Antenatal Detection of Mosaic Trisomy 22 with a Finding of Blake’s Pouch Cyst

The authors report a case of mosaic trisomy 22 diagnosed antenatally by amniocentesis at 19 weeks. The ultrasound finding was an isolated posterior fossa fluid collection in the brain with features possibly suggestive of a Blake’s Pouch cyst with doubtful hypoplasia of cerebellar vermis. The karyotype of the amniocytes was mos47, + 22[6]/46[8] with two separate clones of cells. Trisomy 22 was seen in one clone (43%) while the other clone (57%) had a normal karyotype. On postnatal examination after termination, there were no dysmorphic features. A selective autopsy of the fetal brain was suggestive of normal posterior fossa anatomy with normal cerebellar vermis which retrospectively confirmed the diagnosis of a Blake’s pouch cyst.