Cuillier F, MD*
* Department of Gynecology, Félix Guyon’Hospital, 97400 Saint-Denis ** Sonographer, Sain-Andre ***Department of Gynecology, Intercommunal’s Hospital, 97400 Saint-Benois , Ile de la Réunion, France.
Definition: A cyst-like mass in the posterior fossa has been regarded as pathological finding attributed to Dandy-Walker malformation, vermian cerebellar hypoplasia, mega cisterna magna or Dandy-Walker variant.
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Dandy-Walker malformation: characterized by the ascent of the cerebellar tentorium and the torcular with ventriculomegaly of variable degree, resulting in a large posterior fossa (large cistern magna). This anomaly is frequently associated with chromosome aberrations. A defect in the cerebellar vermis exists and there is a communication with the fourth ventricle.
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Vermian cerebellar hypoplasia: easy detectable, without large cistern magna
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Megacisterna magna: an enlarged cistern magna with integrity of both cerebellar vermis and fourth ventricle.
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Dandy-Walker variant: There is a variable hypoplasia of the cerebellar vermis with or without enlargement of the cistern magna.
Nevertheless, the diagnosis of a cystic lesion of the posterior fossa often leads to difficult diagnosis and prognostic dilemmas.
Case report: A 30-year-old woman, G3P2, was referred at 32 weeks. At 13 weeks, the nuchal translucency was normal. Biochemical test 17 weeks was normal. During anomaly scan at 22 weeks, a moderate ventriculomegaly was discovered (11 mm). At 26 weeks, the cerebellar vermis seemed hypoplastic, but the cerebellar hemispheres were normal. The two lateral ventricles were dilated. The corpus callosum and the artery were normal. In a sagittal view, we could see the vermis malrotation, but without hypoplasia. Amniocentesis was performed. The fetal karyotype was normal, 46 XX. All screening serology were negative (Toxoplasmosis, Rubella, MNI test, CMV, Herpes and Chikungunya). At 27 weeks, the ventriculomegaly were stable. After genetic counseling the parents decided to continue this pregnancy. At 32 weeks, a MRI confirmed the ultrasound findings. According to Pr Kahn (Pediatric Neuro-surgery, Paris), the diagnosis of posterior fossa arachnoid cyst could be possible. Our colleague recommended us to perform serial scans to detect an increase of the ventriculomegaly or severe hydrocephaly.
The baby was born at 39 weeks (3100g). The external morphology analysis of the baby was normal. The neurological examination was normal. The transfontanellar scan confirmed the diagnosis and did not find any posterior fossa arachnoid cyst or any other cerebral anomaly.
Axial image at 26 weeks showing the cisterna magna and the vermis hypoplasia