THE NORMAL CEREBELLUM  

 

 

  • Can be sonographically visualized as early as 9-10 weeks.

  • It grows rapidly in the second trimester having a linear relationship with gestational age. Measurement in mm equals approximate gestational age in weeks.
  • Peanut shaped with a central constriction denoting the vermis and flared ends representing the two hemispheres.
  • Its location in the posterior fossa (surrounded by the dense petrous ridges and occipital bone) makes it more resistant to deformation by extrinsic pressure.
  • It has therefore been proposed that the Transcerebellar diameter (Table / Graph) is a better predictor of gestational age than the BPD when there are variations in the shape of the fetal head (dolicocephaly or brachycephaly).

 

Transcerebellar Diameter

Cisterna magna

·         Sonographic changes in the appearance of the cerebellum throughout gestation.

 

Sonographic appearance of the cerebellum through gestation

Grade 1:

  • Seen predominantly up to 27 wks of gestation.
  • Cerebellar hemisphere is rounded and lacks echogenicity.
  • Vermis poorly developed giving the cerebellum the appearance of an “eyeglass”.

 

 

Grade II:

  • Seen predominantly from 28-32 wks of gestation.
  • Vermis more prominent and appears as an echogenic rectangular tissue connecting both hemispheres.
  • Cerebellar hemisphere is oval and the central portion is more echogenic than the peduncles but less echogenic than the circumferential margin of the hemisphere.
  • Cerebellum has a “dumbbell” appearance.

 

Grade III:

  • Seen predominantly after 32-33 wks of gestation.
  • Hemispheres become triangular or “fan-shaped”.
  • Echo pattern from the central portion of the hemisphere is now similar to the margin of the vermis.
  • Cerebellum now looks more solid than cystic.

 

 

 

 

NORMAL VARIANT – DEFECT IN THE INFERIOR VERMIS

 

Normal Variant - Defect in the Inferior Vermis

During normal embryologic development of the posterior fossa the cerebellar vermis appears to form by fusion of the cerebellar hemispheres superiorly and in the midline during week 9 of gestation (1,2).

Fusion continues inferiorly with complete closure of the vermis being complete at the end of the 15th week. Vermian development is one of the last steps in the formation of the cerebellum occurring when crown-rump length is 150 mm (2).



 

 

In a recent study (3), 56% of fetuses had an open vermis at 14 weeks gestation, decreasing to 23% at 15 weeks and 6% at 17 weeks. Therefore a diagnosis of Dandy-Walker variant should not be made prior to 18 weeks gestational age.

 

 

Normal Variant – Defect in Inferior Vermis created by a steep transducer angle.

 

 

 

Echogenic band of tissue crossing the vermis - artifact



 

REFERENCES
  1. Barkovich AJ, Kjos BO, Norman D et.al. Revised classification of posterior fossa cysts and cystlike malformations based on the results of multiplanar MR imaging. AJR 1989;10:977-988.
  2. Lemire RJ, Loeser JD, Leech RW et.al. Normal and abnormal development of the human nervous system. Hagerstown, Md: Harper and Rowe 1975:144-163.
  3. Bromley B, Nadel AS, Pauker S et.al. Closure of the cerebellar vermis: Evaluation with second trimester US. Radiology 1994;193:761-763.