What is Spinal Muscular Atrophy (SMA)?

What is Spinal Muscular Atrophy (SMA)?


Spinal Muscular Atrophy (SMA) is a congenital neuromuscular disorder characterized most prominently by progressive muscle atrophy and weakness.1 SMA is a neurodegenrative disease and the weakness and loss of muscle tone associated with SMA are the direct result of the death of anterior horn cells in the spinal cord.2 The weakness of SMA is typically proximal more than distal and can involve brainstem motor nuclei in severe forms of the disease.3 The phenotypic spectrum seen in patients with SMA is broad and ranges in age and severity from death in infancy due to absence of any tone including respiratory muscle compromise4 to an insidious loss of muscle mass without functional compromise in adults.5 There are five major subtypes of SMA designated types 0-4 that correspond to the age ranges of onset from prenatal (type 0) through adulthood (type 4).5-7

SMA is a common congenital illness, occurring in an estimated 1 in 11,000 live births.6 SMA is inherited as an autosomal recessive disease, and virtually all patients with SMA have biallelic deletions in exon 7 of the Survival Motor Neuron 1 (SMN1) genes located on chromosome 5q.8 While there is a genotype-phenotype correlation noted in SMA, the presence of other genes can modify the severity of the disease.9 Other diseases can present with similar features to SMA but genetic testing confirms SMA diagnosis and there is increasing advocacy for prenatal testing for early diagnosis and intervention.10,11

Treatment needs for SMA vary by age and phenotype. Infants with SMA require fulltime dedicated care including respiratory, orthopedic, and nutritional support while older children may only physical therapy and require assistive devices for safe ambulation.12

There is one approved therapy for SMA, nusinersen (Spinraza), an antisense oligonuceliotide that increases the production of a homologue of SMN1 and appears to have efficacy as measured by achievement of motor milestones.13

Prognosis in SMA is tied to age of onset and severity of disease. Onsets between prenatal and 6 months of age are associated with severe weakness, respiratory compromise, and early life mortal outcomes.7 Onset of SMA in childhood is not a benign diagnosis but patients can survive into adulthood.3 The adult onset variant of SMA is very rare but appears to confer minimal morbidity and no risk of mortality.5


1. Darras BT. Spinal muscular atrophies. Pediatric clinics of North America. 2015;62(3):743-766.

2. Beattie CE, Kolb SJ. Spinal muscular atrophy: Selective motor neuron loss and global defect in the assembly of ribonucleoproteins. Brain Res. 2018;1693(Pt A):92-97.

3. Yuan P, Jiang L. Clinical characteristics of three subtypes of spinal muscular atrophy in children. Brain Dev. 2015;37(5):537-541.

4. MacLeod MJ, Taylor JE, Lunt PW, Mathew CG, Robb SA. Prenatal onset spinal muscular atrophy. Eur J Paediatr Neurol. 1999;3(2):65-72.

5. Juntas Morales R, Pageot N, Taieb G, Camu W. Adult-onset spinal muscular atrophy: An update. Revue neurologique. 2017;173(5):308-319.

6. Lally C, Jones C, Farwell W, Reyna SP, Cook SF, Flanders WD. Indirect estimation of the prevalence of spinal muscular atrophy Type I, II, and III in the United States. Orphanet journal of rare diseases. 2017;12(1):175.

7. Grotto S, Cuisset JM, Marret S, et al. Type 0 Spinal Muscular Atrophy: Further Delineation of Prenatal and Postnatal Features in 16 Patients. J Neuromuscul Dis. 2016;3(4):487-495.

8. Feldkotter M, Schwarzer V, Wirth R, Wienker TF, Wirth B. Quantitative analyses of SMN1 and SMN2 based on real-time lightCycler PCR: fast and highly reliable carrier testing and prediction of severity of spinal muscular atrophy. American journal of human genetics. 2002;70(2):358-368.

9. Butchbach ME. Copy Number Variations in the Survival Motor Neuron Genes: Implications for Spinal Muscular Atrophy and Other Neurodegenerative Diseases. Frontiers in molecular biosciences. 2016;3:7.

10. Verhaart IEC, Robertson A, Wilson IJ, et al. Prevalence, incidence and carrier frequency of 5q-linked spinal muscular atrophy – a literature review. Orphanet journal of rare diseases. 2017;12(1):124.

11. Sugarman 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 Genet. 2012;20(1):27-32.

12. Qian Y, McGraw S, Henne J, Jarecki J, Hobby K, Yeh WS. Understanding the experiences and needs of individuals with Spinal Muscular Atrophy and their parents: a qualitative study. BMC neurology. 2015;15:217.

13. Maharshi V, Hasan S. Nusinersen: The First Option Beyond Supportive Care for Spinal Muscular Atrophy. Clinical drug investigation. 2017;37(9):807-817.