Understanding Spinal Muscular Atrophy (SMA): Symptoms, Causes & Treatment

Written by Mary Jo DiLonardo, Alicia Racelis, PhD
Medically Reviewed by Christopher Melinosky, MD on April 08, 2023
11 min read

Spinal muscular atrophy (SMA) is a rare genetic syndrome usually caused by changes to the SMN1 gene. It weakens your voluntary muscles. SMA mostly impacts your lower body and sometimes your breathing.

Normally, two working copies of the gene make a protein called survival motor neuron (SMN). SMN protein keeps your lower spinal cord cells (motor neurons) healthy. These motor neurons mainly control your lower limbs. 

With SMA, your genes can't make enough SMN protein. Without the protein, your spinal cord can't send strong electrical signals to move your muscles. If your muscles don't move, they can get weaker and shrink (atrophy). 

SMA usually happens over time (progressive). You can have muscle weakness in your hips, legs, thighs, and upper back. You may have a curve in your spine, trouble sitting, or walking. 

In some cases, you can have trouble swallowing or breathing as your muscles get weaker. And sometimes, SMA can impact your hands or other muscles, too. Keep in mind everyone with SMA has a different experience. But SMA doesn't affect your intelligence. You'll still sense and feel what's around you.

Your symptoms depend on the amount of SMN protein your spinal cord makes. Without the SMN1 gene, your spinal cord must depend on the SMN2 gene. The more SMN2 gene copies you have, the milder your symptoms will be. 

There are five spinal muscular atrophy types, starting at different ages. Here are some common SMA symptoms for each type:

Type 0. You'll likely have one SMN2 gene copy. This is the rarest and most serious form of SMA. Type 0 often starts when your baby is growing in your womb. 

You may notice symptoms while you’re still pregnant . Your baby may:

  • Move less often or stop moving in the womb
  • Be born with serious muscle weakness
  • Have softer, floppier muscles (hypotonia)
  • Lack reflexes
  • Have paralysis on both sides of their face (palsy)
  • Have heart problems at birth (congenital heart issues) 
  • Have weak breathing muscles

Sometimes, type 0 is also called type 1 SMA.

Type 1. Also called Wernig-Hoffman disease, can show up between birth and 6 months old. Many children with type 1 have two to three SMN2 genes. 

Although your baby's muscles may be strong at first, they can get weaker over time. The syndrome mainly impacts your lower limbs. So your baby can frown and smile. 

With SMA type 1, your baby may:

  • Not be able to support their head by themselves
  • Not sit up without help
  • Have floppy arms and legs (hypotonia)
  • Have problems swallowing (dysphagia) or a higher risk for swallowing liquid down their airway
  • Have a weak cry
  • Move their tongue irregularly
  • Have saliva that pools
  • Have breathing trouble
  • Have a bell-shaped chest
  • Develop slower, lose weight, or not eat as much (failure to thrive)

Weaker rib muscles can mean your baby's ribs push in and belly pushes out when breathing (paradoxical breathing). The rib cage can look bell-shaped. But once your baby's muscles get weaker, they may need help to breathe. 

Treatments like a ventilator can help get oxygen to your baby's tissues and organs. Because of breathing problems, many children live until they're about 2 years old. But with respiratory treatment, some children can live longer.

Type 2. It's also called Dubowitz disease or chronic infantile SMA. Your baby may have about three SMN2 gene copies. About 20% of children with type 2 show symptoms between 3 to 15 months old. 

Your child may:

  • Sit up at some point (can be delayed)
  • Not stand or walk without help
  • Have problems swallowing
  • Have breathing trouble
  • Have a curved spine (scoliosis)
  • Have limited joint movement (joint contracture)
  • Get lower jaw fusion causing problems (ankylosis of the mandible)

Your child may lose the ability to sit up by the time they're a teenager. Almost two-thirds of children with type 2 SMA live till they're 25 years old. 

Type 3. About 3 out of every 10 people with type 3 show symptoms between 18 months and adulthood. You'll likely have three to four SMN2 gene copies. Although you'll likely walk, muscle weakness can mean you:

  • Have problems running
  • Have trouble climbing stairs
  • May fall down more often
  • Need help getting up from a chair 
  • May need a wheelchair to get around when you're older
  • Have foot problems
  • Probably won't have breathing trouble
  • Likely don't have scoliosis (curved spine)

Type 3 SMA often means living a normal life span. 

