SMA is classified into several types based on the age of onset and the severity of symptoms:
Type 0: The most severe form, evident at birth or in utero, leading to significant muscle weakness and respiratory failure. Type 1 (Werdnig-Hoffmann disease): Symptoms appear before 6 months of age. Infants exhibit severe muscle weakness, poor head control, and difficulty swallowing and breathing. Type 2 (Dubowitz disease): Onset occurs between 6 to 18 months. Children can sit but cannot stand or walk independently. Type 3 (Kugelberg-Welander disease): Symptoms appear after 18 months and into early adulthood. Patients can walk but may lose this ability over time. Type 4: Adult-onset SMA, which is less common and less severe than the other types.
