LAUSR

OBJECTIVES The disruption or defect of the myometrium in the uterine scar of a cesarean section (CS) has been known by various names, such as uterine niche, isthmocele, deficient uterine scar, scar pouch, or diverticulum. Symptomatology, risk factors for niche development, and available treatment modalities have been recently studied. However, the histologic features of this disease remain unknown. METHODS The histologic features of eight uterine niches are thoroughly described and a summary of the most important aspects of the uterine niche literature is provided. Five cases of CS scars without niche formation are comparatively examined. RESULTS Most uterine niches harbor endocervical mucosa, often cystically dilated and/or an atrophic or disorganized endometrial mucosa of lower uterine segment origin. Regenerative epithelial atypia and fibroblastic stromal reaction are frequent features. No granulomatous reaction, important inflammation, or hemorrhage is seen. CS scars without niche formation do not harbor endocervical mucosa or inclusion cysts, fibroblastic stroma, or regenerative atypia. CONCLUSIONS As more prospective studies of uterine niche development and treatment will be conducted, a detailed pathologic report with the criteria proposed herein can be designed.