Prenatal diagnosis of anencephaly: spectrum of sonographic appearances and distinction from the amniotic band syndrome.

To document the characteristic sonographic abnormalities of anencephaly and to identify potentially confusing sonographic features, we reviewed 20 cases of anencephaly. All of these cases were diagnosed prenatally with sonography after 14 menstrual weeks in patients who were seen at our institution between 1984 and 1988. In all cases, the correct diagnosis was made on the prenatal sonograms and was confirmed pathologically. The sonographic diagnosis was primarily based on the absence of brain and calvarium superior to the orbits on coronal views of the fetal head. This typical appearance was altered by the presence of echogenic tissue superior to the orbits in nine (45%) of 20 cases. Pathologically, the tissue corresponded to angiomatous stroma (area cerebrovasculosa) and appeared quite sizable on sonograms in four fetuses (20%). It may appear solid or mixed solid and cystic. In one fetus, it appeared brainlike. Despite this appearance, the sonologist should not be dissuaded from the diagnosis of classic anencephaly. Hydramnios occurred in seven (35%) of 20 patients, and oligohydramnios occurred in none of the patients. Anencephaly may be distinguished from the cranial defects associated with the amniotic band syndrome (amputation defects that occur as the sequelae of amniotic disruption) on the basis of the symmetry of the cranial defects (100% of anencephalic fetuses in this series) and the absence of limb, body wall, and spinal abnormalities that typically accompany the amniotic band syndrome. Although there may be minor variations in the sonographic appearance of the cranial defect of anencephalic fetuses (i.e., much or little angiomatous stroma), we conclude that this anomaly can be accurately detected and diagnosed on fetal sonograms obtained after 14 weeks menstrual age and distinguished from the amniotic band syndrome.