Understanding the Different Types of Spinal Muscular Atrophy

Understanding the Variants of Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA) is a hereditary condition characterized by progressive weakness and loss of muscle mass due to nerve cell degeneration in the spinal cord and brainstem. This disorder prevents affected individuals from controlling their muscle movements. Globally, approximately 1 in 5,000 to 10,000 newborns are diagnosed with SMA. The primary cause in over 90% of cases is insufficient production of Survival Motor Neuron (SMN) protein, vital for motor neuron health.

The SMN protein is generated by the SMN1 and SMN2 genes located on chromosome 5. Most individuals have two copies of SMN1 and several copies of SMN2 in their motor neurons. In SMA patients, both SMN1 gene copies are missing or mutated, with extra SMN2 copies often resulting in milder symptoms. Here are the main SMA types you should know:

SMA Type 1
Known as Werdnig-Hoffman disease, this is the most severe form, typically evident at birth or within the first months of life. Infants with SMA1 usually display weak trunk control, floppy limbs, and difficulty with swallowing and head support. The disease progresses rapidly, leading to muscle weakening, respiratory infections, and often fatality by age 2.

SMA Type 2
Symptoms usually emerge between 7 and 18 months. The condition primarily affects leg muscles, preventing standing and increasing susceptibility to respiratory infections. With appropriate care, children with SMA2 may survive into adulthood, though severity varies.

SMA Type 3
Also called juvenile SMA or Kugelberg-Welander disease, it is the mildest form, appearing in children aged 18 months or older. These children can stand and walk but may struggle to rise from sitting and often experience muscle weakness.