Endometriosis Associated Striated Muscle Changes: Simulating Tumor

Endometriosis in the abdominal wall can be a diagnostic challenge, both clinically and pathologically. We are describing a case of abdominal wall endometriosis where regenerative muscle cells are florid. The cells are small and polygonal in shape, arranged in groups, and intersecting through differentiated skeletal muscle. Cells have pink cytoplasm and a centrally located nucleus. Immunohistochemistry demonstrates diffuse positivity with S100 and CD56, and they are negative with Ae1/Ae3, Pax8, inhibin, calretinin, and CD68. Cells are also focally positive with actin, vimentin, and myogenin. Electron microscopy reveals the presence of intracytoplasmic myofibrils. These groups of small cells are compatible with early to intermediately differentiated skeletal muscle cells as demonstrated by histomorphology, immunohistochemical pattern, and ultrastructure analysis. Muscle undergoes a regenerative process following injury. Endometriosis associated regenerative changes in muscle are very rare as documented in the English literature. The presence of florid regenerative muscle cells mimics a neoplastic process. In this report, we describe the histomorphological and immunohistochemical pattern of regenerative muscle groups and discuss differential diagnoses. We found electron microscopy to be an important diagnostic tool in this case.