Rudimentary Horn Pregnancy

Rudimentary horn pregnancy (RHP) is defined as the implantation of pregnancy in the rudimentary horn of a unicornuate uterus

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Congenital uterine anomalies occur in less than 5% of all women. The unicornuate uterus comprises about 10-20% of uterine malformations¹. Rudimentary horn pregnancy is an extremely rare form of ectopic pregnancy, with a reported incidence of 1 in 75000– 150 000 pregnancies².

Uterine anomalies result from abnormal development of the paired Müllerian ducts. A unicornuate uterus results from the normal development of one Müllerian duct and incomplete development and failure of fusion with the contralateral side of the other Müllerian duct¹.

Unicornuate uterus is a type 2 Mullerian anaomaly, as classified by American Society of Reproductive Medicine in 1988³. Of recognized cases of unicornuate uterus about one-third  are comprised of   isolated unicornuate uterus with no rudimentary horn, about one-third have a rudimentary horn that does not contain any endometrium, and about one-third have a rudimentary horn with  endometrium. The rudimentary horn is connected to the unicornuate uterus by either a narrow fibrous band of tissue or a broad muscular attachment; the muscular layer of the horn varies from thick and strong to thin and malformed¹. 

Approximately two-thirds of rudimentary horns containing endometrium do not communicate with the endometrium of the contralateral horn. Eighty-five percent of rudimentary horn pregnancies occur in non- communicating rudimentary horns⁴. The mechanism of pregnancy occurrence in the non-communicating rudimentary horn is assumed to be by transperitoneal migration of either the fertilized ovum or the spermatozoon from the contralateral tube followed by  implantation in the rudimentary horn⁴.

The first case of ruptured rudimentary horn pregnancy was reported in 1669 by Mauriceau⁵. Although ultrasound remains the first-line diagnostic tool ,  the sensitivity is poor, approximately 26%, and may be less in  advanced pregnancies⁶.

Tsafrir et al. reported 2 cases of rudimentary horn pregnancy and proposed the following criterias for ultrasonographic diagnosis: (1) a pseudo pattern of an asymmetrical bicornuate uterus; (2) absent visual continuity tissue surrounding the gestational sac and the uterine cervix; (3) absent continuity between the cervical canal and the lumen of the pregnant horn; and (4) the presence of myometrial tissue surrounding the gestation sac⁷.

Mavrelos et al reported that the critical sonographic finding was detection of a single interstitial tube in an empty uterus adjacent to the pregnancy, which could be identified in both early and advanced pregnancies. They also found that free mobility of a rudimentary horn pregnancy and a vascular pedicle joining the gestational sac to the unicornuate uterus were helpful signs⁸. Thin surrounding myometrium and placenta accreta are other clues suggesting a rudimentary horn pregnancy⁷.

Three-dimensional ultrasound and magnetic resonance imaging (MRI) are reproducible methods of diagnosing Müllerian duct anomalies. Therefore, in doubtful cases, three-dimensional ultrasound or MRI should be done to confirm the diagnosis. Three-dimensional ultrasound is feasible and low cost compared with MRI, so it may be the primary imaging test for evaluating both the pregnant and nonpregnant anomalous uterus especially in the early stage. MRI is usually indicated in advanced pregnancies. MRI is offers an  advantage over ultrasound diagnosis  by improving visualization of the unicornuate uterus’s connection to the rudimentary horn and  cavitary communication, and documenting absent continuity between the cervical canal and the lumen of the pregnant horn, and demonstrating the presence of myometrial tissue surrounding the gestation sac^7,9. MRI can also be helpful in evaluating for adherent placenta, delineating the vascular supply of the pregnancy and in surgical planning.

Detailed anatomic evaluation of an enlarging gravid rudimentary horn on 2D ultrasound may be difficult. Interstitial ectopic pregnancies, bicornuate uterus pregnancies, and abdominal pregnancies are  the common sonographic misdiagnoses.

Interstitial ectopic pregnancy implants in the proximal portion of the Fallopian tube and  is surrounded by myometrium so it may mimic a rudimentary horn pregnancy. The interstitial line sign, a highly specific ultrasound sign of interstitial pregnancy can support the diagnosis of an interstitial ectopic pregnancy. A rudimentary horn pregnancy usually demonstrates free mobility versus an interstitial ectopic pregnancy that  is implanted in the cornu of the uterus.

Bicornuate uterus pregnancy refers to intrauterine implantation in an anomalous bicornuate, or septate uterus. The rudimentary horn containing the pregnancy enlarges rapidly, which makes it very difficult to differentiate this anomaly from a normal implantation of the pregnancy in a bicornuate or septate uterus. The only way to reach the correct diagnosis is by demonstrating the lack of communication between the blind rudimentary horn containing the gestational sac and the cervical canal. 

