What are the clinical subtypes of SMA?
SMA subtypes are usually defined by the highest level of motor function achieved by the patient.
• SMA type 1 (Werdnig–Hoffmann disease) is the most severe and common phenotype, accounting for 50% of patients diagnosed with SMA. Children with SMA type 1 are never able to sit independently, and most patients die by 2 years of age from respiratory failure. The diagnosis is usually made before 6 months of age.
• SMA type 2 represents intermediate severity, and the age of onset is between 7 and 18 months. Patients are able to sit but unable to walk independently.
• SMA type 3 (Kugelberg–Welander disease) is variable in its prognosis for motor function. Some patients require a wheelchair for mobility during childhood, while others are able to ambulate independently and only have minor weakness.
• SMA type 4 is the mildest form of the disease and patients are diagnosed during the second and third decade of life. These patients have very mild weakness.