Sir.—Gardner in the Journal (133:253-254, 1979) reviewed case reports showing polysomy-X masquerading as Down syndrome.1 Specifically, persons with XXXX, XXXXX, and XXXXY karyotypes were reported to resemble children with trisomy 21 syndrome. Triplo-X (47XXX) may be added to this list. We have seen a retarded teenage girl who had this karyotype and a suggestive, Downlike facies.1 A. R. Robinson, MD, pointed out in conversation (November 1979) that children with 47XXX ascertained by newborn screening have also shown an increased incidence of epicanthal folds and clinodactyly, and resemble children with Down syndrome.2
Misdiagnosis of polysomy-X is, of course, not the only reason to perform a karyotype on every child with Down syndrome. Approximately 5% of these patients will have a translocation, half of whom have a parent who is a balanced carrier. A karyotype is mandatory for accurate genetic counseling regardless of maternal age.
YEATMAN GW. Triplo-X and Down Syndrome. Am J Dis Child. 1980;134(5):527. doi:10.1001/archpedi.1980.02130170077030