Early diagnosis of spina bifida: the value of cranial ultrasound markers.

Ultrasonography plays an integral part in the prenatal diagnosis of neural tube defects. However, experienced sonographers with careful evaluation are successful in accurately diagnosing spina bifida only 80-90% of the time. This study was conducted to evaluate the accuracy and reliability of certain cranial ultrasound markers--the "lemon sign," cerebellar abnormalities, microcephaly, and ventriculomegaly--in facilitating the diagnosis of spina bifida in patients referred for prenatal diagnosis. Open spina bifida was diagnosed in 24 of 44 fetuses found to have neural tube defects. The lemon sign and cerebellar abnormalities were identified in all 16 fetuses in whom the diagnosis of spina bifida was made between 16-24 weeks' gestation. In four of these fetuses, the lemon sign and cerebellar abnormalities were noted 1-2 weeks before the spinal defect was identified. Microcephaly was present in 69% and ventriculomegaly in 63% of the cases. In the eight cases diagnosed after 24 weeks' gestation, the lemon sign was less reliable, being noted in only 25% of the fetuses. Ventriculomegaly increased in frequency to 75% and cerebellar abnormalities and microcephaly were present in all. Our findings indicate that these cranial ultrasound markers are extremely reliable for the early diagnosis of spina bifida; their identification should alert ultrasonographers at all skill levels to the possibility of open spina bifida.