Spinal muscular atrophy

Spinal muscular atrophy (SMA) is a type of congenital neuromuscular disease affecting anterior horn cells of the brainstem and spinal cord.

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

Genetics

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.

MRI

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.

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 yield a lot of promise to improving the quality of life and prognosis of these patients 7,8

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

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Article information

rID: 10154
Synonyms or Alternate Spellings:
  • Spinal muscular atrophy (SMA)

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Cases and figures

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    SMA : age 10 yrs
    Case 1: type 2, age 10 yrs
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    SMA : age 12 yrs
    Case 1: type 2, age 12 yrs
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