MRI is a useful tool to complement US for imaging of the fetal posterior fossa (PF). In France, the discovery of a PF malformation in the fetus frequently leads to termination of pregnancy (80% in a personal series). However, despite improved accuracy in the diagnosis of PF abnormalities, prognosis remains uncertain. The first objective of this review is to document the normal MRI landmarks of the developing fetal PF. Because of their thinness, the visibility of the cerebellar fissures is dramatically delayed on MRI compared to macroscopic data. An important landmark is identification of the primary fissure of the vermis, normally seen at around 25-26 weeks' gestation (WG) on the sagittal slice, separating the larger posterior lobe from the anterior lobe (volume ratio around 2:1). The prepyramidal and secondary fissures are usually only identifiable after 32 WG and the hemispheric fissures are difficult to see until the end of pregnancy. Considering the signal changes, high signal on T2-weighted (T2-W) sequences is seen from 25 WG in the posterior part of the brain stem (tegmentum and ascending sensory tracts) related to myelination. The low signal intensities seen within the cerebellum on T2-W images correspond to high cellularity of grey matter (deep nuclei), as there is no myelination within the white matter before 38 WG. The second objective is to highlight the signs highly predictive of a poor neurological prognosis. Lack of pontine curvature or vermian agenesis without a PF cyst (small volume of PF) is greatly associated with poor neurological status. The third objective is to propose a diagnostic strategy in difficult cases where prognosis is important, e.g. the Dandy Walker continuum. Analysis of the cerebellum is often impossible if a PF cyst is present (whatever its nature) as the mass effect usually blurs the foliation and even impairs evaluation of the normal ratio between the posterior and anterior lobes of the vermis. Isolated cerebellar hypoplasias raise the question of prognosis and genetic counselling. Such uncertainties require an amniocentesis and a careful search for other anomalies (cerebral and extracerebral). Unilateral abnormalities of a cerebellar hemisphere can be associated with good neurological status if they are isolated. The final objective is to discuss other rare PF fetal abnormalities, such as vascular malformations and tumours.

This study demonstrates that if outcome of isolated ACC is favorable, a long follow-up is necessary: with age, IQ in the lower range and behavioral troubles are linked to difficulties in school.

Although prenatal diagnosis of isolated CCA is reliable, false postnatal diagnoses remain possible (10-20%) even with complete prenatal screening. Outcome is mostly favorable because intelligence is within the normal range for nearly 3/4 of the children. However, they frequently have mild learning difficulties.

Prenatal ultrasonographic (US) detection of congenital adenomatoid malformation (CAM) was made in 18 fetuses at 17 - 36 weeks' gestation and managed in our institution during a 10-year period (1985-1994). The lesion was left-sided in 13 cases, right-sided in 4, and bilateral in 1. According to Stocker's classification, 12 cases were type I, 4 type II, and 2 type III. The prenatal course was followed with serial US examinations in 13 cases; the size of the lesion was stable in 8 and decreased in 5. Mediastinal shift was usually observed, and amniotic fluid volume was increased in 4 cases. One fetus was aborted. Six infants presented with respiratory distress syndrome and required neonatal surgery; delayed surgery was performed in 9 cases. Spontaneous regression of the lesion was observed on follow-up in 2 cases. Surgery consisted in lobectomy in 8 cases and segmentectomy in 6. The presence of fetal hydrops, type III lesions, and bilateral lung involvement are prenatal factors known to be associated with a poor prognosis. However, this series and a review of the literature suggest that caution should be observed with regard to the initial impression when counseling the parents regarding prognosis.

The diagnostic value of amniotic fluid gamma-glutamyl-transpeptidase (GGTP) and intestinal alkaline phosphatase (iALP) was evaluated in 55 patients who underwent amniocentesis for karyotyping because fetal gastric or small bowel dilatation had been detected by ultrasound. Gastrointestinal malformation was confirmed in 46 cases and there was no gastrointestinal anomaly in nine cases. Prenatal ultrasound was suggestive of gastroduodenal dilatation in 34 cases (group I) and small bowel dilatation in 21 cases (group II). In group I, amniotic fluid GGTP above the 99th percentile was 71 per cent sensitive and 100 per cent specific for a true anatomical defect of the digestive tract (mainly duodenal atresia). In group II, high levels of GGTP and/or iALP were 69 per cent sensitive and 83 per cent specific for a fetal digestive tract anomaly. In other words, when digestive tract dilatations were diagnosed by prenatal sonography, abnormal amniotic fluid enzyme activities were strongly suggestive of such an anomaly, the possibility of which was not precluded by normal amniotic fluid iALP and GGTP activities. But amniotic fluid digestive enzyme activities do not help in defining the prognosis.