Adenolipoma of the skin is a recently discovered benign entity first described in 1993 by Hitchcock. It is a variant of lipoma which consists of an admixture of adipose tissue and eccrine sweat glands, hence the name adenolipoma. However, in 1997, the name peri-sudoral lipoma was suggested by Ait-Ourhrouil and Grosshans [rx] since they believed that adenolipoma is a misnomer due to the fact that this lesion originates from adipose tissue around the eccrine glands without proliferation of the glands or ducts themselves. Adenolipomas are typically found in patients aged 25–75 years, with the average being 50 years of age [rx].
Clinically, adenolipomas present as a single, slow-growing, painless lump. They are very similar in presentation to typical lipomas. Adenolipomas are most commonly found arising from the thigh, followed by the arms, shoulders, and chest [rx]. The size typically ranges from 1 to 6 cm.
By gross examination, adenolipomas appear to be soft, lobulated, yellow masses. Histologically, they consist of lobules of adipose tissue that are larger than those of typical subcutaneous tissue. Dispersed amongst these adipocyte clusters are adult-like eccrine glands and ducts. These eccrine glands and ducts may show epithelial hyperplasia, cystic duct dilatation, and squamous or clear cell metaplasia [rx]. Rarely, apocrine glands may also be found as noted in one of our cases [rx]. The first case of apocrine glands visualized within an adenolipoma was reported in 2005 by Antúnez et al. [rx] in an adenolipoma of the elbow. Most reported cases of adenolipomas have an encircling capsule; however, only 1 of the 11 cases in our case study was encapsulated. They typically arise from the dermis or subcutaneous fat. As for adenolipomas which involve the dermis, they are found at a deeper level than the normal eccrine glands. The distance between the dermis and eccrine sweat glands in both patients and adenolipoma-free people was measured by Ait-Ourhrouil and Grosshans [rx]. They concluded that the distance is much smaller in normal skin than in adenolipomas.
Histological features in correlation with clinical presentation are the mainstay for the diagnosis of adenolipoma with adipose cells mixed with eccrine glands and ducts. Ancillary studies such as special stains and immunohistochemistry may help highlight features but are not required to make a diagnosis. Eccrine glands are composed of secretory epithelial cells surrounded by myoepithelial cells. The secretory cells contain intracytoplasmic granules and intraluminal secretions which stain periodic acid-Schiff positive and are positive for epithelial markers such as cytokeratin. The surrounding myoepithelial cells are actin, S-100, and p63 positive [rx].
The most common differential diagnoses of adenolipoma include the typical lipoma, nevus lipomatosus superficialis, eccrine angiomatous hamartoma, spindle cell lipoma, and cutaneous myxolipoma. Adenolipomas and lipomas have a very similar presentation; they both present in around the same age range and tend to spare the more distal regions of the body. However, adenolipomas are typically smaller in size. Occasionally, a common subcutaneous lipoma may compress the dermis giving an appearance very similar to that of an adenolipoma. Yet the presence of eccrine glands around the lesion as opposed to being found within the lesion hints towards a diagnosis of a common lipoma.
Nevus lipomatosus superficialis presents in quite a different manner from adenolipomas. As opposed to being a single lesion it usually presents as multiple papules or nodules with a wrinkled or smooth surface arranged in a linear fashion commonly on the hip or buttock. It presents at birth or during infancy. Rarely, it may present as a solitary mass but they are more likely to be diagnosed as polypoid fibrolipomas. Histologically, collections of adipocytes amidst collagen bundles are found in the dermis. The rest of the dermis appears to be completely normal; however, the number of fibroblasts, the density of the bundles of collagen, and the vascularity are higher than those of normal skin. An abundance of mast cells and perivascular inflammation are also noted.
Eccrine angiomatous hamartomas usually present as a solitary lesion on the lower limbs at birth. On microscopy, the deeper layers of the dermis show a raised number of eccrine structures surrounded or intermingled with capillary channels. Some fatty tissue may also be seen in the rare lipomatous variant [rx].
Spindle cell lipomas are benign lesions typically affecting the posterior neck, upper back, and shoulders. Microscopy shows three main constituents: adipose cells, spindle cells, and collagen. Occasionally, myxoid matrix may be seen, while with cutaneous myxolipoma typical microscopy shows adipocytes with well-developed myxoid areas.
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