The severity of the disease is classified into four types (SMA I-IV) based on SMN2 gene copy number, age of onset and motor function: type II SMA is considered the intermediate form, characterized by an early age of onset (7–18 months), reduced lifespan (10–40 years) and severe loss of muscular tone and strength (D’Amico et al., 2011). Here, SMN2 is linked to proximal spinal muscular atrophy.