- What Is SMA?
- 4 Types
- Diagnostic Tests
Spinal muscular atrophy (SMA) can be identified between weeks 10 and 14 of pregnancy. If both parents are carriers of the SMN1 gene, a chorionic villus sampling test can be done during the 10th and 14th weeks of pregnancy.
Using an ultrasound, the physician will remove a tiny piece of tissue from the placenta. These small tissues may have chorionic villi (tiny little structures), which include fetal DNA. This DNA will be tested for the SMN1 gene.
To identify if one or both parents are carriers, they should undergo blood tests. If one parent is positive, the child’s chances of acquiring the gene are very low. However, if both parents are carriers of the gene, they should meet a genetic counselor to discuss these issues.
What is spinal muscular atrophy?
Spinal muscular atrophy (SMA) is a genetic condition that results in weakness and the wasting of muscles in infants. It causes atrophy (the muscles get smaller).
In SMA, the nerve cells in the brain and spinal cord break down. As a result, the brain cannot send a signal to the muscles to make them move, leading to muscular atrophy.
SMA can affect the child’s ability to:
- Sit up
- Control head movements
Severe SMA can injure the involuntary muscles that enable breathing and swallowing functions.
In most cases, SMA is caused by inadequate production of a protein called survival motor neuron (SMN). Rarely, SMA is caused by mutations in other genes apart from SMN1.
4 types of spinal muscular atrophy
- Type I: The most severe form of spinal muscular atrophy (SMA) and is also called Werdnig-Hoffmann disease. Affected individuals have severe muscle weakness and poor muscle tone. Children with type I SMA have developmental delays.
- Other signs include:
- Breathing problems
- Difficulty swallowing
- Poor growth
- Joint abnormalities
- Other signs include:
- Type II: Characterized by muscle weakness, which typically appears in children aged between six months and two years. They can maintain a seated position without support but are unable to walk.
- Type III: Often diagnosed between 18 months and 3 years. They may not develop muscle weakness until adolescence. Typically, children can walk and stand independently but face a rapid limitation in mobility with increasing age.
- Type IV: Characterized by mild to moderate symptoms that usually develop after 30 years.
- Symptoms include:
- Gradual muscle weakness
- Tremor (involuntary, rhythmic muscle contractions)
- Mild breathing problems
- Symptoms include:
What are some of the tests to diagnose spinal muscular atrophy before pregnancy?
Couples planning for pregnancy and belonging to a high-risk group should consider visiting a genetic counselor and undergoing tests to identify if they are carriers of SMN1 genes.
Consult a physician if you are planning a pregnancy and you have the following:
- You had a previous child with spinal muscular atrophy (SMA)
- You have a history of SMA in your family
- Your partner has a history of SMA in their family
Your physician may ask you to undergo genetic counseling to understand the risk of your child having SMA.
Some of the options which the genetic counselor might suggest include:
- Getting pregnant and waiting to see if your child is born with SMA
- Using donated sperm or eggs to get pregnant
- Pre-implantation genetic diagnosis (eggs are fertilized in the laboratory and the resultant embryo is tested for SMA before implantation in the uterus)
What are the tests to diagnose spinal muscular atrophy during pregnancy?
The physician might suggest the following test for spinal muscular atrophy (SMA):
- Chorionic villus sampling: During 11 to 14 weeks of pregnancy, a sample of cells from the placenta is tested. The doctor uses an ultrasound to remove a tiny piece of tissue from the placenta, consisting of tiny structures called chorionic villi with fetal DNA. This DNA will be tested for the presence of SMN1 genes.
- Amniocentesis: During 15 to 20 weeks of pregnancy, a part of the amniotic fluid is tested. With the help of an ultrasound, the physician will insert a tiny needle to remove a small amount of amniotic fluid that has fetal DNA. The fluid is then tested for SMA.
If these tests show a positive result, talk to your physician about your options.
Note: These tests can slightly increase the risk of miscarriage.
Which tests identify spinal muscular atrophy after a child’s birth?
If you or your child exhibit typical spinal muscular atrophy (SMA) symptoms, a genetic blood test can confirm the condition. In genetic testing, a blood test is carried out to check for the presence of the SMN1 gene.
The physician may use the following to diagnose SMA:
- Physical examination: To look for signs of SMA.
- Electromyography: Involves the insertion of thin needles into a muscle to detect its functioning.
- Muscle biopsy: The physician removes muscle cells through a needle or small cut in the skin.
- Imaging tests: CT or MRI helps visualize the organs and structures inside the body.
- Creatinine kinase test: Helps distinguish other types of neuromuscular disease.
How is spinal muscular atrophy treated?
No treatment cures spinal muscular atrophy (SMA) completely. Many therapies target SMN1 genes, such as:
- Zolgensma (gene replacement therapy): Replaces the defective or missing SMN1 gene and prevents the degeneration of motor neurons and muscles.
- Nusinersen and risdiplam: Increase the survival motor neuron (SMN) protein that the body fails to produce due to the missing SMN1 gene. These drugs create a backup SMN2 gene that looks like the SMN1 gene and produces the needed SMN protein.
These therapies are currently used to treat SMA.
Prenatal Screening for Spinal Muscular Atrophy. https://www.webmd.com/brain/spinal-muscular-atrophy-prenatal-screening
Spinal Muscular Atrophy (SMA). https://kidshealth.org/en/parents/sma.html
Spinal Muscular Atrophy. https://www.babysfirsttest.org/newborn-screening/conditions/spinal-muscular-atrophy
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