Rectus Abdominis Muscle Endometriosis: A Unique Case Report with a Literature Review

Abstract

Introduction and importance: Extrapelvic endometriosis, confined exclusively to the body of the rectus abdominis muscle, is a rare form of abdominal wall endometriosis. While its etiopathology remains unclear, it is often diagnosed in healthy women who present with atypical symptoms and localization unrelated to any incision site, or in the absence of a history of endometriosis or previous surgery. Presentation of the case: Here, we describe a unique case of intramuscular endometriosis of the rectus abdominis muscle in a healthy 39-year-old Caucasian woman. The condition was located away from any prior incisional scars and presented without typical symptoms or concurrent pelvic disease, making diagnostic imaging unclear. After partial surgical resection of the endometriotic foci, the diagnosis was confirmed histologically. Progestogen-based supportive medication was initiated to prevent the need for additional surgeries and to reduce the risk of recurrence. After 6 years of follow-up and continued progestogen treatment, the patient remains symptom-free and has shown no recurrence of the disease. Clinical discussion: Endometriosis of the rectus abdominis muscle exhibits specific characteristics in terms of localization, etiopathology, symptomatology, and diagnostic imaging, suggesting that it should be considered a distinct clinical entity. Conclusions: Although rare, primary endometriosis of the rectus abdominis muscle should be included in the differential diagnosis for women of childbearing age. Early diagnosis is essential to avoid delayed recognition, tissue damage, and to minimize the risk of recurrence or malignant transformation. Given the increasing frequency of gynecologic and laparoscopic surgeries worldwide, it is crucial to establish standardized reporting protocols, follow-up timelines, and imaging assessments during specific phases of the menstrual cycle. Standardization will help raise awareness of this disease, and further our understanding of its pathogenesis, risk factors, recurrence patterns, and potential for malignant transformation-factors that are still not fully understood.

Keywords: abdominal wall endometriosis; case report; incomplete surgical resection; rectus abdominis muscle; unique clinical entity.

Figure 1

Cytologic examination of fine-needle aspiration. Fine-needle aspiration shows cohesive fragments consist of cells with crowding and overlapping of enlarged nuclei and indistinct cytoplasmic borders; May Grunwald-Giemsa stain, magnification ×100 (A), ×200 (B).

Figure 2

Magnetic resonance imaging (MRI) of abdomen and pelvis. Sagittal (A) and axial (B) HASTE sequence on magnetic resonance imaging (MRI) of abdomen and pelvis (like T2 weighted imaging with fat suppression) reveals enlarged left rectus abdominis muscle with ill-defined hyperintense lesion (arrow).

Figure 3

Pathohistological microscopic examination. Pathohistological analysis of the intramuscular node shows clusters of glands and the stroma characteristics of the endometrial mucosa. In the vicinity of the gland with a dilated lumen, the stroma of the endometrium is less noticeable, next to which the remnants of striated muscle tissue can be seen (asterisk) (A). Abundant connective tissue is seen around the glands and the clearly visible stroma of the endometrium, but the transverse striated musculature is not found (B). The focus of endometriosis with the microscopic foci of fresh bleeding is surrounded by connective tissue that permeates the fibers of the striated musculature with reparative changes (C). At high magnification, the epithelium of the gland is without atypia, showing mild proliferation, and there is scarce fresh bleeding in the surrounding stroma (D). Hematoxylin and eosin stain, magnification ×100 (A,B), ×200 (C), ×400 (D).

Figure 4

Alpha-smooth muscle actin (ASMA) immunohistochemical staining. Immunohistochemical staining for ASMA shows myoepithelial cells in the basal layer of the endometrial glands, but also within the stroma (arrows), magnification ×100.

Figure 5

Immunohistochemical staining of inflammatory cells and blood vessels. Immunohistochemical staining using antibodies against specific cluster of differentiation (CD) molecules shows rare scattered B lymphocytes (A) and significantly more T lymphocytes (B) with a higher proportion of cytotoxic (C) than helper lymphocytes (D). Macrophages were present in a smaller percentage than T lymphocytes (E), while the stroma of endometriotic foci shows blood vessel proliferation (F); (A) CD20; (B) CD3; (C) CD8; (D) CD4; (E) CD68; (F) CD31; magnification ×100.

Figure 6

Anterior abdominal wall ultrasound examination. An ultrasound examination of the anterior abdominal wall with a linear 15 Hz probe 5 years after resection with minimal scarring (arrow) of the left rectal muscle without signs of recurrence.