Spinal Muscular Atrophy (SMA) is a genetic disorder characterized by the progressive degeneration of motor neurons, leading to muscle weakness and atrophy. It primarily affects infants and children, with varying degrees of severity.

Causes 

• Genetic mutation: SMA is caused by mutations in the SMN1 (survival motor neuron 1) gene, which is critical for the maintenance of motor neurons. The severity of SMA is influenced by the number of copies of a similar gene, SMN2, which can partially compensate for the loss of SMN1.
• Inheritance: SMA is an autosomal recessive disorder, meaning a child must inherit two defective copies of the SMN1 gene (one from each parent) to develop the disease.

SMA is classified into four types based on the age of onset and severity of symptoms:

• Type 1 (Infantile-Onset or Werdnig-Hoffmann Disease): Onset: Before 6 months of age. Severe muscle weakness, inability to sit without support, breathing and swallowing difficulties.
• Type 2 (Intermediate SMA): Onset: Between 6 and 18 months of age. Ability to sit but not to stand or walk unaided, progressive muscle weakness, potential respiratory issues.
• Type 3 (Juvenile SMA or Kugelberg-Welander Disease): Onset: After 18 months of age, typically in childhood or adolescence. Ability to stand and walk, though walking becomes increasingly difficult over time, muscle weakness primarily affects legs and hips.
• Type 4 (Adult-Onset SMA): Onset: Adulthood. Mild to moderate muscle weakness, typically affecting proximal muscles.

Image credit: Peter Saxon, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons