Fluid area measurements in the posterior fossa at 11-13 weeks in normal fetuses and fetuses with open spina bifida.

OBJECTIVES: To measure the area of the intracranial translucency (IT) (syn: 4th ventricle) and the future cisterna magna (CM) in normal fetuses and to compare with fetuses with open spina bifida.

PATIENTS: In the midsagittal plane of the face of 220 fetuses between 11 and 13 weeks' gestation, the areas of the IT and CM were measured and the sum, defined as the posterior fossa fluid (PFF) area was calculated. Reference ranges were constructed in relation to the crown-rump length. The study group consisted of 21 fetuses with open spina bifida and showed in all cases a single pocket of fluid in the posterior fossa. Fetuses with no fluid in the fossa were excluded. This PFF-area was measured and compared with the reference range of the IT-area and the PFF-area of normal fetuses and Z-scores were calculated.

RESULTS: In normal fetuses, a significant increase of the IT-, the CM- and the PFF-area was found as a sign of the expanding posterior fossa. The mean PFF-area increased from 8.55 to 29.72 mm(2) in the observation period. Fetuses with open spina bifida had reduced fluid in the posterior fossa with values ranging between 2.39 and 5.08 mm(2) and significantly lower Z-scores.

CONCLUSIONS: Fetuses with open spina bifida have an abnormally small posterior fossa at 11-13 weeks' and in cases where the cerebrospinal fluid is still present, the fluid area in the midsagittal plane is reduced when compared to normal fetuses. Area fluid assessment can be an additional useful measurement in suspicious cases for open spina bifida in early gestation.