Hypothesis: Central digit hypoplasia.

Limb deficiencies are a common birth defect. A malformations surveillance program among many newborns, stillborn fetuses, and malformed fetuses in elective terminations can identify a sufficient number of infants with the same set of abnormalities to characterize a specific limb deficiency phenotype. The active malformations surveillance program was carried out among 289,365 births at Brigham and Women's Hospital in Boston over a 41-year period (1972-2012). The research assistants identified the affected infants and fetuses from reading the findings recorded in each newborn's medical record by the examining pediatricians and consultants and by the pathologists in autopsies. One hundred ninety-four newborn infants and fetuses were found to have a limb deficiency either as an isolated abnormality or as one of multiple malformations. We identified three phenotypes of limb deficiency. We present here the seventeen infants and fetuses with "central digit hypoplasia," a term we suggest for this phenotype: hypoplasia of the thumb and fifth finger with nubbins of soft tissue in place of fingers 2, 3, and 4 at the level of the metacarpal-phalangeal joint. Central digit hypoplasia is to be distinguished primarily from the terminal transverse limb defect that ends at the wrist. In symbrachydactyly, the middle and distal phalanges of the fingers and toes are hypoplastic. In addition, central digit hypoplasia should be distinguished from the amniotic band syndrome, the most common and incorrect diagnosis suggested by the pediatricians and the consultants in this survey. The affected infant and her/his parents benefit from more accurate and specific counseling.
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