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Spinal Muscular Atrophy Type 1

Posted on March 05, 2021
Medically reviewed by
Amit M. Shelat, D.O.
Article written by
Bethany J. Sanstrum, Ph.D.

Spinal muscular atrophy (SMA) type 1 is the most common type of spinal muscular atrophy. SMA is a group of genetic diseases that cause muscle weakness (hypotonia) and other symptoms related to the loss of motor neurons, which control voluntary movement. The severity of SMA’s symptoms can range widely and depend on the type of spinal muscular atrophy.

Approximately 45 percent of all SMA diagnoses are type 1, which is also commonly called Werdnig-Hoffmann disease or infantile-onset SMA because it affects infants.

What Are the Symptoms of SMA Type 1?

Babies with SMA type 1 are often described as appearing “floppy” because they lack proper muscle tone. They likely cannot sit or roll over by themselves. They also typically have a weak cry or breathing problems from weak respiratory muscles.

Children with SMA type 1 can also have issues with swallowing or sucking. Difficulty swallowing and sucking can lead to problems with feeding and weight gain. The infant may pass food or liquids into the lungs (aspiration), which can cause infection or pneumonia.

SMA type 1 does not affect the baby’s intelligence, and infants with the condition are typically bright and alert.

How Is SMA Type 1 Different From Other Types of SMA?

Compared to other forms of SMA, SMA type 1 is considered a severe form of this genetic condition. The age of onset is also much earlier compared to other forms of SMA: SMA type 1 is considered an early childhood genetic disorder because it starts as early as 0 to 6 months of age.

Causes and Diagnosis of SMA Type 1

To understand the cause of SMA type 1, it helps to learn what causes most types of SMA — a recessive genetic mutation in the survival motor neuron 1 gene (SMN1 gene). A person needs to have two mutated copies of the SMN1 gene (one from each parent) to be diagnosed with SMA. In these people, there is not enough working SMN protein. The SMN protein is very important to maintain motor function (movement) and make healthy nerve cells in the spinal cord.

Humans also have a second gene, called the SMN2 gene, that can make SMN protein and help replace some of the missing protein. The number of copies of the SMN2 gene in a person’s cells influences how severe the symptoms of SMA will be. It is possible to have up to eight copies of the SMN2 gene in each nerve cell, which will make more SMN protein.

Infants with SMA type 1 have only one or two copies of the SMN2 gene.

Diagnosis can be made through genetic testing. Prenatal screening for SMA and the related genes has become increasingly more common in the United States, particularly for individuals with a family history of SMA. Earlier diagnoses can even be made during pregnancy.

Life Expectancy and Disease Prognosis

In the past, the life expectancy for children diagnosed with SMA type 1 was as little as 2 years. With the advancements of clinical trials, novel gene therapy, and earlier diagnoses, life expectancy and disease prognosis may be improving. Current long-term results are unknown.

Treatments for SMA Type 1

Children with SMA type 1 have several supportive and treatment options that may help.

Therapeutic Treatment Options

The outcomes for children with SMA have recently improved due to the U.S. Food and Drug Administration’s (FDA) approval of new disease-modifying treatments.

The first drug approved for SMA treatment is called Spinraza (nusinersen) and was approved in 2016. Spinraza works by helping to increase the body’s production of SMN protein. Although Spinraza is approved for use to treat SMA type 1, it is typically given to children after infancy (age 2-14) because the drug must be injected directly into the spinal cord, which is a procedure not often performed in younger children. Its administration may limit the practical use of Spinraza for children with SMA type 1.

Another drug that was approved by the FDA for SMA is called Zolgensma (onasemnogene abeparvovec-xioi). This drug was released in 2019 and has a different mechanism of action in the body than Spinraza. Zolgensma is considered a gene therapy, as it delivers a new fully functioning SMN1 gene to replace the mutant copy of the gene. Zolgensma is given by a one-time intravenous injection and can be used in children younger than 2 years old.

In 2020, the FDA approved the newest SMA drug, called Evrysdi (risdiplam). It functions in the body in a similar way as Spinraza. However, the drug is given as an oral liquid and is available for children with SMA types 1 to 3 who are as young as 2 months old.

Respiratory Support

Respiratory failure is the most common cause of death in newborns with SMA type 1. Respiratory support is vital to the care of newborns diagnosed with SMA. Ventilation assistance devices, chest physiotherapy, and other interventions may help these children breathe until they can regain strength and are old enough to begin therapeutic treatments.

Feeding Support

Because swallowing may be difficult with SMA type 1, parents and caretakers have options for feeding support, such as feeding tubes, which help give the child proper nutrition. Gastric or gastrostomy tubes (G-tubes) are very common and can be placed under the skin to reach the stomach. Another type of feeding tube is called a nasogastric tube or NG tube. These tubes are placed through the nose and run to the stomach. In both cases, the child is fed a liquid-based diet.

Find Support

Navigating a diagnosis of SMA type 1 can be difficult, physically and emotionally, but it may be made easier backed by the support of others who understand. On mySMAteam, you can find a community of people with SMA and their loved ones. Here, you can share your experiences and connect with others for advice and support.

Have questions or concerns? Share them in a comment below or start a conversation on mySMAteam.

All updates must be accompanied by text or a picture.
Amit M. Shelat, D.O. is a fellow of the American Academy of Neurology and the American College of Physicians. Review provided by VeriMed Healthcare Network. Learn more about him here.
Bethany J. Sanstrum, Ph.D. holds a doctorate in cell and molecular biology with a specialization in neuroscience from the University of Hawaii at Manoa. Learn more about her here.

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