Type 0 is the most severe form of spinal muscular atrophy (SMA), a rare genetic disease characterized by the progressive loss of motor neurons, or nerve cells that control voluntary movement, leading to muscle weakness and wasting.
Along with the other four main types of SMA, type 0 is caused by mutations in both copies of the SMN1 gene (one inherited from the biological mother and one from the father), which provides instructions to produce a protein essential for motor neuron and muscle health.
Type 0 affects a baby while still in the womb, evidenced by reduced fetal movements in the later stages of pregnancy. At birth, infants with this SMA type typically present with severe weakness, extremely weak muscle tone (lack of tension in the muscles, which can cause floppiness), as well as respiratory failure.
Besides breathing difficulties, these babies commonly have a weak cry and suck, problems in swallowing, facial weakness, absent reflexes, joint contractures, and heart defects. Joint contractures refer to limited range of motion caused by shortening of muscles or tendons around a joint. Involuntary tongue movements may also occur. Usually, the affected babies do not develop any of the motor skills typically seen in infants their age.
This severe type can be fatal before birth or within the first six months of life.
Diagnosis of SMA type 0 can be made prenatally or shortly after birth through genetic testing for mutations in the SMN1 gene. During pregnancy, a placenta sample or the liquid surrounding the fetus can be used for such testing. If both parents carry one mutated copy of SMN1, they will have a 25% chance of having a biological child with one of the most common SMA types. In some countries, such as the U.S., newborn screening for SMA is available for free.
Currently, no cure exists for SMA, but three disease-modifying therapies — Biogen’s Spinraza (nusinersen), Roche’s Evrysdi (risdiplam), and Novartis’ gene therapy Zolgensma — have become available since 2016. While they have the potential to slow or even prevent progression of most of the main types of the disease, the few reports of their use in infants with type 0 highlighted mild improvements only and failure to prevent early death. Symptoms of SMA type 0 can be managed to a certain extent with ventilatory support and/or feeding tubes.
Last updated: Aug. 2, 2021
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