ULTRASOUND OF DANDY-WALKER MALFORMATION

CLASSIFICATION – 3 MAIN TYPES:

1. Absence of the cerebellar vermis.
• Widely separated cerebellar hemispheres.
• V shaped anterior prolongation of the cisterna magna through which the cyst communicates with the fourth ventricle and may reach the cerebral peduncles.
• Increased trans-cerebellar diameter.
• Very large cisterna magna.
• Ventriculomegaly is usually present.

2. Absence of the cerebellar vermis + small cerebellum.
   Findings are similar to above group except:
• Normal trans-cerebellar diameter (due to the small cerebellum),
• Very large cisterna magna.
• Ventriculomegaly is usually present.

3. Defect in the cerebellar vermis (Dandy-Walker variant).
• Normal cerebellar hemisphere.
• Trans-cerebellar diameter is normal.
• Cisterna magna is marginally increased.
• V shaped prolongation of the inferior cisterna magna to reach the 4th ventricle.

PSEUDO DANDY-WALKER VARIANT

• A pseudo-Dandy-Walker variant has been described when a scan is done with steep angulation of the transducer giving the impression that there is a defect in the inferior vermis. This artifact arises as the plane of the ultrasound beam passes between the cerebellum and brainstem, creating the impression of communication between the cisterna magna and fourth ventricle. It has been speculated that it may be caused by fluid within the valleculae cerebelli that tends to expand anteriorly and has a thin membranous roof that cannot be visualized due to current limits in ultrasound resolution The juxtaposition of the vallecula with the adjacent cerebellar tonsils creates the impression of a connection between the fourth ventricle and cisterna magna.

ULTRASOUND

• Retrocerebellar cyst communicating with the 4th ventricle.
• Cisterna magna - enlarged.
• Vermian defect - wide spectrum of severity ranging from complete aplasia to absence of the inferior portion.
• Trans-cerebellar diameter.
• Increased or normal depending on the size of the vermian defect (and therefore the amount of splaying of the cerebellar hemispheres).
• Posterior fossa - Enlarged with upward displacement of the lateral sinus, tentorium and torcular.
• Hydrocephalus
• Previous series found hydrocephalus to be present in 50-70% of cases (2-4), however it has been suggested that frank hydrocephalus is absent at birth in almost 80% of cases.
• Pilu and co workers found that only 50% of fetuses had moderate to severe ventriculomegaly, with most of the remaining fetuses had borderline values of atrial width (10-15mm).
• Macrocrania - usually as a consequence of hydrocephalus rather than an enlarged posterior fossa.
• Occipital encephalocele (<5%).

CT Scan on Day 2 of life

REFERENCES

1. Hirsch JF, Pierre-Kahn A, Reiner D et.al. The Dandy-Walker malformation: a review of 40 cases. J Neurosurg 1984;61:51-52.
2. Pilu G, Romero R, De Palma L et.al. Antenatal diagnosis and obstetric management of Dandy-Walker syndrome. J Repro Med 1986;31:1017-1022.
3. Nyberg DA, Cyr D, Mack LA et.al. The Dandy-Walker malformation. Prenatal sonographic diagnosis and its clinical significance. J Ultrasound Med 1988;7:65-71.
4. Russ PD, Pretorius DH, Johnson MJ. Dandy-Walker syndrome: a review of fifteen cases evaluated by prenatal sonography. Am J Obstet Gynecol 1989;161:401-406.
5. Pilu G, Goldstein I, Reece EA et.al. Sonography of fetal Dandy-Walker malformation: a reappraisal. Ultrasound Obstet Gynecol 1992; :151-