SMA (spinal muscular atrophy) is a disease that robs people of physical strength by affecting the motor nerve cells in the spinal cord, taking away the ability to walk, eat, or breathe. It is the number one genetic cause of death for infants. SMA is caused by a mutation in the survival motor neuron gene 1 (SMN1).
The disorder is caused by a genetic defect in the SMN1 gene, which encodes SMN, a protein widely expressed in alleukaryotic cells and necessary for survival of motor neurons. Lower levels of the protein results in loss of function of neuronal cells in the anterior horn of the spinal cord and subsequent system-wide muscle wasting (atrophy).
Spinal muscular atrophy manifests in various degrees of severity, which all have in common progressive muscle wasting and mobility impairment. Proximal muscles and lung muscles are affected first. Other body systems may be affected as well, particularly in early-onset forms of the disorder. SMA is the most common genetic cause of infant death.
Spinal muscular atrophy is an inherited disorder and is passed on in an autosomal recessive manner.
SMA manifests over a wide range of severity, affecting infants through adults. The disease spectrum is variously divided into 3–5 types, in accordance either with the age of onset of symptoms or with the highest attained milestone of motor development.
- SMA 1 (Infantile) -- Werdnig–Hoffmann disease (0–6 months onset): The severe form manifests in the first months of life, usually with a quick and unexpected onset ("floppy baby syndrome"). Rapid motor neuron death causes inefficiency of the major bodily organs - especially of the respiratory system - and pneumonia-induced respiratory failure is the most frequent cause of death. Babies diagnosed with SMA type 1 do not generally live past two years of age, with death occurring as early as within weeks in the most severe cases (sometimes termed SMA type 0). With proper respiratory support, those with milder SMA type I phenotypes, which account for around 10% of SMA1 cases, are known to live into adolescence and adulthood.
- SMA 2 (Intermediate) -- Dubowitz disease (6–18 months onset): The intermediate form affects children who are never able to stand and walk but who are able to maintain a sitting position at least some time in their life. The onset of weakness is usually noticed some time between 6 and 18 months. The progress is known to vary greatly, some patients gradually grow weaker over time while others through careful maintenance avoid any progression. Scoliosis may be present in these children, and correction with a brace may help improve respiration. Body muscles are weakened, and the respiratory system is a major concern. Life expectancy is somewhat reduced but most SMA2 patients live well into adulthood.
- SMA 3 (Juvenile) -- Kugelberg–Welander disease (>12 months onset): The juvenile form usually manifests after 12 months of age and describes patients who are able to walk without support at some time, although many later lose this ability. Respiratory involvement is less noticeable, and life expectancy is normal or near normal.
- SMA 4 (Adult-onset): The adult-onset form (sometimes classified as a late-onset SMA type 3) usually manifests after the third decade of life with gradual weakening of muscles – mainly affects proximal muscles of the extremities – frequently requiring the patient to use a wheelchair for mobility. Other complications are rare, and life expectancy is unaffected.