Spinal muscular atrophy (SMA) Carrier Screening

Clinical Background

Spinal muscular atrophy (SMA) is an autosomal recessive disease that causes progressive muscle wasting and weakness due to loss of motor neurons in the spinal cord. The disease is often fatal and is the leading genetic cause of death in infants. SMA incidence is approximately 1 in 10,000 live births and the carrier rate is 1 in 50.

Classification of Spinal Muscular Atrophy

SMA is usually subdivided into three clinical groups based on age at onset and degree of motor function:

SMA Type

Age of Onset

Clinical Severity



<6 months

Severe: generalized 
weakness and hypotonia; unable to sit

Patients typically die of 
respiratory failure before 
2 years of age


7–18 months

Intermediate: patients 
typically able to sit but not 

Survival beyond 4 years into adulthood depending on degree of respiratory distress


>18 months~
>20 years

Mild~ Very mild: patients can typically stand and walk
or have motor impairment

Normal lifespan



SMA and carrier screening

SMA is caused by mutations in the survival motor neuron 1 gene (SMN1). Roughly 95% of cases are caused by it. Normal individuals have 1 or more full-length SMN1 copies on each chromosome, affected patients have mutated copies which have no function on both chromosomes. SMA carriers typically have 1 normal copy and 1 mutated copy, and have no symptoms. The SMA Carrier Screen determines the copy number, or gene dosage of the SMN1 gene.