue. This is defined as a subjective impression that might be aggravated by the inability to perform everyday tasks. Perceived fatigue does not always correlate with muscle fatigue in SMA patients.

While it’s not entirely understood what causes muscle fatigue in SMA, previous studies suggest it may be related to the abnormal development and maturation of the neuromuscular junction, the site of communication between a nerve cell and a muscle cell.

Fatigue also may be intensified by daytime tiredness due to hypoventilation during sleep.

Scoliosis

People who develop symptoms of SMA at a young age frequently develop scoliosis, an abnormal and progressive sideways curvature of the spine — which forms an “S” shape when viewed from behind. It is caused by weakness in the back muscles that support the spine’s position.

Other abnormal curvatures also may occur, such as lordosis, an excessive inward bend of the spine, and kyphosis, an outward curve, sometimes called hunchback.

Patients with SMA type 1 or 2 are more likely to develop scoliosis. The condition is usually less severe and progresses more slowly in patients with milder SMA types, with fewer impacts on daily functioning.

Severe scoliosis can cause breathing problems, as the lungs have less room to expand, as well as problems at the joints placed under stress by the bending spine.

Other bone problems

SMA patients also may experience a number of other bone or joint problems including:

• hip dislocation
• joint contractures, which are marked by a limited range of motion due to the shortening of muscles or tendons around a joint
• weak and fracture-prone bones
• deformities of the hands, feet, chest, and joints, which are particularly common in more severe SMA types.

Nutrition

Issues with nutrition are common in SMA due to muscle weakness and impaired ability of the gut to move food along.

Patients with SMA types 0-2 sometimes have difficulties in sucking, swallowing, and chewing due to weakness in the muscles of tongue, jaw, throat, and neck. These problems can prevent patients from getting adequate nutrition.

A number of gastrointestinal problems also can lead to malnutrition. These include vomiting, abdominal swelling, diarrhea, and constipation, which are often due to impaired gut motility. Gastroesophageal reflux disease, known as GERD, also is common in patients with SMA types 1 and 2. This complication, marked by the flow of stomach acid back up the esophagus (which carries food from the mouth to the stomach), can increase the risk of aspiration pneumonia.

Poor nutrition can, in turn, further weaken the muscles, exacerbating muscle weakness and breathing issues. Patients, particularly those with more limited mobility, also may experience stunted growth and weight gain.

Other symptoms

SMA patients may experience a range of other symptoms, many of which are secondary to muscle problems in various parts of the body.

• Tremors — uncontrolled shaking — may occur, particularly in the hands and fingers.
• Reduced or absent deep tendon reflexes also may be a symptom for patients. An example of this type of reflex is the one that occurs when a person’s knee is tapped, causing an automatic knee jerk reaction.
• Involuntary tongue movements can be common; these are especially prevalent among infants with SMA type 1, the most common form of the disease.
• Speech problems may result from weakness in the muscles involved in speaking.

SMA also may cause heart problems, issues with the liver, and metabolic and pancreatic abnormalities.

SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.