Antenatal Ultrasound Findings in Spinal Muscular Atrophy Type 0

Spinal muscular atrophy (SMA), caused by pathogenic variants in the survival motor neuron (SMN) gene, is the most common genetic cause of mortality in children under the age of two. Prior reports of obstetric sonograms performed in pregnancies with severe forms of fetal SMA have discrepant findings that may stem from a failure to account for the SMN2 copy number.

We present a neonate diagnosed with SMA type 0 postnatally (0SMN1/1SMN2 genotype). Antenatally, the fetus was noted to have HLHS (hypoplastic left heart syndrome), 2:1 AV block (atrioventricular), thickened nuchal translucency, polyhydramnios, and perceived maternal decreased fetal movement, and the mother declined genetic testing. A literature search was conducted to analyze potential prenatal findings in severe SMA type 0.

The most common associations from 32 cases of SMA type 0 include cardiac defects, increased NT (nuchal translucency), decreased fetal movement, and contractures noted postnatally. Other associations that were present in the literature and in our case include nonvertex presentation, polyhydramnios, and fractures after birth.

Prenatal onset SMA type 0 with one copy of SMN2 appears to have a distinct phenotype. Cardiac anomalies, increased nuchal translucency, and decreased maternal perception of fetal movement in the third trimester are the most frequent findings, and if found, should prompt SMA testing.