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Supratentorial Abnormalities in the Chiari II Malformation, II

Tectal Morphologic Changes

Department of Radiology, University of California, San Francisco, California USA.

Address correspondence to Roy A. Filly, MD, Department of Radiology, University of California, 505 Parnassus Ave, L374, San Francisco, CA 94143-0628 USA. E-mail: roy.filly{at}radiology.ucsf.edu

Objective. The sonographic diagnosis of fetal myelomeningocele has improved mainly because the diagnostic focus has shifted from observation of spinal manifestations to observation of cranial abnormalities. Intracranial diagnostic criteria rely on abnormalities in the posterior fossa. We describe abnormalities in tectal morphologic characteristics that, although well described on magnetic resonance imaging, have received little attention in the sonographic literature. This study analyzed the frequency of this observation and technical aspects that improve its visualization. Methods. From a database of obstetric sonograms, we identified all cases of sonographically detected myelomeningocele. The search covered the years 1999 to 2007. We retrospectively reviewed the fetal intracranial findings with special attention to elongation or "beaking" of the tectum. Results. A total of 89 fetuses were identified. The mean and median gestational ages were 22 weeks 4 days and 22 weeks 2 days, respectively. Of the 89 cases, 59 (66%) had an abnormal tectal shape. The abnormality was seen in 77% of cases judged to be suboptimally visualized and 62% of cases with good visualization. Tectal abnormalities were seen equally well in fetuses before and after 24 weeks. Finally, tectal abnormalities were seen more frequently as the severity of posterior fossa findings increased. Conclusions. Tectal morphologic alteration is a common supratentorial feature of the Chiari II malformation on prenatal sonography both before and after 24 weeks’ gestation. Its frequency increases with the severity of posterior fossa abnormalities. Therefore, it may be useful as a supratentorial indicator of both the presence and, potentially, the severity of the Chiari II malformation.

Key Words: Chiari II malformation • fetal sonography • myelomeningocele • tectum shape

Abbreviations: MRI, magnetic resonance imaging

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