What is spinal muscular atrophy and how is it recognized?

Spinal muscular atrophy (SMA) is a rare, inherited neuromuscular disorder caused by a mutation in the SMN1 gene, leading to the gradual degeneration of motor neurons in the spinal cord and progressive muscle weakness. Although it is most commonly diagnosed in early childhood, symptoms may also appear later in life. The condition does not present immediately in all affected individuals, which means early signs can often go unrecognized. Muscle weakness, difficulty walking, and problems with breathing or swallowing may be the first indicators that further medical evaluation is needed. Early detection of SMA can significantly improve disease management and quality of life. In the following text, you will learn what causes SMA, its different types, how it is diagnosed, and what modern treatment options are available.

What is spinal muscular atrophy (SMA)

Spinal muscular atrophy (SMA) is a rare, inherited neuromuscular disorder that causes the gradual weakening and deterioration of muscles. The condition is caused by damage to the motor neurons in the spinal cord—nerve cells responsible for controlling voluntary muscle movements. When these neurons do not function properly, the muscles no longer receive signals, leading to progressive weakness and muscle atrophy over time.

What causes SMA?

The primary cause of spinal muscular atrophy (SMA) is a mutation in the SMN1 gene, which is responsible for producing the SMN (Survival Motor Neuron) protein. This protein is essential for the survival of motor neurons. When there is not enough of it, motor neurons begin to deteriorate and die.

SMA is inherited in an autosomal recessive pattern, meaning that a child must inherit one mutated copy of the gene from each parent in order to develop the condition. Most parents are healthy carriers and are unaware that they can pass the gene on to their children.

Types of spinal muscular atrophy

Spinal muscular atrophy (SMA) is classified into four main types, based on the age at which symptoms appear and the severity of the condition. Although all types share the same genetic cause, their clinical progression can vary significantly.

SMA Type 1 (Werdnig-Hoffmann disease)

This is the most severe form of SMA, typically appearing within the first few months of life. Infants are unable to hold up their heads, sit independently, or breathe properly. Without treatment, the disease progresses rapidly and often leads to serious complications.

SMA Type 2

This type usually appears between 6 and 18 months of age. Children with Type 2 can sit independently but are unable to walk without assistance. Breathing and swallowing may also be affected, although the disease progresses more slowly than Type 1.

SMA Type 3 (Kugelberg-Welander disease)

This milder form typically develops in childhood or adolescence. Children with Type 3 are initially able to walk but may lose this ability over time. With appropriate support and treatment, life expectancy is usually normal.

SMA Type 4

The mildest form of spinal muscular atrophy, it appears in adulthood, usually after age 30. Symptoms are generally mild and may include gradual muscle weakness in the legs and slight difficulty walking.

Symptoms and early signs of SMA

The symptoms of spinal muscular atrophy (SMA) vary depending on the type of the disease and the age at which they appear. However, certain signs can emerge in early childhood and may be the first indicators of delayed or impaired muscle development.

One of the most common early symptoms is muscle weakness, particularly in the arms and legs. In infants, this may appear as floppy limbs, low muscle tone, or the inability to lift their head while lying on their stomach.

Common early signs of SMA include:

• Difficulty with sitting, walking, or lifting the head
• Delayed development of motor skills such as rolling over, crawling, or standing
• Trouble with breathing and swallowing, especially in more severe forms
• Weak or completely absent reflexes in infants

If any of these symptoms are noticed at an early age, it is important to consult a neurologist as soon as possible and undergo appropriate testing.

How is the diagnosis made

Spinal muscular atrophy (SMA) is diagnosed through a combination of clinical examination, symptom observation, and precise genetic testing. Early detection enables faster access to treatment and helps preserve motor function, especially in children.

The first step is a clinical evaluation, during which the doctor assesses muscle strength, reflexes, tone, and developmental abilities. If SMA is suspected, additional diagnostic methods may be used:

• EMG (electromyography): measures electrical activity in the muscles and can indicate damage to motor neurons
• Genetic testing: the most reliable method for confirming the diagnosis

In addition to diagnosing affected individuals, genetic counseling is important for couples planning a family. If both partners are carriers of the mutated SMN1 gene, there is a risk that their child could develop the disease.

In such cases, preventive options are available:

• SMA (SMN1 gene, exon 7) – carrier screening performed before pregnancy to assess genetic risk in partners.
• Prenatal testing for spinal muscular atrophy is a non-invasive blood test performed during pregnancy that can detect the presence of SMA mutations in the fetus at an early stage
• Preimplantation genetic diagnosis (PGD), as part of the IVF process, allows for the selection of genetically healthy embryos

Treatment of spinal muscular atrophy

Although there is no cure for spinal muscular atrophy (SMA), several therapies are now available that can significantly slow the progression of the disease and improve quality of life.

Available treatment options include:

• Nusinersen (Spinraza): an intrathecal therapy that increases the production of SMN protein in patients with a mutation in the SMN1 gene
• Onasemnogene abeparvovec (Zolgensma): a one-time gene therapy that delivers a functional copy of the SMN1 gene
• Risdiplam (Evrysdi): an oral medication that stimulates the production of SMN protein

In addition to drug therapy, physical therapy and respiratory support play a critical role, especially for patients with more severe forms of the disease. Daily care requires a multidisciplinary approach involving a neurologist, physiatrist, pulmonologist, speech therapist, and other specialists, depending on the patient’s needs.

Frequently asked questions

Can healthy parents have a child with SMA?

Yes. If both parents are carriers of a mutation in the SMN1 gene, there is a 25% chance that their child will inherit the condition.

How common is spinal muscular atrophy?

SMA occurs in approximately 1 in 6,000 to 10,000 live births. About 1 in 50 people is a carrier of the gene mutation.

What is the life expectancy for individuals with SMA?

Without treatment, children with Type 1 rarely survive beyond 2 to 3 years of age. With therapy, individuals with milder forms (Types 3 and 4) can have a normal life expectancy.

What is the difference between SMA and muscular dystrophy?

SMA is a neurological disorder that affects motor neurons in the spinal cord, while muscular dystrophy primarily affects the muscle tissue itself. Although both conditions cause muscle weakness, their underlying causes and mechanisms are different.

Life with SMA is possible with support and timely treatment

Spinal muscular atrophy is a serious but increasingly well-recognized genetic disorder that requires early diagnosis and comprehensive care. While there is no permanent cure, modern therapies can significantly slow the progression of the disease and improve quality of life. Early detection of symptoms, access to genetic testing, and a multidisciplinary approach are key factors in achieving better outcomes.

If SMA is suspected in a family, IntroLab offers reliable genetic testing that can support informed decision-making. Understanding the condition and taking timely action are the first and most important steps toward living a life with SMA that is dignified, active, and fulfilling.