What is an Endometrial Cast?
An endometrial cast (also called a decidual cast) is the spontaneous sloughing of the entire endometrial lining in one piece, maintaining the triangular shape of the uterine cavity, rather than the typical fragmented shedding that occurs during normal menstruation 1, 2.
Pathophysiology
The endometrium undergoes decidualization—a progesterone-mediated hyperproliferation that naturally occurs during the menstrual cycle. Normally, this tissue dissolves and passes in fragments during menstruation. In rare cases, the natural dissolution fails to occur, and the entire endometrial lining detaches as a single membranous piece that retains the exact contour of the uterine cavity 3.
Primary Causes
The overwhelming majority of endometrial casts are caused by hormonal contraceptive use, specifically:
- Initiation of progesterone-containing contraceptives (combined oral contraceptive pills or progestin-only methods) 2, 4, 5
- Cessation or poor compliance with hormonal contraceptives, particularly:
The common thread is fluctuations in progesterone levels—either a sudden increase when starting hormones or a sudden withdrawal when stopping them 5.
Less Common Causes
While hormonal contraception dominates the etiology, rare cases have been associated with:
- Ectopic pregnancy
- Miscarriage or retained products of conception 3
- Postpartum hemorrhage
- Uterine anomalies (didelphys/bicornis)
- Spontaneous occurrence (extremely rare) 5
Clinical Presentation
Patients typically present with:
- Severe dysmenorrhea (termed "membranous dysmenorrhea" when associated with cast passage) 4
- Heavy vaginal bleeding
- Passage of large tissue per vagina
- Lower abdominal pain that may mimic genital tract obstruction 1
Critical pitfall: The combination of bleeding and obstructive symptoms can create a confusing clinical picture that may lead to extensive workup for other causes of abnormal uterine bleeding 1.
Proposed Mechanism
The pathophysiology likely involves dysregulation of matrix metalloproteinases (MMPs) or their tissue inhibitors, which normally control the orderly breakdown of endometrial tissue during menstruation 5. Progesterone fluctuations may disrupt this enzymatic balance, causing the endometrium to detach en masse rather than fragmenting.
Management Considerations
Patient education before initiating hormonal contraceptives is essential, as this rare but dramatic side effect can be alarming 2. Symptoms typically resolve spontaneously after passage of the cast 2. Histopathologic confirmation shows decidualized endometrium without malignant features 5.