What is SMA

Spinal Muscular Atrophy – S.M.A

Spinal muscular atrophy (SMA) is an incurable autosomal recessive disease caused by a genetic defect in the SMN1 gene which codes SMN, a protein necessary for survival of motor neurons, and resulting in death of neuronal cells in the anterior horn of spinal cord and subsequent system-wide muscle wasting (atrophy).

Spinal muscular atrophy manifests in various degrees of severity which all have in common general muscle wasting and mobility impairment. Other body systems may be affected as well, particularly in early-onset forms. SMA is the most common genetic cause of infant death.

The most severe form of SMA type I is sometimes termed SMA type 0 (or severe infantile SMA) and is diagnosed in babies that are born so weak that are able to survive only a few weeks even with intensive respiratory support. SMA type 0 should not be confused with SMARD1 which may have very similar symptoms and course but has a different genetic cause than SMA. Development milestone attainment is commonly measured using a specially modified Hammersmith Functional Motor Scale Symptoms

The symptoms vary greatly depending on the SMA type involved, the stage of the disease and individual factors and may include:

  • Areflexia, particularly in extremities
  • Marked hypotonia in legs, arms, rib, chest, and bulbar (facial) muscles, limpness or a tendency to flop
  • Difficulty achieving developmental milestones, difficulty sitting/standing/walking
  • Adopting of a frog-leg position when sitting (hips abducted and knees flexed)
  • Respiratory distress, weak cough/cry
  • Bell-shaped torso (caused by using only abdominal muscles for respiration)
  • Difficulty sucking or swallowing, poor feeding
  • Fasciculations (twitching) of the tongue
  • Arthrogryposis (multiple congenital contractures)


Prenatal screening is controversial, because of its cost on the one hand, and the severity of the disease on the other hand. Some researchers have concluded that population screening for SMA is not cost-effective, at a cost of $5 million per case averted in USA. Others conclude that SMA meets the criteria for screening programs and relevant testing should be offered to all couples. Very severe SMA (type 0/I) can be sometimes evident before birth – reduction in fetal movement in the final months of pregnancy; else, it manifests within the first few weeks or months of life when abnormally low muscle tone is observed (the “floppy baby syndrome”).

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