The isolated mildly enlarged cisterna magna in the third trimester: much ado about nothing?

ase—A 30-year-old woman, gravida 1, presents at 32 weeks for sonography to assess fetal growth. She is otherwise healthy, and her pregnancy has been uncomplicated. Her 18-week anatomy scan results were normal. Her serum screen for aneuploidy was low risk. On this sonographic examination, the fetus is found to have normal growth and fluid. The anatomic survey results are normal aside from a cisterna magna that measures 11 mm. The remainder of the intracranial anatomy is normal. How do you counsel her? With advances in fetal sonography, variations in fetal anatomy and structure are being identified. In particular, posterior fossa abnormalities pose a challenging diagnostic and management dilemma, as they are often difficult to interpret, and the neonatal outcome is difficult to accurately predict. Outcomes for neonates with posterior fossa abnormalities diagnosed antenatally can range from normal to severe developmental disabilities. The cisterna magna contains cerebrospinal fluid and is located behind the cerebellum in the posterior fossa of the brain. It communicates with the fourth ventricle via the foramina of Magendie and Luschka. Measurement of the anteroposterior diameter of the cisterna magna can be useful in diagnosing posterior fossa abnormalities. An enlarged cisterna magna has been defined as measuring greater than 10 mm.1 The optimal time to assess the posterior fossa for diagnosis of abnormalities has been debated in the literature. Embryologically, complete posterior fossa development is thought to occur between 18 and 22 weeks. Therefore, caution in making a diagnosis of cerebellar dysgenesis before 18 weeks is warranted.2 However, false-positive findings have also been reported at later gestational ages, and some authors have recommended that the diagnosis of the different forms of vermian hypoplasia should not be made before 24 weeks’ gestation.3 If a posterior fossa abnormality is suspected, further characterization by follow-up sonography is usuAsha J. Heard, MD Adam C. Urato, MD