Spinal Muscular Atrophy Carrier Screening
Spinal muscular atrophy (SMA) is a genetic condition that affects the strength and movement of your muscles. While some people with SMA don’t experience symptoms until adulthood, it usually affects children very early in life. Babies born with the most common type of SMA may be unable to sit or stand, and they often don’t live past 2 years old.
Even if you don’t have the condition yourself, you could be a spinal muscular atrophy carrier. This means if you have children, they have a higher risk of developing SMA or being a carrier themselves. About 1 in 50 Americans are carriers of SMA, according to the SMA Foundation.
“A genetic carrier screening test, which is a blood test taken before or during pregnancy, helps you understand your chances of having a baby with SMA,” says Louise E. Wilkins-Haug, MD, PhD, a Mass General Brigham obstetrician-gynecologist. “Having the father of the baby take the same test can further help you understand your child’s risk.”
Dr. Wilkins-Haug, who cares for pregnant persons at Brigham and Women’s Hospital, explains more about the condition, the carrier screening test, and what to do if you’re a carrier.
What is spinal muscular atrophy?
SMA is an autosomal recessive genetic disorder, which means a child may inherit the condition if each parent is a carrier. It affects the nerves in the spinal cord that control most of the muscles in your body (called motor neurons).
With SMA, these nerves have trouble sending signals to your muscles, which prevents them from moving well. The muscles get smaller (atrophy) and become weaker over time. It affects muscles in your arms and legs, as well as those involved in swallowing and breathing.
Depending on the type of SMA, there’s a range of how much impact it has on a person’s life. Symptoms might appear very early when a baby is less than 6 months old, or they may appear when the person is an adult.
Spinal muscular atrophy symptoms
SMA affects everyone differently, and symptoms range from mild to severe. There are several types of SMA which differ by what age symptoms begin and how much it affects the muscles. Genetic studies can help determine which type of SMA will occur. People with SMA often have weakness in the muscles closest to the center of their body. It usually affects the lower part, like the hips and thighs, but can also affect the neck, shoulders, and upper back.
Symptoms of SMA may include:
- General muscle weakness
- Difficulty swallowing or breathing
- Being unable to or slowly losing the ability to walk, stand, sit, and/or move
- Respiratory illnesses, like pneumonia
- Scoliosis, or a curving of the spine
Types of SMA
There are 5 types of spinal muscular atrophy:
Type 0 is a newer classification of SMA where the baby has symptoms before birth. These may include moving less in the womb due to stiff joints, developing smaller lungs, or having more fluid surrounding them (called polyhydramnios).
Type 1 includes babies and infants that show symptoms of SMA when they’re born or are less than 6 months old. The infant may appear normal at birth, but symptoms appear soon. They usually have trouble swallowing, sucking, and breathing, have a weak cry, and are unable to sit up on their own when they’re old enough. This is the most common SMA type with the most severe symptoms.
Type 2 includes infants that show symptoms of SMA between 6 and 18 months. They may have muscle weakness close to the center of their body, usually in the legs, and may need help walking or standing when they’re old enough.
Type 3 is a milder form of SMA and usually starts to affect children older than 18 months through early adulthood. As they get older, they often have difficulty walking and mild muscle weakness that may result in becoming a wheelchair user.
Type 4 is a less common form of SMA where symptoms don’t appear until adulthood, usually after 30 years old. These symptoms are mild and progress slowly, so most people are able to keep their mobility.
Spinal muscular atrophy life expectancy
Unfortunately, babies born with spinal muscular atrophy type 0 may not survive through the pregnancy or pass away shortly after birth. With spinal muscular atrophy type 1, children often don’t live past 2 years old. Their symptoms get worse quickly and may cause them to be unable to breathe properly. However, some live much longer depending on their treatment and the severity of their SMA.
Children with SMA type 2 typically also have a shortened lifespan, but people with SMA types 3 or 4 have milder symptoms and live a normal lifespan.
How is SMA genetic?
Genes are like instruction manuals for your body. They determine how a person grows and how the body functions throughout your life. Most cases of SMA are caused by changes in a gene called SMN1.