Type 4. About 1 out of every 20 people with SMA have type 4. Type 4 SMA starts as an adult. It's still unclear, but some doctors combine juvenile SMA with type 4. With type 4 SMA, your symptoms start around 30 years old. Type 4 symptoms can mean you:

  • Reach all your milestones, as usual
  • May walk and never need a wheelchair
  • Have lower leg and limb muscle weakness
  • Have twitching
  • Don't have breathing problems

Usually, your upper arms and legs have symptoms. You'll have 4-8 SMN2 gene copies. The more gene copies you have, the milder your symptoms will be. 

You can keep moving with exercise treatments. And you may get stronger, too. You'll work with a physical therapist to practice your exercises. Many people with type 4 keep working for many years.

SMA is a rare genetic syndrome passed down through families. With SMA, you'll have changes in your SMN1 gene on chromosome 5 (5q13.2) from both parents (autosomal recessive). 

About 94% of people with SMA have lost copies of the SMN1 gene. That means your spinal cord cells must rely on your SMN2 gene to make enough SMN protein. 

Without enough SMN protein, your spinal cord can't send strong signals to control your lower body muscles. And if your muscles don't get nerve signals, the muscle cells can slowly die. With more SMN2 gene copies, you'll likely have fewer symptoms, later in life.

Can other genes cause SMA?

In rare cases, changes to the UBE1 gene can lead to SMA. This gene makes the ubiquitin-activating enzyme 1. This enzyme helps mark certain proteins for your cells to destroy.

Another rare cause of SMA are changes to your cytoplasmic dynein 1 heavy chain 1 (DYNC1H1) gene. Sometimes called SMA-LED, symptoms show up in early infancy to childhood. With SMA-LED, you may notice your child has:

  • Very weak thigh muscles
  • Problems waddling
  • Trouble walking or climbing stairs
  • Problems standing up

What does it mean to be a SMA carrier?

If your child only has changes in one gene from a single parent, they won't get SMA. That means they carry the gene change (carrier), but don't have SMA. When your child grows up, they could pass the gene changes to their children, too.

Spinal muscular atrophy is usually diagnosed at birth. SMA is part of the Recommended Uniform Screening Panel (RUSP). In fact, newborn screening detects about 95% of newborns with SMA.

As you get older, SMA can be harder to diagnose. That's because the symptoms can be similar to other conditions. To help figure out what's going on, your doctor may ask you:

  • Has your baby missed any developmental milestones, such as holding their head up or rolling over?
  • Does your child have trouble sitting or standing on their own?
  • Have you seen your child have trouble breathing?
  • When did you first notice the symptoms?
  • Has anyone in your family had similar symptoms?

Your doctor may also order tests to help diagnose SMA. They may take a blood sample to check for missing or broken genes causing SMA. 

Your doctor may also perform a blood test to check your creatine kinase (CK). It’s an enzyme that can leak out of weakening muscles. High blood CK levels aren’t always harmful, but can show possible muscle damage.

Other tests can rule out conditions that have similar symptoms:

Nerve tests, such as an electromyogram (EMG). Your doctor puts small patches on your child's skin and sends an electrical impulse (message) through the nerves to the muscles.

CT. This powerful X-ray makes detailed pictures of the inside of your child's body.

MRI. These powerful magnets and radio waves make pictures of organs and structures inside your child.

Muscle tissue biopsy. In this test, the doctor removes muscle cells through a needle or a small cut in your child's skin.

  • Have you treated others with this condition?
  • What treatments do you recommend?
  • Is there therapy that can help keep my child's muscles strong?
  • What can I do to help my child become more independent?
  • How can I get in touch with others who have family members with SMA?

You and your medical team will create a treatment plan. Key ways to manage your symptoms may include:

Breathing. With SMA, especially types 1 and 2, weak muscles can keep air from moving easily in and out of your lungs. If this happens, you may need a special mask or mouthpiece. For serious problems, a machine can help you breathe.

Swallowing and nutrition. SMA can weaken muscles in your mouth and throat. And babies and children with SMA can have a harder time sucking and swallowing. So your child may not get good nutrition or have trouble growing. Your doctor may suggest working with a nutritionist. Some babies with SMA may need a feeding tube, too.

Movement. Physical and occupational therapy use exercise and regular daily activity to help protect your joints and keep muscles strong. A therapist may suggest leg braces, a walker, or an electric wheelchair. Special tools can control computers and phones. The tools can help with writing and drawing, too.

Back issues. When SMA starts in childhood, children can get a curve in their spine. Your doctor may suggest wearing a back brace while your child's spine is still growing. Once the spine is done growing, your child can have surgery to fix the problem.

