It is a genetic disorder in which a person cannot control the movement of their muscles.
It is due to the loss of nerve cells in the spinal cord and brainstem.
Muscle wasting and weakness result in it being difficult to stand, walk, control head movements, and even, in some cases, breathe and swallow.
Spinal muscular atrophy (SMA) is not just one condition, but a variety of diseases. Together, the different types of SMA form the second leading cause of neuromuscular disease.
SMA is present in 1 in 6,000 to 10,000 live births. It is the second most common fatal autosomal recessive disorder after cystic fibrosis.
If SMA affects a baby younger than 2 years old, 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.
There is no cure for SMA, but in December 2016, the first drug to treat it was approved: Spinraza.
Important points about spinal muscular atrophy (SMA)
- There are different types of SMA, which vary in severity.
- It is a genetic condition.
- The main symptom is muscle weakness and muscle loss.
- In severe cases, breathing problems can occur.
- Treatments include assistive devices to help the patient breathe and a new drug, called Spinraza.
- SMA cannot be prevented, but prospective parents can request genetic testing if they can be carriers.
Types of spinal muscular atrophy
There are different types of SMA. They vary in terms of when they start to appear in an individual and the life expectancy that a person with the disorder will have.
SMA type I
Muscle atrophy in the spine
Spinal muscular atrophy affects physical function but not necessarily intellectual capacity.
Type I SMA is a serious condition. Children with this disorder are never able to sit or stand. This type can be fatal within the first 24 months of life.
It can be detected before birth, as there may be a reduction in fetal movement during the last months of pregnancy. If not, it will become apparent in the first months of life.
Babies with SMA type never sit up or stand up, and generally do not survive until the age of two. SMA type I is also known as Werdnig-Hoffmann disease.
SMA type II
Type II SMA usually appears between 3 and 15 months. The baby can learn to sit up, but will never be able to stand or walk.
Life expectancy depends on whether or not the patient develops respiratory problems. Most people with type II SMA survive to adulthood.
SMA type III
There may be a slight trembling of the fingers. Some people may lose the ability to walk and may also develop scoliosis. Complications 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 the ages of 20 and 40, but can appear later in life.
Women carry the gene, but only one child 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 the 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 droopy upper eyelids.
SMA in adults
Adult SMA, or IV SMA, begins after age 18. People with this condition can walk and have no trouble breathing or eating.
Researchers have also found links between SMA and amyotrophic lateral sclerosis (ALS), commonly called Lou Gehrig’s disease.
Symptoms of spinal muscular atrophy
The symptoms of SMA depend on its severity and the age of the person when it begins. With SMA type III, symptoms may not appear until the second year of life.
In all its forms, the main feature of SMA is muscle weakness, accompanied by muscle atrophy. This is the result of denervation, or the loss of the signal to contract, which is transmitted from the spinal cord.
This signal is normally transmitted from the motor neurons of the spinal cord to the muscle through the axon of the motor neuron. In SMA, the motor neuron with its axon, or the axon itself, stops working. It stops working.
Many of the symptoms of SMA are related to secondary complications of muscle weakness. These can be alleviated in part by therapy.
Causes of spinal muscular atrophy
SMA occurs when motor neurons in the spinal cord and brain stem do not work or stop working due to genetic changes. Motor neurons are the nerve cells that control movement.
Every human cell contains a part that receives instructions from genes, and when the instructions contain an error, this is called deletion. The part that receives the instructions is normally a protein.
In SMA, the instructions given to motor neurons, or nerves that control movement, contain a deletion that causes a protein deficiency. The gene responsible for instructing motor neurons is a survival motor neuron, usually SMN 1.
Other diseases, such as SMA, will only arise if there is an error in the genes inherited from the mother and father.
For a child to have SMA, both parents must contribute an SMN 1 with faulty instructions.
However, even if both parents have the faulty gene, the child will not always inherit it. Even among this population, the chance that the child will have SMA is only 1 in 4 per pregnancy. One in 40 adults carries the gene that causes SMA.
Diagnosis of spinal muscular atrophy
Diagnosis usually begins when parents or caregivers notice symptoms of SMA in a child.
A doctor will conduct a detailed medical history, family history, and physical exam. They will see if the muscles are loose or flabby, to check for deep tendon reflexes and muscle twitching of the tongue muscle.
Tests used to diagnose SMA include blood tests, muscle biopsy, genetic testing, 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 pregnancy.
Muscular dystrophy is another progressive condition that causes weakness. Know more.
Spinal muscular atrophy treatment
There is, as of yet, no cure for SMA, and there is no way to prevent it, as it is an inherited condition. However, treatment can help people live more fulfilling lives.
In December 2016, the United States (US) Food and Drug Administration (FDA) approved a drug, nusinersen (Spinraza) to treat SMA. It is the first drug approved for this condition.
It is given by injection, the first three doses at 14-day intervals, the fourth after 30 days, and then every 4 months.
Spinraza targets the underlying defect in SMA, so it can help delay, prevent, or even reverse symptoms.
Common side effects include an increased risk of respiratory tract infection and constipation. There may also be a risk of bleeding and kidney problems.
Assistive technology, such as fans, electric wheelchairs, and modified computer access, enables people with SMA to live longer, be more active, and participate in the community.
Ventilation is especially important. The severity of the individual’s weakness directly affects the course of the disease. Babies with severe SMA may experience respiratory illness because the muscles that support breathing are weak.
Children with milder forms of SMA can expect to have a longer life span, although they may need extensive medical support.
Molecular biology has improved our understanding of SMA. Many experimental treatments are being tested, including gene replacement, motor neuron stem cell replacement, and therapies to increase SMN 2 gene expression.
SMA is genetic, and there is no way to prevent it.
Parents with a family history of SMA are advised to seek genetic counseling before starting a family.