Spinal Muscular Atrophy (SMA): Complete Guide

What is Spinal Muscular Atrophy (SMA)?

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder that targets the motor neurons in the spinal cord, which control muscle movements. In spinal muscular atrophy (SMA), progressive loss of these neurons leads to increasing muscle weakness and atrophy. The condition is caused by a defect in the SMN1 gene, which produces the SMN protein necessary for motor neuron survival. People with SMA may have several types that differ in symptom severity and age of onset. Common symptoms include muscle weakness, movement difficulties, respiratory and swallowing challenges, and developmental delays.

What is SMA?

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder that targets the motor neurons in the spinal cord, responsible for controlling muscle movements. In spinal muscular atrophy (SMA), the progressive loss of these neurons causes increasing muscle weakness and atrophy (muscle mass reduction). The condition is due to a defect in the SMN1 gene, which produces the SMN protein vital for neuron survival. Individuals with spinal muscular atrophy (SMA) have either absent or mutated SMN1 genes, leading to insufficient SMN protein. Types of spinal muscular atrophy (SMA) differ by symptom severity and age at onset. Common symptoms include weakness, muscle wasting, movement issues, and developmental delays.

What Causes Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is caused by a genetic mutation. The primary cause is a defect in the survival motor neuron 1 (SMN1) gene, which produces a protein essential for motor neuron health. These neurons send signals from the brain and spinal cord to muscles, allowing movement.

In people with spinal muscular atrophy (SMA), mutations or deficiencies in the SMN1 gene prevent the body from producing enough SMN protein. This deficiency leads to damage and dysfunction in motor neurons, causing muscles to weaken and waste away.

Spinal muscular atrophy (SMA) follows an autosomal recessive inheritance pattern, meaning both parents must carry the defective gene for a child to be affected. If only one parent is a carrier, the child will not have spinal muscular atrophy (SMA) but may still be a carrier.

Types of SMA

Spinal muscular atrophy (SMA) is divided into several types, depending on the age of onset and symptom severity:

• Type 1 (Werdnig-Hoffman Disease): The most severe and common form of spinal muscular atrophy (SMA), appearing at birth or within the first months of life. Infants cannot hold up their heads or sit and often experience severe respiratory issues.
• Type 2: A moderate form of spinal muscular atrophy (SMA) that begins between 6 and 18 months. Children can sit but cannot walk unaided.
• Type 3 (Kugelberg-Welander Disease): A milder type of spinal muscular atrophy (SMA) that develops after 18 months. Children can walk but may lose this ability later.
• Type 4: The mildest adult-onset spinal muscular atrophy (SMA), developing around age 30, with gradual muscle weakness.

Symptoms of SMA

Symptoms of spinal muscular atrophy (SMA) vary by type but include:

1. Muscle Weakness and Atrophy: The main sign of spinal muscular atrophy (SMA) is progressive muscle weakness, beginning in the spine and hips, then spreading to limbs.
2. Movement Problems: Weak muscles lead to coordination issues and frequent falls.
3. Respiratory Problems: Spinal muscular atrophy (SMA) weakens breathing muscles, sometimes requiring ventilation.
4. Swallowing and Feeding Problems: Difficulty swallowing and poor nutrition are common.
5. Other Symptoms: Scoliosis, joint stiffness, and fatigue often accompany spinal muscular atrophy (SMA).

How is SMA Diagnosed?

Diagnosis of spinal muscular atrophy (SMA) involves:

• Clinical Evaluation: Assessment of muscle strength, reflexes, and family history.
• Electromyography (EMG): Measures muscle electrical activity, which decreases in SMA.
• Genetic Testing: Detects SMN1 gene mutations, confirming SMA in 95% of cases.
• Additional Tests: Muscle biopsy, creatine kinase tests, and lung function evaluations may be used when necessary.

SMA Test

The spinal muscular atrophy (SMA) test is a genetic test to detect the SMN1 gene mutation. It identifies carriers and diagnoses the condition.

Why it’s done:

• To identify SMA carriers
• To confirm SMA diagnosis
• To assess risk before marriage or pregnancy

How it’s done:

A blood sample is taken and analyzed in a lab to check for SMN1 gene mutations.

Results:

• Negative: No SMA mutation found.
• Positive: Carrier or affected by spinal muscular atrophy (SMA).
• Unclear: Additional testing required.

What Do SMA Test Results Mean?

There are three possible outcomes for SMA test results:

• Negative: The person does not carry the SMA-causing genetic mutation.
• Positive: The person carries the SMA gene mutation. Although carriers typically do not develop SMA, they can pass the mutation on to their children.
• Unclear: The test results are inconclusive; additional testing may be needed for a definitive answer..

Who Should Consider Taking the SMA Test?

Testing for SMA is recommended if:

• You have a family history of SMA.
• You’re planning a family or are pregnant.
• You show symptoms of SMA.

How is SMA Treated?

There are now multiple treatment options for spinal muscular atrophy (SMA) that improve muscle function and slow disease progression:

1. Gene Therapy (Zolgensma): Replaces the faulty SMN1 gene.
2. Splicing Modifier Therapy (Spinraza): Boosts SMN2 gene activity.
3. SMN Protein Enhancer (Evrysdi): Increases SMN protein through oral medication.
4. Supportive Therapies:
   • Physical and respiratory therapy
   • Nutritional and orthopedic support
   • Psychological and speech therapy

Treatment plans for SMA are individualized, considering patient age, symptoms, and overall health.

Choosing Treatment:

SMA treatment is tailored to each patient's age, specific disease type and severity, symptoms, and overall health. Treatment decisions are made together by the patient's family and their doctor.

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