In the natural history of spinal muscular atrophy (SMA), all patients experience a progressive loss of motor function over time1

SMA is a rare neuromuscular disease that is characterized by the degeneration of motor neurons in the spinal cord and brainstem, leading to skeletal muscle atrophy and general weakness.1,2

SMA is the leading genetic cause of infant death, with an estimated incidence of 1 in 11,000 live births in the United States.6,7


*SMN2 copy number of 2.6
Event-free survival was defined as death or requiring at least 16 hours/day of noninvasive ventilatory support (for at least 14 days in the absence of an acute reversible illness or perioperatively).6


Progression can be a series of gradual, cumulative changes.8

Many with later-onset SMA may lose function in adulthood8-11

Muscle strength or function loss occurs for many during adulthood, despite apparent plateaus.

Function loss cannot be predicted11,12

It is not currently possible to predict when motor function losses will occur, or who will experience them.

Consistent monitoring of motor function is key for assessing disease progression10,12,13

Clinical strength tests correlate with patients’ functional abilities.

No-charge genetic testing for your patients

Find out more about genetic testing, sponsored by Biogen and offered through Invitae, at no charge to patients.

Understanding SMA

Watch a video about the clinical manifestation and mechanism of disease in SMA.

Survival motor neuron (SMN) protein in SMA

Learn about the production of SMN protein.


1. Darras BT, Royden Jones H Jr, Ryan MM, De Vivo DC, eds. Neuromuscular Disorders of Infancy, Childhood, and Adolescence: A Clinician’s Approach. 2nd ed. London, UK: Elsevier; 2015.

2. Lunn MR, Wang CH. Spinal muscular atrophy. Lancet. 2008;371(9630):2120-2133.

3. De Sanctis R, Coratti G, Pasternak A, et al. Developmental milestones in type I spinal muscular atrophy. Neuromuscul Disord. 2016;26(11):754-759.

4. Prior TW, Finanger E. Spinal muscular atrophy. NCBI Bookshelf website. Updated December 22, 2016. Accessed September 12, 2018.

5. Rouault F, Christie-Brown V, Broekgaarden R, et al. Disease impact on general well-being and therapeutic expectations of European type II and type III spinal muscular atrophy patients. Neuromuscul Disord. 2017;27(5):428-438.

6Finkel RS, McDermott MP, Kaufmann P, et al. Observational study of spinal muscular atrophy type I and implications for clinical trials. Neurology. 2014;83(9):810-817.

7Sugarman EA, Nagan N, Zhu H, et al. Pan-ethnic carrier screening and prenatal diagnosis for spinal muscular atrophy: clinical laboratory analysis of >72,400 specimens. Eur J Hum Gen. 2012;20(1):27-32.

8Wadman RI, Wijngaarde CA, Stam M, et al. Muscle strength and motor function throughout life in a cross-sectional cohort of 180 patients with spinal muscular atrophy types 1c–4. Eur J Neurol. 2018;25(3):512-518.

9Montes J, McDermott MP, Mirek E, et al. Ambulatory function in spinal muscular atrophy: age-related patterns of progression. PLoS One. 2018;13(6):e0199657.

10Werlauff U, Vissing J, Steffensen BF. Change in muscle strength over time in spinal muscular atrophy types II and III: a long-term follow-up study. Neuromuscul Disord. 2012;22(12):1069-1074.

11Zerres K, Rudnik-Schöneborn S, Forrest E, Lusakowska A, Borkowska J, Hausmanowa-Petrusewicz I. A collaborative study on the natural history of childhood and juvenile onset proximal spinal muscular atrophy (type II and III SMA): 569 patients. J Neurol Sci. 1997;146(1):67-72.

12Deymeer F, Serdaroglu P, Parman Y, Poda M. Natural history of SMA IIIb: muscle strength decreases in a predictable sequence and magnitude. Neurology. 2008;71(9):644-649.

13Querin G, Lenglet T, Debs R, et al. The motor unit number index (MUNIX) profile of patients with adult spinal muscular atrophy. Clin Neurophysiol. 2018;129(11):2333-2340.