Background We present the case of a female infant with mosaic trisomy 21/Turners syndrome (45 X0, 47 XX +21). Phenotypic features at birth were consistent with Trisomy 21. Her postnatal course was complicated by transient abnormal myelopoiesis which has resolved on follow up. This case presents an interesting challenge in the management and follow up of mosaic Trisomy 21/Turners including the dual endocrine issues and developmental follow up.

Case A female infant was born at 37⁺³ weeks gestation following an spontaneous vaginal delivery. Examination was significant for upslanting palpebral fissures, epicanthic folds and low set ears. A third fontanelle was also present. Examination of the peripheries revealed a single palmar crease on the right hand and a bilateral sandal gap. There was hypotonia on ventral and vertical suspension with mild head lag.

Demonstrated was an abnormal female karytotype of 45X, 47XX + 21 with presence of 2 cell lines. Twenty one metaphases had 45x while 19 had 47xx+21. A buccal smear revealed two abnormal cell lines – 65% trisomy 21 and 34% XO disomy 21. Also significant in post-natal course was thrombocytopaenia and haematology opinion was sought. Investigations displayed a population of immature myeloid cells (CD7 positive), thrombocytopenia on blood film and atypical mononuclear cells with circulating myeloblasts. A diagnosis of transient abnormal myelopoiesis was made. This has resolved with follow up.

We were pleased to see her growing and thriving appropriately at six week follow up. Developmentally, she had normal tone but had yet to display a social smile.

Discussion The incidence of Trisomy 21 in Ireland is the highest in Europe at 1 in 546 live births whereas the incidence of Turner’s syndrome is 1 in 2500 live female births. Double aneuploidy of both autosomal and sex chromosomes, is, however a rare occurrence. It presents an interesting challenge in interpreting clinical features and in follow up, investigation and management of associated co-morbidities. Patients with mosaic Trisomy 21 typically demonstrate a milder phenotype than non-mosaic individuals. This individual is likely to demonstrate both features of Turner’s syndrome and Trisomy 21 although previous reports on Turner/Trisomy 21 polysyndrome mosaicism suggest that phenotypic features of Downs Syndrome predominates.