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Prevalence is estimated at around 1/80,000 and annual incidence is estimated at around 1/10,000. The disease is slightly more frequent in males than in females.

Disease onset occurs before 6 months of age (usually before 3 months). The severe muscle weakness (almost always symmetrical) first affects proximal limbs and then progresses to the extremities (hands and feet). Cries are weak. Poor sucking ability and reduced swallowing are frequent, leading to feeding difficulties. Deep tendon reflexes are absent. Respiratory failure is common. Mild contractures (of the knees and, more rarely, of the elbows), and scoliosis may be present. Patients are not able to sit without support and will never be able to walk.

As in the other forms of SMA, SMA1 is primarily caused by homozygous deletions in the SMN1 gene (5q12.2-q13.3) encoding the SMN (survival motor neuron) protein. Although there is some variation, disease severity in SMA is inversely correlated with the number of copies of the second SMN gene (SMN2; 5q13.2), with patients with MSA1 having a low number (1 or 2) of SMN2 copies. Deletions of the NAIP (5q13.1) gene have also been identified in SMA1 patients and may play a role in modifying disease severity.

The diagnosis is based on clinical history and examination and can be confirmed by genetic testing.

Differential diagnoses include SMA2, congenital muscular dystrophies, congenital myopathies, some early-onset mitochondrial disorders, and carbohydrate metabolism disorders (see these terms).

Antenatal diagnosis is possible through molecular analysis of amniocytes or chorionic villus samples.

Transmission is autosomal recessive but around 2% of cases are caused by de novo mutations. Genetic counseling should be offered to affected families.

Clinical trials are ongoing to identify potential drug treatments for SMA1, mainly targeted towards increasing the levels of the full length SMN protein. However, at present, management remains symptomatic, involving a multidisciplinary approach and aiming to improve quality of life. Respiratory support is necessary and physiotherapy is recommended. Noninvasive ventilation and gastrostomy may be useful. Antibiotic therapy is required in case of pulmonary infection and chronic hunger must be prevented.

The prognosis is generally poor with most patients dying within the first two years of life due to respiratory failure. However, in some cases, manifestations are stable or even regress, and patients may live longer.