Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a rare genetic neuromuscular disease. The most obvious spinal muscular atrophy symptoms are muscle weakness and floppiness in the legs and arms; movement difficulties; muscle tremors and tongue fasciculation; bone and joint abnormalities such as scoliosis or abnormal sideways curvature of the spine; and breathing, chewing, and swallowing difficulties.1

SMA does not affect intellect or cause learning disabilities.

Muscle Weakness

Muscle weakness is the most prevalent SMA symptom. It is caused by muscles failing to receive electrical signals from motor neurons since the patient’s body lacks the SMN protein, which is essential for the survival of motor neurons.2 This is because there is a mutation in the SMN1 gene, which encodes for most of the functional SMN protein in the body. Without electrical signals coming from motor neurons, muscles atrophy over time.

Muscle weakness occurs equally on both sides of the body and affects proximal muscles more than distal ones.3 As a result, patients have difficulty holding their head up, sitting, standing, crawling, or walking, depending on the severity of their disease. Adults with the condition can also experience fatigue.4

Muscle Tremors and Tongue Fasciculations

Some patients with SMA have tongue fasciculations.5 Tongue fasciculations are pathognomonic for SMA because they are not seen in other neuromuscular conditions in infancy. Tremor or trembling of the fingers is present in almost all patients with SMA type 2 and can also be a useful diagnostic feature.3

Bone and Joint Abnormalities

Other typical SMA symptoms are bone and joint abnormalities caused by muscle weakness. 

Weakness of muscles on either side of the spine can mean that the spine is not supported properly. 

The main bone abnormality caused by this lack of support is scoliosis.6 This abnormal curvature can cause pain and compromise breathing ability.

About 1 in 3 patients have kyphosis, lordosis, and/or pelvic tilt coupled with scoliosis.7 Joint abnormalities include contracture, with the joints in the legs and arms becoming too stiff and limiting movement.8,9

Breathing Difficulties

Patients with SMA often have breathing difficulties due to the weakness of the intercostal muscles. Due to this weakness, patients increasingly have to rely on their diaphragm for breathing. Spine abnormalities can also contribute to breathing difficulties and patients may need ventilatory support to aid in breathing.

Muscle weakness can also cause patients not to be able to cough effectively, which makes them more prone to recurrent chest infections.10

In some types of SMA, such as SMA with respiratory distress type 1 (SMARD1), diaphragm paralysis occurs and patients need lifelong ventilation.11

Chewing and Swallowing Difficulties

SMA can also cause weakness in the muscles used for swallowing. This can lead to drooling and aspiration pneumonia, where gastric content enters the airways, causing infections.12,13 

Swallowing difficulties can mean that patients are at risk of being unable to get the nutrition they need. In more severe cases, they may need feeding tubes that directly connect to the stomach or intestine either via the nose or through the skin on the abdominal wall.14

Age of Symptom Onset in SMA

In patients with SMA type 0, the most severe form of the disease, SMA symptoms are apparent even before birth and characterized by reduced fetal movement. 

In those with SMA type 1, muscle weakness and floppiness become obvious before age 6 months. Babies are never able to sit unaided. The condition is sometimes referred to as floppy baby syndrome.

In patients with SMA type 2, symptoms usually appear between ages 6 months and 1 year. Affected children can sit independently but usually lose this ability by their mid-teens. These children are never able to walk on their own.

The symptoms of SMA type 3 become apparent when a child is aged 18 months to 3 years. Muscle weakness affects the legs more than the arms. Affected children can stand and walk independently but experience frequent falls. They usually find it difficult to go up and down stairs.

Symptoms of SMA type 4 usually do not appear until the third decade of life. Patients maintain the ability to walk until age 60. The respiratory muscles and those used for swallowing are usually not affected by the disease.

Reviewed by Michael Sapko, MD on 7/1/2021


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  14. Tube feeding and SMA: recommendations and practices. Cure SMA. June 16, 2018. Accessed June 2, 2021.