Conditions Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a group of inherited conditions that affect the motor neurons of the spinal cord. Motor neurons are specialized nerve cells that control the muscles used for activities such as breathing, crawling, and walking. In people affected by SMA, the loss of motor neurons leads to progressive muscle weakness and atrophy (wasting).

There are four primary forms of SMA which are classified based on the severity of the condition and the age at which symptoms begin. The symptoms and long-term outlook of each form vary widely. In general, forms of SMA with an earlier age of onset are more severe and have a greater impact on motor function. Early detection and treatment of SMA is important since studies suggest that therapy is most effective when started in the first few months of life.

Condition Type

Other Disorders

Frequency

Approximately one of every 10,000 babies is born with SMA.

Also known as

  • SMA

Follow-Up Testing

Because early testing and treatment of spinal muscular atrophy (SMA) may improve quality of life and slow disease progression, follow-up testing must be conducted as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve collecting a sample of blood for genetic testing. People affected by SMA have a change (mutation) in each copy of the SMN1 gene.

Your doctor may also order genetic testing to determine how many copies of the SMN2 gene are present. The SMN2 copy number modifies the severity of SMA. In general, people with three or more copies of the SMN2 gene tend to have less severe forms of the condition.

About Spinal Muscular Atrophy

Early Signs

The age of onset and signs of spinal muscular atrophy (SMA) differ depending on the form. 

SMA type I is the most common and severe form of the condition. Signs and symptoms often begin within the first six months of life. Affected infants have severe muscle weakness and poor muscle tone which leads to significant developmental delay. Most are unable to support their heads or sit unassisted. Other signs of SMA type I many include:

  • Breathing problems
  • Difficulty swallowing
  • Poor growth
  • Joint abnormalities

SMA type II is generally characterized by muscle weakness that develops between six months and two years of age. Affected children can typically maintain a seated position without support but are unable to walk.

SMA type III is often diagnosed between 18 months and three years of age. However, some affected people may not develop muscle weakness until adolescence. People with SMA type III are able to stand and walk independently, but have increasingly limited mobility with age.

SMA type IV is characterized by mild to moderate symptoms that usually don’t develop until adulthood (often after age 30). Affected people may experience mild motor impairment such as:

  • Gradual muscle weakness
  • Tremor (involuntary, rhythmic muscle contractions)
  • Twitching
  • Mild breathing problems
Treatment

Although there is currently no cure for spinal muscular atrophy (SMA), treatments are available to manage associated symptoms and improve quality of life. Certain therapies may even stabilize or slow disease progression in some affected people.

Medications

In December 2016, nusinersen (SPINRAZA™) became the first FDA-approved drug therapy for children and adults affected by SMA. Studies suggest that SPINRAZA may help some people with SMA:

  • Achieve milestones such as the ability to sit independently, stand or walk
  • Maintain motor milestones at ages when they are typically lost
  • Survive longer than expected based on the diagnosed form of SMA

SPINRAZA is given through intrathecal injection. During this procedure, a needle is inserted between two lumbar bones in the lower back (also called a lumbar puncture) and medication is administered directly into the central nervous system.

Management of Symptoms

Other treatments aim to address the signs and symptoms of the condition. For example, a feeding tube may be necessary to help people who cannot chew or swallow maintain proper nutrition. In some forms of SMA (type I, type II and rarely, type III), respiratory support may be required to address progressive issues with lung function. Physical therapy, occupational therapy, and rehabilitation may improve joint mobility and slow muscle weakening and atrophy. Palliative care can help provide relief from pain, fatigue, stress and other distressing symptoms with the goal of improving comfort and quality of life.

Expected Outcomes

The long-term outlook for children and adults with spinal muscular atrophy (SMA) varies significantly based on the form, the symptoms present, and the response to treatment. Although there is currently no cure for SMA, treatments are available to improve the overall quality of life, and some therapies may even stabilize or slow disease progression. Studies suggest that therapy for SMA is most effective when started in the first few months of life before significant loss of motor neurons occurs. This is why newborn screening for SMA is so important. 

The more severe forms of SMA are usually associated with a shortened lifespan, although newer therapies may improve survival. In general, most babies affected by SMA type I die in early childhood. SMA type II is associated with a variable life expectancy. Some affected people live into adolescence and others into their 30s or 40s.

People with SMA type III may be more susceptible to respiratory infections, but with appropriate care, most have a normal lifespan. SMA type IV is associated with a normal life expectancy.

Causes

Motor neurons are specialized nerve cells that control the muscles used for activities such as breathing, crawling, and walking. Certain proteins in the body (called survivor motor neuron protein) are important for the maintenance and continued health of motor neurons. 

People affected by spinal muscular atrophy (SMA) have changes in the SMN1 gene which lead to reduced levels of survivor motor neuron protein. Without this protein, motor neurons die causing the many signs and symptoms associated with SMA.

SMA is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition.

Although most functional survivor motor neuron protein is made by the SMN1 gene, a small amount is produced based on instructions from the SMN2 gene. For this reason, extra copies of the SMN2 gene can modify the severity of spinal muscular atrophy. In general, people with less severe forms of SMA have three or more copies of the SMN2 gene.

While having a child with SMA is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for Spinal Muscular Atrophy

Support Services

Support groups can help connect families who have a child or other family member affected with spinal muscular atrophy with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates.

Accessing Care

Work with your baby’s health care provider to determine the next steps for your baby’s care. Your baby’s doctor may help you coordinate care with physicians that specialize in the treatment of neuromuscular conditions and other medical resources in your community. Children affected by spinal muscular atrophy (SMA) often have developmental delays. If you think that your baby is not meeting his or her developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because SMA is a genetic condition, you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for SMA, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

Many families living with spinal muscular atrophy share their experience with disease progression and day to day life on the Learn About Spinal Muscular Atrophy website.

Cure SMA has created a booklet entitled The Voices of SMA that shares the experiences of individuals and families and their views on living with SMA.

References & Sources

GeneReviews offers detailed information regarding the diagnosis, clinical characteristics, and treatment of spinal muscular atrophy. 

The National Human Genome Research Institute's (NHGRI) website has information on spinal muscular atrophy. The NHGRI is part of the National Institute of Health and supports studies aimed at understanding the structure and function of the human genome and its role in health and disease.

The National Organization for Rare Disorders (NORD) offers an article about spinal muscular atrophy for patients and families. NORD is a patient advocacy organization dedicated to people with rare diseases and the organizations that serve them.

Cure SMA offers a booklet called Understanding Spinal Muscular Atrophy which is intended to serve as a source of information and support for children, adults, and families living with Spinal Muscular Atrophy (SMA).

Genetics Home Reference offers an information page on spinal muscular atrophy. Genetics Home Reference provides consumer-friendly information about the effects of genetic variation on human health.

The Genetic and Rare Diseases Information Center (GARD) offers information and resources regarding spinal muscular atrophy. GARD is a program of the National Center for Advancing Translational Sciences (NCATS) to provide access to current, reliable, and easy to understand information about rare or genetic diseases in English or Spanish.

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Last Reviewed - 2/8/2018