Synonyms: Siamese twins.

Definition: Monozygotic twins with common or shared organ system(s). Excludes: acardiac twins.

Prevalence:  0.2-0.6:10,000 births. M0.2-0.3:F1

Embryology: Incomplete division of the embryonic disc after the formation of the disk and primitive amniotic sac.

Associated anomalies: Omphalocele, meningomyelocele, facial clefts, imperforate anus, diaphragmatic hernias,  shared organ system(s).

Differential diagnosis: Multiple gestation.

Prognosis: Function of shared organ system(s), anomalies, and prematurity. Historically the prognosis was poor but has improved in the last decade.

Recurrence risk: Not increased.

Management: Based on viability. Operative abdominal delivery at maturity provides best perinatal outcome.

MESH Twins;-conjoint BDE 0202 MIM 27641 ICD9 759.4 CDC 759.430

Address correspondence to Charles M. McCurdy, Jr., MD, Division of Maternal-Fetal Medicine, Dept. of Obstetrics and Gynecology, University of Arizona, 1501 North Campbell, Tucson, Arizona 85724-1000 Ph: 602-626-6043; Fax: 602-626-2514. ¶Dept. of Obstetrics and Gynecology, Medical College of Georgia.

Introduction

Conjoined twins are an uncommon complication of monozygotic twins, accounting for 0.2 in 10,000 births and 40:10,000 twin births. The emphasis on antenatal diagnosis is largely due to improved perinatal options of management¹. Unfortunately, diagnosis prior to the second stage of labor has been rare prior to recent advances in ultrasound technology. Few reports exist of early antenatal sonographic diagnosis of this entity, with only four confirmed by ultrasound between 11 and 24 weeks². We present a case report describing early second trimester ultrasound diagnosis of fetal demise of omphalopagus twins.

Case report

A 20-year-old G₁P₀ with an uncertain last menstrual period presented to the antenatal clinic. Clinical evaluation found a 16-18 weeks fundal height but no auscultable fetal heart. Sonographic evaluation with a 3.5 MHz sector and 5 MHz linear array realtime ultrasound (ADR Ultramark 4) showed a twin gestation with demise of both twins.

Cephalic, abdominal, and long bone parameters were consistent with a gestational age of 14.5 weeks. Ultrasound evaluation also showed close and constant approximation of the fetal skulls, no separating membrane, and apparent union at the level of the abdominal wall (fig. 1, 3).

Fig. 1: Transverse plane demonstrating frontal-frontal apposition of fetal cranium.

Fig. 2: Transverse plane through fetal thoraces at the level of the clavicles (arrows).

Fig. 3: Transverse plane through the lower thoraces, cephalad to the umbilical cord insertion, demonstrating the ventral wall fusion (large arrow).

The patient was admitted and counseled with regard to therapeutic options, including expectant management and surgical or medical abortion.

The patient opted for pregnancy termination. Laminaria tents were placed, and prostaglandin E₂ suppository induction of labor was begun the following morning. After 7 hours, the patient delivered omphalopagus twins in the breech/breech presentation.

The autopsy documented:

1)      a common inferior thoracic wall, abdominal wall, and pelvis;

2)      multiple shared alimentary organs (e.g. liver, GI tract, rectum and anus);

3)      two separate hearts;

4)      musculoskeletal anomalies of three rudimentary lower extremities;

5)      genitourinary mal-development; and

6)      duplication of rudimentary external male genitalia (fig. 4).

Examination of the placenta demonstrated a single three-vessel umbilical cord and monochorionic placentation.

Fig. 4: Pathology specimen.

Discussion

Diagnosis

This case illustrates the early diagnosis of conjoined twins at 14 weeks gestation. Koontz et al² have summarized the sonographic findings associated with conjoined twins which include:

·        the lack of a separating membrane;

·        inability to separate the fetal bodies;

·        detection of other anomalies;

·        three or more vessels present in the umbilical cord; and

·        any of the classic radiographic signs as described by Gray³. These radiographic signs are:

·        fetal heads persistently at the same level,

·        dorsiflexed cervical spine,

·        a narrow space present between the lower cervical and upper thoracic spine, and

·        no change in apparent fetal position after maternal manipulation.

The diagnostic criteria in this case were confirmed by the noted approximation of the fetal skulls, approximation of the fetal bodies, absence of a separating membrane, and the strong suspicion of co-twin anomalies. No alternate diagnostic modality was used for diagnostic confirmation.

Embryology

This anomalous sequence results from incomplete division of the embryonic disc during twinning that occurs after the formation of the disc and primitive amniotic sac⁴. Male-to-female ratios range from 0.3:1 to 0.2:1.

Pathogenesis

The frequent types of conjoined twins are illustrated in fig. 5. The most severe forms of this anomalous sequence are the duplicata incompleta type, where few organ systems are duplicated. Thoracopagus twins are the most frequent type and represent about 29% of all affected twins^6,7. The types of conjoined twins, as well as frequency and perioperative mortality, are included in Table 1.

Table 1: Incidence and perioperative mortality of conjoined twins⁷

Associated anomalies

Other than duplication, the most frequently reported anomalies associated with conjoined twinning are omphalocele, facial clefts, menin­go­myelocele and imperforate anus⁶.

Management

The options for management of this case are typical for mid-trimester demise with a large uterus, and include surgical evacuation or termination with an abortofacient. The trial of prostaglandin E₂ preceded by laminaria dilation of the cervix was believed to represent the safest alternative in this case with increased uterine size and fetal mass.

Each antenatal diagnosis of twin gestation should be followed by diligent efforts to identify a separating membrane and, where none is found, to exclude conjoined twinning. Ultrasound evaluation of fetal anatomy to assess for associated fetal anomalies and extent of shared organs is paramount. Term or near term abdominal delivery provides an optimal situation for reduced maternal morbidity and perinatal survival of conjoined twins⁷.

Prognosis

Historically, the possibility of fetal survival in conjoined twins has been poor, with a 40% stillbirth rate. Close antenatal surveillance and a “controlled” postnatal surgical repair provide the best prognosis, however. Consequently, the care of diagnosed conjoined twins should be referred to a tertiary center with an available skilled perinatal team^2,4,5. Surgical separation of conjoined twins should be considered in virtually all patients in the absence of severe prematurity or other anomalies. Survival rates in the last decade have increased dramatically⁷(Table 1).

References

1. Koontz, WL, Herbert, WN, Seeds, JW, Cephalo, RC: Ultrasonography in the diagnosis of conjoined twins; A report of two  cases. J Reprod Med 28:627, 1983.

2. Maggio, M, Callan, NA, Hamod, KA, Sanders, RC: The first-trimester ultrasonographic diagnosis of conjoined twins. Am J  Obstet Gynecol 152:833, 1985.

3. Gray, CM, Nix, HG, Wallace AJ: Thoracopagus twins: Prenatal  diagnosis. Radiology 54:398, 1950.

4. Cunningham, FG, MacDonald, PC, Gant, NF: Williams Obstetrics. 18th ed., Norwalk, CT, Appleton & Lange, 1989, pp 634.

5. Seeds, JW, Azizkhan, RG: Congenital Malformations: Antenatal diagnosis, Perinatal management, and Counselling. Rockville, MD, Aspen, 1990, pp 3.

6. Romero R, Pilu G, Jeanty P et al: Prenatal  Diagnosis of Congenital Anomalies. Norwalk, CT, Appleton &  Lange, 1988, pp 405-409.

7. Hoyle RM. Surgical separation of conjoined twins. Surg Obstet Gynecol 170:549, 1990.