Gene therapy for SMA

The FDA has approved three medications to treat SMA: nusinersen (Spinraza), onasemnogene abeparvovec-xioi (Zolgensma), and risdiplam (Evrysdi). Both are forms of gene therapy that affect the genes involved in SMA. 

The SMN1 and SMN2 genes give your body instructions for making a protein that helps with controlling muscle movement.

Nusinersen (Spinraza). This treatment adjusts the SMN2 gene to make more SMN protein. This therapy helps children and adults with SMA. Your medical team can inject the drug into the fluid around your spinal cord. 

Including preparation and recovery time, therapy can take at least two hours. Treatments are usually done several times, followed by another dose every four months. Studies show the medication may help about 40% of people with SMA get stronger and slow down symptoms.

Onasemnogene abeparvovec-xioi (Zolgensma). This involves replacing the problem SMN1 gene. It's used for children under 2 years old. Your child's medical team will put a tiny tube (catheter) directly into a vein in their arm or hand (an IV). 

Then, they'll send a healthy copy of the SMN1 gene through the tube. The gene goes directly to a specific group of spinal cord cells. Studies showed the medicine may help your child reach certain milestones faster. The gene therapy may also help them control their head and sit without help, too.

Risdiplam (Evrysdi). This treatment works to stop the SMN2 gene from being turned off. The medicine raises SMN protein amounts inside your nerve cells. 

Your child takes it once a day by mouth, after a meal. The dose is determined by their weight. In the clinical trials, about 41% of the children with SMA who took risdiplam showed better muscle function after a year.

There's a lot you can do as a family to help your child with some of the basic tasks of daily life. A team of doctors, therapists, and support groups can help you with your child's care. They can also help you keep up with friendships and activities with your family.

Your child will probably need lifelong care from different types of doctors. They may need to see:

  • Pulmonologists, doctors who treat lungs
  • Neurologists, specialists in nerve problems
  • Orthopedists, doctors who treat bone problems
  • Gastroenterologists, specialists in stomach disorders
  • Nutritionists, experts in the way food affects your child's health
  • Physical therapists, people trained in using exercise to improve your child's motion

This team can help you make decisions about your child's health. It's important not to let yourself get overwhelmed by the task of managing care. Check for support groups that allow you to share your experiences with others who are in similar situations.

Keep in touch with your medical team, family members, and clergy. Stay in touch with others who can help give you the emotional support you need while your child lives SMA. 

Ask your doctor about ways to meet with others who have the same condition. It can help to talk and meet others who know what you're going through, too.

Spinal muscular atrophy (SMA) is a rare genetic syndrome that can weaken your voluntary muscles. Usually SMA slowly impacts your lower body and sometimes your breathing. Often caused by lower SMN protein amounts, your outlook depends on when symptoms start and how serious they are. Talk to your medical care team about SMA treatments to improve your quality of life.

Is there a cure available for SMA?

There's not a complete cure for SMA. But some SMA treatments may improve your symptoms. In some cases, SMA therapy can help your child live longer. Some treatments work to replace the broken gene or raise levels of the missing SMN protein.

What is the life expectancy for someone with SMA?

Your life expectancy can vary depending on your SMA type. That means the age your symptoms start and how serious they are. In general, you can live:

  • Type 0, from birth to one month old
  • Type 1, up to 2 years old
  • Type 2, until 25 years old but sometimes till your 30s
  • Type 3, a normal life span
  • Type 4, a normal life span

 But other factors also include your treatments, how much SMN protein you have, and other medical conditions. 

What does spinal muscular atrophy feel like?

It depends on the type of SMA you have. SMA usually impacts your lower limbs like your legs and hips. With type 0 SMA, you may notice your baby moves less in the womb. But with other SMA types, you may feel: 

  • Little control of your baby's head
  • Swallowing problems
  • A loss of reflexes
  • Floppy legs or arms (weak muscles)
  • Not being able to sit
  • Breathing difficulty and chest muscle weakness (bell-shaped chest)
  • A progressive loss of lower limb strength like trips, falls, or a need for help to stand or walk
  • Jerky or twitchy hand movements
  • Stiff jaw because of a fused bone
  • Joint pain (contractures)
  • Curved spine (scoliosis)

Organizations like these can offer you support:

Cure SMA. The organization funds research to treat and cure SMA. Its SMArt Moves program focuses on early SMA diagnosis and treatment. Cure SMA also offers local chapter support group information for people with SMA and their families. The website also contains details about SMA.

Muscular Dystrophy Association. This group has information and links to services and support groups in your area.

SMA Foundation. The foundation aims to boost SMA treatments, work with researchers, and develop clinical trials. The site also helps educate others about this genetic condition.