This disorder affects 1 in 6000-10000 infants 1.
The condition typically affects infants and young children, presenting with progressive, symmetrical, proximal-predominant muscle atrophy and weakness of varying severity 1,2. Although lower extremities can have greater involvement, axial, intercostal, and bulbar musculature are also frequently involved 1,2.
In infants, the most common demographic affected, this classically manifests as difficulties sitting and rolling, assuming a frog-leg position, having a weak cry, and having increased respiratory effort with paradoxical breathing 1,3. Rarely is intellectually disability a feature 3.
SMA is classified into four types with decreasing clinical severity and age of onset 1,2:
- type 1 (Werdnig-Hoffman disease): infantile form, most severe and most common form, onset before six months
- type 2: intermediate form, onset between six and twelve months
- type 3 (Kugelberg-Welander disease): juvenile form, onset after twelve months
- type 4: adult form
Some authors suggest an additional type 0, also termed 'severe infantile' form with a prognosis (without novel therapy) of just a few weeks, although this has not been widely adopted in the literature 4.
SMA has an autosomal recessive mode of inheritance due to mutations to the SMN1 (survival motor neurone 1) gene on chromosome 5 1,2. This mutated gene has a carrier frequency of 1 in 40 1,2.
In muscles, the predominant features are those of muscle atrophy 5,6. Mild types tend to demonstrate fatty infiltration of muscular bundles with increased intramuscular fat planes 5,6. Intermediate forms may show ragged atrophy of muscles, while the severe forms often show gross atrophy 5,6.
In the spinal cord, MRI may reveal T2 hyperintensities in the anterior horns of the cervical cord, and less so in the thoracolumbar cord, that are thought to histopathologically correspond to motor neuron loss in these regions 3. These features may resemble the owl's eye sign 3.
Treatment and prognosis
Although management was once considered to be mainly supportive, recent advances (as of June 2017) have led to the development of oligonucleotide drugs such as nusinersen which hold promise in improving the quality of life and prognosis of these patients 7,8.
History and etymology
Spinal muscular atrophy was originally described in two infant brothers by Guido Werdnig (1844-1919), an Austrian neurologist, in 1891, and also in seven additional cases by Johan Hoffmann (1857-1919), a German neurologist, from 1893 to 1900 2.
- 1. Stephen J. Kolb, John T. Kissel. Spinal Muscular Atrophy: A Timely Review. Archives of Neurology. 68 (8): 979. doi:10.1001/archneurol.2011.74
- 2. Pearn J. Classification of spinal muscular atrophies. Lancet (London, England). 1 (8174): 919-22. Pubmed
- 3. Hsu CF, Chen CY, Yuh YS, Chen YH, Hsu YT, Zimmerman RA. MR findings of Werdnig-Hoffmann disease in two infants. American Journal of Neuroradiology. 19 (3): 550. Pubmed
- 4. Dubowitz V. Very severe spinal muscular atrophy (SMA type 0): an expanding clinical phenotype. European journal of paediatric neurology : EJPN : official journal of the European Paediatric Neurology Society. 3 (2): 49-51. doi:10.1053/ejpn.1999.0181 - Pubmed
- 5. Chan WP, Liu GC. MR imaging of primary skeletal muscle diseases in children. AJR Am J Roentgenol. 2002;179 (4): 989-97. AJR Am J Roentgenol (full text) - Pubmed citation
- 6. Murphy WA, Totty WG, Carroll JE. MRI of normal and pathologic skeletal muscle. AJR Am J Roentgenol. 1986;146 (3): 565-74. AJR Am J Roentgenol (abstract) - Pubmed citation
- 7. Chiriboga CA, Swoboda KJ, Darras BT, Iannaccone ST, Montes J, De Vivo DC, Norris DA, Bennett CF, Bishop KM. Results from a phase 1 study of nusinersen (ISIS-SMN(Rx)) in children with spinal muscular atrophy. Neurology. 86 (10): 890-7. doi:10.1212/WNL.0000000000002445 - Pubmed
- 8. Corey DR. Nusinersen, an antisense oligonucleotide drug for spinal muscular atrophy. Nature neuroscience. 20 (4): 497-499. doi:10.1038/nn.4508 - Pubmed