Prenatal sonographic diagnosis of vermal agenesis.

Agenesis of the vermis as detected during gestation by ultrasonography may indicate the existence of various malformation arrays or syndromes. We report on our observations of five cases of complete vermal agenesis that were detected at 22-31 weeks of gestation. All had a vertex presentation and transvaginal sonography established the diagnosis of vermal agenesis. Two of the vermal agenesis cases had no associated anomalies outside the central nervous system (CNS). In one, the cerebellar cleft was the only abnormality present and the other also had lobar holoprosencephaly. The three remaining fetuses had trisomy 13 and featured various additional extra-CNS anomalies. The association of complete vermal agenesis and trisomy 13 has not been previously reported. Our experience with this series suggests that supplementation with vaginal fetal sonography is a valuable tool for obtaining a more accurate view of the posterior fossa whenever a cyst or a cyst-like abnormality is detected by transabdominal sonography. A finding of isolated vermal agenesis appears to mandate a careful search for additional anomalies and the performance of karyotype analysis.