What Is Spinal Muscular Atrophy (SMA) in Babies?

Spinal muscular atrophy (SMA) is a serious genetic disease that causes the muscles to become weaker and waste away over time. This inherited neuromuscular disease is caused by a missing or mutated gene that leads a baby to lose motor neurons, nerve cells in the spinal cord that control voluntary muscle movement. Without an adequate supply of motor neurons, the muscles become weak and atrophy. 

Spinal muscular atrophy most often affects babies and young children but may present in adulthood (though this is rare). SMA affects about 1 in 8,000 to 10,000 people in the world. Type 1, which presents at birth or early infancy, makes up over half of all cases. Spinal muscular atrophy is a devastating diagnosis that affects a baby’s quality of life and life expectancy.

This article will describe the symptoms, treatment, and prognosis of spinal muscular atrophy in babies. 

Types of Spinal Muscular Atrophy

There are five different types of spinal muscular atrophy, and they can present at different times throughout the lifespan:

• Type 0 is the most severe and rarest type of SMA. It can be detected before birth because the baby will usually move less in utero than healthy babies. Because of this, the baby may be born with joint deformities and contractures. Infants with type 0 have very weak muscle tone (hypotonia) at birth, as well as weak respiratory muscles. Babies with type 0 usually do not survive past infancy due to respiratory failure.
• Type 1 is also severe and is the most common type of SMA. Also known as Werdnig-Hoffman disease, type 1 presents at birth or within the first six months of life. Babies with type 1 usually have difficulty holding their heads up. Most experience feeding problems because of sucking and swallowing problems. A baby with type 1 SMA does not meet expected milestones and is at an increased risk of respiratory infections. Babies with type 1 usually do not live past their second birthday because of respiratory infections or a collapsed lung. 
• Type 2 is an intermediate type of spinal muscular atrophy and is also referred to as Dubowitz disease. Symptoms usually start between 6 and 18 months and mostly affect the lower limbs. Infants and young children with this type of SMA may be able to sit up but cannot walk. They often experience scoliosis and involuntary tremors in their fingers. Babies with this type of SMA can live into their 20s or 30s. 
• Type 3 is a more mild type of SMA. Also called Kugelbert-Welander or juvenile-onset SMA, symptoms usually start after 18 months. Sometimes, symptoms do not present until early adulthood and include muscle weakness, difficulty walking, and respiratory infections. Those with type 3 can usually walk unassisted but may find stairs too difficult to manage. Individuals with type 3 may need a wheelchair as the disease progresses. Fortunately, type 3 does not shorten one’s life expectancy. 
• Type 4 is a rare type of SMA that appears in an adult’s mid-30s. In this type, the muscle weakness progresses slowly, and individuals are usually able to remain independently mobile. This type does not affect life expectancy.

Spinal Muscular Atrophy Symptoms

Symptoms of SMA in babies depend on the type and may include: 

• Progressive muscle weakness
• Loss of muscle control 
• Weakness in the muscles closest to the torso
• Inability to sit, walk, or stand
• Feeding difficulties 
• Frequent respiratory infections 

Causes

SMA is an inherited disease. This means that it is caused by a genetic abnormality that babies inherit from their parents. 

A baby with spinal muscular atrophy inherits two copies of a missing or mutated motor neuron (SMN1) gene from their parents, one from each parent. 

SMN1 genes produce SMN protein, which is a protein that motor neurons need to function. Without enough of this protein, motor neurons shrink and die. When this happens, the brain cannot control voluntary muscle movements. This especially affects muscles in the head, neck, arms, and legs. 

SMA is inherited through an autosomal recessive pattern. This means that both copies of the baby’s SMN1 gene have mutations. In most cases, the parents each have one copy of a mutated gene and do not have any symptoms themselves. 

Diagnosis

The process to diagnose SMA begins with a physical exam and medical history. Diagnostic tests include: 

• Blood test: High levels of creatine kinase in the blood indicates muscle wasting. Muscles release this substance when they deteriorate, so a high level can indicate SMA. 
• Genetic testing: A blood test can determine if the SMN1 gene is missing or mutated. This test is included in the standard newborn screenings in certain states. 
• Electromyogram (EMG): A nerve conduction test measures the electrical activity of nerves and muscles. This test can detect if there is an abnormality in nerve conduction in the muscles. 

Treatment

There is no cure for SMA. Treatment options will depend on the type, symptoms, and your baby’s overall health. Babies with spinal muscular atrophy require a multidisciplinary care team that involves treatment from respiratory, orthopedic, gastroenterological, and nutritional providers. Occupational and physical therapists are often involved in care as well. As your child grows, they may require assistive devices like braces, crutches, or a wheelchair. 

Depending on your child’s age and prognosis, your healthcare provider may recommend medications to treat SMA as well. Drugs used to stimulate the production of the SMN protein include:

• Spinraza (nusinersen)
• Evrysdi (risdaplam)

Your medical team may also consider using gene replacement therapy for infants and young toddlers with SMA. The medication Zolgensma (onasemnogene abeparvovec-xioi) is used to replace the missing or mutated SMN1 gene with a healthy one. 

Prognosis

The prognosis for SMA largely depends on which type your child has. Babies with type 1, the most common type, usually do not live past their second birthday. This is a devastating diagnosis for parents and families. Talk with your medical team to be sure you understand your child’s type and prognosis. 

Possible complications of spinal muscular atrophy in babies include:

• Aspiration 
• Pneumonia and respiratory infections 
• Respiratory failure
• Scoliosis
• Bone fractures
• Malnutrition 

Coping

Babies with SMA and their families need as many resources as possible. Resources to support your child’s physical health include:

• Breathing support: Many babies with SMA experience respiratory problems because of weakness in the breathing muscles. Your child may require a tracheostomy or other breathing support. 
• Feeding support: Many babies experience difficulties with eating and swallowing and may need a feeding tube. Your healthcare provider may recommend meeting with a dietician to help with nutritional goals.  
• Physical therapy: Physical therapists can teach you how to stretch your infant’s muscles to help prevent joint contractures. 

Parents of children with SMA undergo tremendous levels of stress and need their own resources. A study found that parents with a baby with SMA experienced significant stress related to:

• Financial concerns
• Lost employment opportunities 
• All-consuming nature of care
• Social isolation
• Mental fatigue
• Physical pain

Organizations that can help connect your family with support groups and other needed resources include: 

• Cure SMA
• Muscular Dystrophy Association
• Gwendolyn Strong Foundation
• SMA Foundation

Summary 

Spinal muscular atrophy (SMA) is a serious inherited disease that causes the weakening and wasting away of muscles. The most common type is usually diagnosed at birth or within the first six months of life. Symptoms of SMA in babies include progressive weakening of muscles, inability to sit, feeding difficulties, and frequent respiratory infections. There is no cure for SMA, and this disease can significantly impact a child’s quality of life and lifespan. 

A Word From Verywell

Spinal muscular atrophy is a devastating disease for babies and their families. If you have been struggling with the news of this diagnosis, know that you are not alone. Talk with your child’s medical team about how to access services for mental health, financial resources, and care support. This diagnosis is completely overwhelming for any parent, and there are resources available.