Athelia, the absence of one or both nipples, can occur in both females and males. Though rare, it is more frequently found in conjunction with certain other conditions. Typically, athelia appears on one side (unilaterally) in children with Poland syndrome and on both sides (bilaterally) in specific types of ectodermal dysplasia.Poland syndrome, named after the renowned 19th-century British surgeon Alfred Poland, involves a distinct pattern of unilateral malformations. It is marked by the absence of the chest (pectoralis) muscle on one side and webbed fingers (syndactyly) on the same side of the body. This condition is three times more likely to affect the right side than the left. It is classified as a "nonspecific developmental field defect," usually arising around the sixth week of fetal development, though its exact cause remains unknown. In those with Poland syndrome, the sternal head of the pectoralis major (the part of the main chest muscle that normally attaches to the breastbone) is missing. Additionally, nearby chest muscles, such as the serratus and latissimus dorsi, might also be absent, and there may be a lack of armpit hair. In females, the breast on the affected side is typically missing, and athelia is present.Ectodermal dysplasia encompasses a range of hereditary conditions characterized by abnormal skin development, lack of sweat glands, dry eyes, and unusual tooth formation. These features arise from abnormalities in the ectoderm, the outermost layer during early embryonic development. The absence of sweat glands leads to an inability to perspire, resulting in heat intolerance. Athelia is also associated with premature aging syndrome and Yunis-Varon syndrome, a multiple malformation syndrome first identified in 1980.It is important to distinguish athelia from amastia, where the entire breast is absent, and from amazia, where the breast tissue is missing but the nipple is still present.