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Type 3 spinal muscular atrophy (SMA3) is a milder form of this inherited disease. It’s also called juvenile SMA because it affects children. Another name is Kugelberg-Welander syndrome.

When Do Signs First Appear?

Type 3 SMA symptoms often first appear when a child is between 12 and about 18 months of age or shortly after. But some people don’t have any signs of SMA3 until after age 3, and others don’t have any physical changes until they’re teenagers or young adults.

Muscle weakness

SMA3 causes muscle weakness in a child’s legs, hips, shoulders, and arms. Symptoms of this disease are all related to muscle weakness and loss of muscle mass (wasting).

In SMA3, muscles that are closer to the center of your child’s body tend to weaken before the muscles located farther away from their core. For example, their thigh muscles may weaken before the muscles in their ankles or feet. Leg muscles weaken before muscles in their arms, shoulders, or hands.

Usually, a child’s leg muscles are weaker than muscles in their arms. Your child’s hands may weaken at some point, but most of the time, their hand muscles are still strong enough to do basic tasks like use a computer. They may not be able to do more strenuous activities or chores.

What Are the Early Symptoms of Type 3 SMA?

Children often show their first signs of SMA3 around the time they learn to walk. Some kids may be able to learn to walk or stand without help but gradually develop these signs:

  • Difficulty walking or running
  • Trouble walking up or down a flight of stairs
  • Poor balance
  • Frequent slips and falls
  • Needing help to stand up
  • Fatigue
  • Less endurance during activity
  • Struggling to rise when they’re bent over or lying flat on their back or side
  • Poor or worsening posture
  • Clumsiness

Can’t keep up. Another possible early sign of SMA3 is that you notice that your young child cannot keep up with their friends of the same age during active play or running.

Joint contractures and foot changes. Type 3 SMA could cause your child to have tightened, stiff joints. They may have a loss of full range of motion in some of their joints, also called contractures. Most children with type 3 SMA develop foot deformities.

Lose the ability to walk. Some kids with SMA3 may gradually lose their ability to walk or stand on their own. But other children with this disease are able to learn to walk and stay independent into their teens and adult years.

Breathing Problems

After a few years of living with SMA3, many children develop breathing problems because their intracostal muscles weaken. These are the muscles between the ribs. They expand when your child breathes in air to fill their lungs and contract when they breathe out.

As these muscles weaken, your child may not be able to take full, normal breaths. They may have a weak cough. Their lungs may be underdeveloped.

Weak ribcage muscles may also cause your child to occasionally gag on chewed bits of food or drinks after swallowing. They also may have shallow breathing when they’re asleep.

Breathing problems can cause children with SMA3 to have frequent, serious respiratory infections like pneumonia. Normally mild respiratory infections could be more severe in kids with this disease.

Other possible symptoms

Children with SMA3 may have weak or trembling fingers. They also may not have a knee-jerk reaction when their knee joint reflex is tested.

Weakened muscles could cause someone with SMA3 to develop kidney problems like proteinuria, or high protein in their urine. One sign of proteinuria is swelling around their ankles, also called edema.


About half of children with SMA3 develop scoliosis, or curvature of the spine from side to side in an “S” shape. Scoliosis in SMA3 is caused by weakening of muscles that support each side of your child’s spine. Severe scoliosis can affect breathing. Children with SMA3 typically show signs of spine curvature early.

Show Sources

Photo Credit: JGI / Tom Grill / Getty Images


Cleveland Clinic: “Spinal Muscular Atrophy.”

Muscular Dystrophy Association: “Spinal Muscular Atrophy: SMA.”

Genetic and Rare Diseases Information Center: “Spinal muscular atrophy type 3.”

National Organization for Rare Disorders: “Kugelberg Welander Syndrome.”

Spinal Muscular Atrophy UK: “Symptoms, Diagnosis, and Effects of 5q Spinal Muscular Atrophy.”

Spinal Muscular Atrophy Clinical Research Center, Columbia University Department of Neurology: “About SMA: What Is SMA?”

Mayo Clinic: “What Is Spinal Muscular Atrophy?”

Mott Children’s Hospital: “Spinal Muscular Atrophy.”

Shriners Children’s: “Spinal Muscular Atrophy.”

Children’s Health of Orange County: “Spinal Muscular Atrophy.” “Breathing Risks and Care.”

American Journal of Case Reports: “Monitoring Kidney Dysfunction in Kugelberg-Welander Syndrome.”