Late abdominal pregnancies can resemble rudimentary horn pregnancies. Careful sonographic evaluation of the myometrium can help distinguish a late-presenting abdominal ectopic pregnancy from a rudimentary horn pregnancy. Myometrial tissue around the gestational sac  is not seen in abdominal ectopic pregnancies.

Three-dimensional sonography and MRI are very helpful for differential diagnosis.

Ectopic pregnancy remains  the leading cause of hemorrhage-related maternal mortality in the first trimester, especially when the implantation occurs in an unusual location such as a  rudimentary horn pregnancy. Rudimentary horn rupture is described as the most significant threat to pregnancy and a life-threatening situation.

Despite advances in diagnostic modalities, pre-rupture diagnosis remains challenging. The diagnosis can be missed even in experienced hands, probably due to resembling ultrasonographic findings of this potentially lethal condition with other extrauterine ectopic pregnancies and  rarity of the disease. Early diagnosis before uterine rupture can be managed successfully by laparoscopy and help to reduce remarkable maternal morbidity and mortality⁷. It is important to raise the awareness of this life-threatening condition especially in developing countries where the possibility of detection before pregnancy or before rupture is improbable.

Almost all pregnancies in a rudimentary horn end with rupture before the third trimester. However,  there are also reports of some full-term rudimentary horn pregnancies resulting in live births after cesarean section [9,10]. Neonatal mortality is very high as most cases are emergency laparotomies after uterine rupture at premature gestational age. Maternal mortality has improved to as low as 0.5% but morbidity remains considerable because of massive blood loss and morbidly adherent placentation⁷. The remarkable improvement in maternal mortality rates is likely related to an increase in pre-rupture diagnosis.

It is important to emphasize that, although some full-term rudimentary horn pregnancies have been documented, life-threatening uterine rupture and massive hemorrhage remain the most likely outcomes, and neonatal survival are extremely rare⁴.

Immediate surgery is recommended after the diagnosis even in un-ruptured cases⁴. The surgical approach consists of the total excision of the rudimentary horn and the ipsilateral fallopian tube in order to avoid the risk of a further ectopic tubal pregnancy. In case of emergency it has been traditionally managed by laparotomy. If diagnosed in the first trimester and mother in a stable hemodynamic condition, laparoscopic excision is also described as a safe and successful approach. Conservative management until fetal viability achieved, has been advocated in a few selected cases if the patient expects to continue pregnancy and emergency surgery can be performed immediately as uterine rupture is suspected⁹.

1. Troiano RN, McCarthy SM. Müllerian duct anomalies: imaging and clinical issues. Radiology 2004; 233:19–34
2. Tesemma MG. Pregnancy in noncommunicating rudimentary horn of unicornuate uterus: a case report and review of the literature. Case Rep Obstet Gynecol 2019;2019:1–3.
3. The American Fertility Society classifications of adnexal adhesions, distal tubal occlusion, tubal occlusion secondary to tubal ligation, tubal pregnancies, müllerian anomalies and intrauterine adhesions. Fertil Steril. 1988 Jun; 49(6):944-55.
4. Nahum GG. Rudimentary uterine horn pregnancy: the 20th-century worldwide experience of 588 cases. J Reprod Med 2002; 47:151–163
5. F. Mauriceau, Traite des maladaies des femmes grosses, vol. 1, Compaigne des libraries, Paris, France, 1721.
6. Y. Jayasinghe, A. Rane, H. Stalewski, and S. Grover, “The presentation and early diagnosis of the rudimentary uterine horn,” Obstetrics and Gynecology, vol. 105, no. 6, pp. 1456–1467, 2005.
7. A. Tsafrir, N. Rojansky, H. Y. Sela, J. M. Gomori, and M. Nadjari, “Rudimentary horn pregnancy: first-trimester pre-rupture sonographic diagnosis and confirmation by magnetic resonance imaging,” Journal of Ultrasound in Medicine, vol. 24, no. 2, pp. 219–223, 2005.
8. Mavrelos D, Sawyer E, Helmy S, Holland TK, Ben-Nagi J, Jurkovic D. Ultrasound diagnosis of ectopic pregnancy in the noncommunicating horn of a unicornuate uterus (cornual pregnancy). Ultrasound Obstet Gynecol 2007; 30:765–770
9. Cheng C, Tang W, Zhang L, et al. Unruptured pregnancy in a noncommunicating rudimentary horn at 37 weeks with a live fetus: a case report. J Biomed Res 2015;29:83–6.

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