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What is Spinal Muscular Atrophy (SMA)?

Spinal muscular atrophy (SMA) is a genetic disorder that affects the control of muscle movement. It is caused by a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord. It leads to weakness and wasting (atrophy) of muscles. In severe cases of spinal muscular atrophy, the muscles used for breathing and swallowing are affected. There are many types of spinal muscular atrophy distinguished by the pattern of features, the severity of muscle weakness, and age when the muscle problems begin. Spinal muscular atrophy affects one in 6,000 to one in 10,000 people.

How is Spinal Muscular Atrophy Diagnosed?

The first steps in the diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with some simple tests to distinguish spinal muscular atrophy (SMA) from similar conditions, such as muscular dystrophy. Genetic testing will be recommended if SMA is suspected because this is the least invasive and most accurate way to diagnose. In rare cases, doctors may order a muscle biopsy.

How is Spinal Muscular Atrophy Inherited?

Types I, II, III, and IV spinal muscular atrophy are inherited in an autosomal recessive pattern, which means the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition. SMA-LED and the adult-onset form of spinal muscular atrophy caused by gene mutations are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. X-linked spinal muscular atrophy is inherited in an X-linked pattern. The UBA1 gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected much more by X-linked disorders than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Types of Spinal Muscular Atrophy

Type I – Werdnig-Hoffman disease is a severe disorder that is evident at birth or within the first few months of life. Affected infants are developmentally delayed, unable to support their head, with breathing and swallowing problems that may lead to choking or gagging.


Type II – Characterized by muscle weakness that develops in children between ages 6 and 12 months. Children can sit without support but cannot stand or walk unaided.


Type III – Called Kugelberg-Welander disease, has milder features that typically develop between early childhood and adolescence. Individuals can stand and walk unaided, but walking and climbing stairs may become increasingly difficult, with many requiring a wheelchair later in life.


Type IV – Often occurs after age 30. Affected individuals usually experience mild to moderate muscle weakness, tremors, twitching, or mild breathing problems. Typically, only muscles close to the centre of the body, such as the upper arms and legs, are affected.


X-Linked – Appears in infancy and includes severe muscle weakness and difficulty breathing. Children of this type often have joint deformities that impair movement. In severe cases, affected infants are born with broken bones. Poor muscle tone before birth may contribute to these children’s contractures and broken bones.


SMA-LED – Characterized by leg muscle weakness that is most severe in the thigh muscles. This weakness begins in infancy or early childhood and progresses slowly. Affected individuals often have a waddling or unsteady walk and have difficulty rising from a seated position and climbing stairs.


Adult-Onset – Begins in early to mid-adulthood, characterized by muscle cramping of the limbs and abdomen, weakness in the leg muscles, involuntary muscle contractions, tremors, and a protrusion of the abdomen thought to be related to muscle weakness. Some affected individuals experience difficulty swallowing and problems with bladder and bowel function.

The prognosis for Spinal Muscular Atrophy

Recently, survival has increased in severe spinal muscular atrophy patients with aggressive and proactive supportive respiratory and nutritional support. With proper care, milder spinal muscular atrophy type I cases (which account for approx. 10% of all SMA1 cases) live into adulthood. In spinal muscular atrophy type II, the course of the disease is slower to progress, and life expectancy is less than the general population. Death before age 20 is frequent, although many people with spinal muscular atrophy live to become parents and grandparents. Spinal muscular atrophy type III has normal or near-normal life expectancy if standards of care are followed. Type IV, adult-onset spinal muscular atrophy usually means only mobility impairment and does not affect life expectancy.

Mobility for Those With Spinal Muscular Atrophy

Many with spinal muscular atrophy experience the progressive loss of the ability to walk and position themselves, necessitating the use of a specialized power wheelchair. Power chairs incorporate power-adjustable seating for user repositioning and comfort, speciality drive controls, including those requiring minimal hand strength, and a highly-adaptable design to meet an individual’s current and future needs.


Power Chairs feature the latest advanced technologies to increase the independence of those living with spinal muscular atrophy. iLevel seat elevation technology allows users to operate the power chair at a seated or standing height. Bluetooth is also integrated into Quantum’s Q-Logic 3 electronics, so those with spinal muscular atrophy can operate much of their environment with the power chair drive control itself. Power Chairs are designed to provide optimal medical comfort and maximum independence for those with spinal muscular atrophy.


The Quantum Edge 3 with industry-first 4.5 mph at iLevel, offers the most advanced power chair experience ever. Q6 Power Chair Series, which includes the Edge 3 and Q6 Edge 2.0 power chairs, provide highly adjustable mid-wheel drive power bases. The Q6 Edge 2.0 all accept our optional iLevel technology, which offers up to 12 inches of lift at 4.5 mph. The 4Front is a quiet, more responsive front-wheel drive power chair that features automotive-grade suspension with unprecedented comfort and rides quality.