Background
Supernumerary nipples are common minor congenital malformations that consist of nipples and/or related tissue in addition to the nipples normally appearing on the chest. Supernumerary nipples are located along the embryonic milk lines. Ectopic supernumerary nipples are found beyond the embryonic milk lines. The embryonic milk line is the line of potentially appearing breast tissue as observed in many mammals. In humans, the embryonic milk line extends bilaterally from a point slightly beyond the axillae on the arms, down the chest and the abdomen toward the groin. It is generally thought to end at the proximal inner sides of the thighs, although supernumerary nipples have been described on the foot. [1] The axilla is the most common location accounting for 60%-70% of all cases. [2] Supernumerary nipples can appear complete with breast tissue and ducts and are then referred to as polymastia, or they can appear partially with either of the tissues involved.
The classification established by Kajava in 1915 remains valid [3] :
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Complete supernumerary nipple - Nipple and areola and glandular breast tissue (polymastia)
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Supernumerary nipple - Nipple and glandular tissue (no areola)
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Supernumerary nipple - Areola and glandular tissue (no nipple)
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Aberrant glandular tissue only
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Supernumerary nipple - Nipple and areola and pseudomamma (fat tissue that
Pathophysiology
Saint-Hilaire in 1836 and Darwin in 1871 supported the notion that the supernumerary nipple is an atavistic structure derived from the milk line of mammals. Conceivably, even the ectopic supernumerary nipple falls in line with Darwin's theory of atavism. Supernumerary nipples on the vulva are consistent with the location of breasts in dolphins and whales; whereas, ectopic supernumerary nipples on the back, scapula, and shoulder [38, 39, 40] are reminiscent of breast tissue in nutria and hutia (rodents).
Between the fourth and fifth weeks of embryogenesis, an ectodermal thickening forms symmetrically along the ventral lateral sides of the embryo. This epidermal ridge extends from the axillary region to the inner side of the thigh to form the embryogenic milk (or mammary) line. During the second and third months of embryonic development, the glandular elements of the breast are formed near the fourth and fifth ribs, with regression of the rest of the thickened ectodermal streaks. In the case of failure of a complete regression, some foci may remain, resulting in a supernumerary nipple. This can develop into a supernumerary complete breast (polymastia) or into any other supernumerary nipple variant according to the Kajava classification.
Etiology
Most supernumerary nipples are sporadic, although familial cases have been reported, including 1 report of a family who had supernumerary nipples in 4 successive generations. Familial cases are typically inherited in an autosomal dominant fashion with incomplete expressivity. [41]
Epidemiology
Frequency
The prevalence of supernumerary nipples varies geographically. The prevalence is 0.22% in a Hungarian population, [12] 1.63% in African American neonates, [26] 2.5% in Israeli neonates, [27] 4.7% in Israeli Arabic children, [42] and 5.6% in German children. [35]
Sex
The incidence of supernumerary nipples is similar in males and females. Some studies have supported a slight male predominance with estimates of male-to-female ratio as high as 1.7:1.
Prognosis
Most isolated supernumerary nipples persist without complication. Ectopic breast tissue can be associated with the same inflammatory and neoplastic conditions that affect normal breast tissue. Ectopic breast tissue does not have an increased malignant potential compared with normal breast tissue.
Patient Education
Once a supernumerary nipple is diagnosed, inform the parents or the patient that it is a benign skin lesion in an otherwise healthy individual. Patients should be made aware that supernumerary nipples can go through changes like any regular nipple or breast; these changes may be physiological during puberty or pathological, such as inflammation, mastitis, abscess formation, cysts, adenomata, fibroadenoma, carcinoma, melanoma, or Paget disease.
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Supernumerary nipple in a neonate. Courtesy of Dr P. Merlob.
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Supernumerary nipple (bilateral) in an adolescent girl.
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Supernumerary nipple in an adolescent boy. Courtesy of Dr B. Fisher.
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Ectopic supernumerary nipple on the shoulder. Courtesy of Dr B. Fisher.
