Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA) is a rare genetic disorder causing motor neuron deterioration, leading to progressive muscle weakness, impaired movement, and challenges with voluntary muscle control

What is SMA?

Spinal Muscular Atrophy (SMA) is a genetic neuromuscular disorder that causes progressive muscle weakness and loss of movement due to the deterioration of motor neurons, nerve cells in the spinal cord that control voluntary muscle activity.

SMA is one of the most common genetic causes of death in infants and young children worldwide, but with early detection and access to treatment, many individuals now live longer and healthier lives. Awareness, especially in families with known cases or where cousin marriages are practiced, plays a vital role in preventing transmission and improving outcomes.

What Causes SMA?

Spinal Muscular Atrophy (SMA) is a genetic condition passed down from parents to their child. It happens when a child inherits a faulty gene from both parents. This gene is responsible for keeping the nerves that control muscles healthy. When it doesn’t work properly, these nerves stop working, and the muscles become weak over time.

Parents who carry the faulty gene usually don’t have any symptoms themselves, but they can still pass it on to their children.

SMA Signs and Symptoms

Symptoms vary depending on the type of SMA, but they all involve muscle weakness that worsens over time. Early signs include:

  • Floppy limbs and poor muscle tone in infants
  • Delayed motor milestones (not sitting, crawling, or walking at expected age)
  • Difficulty feeding, swallowing, or breathing
  • Fatigue with physical activity in children or adults
  • Frequent chest infections due to weak breathing muscles
  • Progressive loss of mobility

SMA does not affect intelligence, hearing, or emotions.

How Is SMA Inherited?

SMA follows an autosomal recessive inheritance pattern:

  • Both parents must be carriers of the mutated SMN1 gene
  • Carriers have no symptoms, making the condition difficult to detect without testing
If both parents are carriers:
  • 25% chance with each pregnancy that the child will have SMA
  • 50% chance the child will be a carrier
  • 25% chance the child will not inherit the faulty gene at all

SMA affects males and females equally and can appear even in families with no known history, especially where cousin marriages are common, as both partners may unknowingly carry the same mutation.

Types of SMA

SMA is classified based on age of onset and motor milestones achieved. The earlier the onset, the more severe the condition typically is.

1. SMA Type 1 (Werdnig-Hoffmann Disease)
  • Appears in infants before 6 months of age
  • Babies are floppy ("hypotonia"), have poor head control, weak cries, and trouble feeding or swallowing
  • Many cannot sit without support
  • Without treatment, respiratory failure may occur within the first two years
2. SMA Type 2
  • Onset between 6 and 18 months
  • Children can sit but usually cannot walk unaided
  • May develop curvature of the spine (scoliosis), breathing difficulty, and nutritional issues
  • Life expectancy is variable depending on care
3. SMA Type 3 (Kugelberg-Welander Disease)
  • Onset after 18 months, often during early childhood
  • Children are able to walk but may lose this ability over time
  • Muscle weakness affects hips, legs, and back
  • Life expectancy is near normal with good medical support
4. SMA Type 4
  • Adult-onset SMA (after age 21)
  • Causes mild to moderate muscle weakness, often in the lower limbs
  • Does not significantly affect life expectancy

SMA Testing and Diagnosis

SMA is diagnosed using genetic testing 

Testing includes:

  • Diagnostic testing for children with symptoms of muscle weakness
  • Carrier screening for individuals with a family history or planning to marry within the family
  • Prenatal testing for pregnancies at risk
  • Newborn screening, where available, allows treatment before symptoms appear

In many countries, SMA is now part of national newborn screening programs due to the importance of early treatment.

The Role of Pre-Marital and Carrier Screening

Carrier screening is a simple blood or saliva test that checks if a person carries the SMA gene mutation. It’s especially important for:

  • Individuals with a family history of SMA or unexplained infant deaths
  • Couples related by blood (e.g., cousins)
  • Communities with higher rates of inherited genetic conditions
Why It Matters
  • Two healthy-looking carriers have a 1 in 4 chance with every pregnancy of having a child with SMA
  • Knowing carrier status before marriage allows couples to explore reproductive options and make informed choices
  • Reduces the emotional, financial, and medical burden on families and children
Pregnancy and Prenatal Testing

If both parents are found to be carriers, there are options during pregnancy to determine if the fetus is affected:

  • Chorionic Villus Sampling (CVS) – done between 10–13 weeks of pregnancy
  • Amniocentesis – done between 15–20 weeks

These tests analyze fetal DNA to detect SMA. Couples may also explore pre-implantation genetic diagnosis (PGD) with in-vitro fertilization (IVF) to avoid passing the condition to their child.

Planning early makes it possible to prepare for the child’s medical needs or explore all reproductive options.

SMA Treatment and Management

In recent years, SMA treatment has transformed due to medical breakthroughs. Early diagnosis allows access to these treatments before irreversible nerve damage occurs.

Approved Treatments

1. Gene Replacement Therapy

A one-time intravenous treatment designed to introduce a healthy copy of the gene responsible for SMA. This approach only works when given early in life (before 2 years of age), before significant muscle damage occurs.

2. Spinal Injections

This treatment is delivered directly into the spinal fluid every few months. It helps the body use a backup gene to produce more of the survival motor neuron (SMN) protein, which is essential for muscle function.

3. Oral Medication

A daily liquid medicine taken by mouth that also boosts SMN protein production. It is often used in ongoing treatment plans to help maintain or improve motor function.

Supportive Care Includes:
  • Physical and occupational therapy
  • Respiratory support (e.g., suction machines, ventilators)
  • Nutritional support (e.g., feeding tubes if swallowing is difficult)
  • Regular follow-up with a multidisciplinary team

With access to treatment and care, many children with SMA now attend school, participate in activities, and live longer than ever before.

Raising SMA Awareness and Reducing Risk

Though SMA is a genetic condition, its impact can be greatly reduced through:

  • Pre-marital and carrier testing, especially in high-risk families
  • Prenatal and pre-implantation genetic diagnosis
  • Newborn screening
  • Access to early treatment
  • Community education to break the silence around genetic testing

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