Polythelia: simple atavistic remnant or a suspicious clinical sign for investigation?

Supernumerary nipples (or polythelia) usually appear along the embryonic milk lines or in other sites including the back, thigh, vulva, neck etc. The frequency of polythelia ranges from 0.2% to 5.6%. Despite the plethora of published cases concerning its association with other congenital malformations or syndromes with different patterns of inheritance, polythelia still remains a controversial and theoretical issue. Although most reports describe a link between supernumerary nipples and kidney/urinary tract anomalies, a potential relationship with other congenital anomalies or malignancies has also been speculated. Additionally, polythelia has been associated with genodermatoses, thus being related with an increased malignant potential, as well as with an increased risk for solid tumors such as renal adenocarcinoma, testicular cancer, prostate cancer, and urinary bladder carcinoma. The fact that the Scaramanga (ska) mutant mice presented with ectopic breast tissue imply that misregulation of the neuregulin-3 signaling pathway may be critical in the occurrence of polythelia. This is an attempt to review existing literature in order to (a) draw reliable conclusions whether polythelia is a manifestation of simple atavism or may be associated with concomitant severe conditions needing further investigation and/or management, (b) elucidate its aetiology and (c) establish appropriate clinical and laboratory approach.