Spinal muscular atrophy is classified into types based on age of symptom onset and the highest motor milestone achieved. This classification also broadly correlates with SMN2 copy number, though exceptions exist.

*Type 0: Onset is prenatal to birth, and affected infants never achieve any independent motor milestone. Features include severe weakness at birth, reduced fetal movements, and often a need for ventilator support from birth; historically associated with very early mortality.
*Type 1 (Werdnig-Hoffmann disease): Onset is before 6 months of age, and affected children never sit independently. This is the most common and historically most severe postnatal form, marked by significant hypotonia and feeding and breathing difficulties.
*Type 2: Onset is between 6 and 18 months of age. Children sit independently but do not walk unaided. This intermediate form is associated with scoliosis and joint contractures developing over time.
*Type 3 (Kugelberg-Welander disease): Onset is after 18 months of age, and affected individuals walk independently at some point. This is a milder course, though the ability to walk may be lost later in childhood or adulthood in some individuals.
*Type 4: Onset is in adulthood, typically after age 21, with affected individuals walking independently. This is the mildest form, with slowly progressive proximal weakness that generally does not affect life expectancy.

This classification remains clinically useful for prognosis and care planning, but with the availability of newer disease-modifying therapies, the distinctions between types are becoming less rigid, since early treatment can shift the natural trajectory a child would otherwise have followed.