Discordant malformation in a monozygotic pregnancy: abdominoschisis and sirenomelia

Introduction

Congenital malformations occur in up to 10% of monochorionic twins. In this type of twinning, there is a higher incidence of loss of chromosomal material and midline defects. Fetal body wall defects syndrome (BWD) is thought to be related to amniotic membrane rupture [1]. BWD is a rare entity of very low prevalence, defined as a combination of various malformations, including body wall defects with mutilations of thoracic and/or abdominal organs, depending on the gestational age at which the insult occurs. Amniotic bands leading to BWD are probably caused earlier in embryo development than those leading to fetal amputations or constrictions [1].

Sirenomelia (mermaid syndrome) occurs in the 3^rd–4^th week of gestation when the development of the structures derived from the caudal axis is incomplete. The causes of sirenomelia are unknown; it is possibly one end of the spectrum of caudal regression syndrome [2].

Twin pregnancies are a risk factor for this anomaly and maternal diabetes could also be a predisposing factor [3]; both twinning and diabetes are related to higher maternal age.

To the best of our knowledge, the association of BWD in one twin and mermaid syndrome in the other has not been described previously in monochorionic diamniotic twins. The evaluation of the discordant monozygotic twins could be useful to clarify the underlying pathogenesis of this condition.

Case report

A 31-year-old patient, primigravida with a monochorionic diamniotic twin pregnancy was referred to our institution for further evaluation after an abnormal first trimester ultrasound in a private clinic. The patient had uncontributing personal or family history, no history of genetic diseases and a non-consanguineous marriage.

The patient booked for evaluation at 16 weeks and an ultrasound scan revealed a monochorionic diamniotic pregnancy with abdominal wall defect in the first twin, while fused legs was observed in the second twin (Figure 1 and Figure 2). Amniotic fluid was normal and no amniotic bands were seen.

Figure 1:

Second trimester ultrasound showing fetal body wall defect (BWD).

Figure 2:

Second trimester ultrasound showing sirenomelia: (A) sagittal view, (B) transverse view.

Amniocentesis showed normal karyotype (46 XY). After counseling, the couple asked for termination of pregnancy and abortion was induced with prostaglandins. Postnatal features of the fetuses are shown in Figure 3.

Figure 3:

Abortion specimen.

Necropsy showed a 148-g male twin (intra-abdominal testes) with protruding abdominal viscera attached to the placenta, abnormal vertebral alignment, imperforated anus, pulmonary hypoplasia and complex cardiac malformation (univentricular heart and abnormalities of outflow tracts). The necropsy of the heart was of limited value due to postmortem lysis. Also, the first fetus lacked an abdominal wall, had a rudimentary bladder and left kidney dysplasia. The trajectory of the umbilical cord was adherent to the fetal side of the placenta.

The second twin also showed intra-abdominal testes and a single umbilical artery. This fetus had a single inferior limb (Figure 4) with one abnormal femur, knee and no foot, bilateral kidney agenesis, imperforated anus and absent urinary bladder.

Figure 4:

Sirenomelia, frontal view X-ray.

The patient became pregnant the following year and gave birth to a healthy boy.

Discussion

In monozygotic twins, the prevalence of structural defects per fetus is 2–3 times higher than in singletons and concordance is uncommon (20%) [4]. The association of BWD and sirenomelia with monozygotic twins raises the question of a possible common pathogenetic mechanism at an early stage of monozygotic twin embryonic development.

A known teratogenic mechanism was ruled out, as was chromosomal aneuploidy. Orioli et al. [5] found no cases of abnormal karyotype among 12 cases of sirenomelia in a large population study.

Some authors postulate a suppression in caudal vascularization [6] as responsible for sirenomelia and Van Allen suggested that BWD results also from a vascular disruption. According to his theory, the vascular disruption is secondary to amnion rupture and leads to necrosis and/or adhesions of fetal parts [1]. Hartwig et al. [7] proposed a pathogenic theory for those BWD phenotypes with placenta attachment as the present case: a disturbance of the embryonic folding process. Furthermore, Torpin [8] collected around 400 cases of amniotic band syndrome (ABS) from historical data and personal observation and postulated in 1968 the exogenous theory for BWD. He suggested that amnion rupture can uncover the chorion, with the fetus entering the chorionic cavity followed by formation of mesoblastic fibrous strings detaching from the chorion or from the base of the umblical cord where the amnion is normally attached. As a consequence of its movements, the fetus is at risk of entanglement in these strings, leading to malformations. It has to be considered that in our case, the same disruption affected the body wall of twin A and the caudal vasculature of twin B, as no amniotic bands were found at necropsy.

Discordancy for sirenomelia in monochorionic diamniotic twins was described by Di Lorenzo and Veilleux in 1991 in a gestation where the sibling was normal [2]; they assumed that a genetic mechanism seemed unlikely due to the absence of familial cases. On the contrary, familial cases of BWD and concordant monozygotic affected twins, supporting the theory of a defective developmental program, have been described [9], but this is not in accord with our case and some others. Smrcek et al. [10] reported a monochorionic diamniotic twin gestation where the other twin was unaffected, and a discordant triplet gestation where a selective feticide of the fetus with BWD was performed in early gestation and the remaining two fetuses reached term. Lockwood et al. [11] described 14 cases of twin gestations associated with ABS, 11 cases belonging to monozygotic twinning. In seven of them, there was discordancy in the fetuses’ phenotype, with only one affected fetus. These observations support Torpin’s theory of an exogenous insult causing amniotic disruption and exposure of the chorion with amniotic strings formation.

Data arising from twin pregnancies have a role in the pathogenetic discussion regarding these conditions. The issue of a common injury leading to defective embryonic disc folding before the 4^th week of pregnancy could account for both BWD and caudal regression syndrome.