Spinal Muscular Atrophy (SMA) symptoms & causes in children

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Contact the Spinal Muscular Atrophy Program

Here at Boston Children’s Hospital, we specialize in innovative, family-centered care. From your first visit, you’ll work with a team of professionals who are committed to supporting all of your family’s physical and emotional needs. We understand that you probably want to learn more about your child’s diagnosis of SMA in order to be as informed as possible, and to keep your child safe and healthy.

What is SMA?

Spinal muscular atrophy (SMA) is a genetic condition that results in poor motor skills as a result of weakened muscles. Muscles are weakened due to dysfunctional spinal cord cells and motor nerves. SMA impacts voluntary and involuntary muscles (muscles that are not consciously controlled), such as chest muscles, which can affect breathing.

What is the life expectancy of children with SMA?

The life expectancy for SMA depends on the severity of the disease. Children with Type I, also known as Werdnig-Hoffmann Disease, usually develop respiratory failure by 2 years of age without breathing support. Children with Type II may live into their mid- to late-twenties or much older, and for individuals with Type III, also known as Kugelberg-Welander disease, life span of most individuals is not significantly different compared with the normal population.

Is it curable?

Currently there is no known cure for SMA. Treatment consists of managing the symptoms and preventing disease complications. The Spinal Muscular Atrophy Program at Boston Children’s Hospital brings together a comprehensive team of specialists who are experienced in caring for children with SMA. However, there is reason for optimism. The National Institute of Health has selected SMA as the disease closest to treatment from over 600 neurologic disorders. Researchers now understand the genetic cause of SMA, and are working towards developing treatments and cures.

Who’s at risk?

Because SMA is usually a genetically inherited disease, if someone in your family actually has SMA, your chance of being an SMA carrier significantly increases. In most instances, both parents must be carriers for the child to be at risk for having SMA.

How common is SMA?

SMA affects about 10 to 16 out of every 100,000 children born. SMA Type I is diagnosed in children 6 months old and younger, SMA Type II is diagnosed in children 6 to 18 months old, and SMA Type III is diagnosed in children as early as 18 months of age, but can surface as late as adolescence. SMA Types I and II are most prevalent.

If my child has SMA, what are the odds that I will have another child with SMA?

When both parents are carriers of the gene mutation that causes SMA, there is a 25% chance, with each pregnancy, that the child will be affected with SMA; a 50% chance of having a child who would be a SMA carrier (has the gene mutation but never develops the disease); and a 25% chance of having a child who would not have SMA and would not be a SMA carrier.


Symptoms of SMA vary greatly depending on SMA Type. More severe forms of SMA affect more essential functions that are very apparent during the first 6 months of life, and less severe forms of SMA have less severe symptoms that are not noticed until later in life.

  • Muscle weakness and decreased muscle tone are the main symptoms of SMA.
  • Weakness tends to be equal between the right and left sides of the body.
  • Legs are affected more than arms, and proximal muscles (those closest to the body like hips and shoulders) are affected more than distal muscles (those farthest from the body like fingers and toes).
  • Individuals may have very limited mobility since their muscles are weak.
  • Due to a lack of weight bearing, bones may be fragile and at higher risk of fractures. Care should be taken when moving and positioning children to not exert excessive force on any bone.
  • In many children, muscles used for breathing are also affected, leading to breathing problems which can be life threatening, especially in younger children.  
  • Weak tongue and muscles used for swallowing can lead to eating and swallowing difficulties.
  • Scoliosis (curvature of the spine) occurs at some point in many children with SMA Type I and II and some with Type III.

How severe are the different types of SMA?

There are three types of SMA: type I, II, and III, and the severity of the diagnosis varies greatly between types and from one person to another. The older and more physically developed your child is at the time of diagnosis, the better his or her prognosis will be. If your child is diagnosed later in life, it is because the symptoms are not as obvious and are less severe, meaning your child has a greater chance for better motor function.  Non-life threatening symptoms (such as mild weakness in shoulders and muscles around the hips) may not be diagnosed as symptoms of SMA until a later age.  SMA that is diagnosed early in life is diagnosed within months after birth because essential functions such as crying, head control and muscle strength are affected. Since a baby diagnosed with type I early in life would have problems with essential functions, such as breathing and swallowing, their life expectancy would be shorter, without respiratory interventions.

SMA type I, also known as Werdnig-Hoffmann Disease:

  • is the most severe form of SMA
  • is the most common form of SMA
  • is characterized by an inability to sit independently (without support).
  • can usually be detected in children younger than 6 months because of their severe low    muscle tone, delayed gross motor skills, breathing difficulties, lack of reflexes, and spontaneous tongue movements (referred to as tongue fasciculations). Babies also have weak cries, and difficulty feeding and swallowing.
  • often leads to a shortened life span (around 2  years) due to respiratory complications.

SMA type II:

  • is an intermediate form of SMA.
  • is the second most common form of SMA.
  • is characterized by an ability to sit unassisted (but not walk or stand without support).
  • affects the legs more than the arms.
  • can lead to a higher body fat percentage.
  • can usually be detected in children younger than 18 months because of low muscle tone and       delayed gross motor skills.
  • can also cause the same tongue movements (fasiculations) that children with type I have, and/or a minor tremor in both hands.
  • can cause varying degrees of breathing difficulties.
  • can lead to a shortened lifespan.
  • Survival rate is at 98.5 percent among 5 year olds
  • Survival rate is 68.5 percent among 25 year olds

SMA type III, also known as Kugelberg-Welander disease:

  • is the least severe form of SMA.
  • is characterized by the ability to stand and walk unassisted, but with progressive weakness over a number of years.
  • may lead to need for a wheelchair over time.
  • may cause hand tremors.
  • is not characterized by a shortened life span.
  • sometimes results in minor breathing problems.
  • resembles muscular dystrophy (MD).

What causes spinal muscular atrophy?

SMA is an autosomal recessive disease. This means that, most of the time, both parents must carry the nonworking gene, and that two copies of the gene (one inherited from each parent) are necessary for a child to have the condition.

Individuals have two copies of a gene called “survival of motor neuron” (SMN1 and 2), and in 95 percent of cases, the SMN1 gene is missing (genetic deletion).

People of all ages who show symptoms can be tested through genetic studies, which are typically done from a blood sample, to detect the absence of the SMN1 gene and to confirm the diagnosis of SMA.  

When both parents are carriers, there is a one in four (25 percent) chance, with each pregnancy, that they will have a child with SMA. A test to see if parents carry the gene mutation is available in most countries .

Questions to ask your care team

After your child is diagnosed with SMA, you may feel overwhelmed with information. It can be easy to lose track of the questions that occur to you.

Lots of parents find it helpful to jot down questions as they arise- that way, when you talk to your child’s team members you can be sure that all of your questions are answered. If your child is old enough, you may want to suggest that he or she writes down questions to ask your health care provider, too.

Here are some questions to help get you started:

  • How did the doctor arrive at this diagnosis?
  • How could the recommended SMA treatment impact my child’s life, including academic performance?
  • What resources are available to help our family cope with stress of living with SMA?
  • How should I talk to my child about the SMA diagnosis?
  • How can you coordinate with my child’s school nurse to continue care at school?
    Communication and care

    Sarah Teasdale, MD, is a pediatrician at Boston Children’s who understands that communication is a critical part of care. Medical jargon can be intimidating for parents, and Dr. Teasdale encourages parents to speak up if they need clarification. She encourages other doctors to explain medical jargon, without “dumbing down” the information. Learn more, and join the conversation on Children’s Thriving blog.

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