First-trimester cystic posterior fossa: reference ranges, associated findings, and pregnancy outcomes.

Objective: To report reference ranges for fetal cerebral posterior fossa measurements and to describe the sonographic findings, karyotype results, and pregnancy outcomes in fetuses presenting with cystic posterior fossa (CPF) in the first trimester of pregnancy.

Methods: Two groups of patients undergoing first-trimester sonographic screening at 11-13 weeks' gestation were studied. The first (control group) consisted of 253 consecutive fetuses with normal posterior fossa, in which the brainstem (BS), fourth ventricle, cisterna magna, and BS-occipital bone (BS-OB) diameter were prospectively measured and the BS/BS-OB diameter ratio was calculated. The second (study group) consisted of 14 fetuses in which a CPF was detected. Information on sonographic findings, prenatal karyotype results, and pregnancy outcomes was obtained by reviewing ultrasound reports and medical records. The results from the two groups were then compared.

Results: In the control group, the size of all posterior fossa structures increased and the BS/BS-OB diameter ratio slightly decreased as the pregnancy progressed. In the study group, the BS diameter did not differ significantly from the measurements obtained in the control group. However, the BS-OB diameter and the fourth ventricle were significantly larger ( < .05 and  < .001, respectively) in the study group than in the control group. Additionally, the cisterna magna was not identified in 13 of the 14 fetuses (93%) in the study group, in comparison to zero out of the 253 fetuses in the control group ( < .001). Finally, the BS/BS-OB diameter ratio was significantly smaller in the study group when compared with the control group ( < .05). Regarding pregnancy outcomes, 12 of the 14 (86%) affected pregnancies underwent elective termination ( = 11) or ended in an early intrauterine demise ( = 1) due to the associated chromosomal abnormalities or structural defects. The two fetuses with isolated CPF had a normal second-trimester scan and resulted in the delivery of healthy newborn infants.

Conclusions: The detection of a CPF in the first trimester is associated with a high rate of chromosomal and structural defects. By using normative data, early sonographic screening and detection of mildly and moderately abnormal cases is possible. Fetuses with isolated CPF require further study with a detailed second-trimester scan. This is essential in order to differentiate cases with poor and good perinatal outcomes. Finally, our data also demonstrate that the main sonographic tool when screening for CPF in the first trimester is the assessment of the fourth ventricle, which is significantly larger in abnormal cases as the result of the wide communication between the fourth ventricle and the cisterna magna.