Spinal Muscular Atrophy (SMA) type 2 is an intermediate severity infantile onset SMA and as the second most common diagnosed form of the disease, encompasses approximately 32% of patients with SMA.1,2 Patients with type 2 SMA will experience their first symptoms between 7 and 18 months of age.1
Though patients with type 2 SMA will progress to have significant weakness and disability, there is an expectation of a broader range of function and far lower childhood mortality than is seen in the more severe type 0 and type 1 forms of SMA.3 Patients with type 2 SMA share the common features of proximal muscle weakness and low tone, but, differentiating them from patients with type 1 SMA, patients with type 2 disease may sit without support.3,4 Patients with type 2 SMA will not ambulate independently, however.4 Electromyography in patients with type 2 SMA typically demonstrates a range of low compound motor action potential amplitudes.3
Categorically, patients with type 2 SMA can be divided into groups of patients who achieve and maintain sitting without assistance and those who do not achieve independent sitting.2,3 Weakness and reduction in tone are progressive from disease onset with some patients following a more aggressive disease course than others, and patients who never sit without support have the poorest prognosis.3,5 Survival past age two years in patients with type 2 SMA is correlated with higher retention of developmental gains.3,5 In a large database of patients with SMA, the median survival for patients with type 2 SMA was 59.9 years.2 As with other forms of SMA, cognitive function is preserved in patients with type 2 SMA.3
References
1. Mercuri E, Bertini E, Iannaccone ST. Childhood spinal muscular atrophy: controversies and challenges. The Lancet Neurology. 2012;11(5):443-452.
2. Belter L, Cook SF, Crawford TO, et al. An overview of the Cure SMA membership database: Highlights of key demographic and clinical characteristics of SMA members. J Neuromuscul Dis. 2018;5(2):167-176.
3. Chabanon A, Seferian AM, Daron A, et al. Prospective and longitudinal natural history study of patients with Type 2 and 3 spinal muscular atrophy: Baseline data NatHis-SMA study. PLoS One. 2018;13(7):e0201004.
4. Munsat TL, Davies KE. International SMA consortium meeting. (26-28 June 1992, Bonn, Germany). Neuromuscular disorders : NMD. 1992;2(5-6):423-428.
5. Phan HC, Taylor JL, Hannon H, Howell R. Newborn screening for spinal muscular atrophy: Anticipating an imminent need. Seminars in perinatology. 2015;39(3):217-229.