Spinal muscular atrophy is a genetic disorder in which a person cannot control the movement of their muscles, due to a loss of nerve cells. Muscle wasting and weakness results.
The loss of muscle strength makes it difficult to stand, walk, control head movements and even, in some cases, to breathe and to swallow.
Spinal muscular atrophy (SMA) is not just one condition but a range of diseases. Grouped together, the different types of SMA form the second leading cause of neuromuscular disease.
If SMA affects an infant under the age of 2 years, it is usually fatal. If the patient is older when the disease appears, they will have a different type of SMA, and life expectancy may be normal.
SMA is the number one genetic cause of fatality in children under the age of 2 years, but it can affect people of any age.
Contents of this article:
Types of spinal muscular atrophy
People with some types of SMA are unable to walk.
There are different types of SMA. They vary in terms of when they start to appear in an individual, and in the life expectancy that a person with the disorder will have.
SMA type I
SMA type I is a serious condition. Children with this disorder never manage to sit or stand. It is usually fatal before the age of 2 years.
It can be detected before birth, as there may be a reduction in fetal movement during the final months of pregnancy. If not, it will become evident within the first few months of life.
Infants with SMA type I never sit or stand, and they do not usually survive to the age of two years. SMA type I is also known as Werdnig-Hoffmann disease.
SMA type II
SMA type II usually becomes apparent between the ages of 3 and 15 months. The infant may learn to sit, but will never be able to stand or walk.
Life expectancy depends on whether the patient develops primary pulmonary problems or not. Most people with SMA type II survive into adulthood.
SMA type III
SMA type III, or Kugelberg-Welander disease, appears between 2 and 17 years of age.
Symptoms include an unusual gait and difficulty running, climbing steps, or rising from a chair. There may be a slight tremor of the fingers. Some people may lose the ability to walk, and they may also develop scoliosis. Complications for these patients include obesity and osteoporosis.
Kennedy syndrome is also known as progressive spinobulbar muscular atrophy. Kennedy syndrome is a slowly progressive, inherited condition that usually appears between 20 and 40 years of age, but it can appear later.
Women carry the gene, but only a son will inherit the disorder.
Congenital SMA with arthrogryposis
Congenital SMA with arthrogryposis is a rare disorder. People with this condition will have a persistent contracture of joints, known as arthrogryposis.
The condition is evident at birth. Features include severe contractures, curvature of the spine, chest deformity, respiratory problems, an unusually small jaw, and drooping upper eyelids.
Adult SMA, or SMA IV, begins after the age of 18. People with this condition can walk, and they do not have problems with breathing and eating.
Symptoms of spinal muscular atrophy
Infants with SMA type I are born with very little muscle tone, weak muscles, and feeding and breathing problems. With SMA type III, symptoms may not appear until the second year of life.
In all of its forms, the primary feature of SMA is muscle weakness, accompanied by atrophy of muscle. This is the result of denervation, or loss of the signal to contract, that is transmitted from the spinal cord.
This signal is normally transmitted from motor neurons in the spinal cord to muscle via the motor neuron's axon. In SMA, either the motor neuron with its axon, or the axon itself, becomes non-functioning. It stops working.
The symptoms of SMA depend on its severity and the person's age when it starts.
Many of the symptoms of SMA relate to secondary complications of muscle weakness, and these can be at least partially remediated by therapy.
Causes of spinal muscular atrophy
SMA happens when motor neurons in the spinal cord and the brainstem either do not work or stop working, because of genetic changes. Motor neurons are the nerve cells that control movement,
In SMA, a genetic problem in the motor neurons leads to a weakening and wasting of muscles.
Every human cell contains a part that receives instructions from genes, and when the instructions contain a mistake, this is called a deletion. The part that receives the instructions is normally a protein.
In SMA, the instructions that are given to the motor neurons, or nerves that control motion, contain a deletion that causes a protein deficiency. The gene responsible for the instruction to motor neurons is a survival motor neuron, normally SMN 1.
Each person has 2 pairs of genes for each instruction given. One gene is inherited from the mother and one from the father. Some diseases will appear if only one of the inherited genes contains an instruction error. Other diseases, such as SMA, will only emerge if there is a mistake in both the mother's and the father's inherited genes.
For a child to have SMA, both parents must have contributed an SMN 1 with faulty instructions.
However, even if both parents have the faulty gene, this does not mean that the child will inherit it. Even among this population, the chance is only 1 in 4 per pregnancy that the child will have SMA. One in 40 adults are carriers of the gene that causes SMA.
Diagnosing spinal muscular atrophy
Diagnosis usually starts when parents or caregivers notice symptoms of SMA in a child.
A physician will carry out a detailed medical history, a family history, and a physical exam to see if the muscles are floppy or flaccid, to check for deep tendon reflexes and muscle fasciculation of the tongue muscle.
Tests used to diagnose SMA include blood tests, muscle biopsy, genetic tests, and potentially electromyography (EMG). EMG is used to assess the health of muscles and the nerve cells, or motor neurons, that control them. Amniocentesis or chorionic villus sampling can evaluate the fetus during gestation.
Treating and preventing spinal muscular atrophy
Assistive technology such as ventilators, power wheelchairs, and modified access to computers are enabling individuals with SMA to live longer and fuller lives, and to participate in the community.
Ventilation is especially important. The severity of the individual's weakness directly affects the course of the disease. Infants with severe SMA may experience respiratory disease, because the muscles that support breathing are weak.
Children with milder forms of SMA can expect to have a longer lifespan, although they may need extensive medical support.
Molecular biology has created a better understanding of SMA. Many experimental treatments are being tested, including gene replacement, stem-cell replacement of motor neurons, and therapies to increase the expression of the SMN 2 gene.
Also in the pipeline are drugs to improve SMN function, or to compensate for its loss.
SMA is genetic, and there is no way to prevent it.
Parents with a family history of SMA are encouraged to seek genetic counseling before starting a family.