Overview

Incidence: 4–10 per 100,000 live births

Sex: ♂ > ♀

Autosomal recessive: defect in the SMN1 gene on chromosome 5q → defect in the assembly of small nuclear ribonucleoprotein (snRNP) to form the SMN complex → apoptosis of lower motor neurons (especially the anterior horn cells in the spinal cord) in the later stages of fetal development and postnatal period

Congenital motor neuron disease that only involves the lower motor neurons (spinal ± bulbar motor neurons) → muscle weakness, hypotonia, bulbar symptom

Sensations are not affected

Motor neurons of cranial nerves III, IV, and VI, and sacral motor neurons are not affected → preserved eye movement and continence