Decidual Reaction and Arias-Stella Reaction
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Direct answer
Decidual reaction is the transformation of endometrial stromal cells into large, polygonal cells with pale, glycogen-rich cytoplasm during pregnancy. It can also appear in the fallopian tube in ectopic pregnancy. The Arias-Stella reaction is a hormone-driven change in endometrial glands, with enlarged, hyperchromatic and atypical nuclei that can mimic endometrial carcinoma. Recognising both on histology prevents false diagnosis of malignancy and points towards pregnancy.
What you must remember
- Decidual cells: large, polygonal, with pale or eosinophilic cytoplasm and round, vesicular nuclei.
- Sites: the endometrium in intrauterine pregnancy, and the tubal stroma in ectopic pregnancy.
- Arias-Stella cells: enlarged glandular epithelial cells with hyperchromatic, pleomorphic nuclei, prominent nucleoli and cytoplasmic clearing.
- Causes of Arias-Stella change: pregnancy, intrauterine or ectopic, exogenous progestins and gestational trophoblastic disease.
- Main mimic: Arias-Stella change can be mistaken for endometrial carcinoma, especially clear cell carcinoma, on a curettage specimen.
- Ectopic clue: decidual reaction without chorionic villi on curettage raises suspicion of ectopic pregnancy.
- Resolution: the clinical history, beta-hCG and the presence of villi resolve most diagnostic confusion.
A curettage that shows decidua but no villi
A pathologist reports curettage tissue with large epithelial nuclei, but the patient has a positive pregnancy test and no chorionic villi are seen. The two changes must be separated from neoplasia, and the absence of villi is itself informative. The table compares the key features.
| Feature | Decidual reaction | Arias-Stella reaction |
|---|---|---|
| Cell type | Endometrial stromal cells | Glandular epithelial cells |
| Cell appearance | Large, polygonal, pale cytoplasm | Enlarged, hyperchromatic nuclei with clearing |
| Typical setting | Pregnancy; tubal decidua in ectopic pregnancy | Pregnancy, exogenous progestins, trophoblastic disease |
| Main pitfall | Confused with tumour cells | Mimics endometrial carcinoma |
Both changes are hormone-driven and reversible, so they indicate hormonal stimulation rather than neoplasia. Decidua with absent villi, in the right clinical setting, makes ectopic pregnancy the leading concern. Villi on the same slide would instead point to an intrauterine pregnancy, so the presence or absence of villi is the decisive feature. The pathologist should request the clinical history and beta-hCG before the report is finalised.
Where the exam tries to catch you
Histology questions test recognition of the mimic and the ectopic clue.
- Trap: reporting an Arias-Stella change as carcinoma Correct: check the clinical history and beta-hCG, since the reaction is hormonal and benign.
- Trap: dismissing decidua in curettage without villi Correct: this combination raises suspicion of ectopic pregnancy.
- Trap: assuming decidual cells are malignant Correct: decidual cells are the normal stromal change of pregnancy.
Frequently asked questions
What is the Arias-Stella reaction?
The Arias-Stella reaction is a benign, hormone-driven change of endometrial glandular epithelium, seen in pregnancy and other states of progestogenic stimulation. It shows enlarged, hyperchromatic nuclei with cytoplasmic clearing. It is a key mimic of endometrial carcinoma on biopsy.
What histological clue suggests ectopic pregnancy on curettage?
Decidual reaction without chorionic villi on curettage raises suspicion of ectopic pregnancy, because the tubal tissue is decidualised while the embryo implants outside the uterus. Clinical correlation with beta-hCG and ultrasound is essential. The finding should prompt urgent evaluation.
Why can the Arias-Stella reaction mimic cancer?
The Arias-Stella reaction mimics cancer because its glandular cells show enlarged, hyperchromatic nuclei, prominent nucleoli and clear cytoplasm, similar to endometrial carcinoma. The clinical history of pregnancy or hormone use and the pregnancy test usually resolve the confusion.
Which decidual cells are normal in pregnancy?
Decidual cells are the normal transformed endometrial stromal cells of pregnancy, large and polygonal with pale, glycogen-rich cytoplasm. They support implantation and placental development. Their presence in a pregnant uterus is a normal finding.
Can decidual reaction occur outside the uterus?
Decidual reaction can occur outside the uterus, most commonly in the fallopian tube in ectopic pregnancy. Ovarian stroma may also show decidual change in pregnancy. Its presence outside the uterus supports extrauterine pregnancy when the clinical picture fits.