Spinal muscle atrophy is a group of genetic disorders in which, due to a loss of nerve cells in the spinal cord and brain stem, a person can not regulate the movement of his muscles. It is a neurological disorder, and a form of neuronal motor disease.
Spinal muscular atrophy ( SMA) causes fatigue and muscle wasting. An individual with SMA may have trouble standing, walking, managing their head movements and even breathing and swallowing, in some cases. Some forms of SMA occur from birth, while other types appear later in life. Some forms have an effect on life expectancy.
According to Genetics Home Comparison, SMA affects one in every 8,000–10,000 people globally.
There is no cure for SMA but new drugs like nusinersen (Spinraza) and onasemnogene abeparvovec-xioi (Zolgensma) can delay its development.
There are various forms of SMA. They differ in terms of when symptoms begin to show up and how they affect life expectancy and quality of life.
The National Institute of Stroke and Neurological Disorders outlines four types:
SMA type I
SMA type 1, or Werdnig-Hoffmann disease, is a severe illness that typically develops before age 6. A child can be born with breathing difficulties, which can be fatal without treatment within one year.
Symptoms include muscle fatigue and twitching, inadequate ability to move the arms, trouble swallowing, and, with time, spine curvature. Some children with type 1 SMA will now learn to sit or walk with newer interventions, such as disease modifying therapies (DMTs).
A doctor can diagnose type 1 SMA prior to birth as tests can show low fetal movement levels during the final months of pregnancy. If not, within the first few months of existence it will become clear.
SMA type 2
Symptoms of type 2 SMA typically turn up at 6–18 months of age. The infant can learn to sit down, but can never stand or walk. In certain cases , the person can lose their ability to sit, without treatment.
Life expectancy depends on whether there are issues with the breathing. Most people with type 2 SMA live until puberty or puberty. DMT therapy can improve.
SMA type 3
SMA type 3, or Kugelberg-Welander disease, shows up after 18 months of age. Individuals may have scoliosis or contractures, a shortening of muscles or tendons, which may hinder free movement of joints.
Most people may still be able to walk, but they may experience an odd gait and difficulties running, ascending stairs or rising from a chair. The fingers may also get a slight tremor. Complications include an increased risk of airborne infections.
With appropriate treatment, including with DMTs, a person can have a normal life expectancy.
SMA for adults, or type 4 SMA, is rare. It starts out after the age of 21. The person will have mild to moderate proximal weakness, which means that the condition affects the muscles closest to the center of the body.
Usually it won’t impact life expectancy.
SMA’s symptoms depend on its type and severity, and the age it occurs at.
Common symptoms include:
- muscle weakness and twitching
- difficulty breathing and swallowing
- changes in the shape of the limbs, spine, and chest due to muscle weakness
- difficulty standing, walking, and possibly sitting
Infants with type 1 SMA have poor muscles at birth, minimal muscle tone, and difficulty feeding and breathing. Symptoms may not show up with SMA type 3 until the second year of existence.
SMA’s main characteristics in all its forms are muscle fatigue and muscle wasting. This occur because motor neurons, the nerves that regulate movement, are unable to give the muscles the signal to contract. The weakness tends to affect the muscles that are closer to the center of the body.
Motor neurons usually transmit such signals along the axon of the motor neuron from the spinal cord to the muscles. In SMA, either the neuron of the motor or the axon itself does not operate or may stop working.
SMA is a degenerative condition and over time, the symptoms begin to get worse.
Many of the modifications are not simply SMA symptoms but complications of the resulting weakness in the muscles.
Partially, counseling will alleviate these problems.
SMA occurs when motor neurons in the spinal cord and brain stem either malfunction or stop functioning as a result of gene changes known as survival motor neurons 1 (SMN1) and SMN2.
The nerve cells that control movement are the motor neurons.
The genes SMN1 and SMN2 provide instructions for making the protein required to act in motor neurons.
A SMN1 problem will result in SMA, while an SMN2 problem will affect the form and severity of SMA.
One in 40–60 adults has a genetic disorder which can lead to SMA.
A person may have SMA only if both of their parents have an issue with this gene. But even if both parents have this flaw, there’s only one chance in four that the child will inherit it.
Typically diagnosis begins when parents or carers detect signs of SMA in a child.
A doctor may conduct a comprehensive medical history, family history and physical examination. They can see whether the muscles are relaxed or flaccid, looking for deep reflexes of tendons and any twitching of the muscle of the tongue.
Tests for diagnosing SMA may include:
- blood tests
- a muscle biopsy
- genetic tests amniocentesis or chorionic villus sampling during gestation
- electromyography (EMG)
EMG may measure the health of the muscles and nerve cells that power them, or the motor neurons. Amniocentesis or the sampling of chorionic villus can allow the physician to evaluate the womb fetus.
Some states recommend routine genetic screening for SMA at birth. Detection at this stage of the condition can allow treatment before symptoms occur.
Currently there is no remedy for SMA and no way to avoid it, since it is an inherited disease. Treatment, however, will help a person live a full life.
DMTs can help to minimize symptom severity and to improve outlook for people with some forms of SMA.
Two drugs for SMA have been approved by the Food and Drug Administration ( FDA), and more are in the pipeline.
This drug is capable of treating any form of SMA. A doctor injects four initial doses over approximately 2 months , followed by a maintenance dose every four months.
It belongs to a class of drugs called antisense oligonucleotides (ASOs), which aim by manipulating RNA production to address the underlying problem.
Spinraza tends to delay SMA progression and may minimize muscle fatigue but its efficacy can vary from person to person.
This drug is a form of gene therapy used to treat babies below 2 years of age. A doctor will prescribe it in a single dose as an infusion.
Both medications may have adverse effects, and a boxed warning is provided by Zolgensma.
Researchers are focusing on other therapeutic methods, such as gene transfer therapy and the use of stem cells to replace damaged motor neurons.
Assistive devices and therapy
With SMA, various types of assistive devices can increase the life expectancy and quality of life of an individual.
Assistive technology — including ventilators, powered wheelchairs, and updated computer access — enables people with SMA to live longer, become more involved, and engage more in the community.
Physical exercise is a part of the procedure as well. Water therapy and wheeelchair sports are alternatives.
SMA is a genetic disorder that depends on the form, may affect children or adults.
The outlook for a person will depend on the severity of the symptoms. Children with extreme SMA can have respiratory illness because the muscles that sustain breathing are weak. In certain cases, the complication may be fatal.
Many individuals with milder types of SMA may expect to live as long as an individual without SMA, but some will require additional medical care.
SMA can not be avoided, but medication, physical therapy, and other interventions can help a person live an healthy and productive life.
As researchers learn more about SMA, new and creative treatment approaches show promise for potential care, prevention or even cure of the disease.
People with a SMA family background who intend to start a family may want first to pursue genetic counseling.