The SMN1 gene is responsible for creating a protein called SMN, which stands for “survival of motor neuron.” This protein is important for developing the motor neurons in your spine, which signal your muscles to move. When there’s not enough SMN protein in your body, the motor neurons are weaker and can’t control your muscles well.
“Everyone has two SMN1 genes. If both are faulty, you have SMA,” says Dr. Wilkins-Haug. “If just one of your SMN1 genes is faulty, this means you don’t have SMA but are a carrier. Being a carrier means you have a higher risk of passing that gene change down to your child, and also of having a child with SMA.”
The risk of a child having SMA is 1 in 4 if both parents are carriers. That risk also increases if you have a family history of the condition.
Testing for SMA carrier status is technically complex. Once the testing identifies one person of a couple as a carrier, the couple can consult with a genetic counselor to understand if testing the partner is appropriate.
Genetic carrier screening test for SMA
A genetic carrier screening test tells you if you carry a gene change for certain genetic conditions, like SMA. The test shows if you carry an SMN1 gene change that increases your chances of having a child with SMA. It won’t tell you for certain that your child will have the condition, but it helps you understand the risk.
You can choose to do a carrier screening test before or during pregnancy. A provider takes a sample of blood from you and tests it to see if you carry any gene changes.
“You don’t need to have a carrier screening test if you’re pregnant or planning to be — it’s your choice to do so,” Dr. Wilkins-Haug emphasizes. Your test results are confidential, and genetic counselors help you understand your results and options.
What happens if I and/or my partner are a spinal muscular atrophy carrier?
A genetic counselor helps you understand the complex testing for SMA. If one parent is a carrier of SMA, then the second parent usually has a screening test to see if they’re also a carrier. Carriers don’t have signs or symptoms of SMA.
If your partner isn’t a carrier, then the chances of your child having SMA is low. If you’re both carriers, then the chances of your child having SMA is higher (1 in 4) — and the child’s risk of being a carrier themselves is 1 in 2.
SMA genetic testing during pregnancy
You may find out that you and your partner are carriers when you’re already pregnant. You can choose to have another test to see if the baby has SMA, such as:
- Amniocentesis. A provider uses a thin needle to take a small amount of fluid and cells from the sac in your body that holds the baby.
- Chorionic villus sampling (CVS). A provider takes a small sample of tissue from your placenta.
SMA genetic testing before pregnancy
If you and your partner find out you’re both carriers of SMA before you get pregnant, you have several options:
- Choosing to get pregnant and have a test to see if the baby has SMA.
- In vitro fertilization, or IVF. This is an infertility treatment where a fertility specialist creates an embryo in a lab, using either your own eggs or sperm, or donor eggs or sperm. They test the embryo to see if it has SMA before transferring it to the parent with a uterus.
- Intrauterine insemination, or IUI. A fertility specialist transfers sperm from a donor who doesn’t carry SMA to the parent with a uterus so they can become pregnant.
- Adoption
- Choosing not to get pregnant
You may have heard of fetal therapies where doctors can treat your baby for certain conditions while it’s still inside you, or “in utero.” For example, the Fetal Therapy Program at Brigham and Women’s Hospital treats conditions like congenital heart disease before babies are born.
While there’s currently no fetal therapy for SMA, research is rapidly evolving in disease modifying therapies in infants, and researchers are considering treatment before birth. They’re considering gene therapy approaches and giving the in utero baby the same medications that help infants after birth.
Does newborn screening test for SMA?
Once your baby is born, a newborn screening test identifies certain conditions that may affect your child’s wellbeing. This includes SMA, cystic fibrosis, sickle cell disease, hearing loss, and more.
For a newborn screening test, a provider usually takes a small sample of blood from your child’s heel. They send the blood sample to a lab for testing. This is important for doctors to detect conditions early so they can help your child reach their full potential.
“If the newborn screening shows your child has SMA, a health care team works with you to start treatment early and helps care for them throughout their life,” says Dr. Wilkins-Haug. “And if you choose to have a carrier screening test before or during pregnancy, we’re here to help you understand your options. Talk to your OB/GYN or primary care provider to learn more about what genetic testing may be right for you.”
