Abstract
Background. During prenatal ultrasonography, characteristic malformations including the lemon sign, the banana sign, and obliteration of the cisterna magna are widely used for diagnosis of Chiari type 2 malformation (Arnold Chiari malformation). However, these signs are often obscured by 25 weeks’ gestation. Here, we report an investigation of ultrasonographic markers of Chiari type 2 malformation for diagnosis after 24 weeks’ gestation. Methods. Twenty-two cases of 24–37 weeks’ gestation referred as fetal ventriculomegaly were analyzed prospectively. Fetuses were examined with ultrasonography by axial section of the head to detect conventional signs, and then by coronary section of the posterior horn of the lateral ventricle to detect two prospective new signs: bilateral downward-triangle shape (triangle sign) and quadrilateral angular shape (square sign). Results. From the axial sections, three cases were diagnosed with holoprosencephaly, but the remaining 19 cases did not show the lemon sign. In the coronar sections, the eight cases that showed the triangle sign were associated with open spina bifida at the lower vertebral site. The four cases that displayed the square sign showed upper spinal lesions. Chiari type 2 malformation was suspected in these 12 cases. Four of the other seven cases were suspected to be agenesis of the corpus callosum, and the remaining three were normal. All ultrasonographic findings were consistent with the subsequent MRI and postpartum definitive diagnoses. Conclusions. At 24 weeks’ gestation or later, Chiari type 2 malformation can be precisely and efficiently diagnosed by two new characteristics detected during ultrasonography: triangular shape and quadrilateral angular shape.