complications of monochorionic twins

OccasiOnal Review

Complications of monochorionic twinsJanice Gibson

alan cameron

AbstractMonochorionic twins account for approximately 20% of twin pregnan-

cies. They are at significantly increased risk of fetal and neonatal loss

and neonatal morbidity than dichorionic twins. This can be attributed to

a number of pathological conditions that can arise due to the shared

monochorionic placenta, which contains a number of inter-fetal vascular

anastomoses. specific complications which can arise include; Twin-twin

Transfusion syndrome, Twin Reversed arterial Perfusion syndrome, single

intrauterine death and the complications arising from this for the sur-

viving twin. Monochorionic twin pregnancies may also be complicated

by pathological conditions arising from increasingly late embryological

division such as monoamniotic monochorionic twins and conjoint twins.

These complicated pregnancies are discussed in this review and the

obstetric management of these pregnancies is outlined.

Keywords conjoint twins; fetal structural anomaly; monoamniotic

twins; monochorionic twins; obstetric management; placental vascular

anastomoses; single intrauterine demise; twin reversed arterial perfusion

syndrome; twin-twin transfusion syndrome

Introduction

In the UK, twin pregnancies have an incidence of approximately 1 in 80 pregnancies, of which 20% will be monochorionic (2.5 per 1000 pregnancies). Monochorionic twins are at risk of all the typical complications of twin gestations – i.e. spontaneous prema-ture delivery, foetal growth restriction, maternal pre-eclampsia, antepartum haemorrhage and delivery complications. In addi-tion, monochorionic twins are at risk of specific complications, which greatly increase their rate of foetal and neonatal loss, and neonatal morbidity. Inter-foetal vascular communications exist in monochorionic placentae linking one twin’s viability and well being to the other’s. Thus, there is a low threshold for interven-tion and iatrogenic preterm delivery in this subset of twins when complications occur during pregnancy.

Janice Gibson MB ChB MD MRCOG is a Consultant Obstetrician at the

Department of Obstetrics, The Southern General Hospital, Glasgow, UK.

Alan Cameron MB ChB MD FRCOG is a Consultant Obstetrician at the

Department of Obstetrics, The Queen Mother’s Hospital, Glasgow, UK.

PaeDiaTRics anD cHilD HealTH 18:12 56

The embryology of monochorionic twins

Twins can be classified by their zygosity (number of fertilised ova, i.e. non-identical dizygotic twins or identical monozygotic twins) or by their chorionicity (the number of placentae – dichorionic or monochorionic/one shared placenta; see Table 1). The latter clas-sification is of clinical relevance. It predicts the risk of obstetric complications, and as such allows an optimal management plan for antenatal care to be formed. Approximately 30% of twins are identical or monozygotic of which 70% are monochorionic. This occurs when a single fertilised ovum splits into identical twins after 3 days of ova fertilisation. At this time point the outer chorion has started to differentiate and therefore, although embryo division occurs, the resultant foetuses will share an outer chorion (placenta and chorionic membrane). Division of the embryo after 8 days will occur after amnion differentiation, the resultant twins will be monoamniotic as well as monochorionic. Division beyond 13 days occurs after the embryonic disk has started to differentiate and these twins will be conjoint, monoamniotic monochorionic twins.

Division of a single fertilised ovum before 3 days post-fertilisation will result in identical twins that will each form their own chorion. These twins will be dichorionic diamniotic and be indistinguishable from (same sex) dizygotic (obligatory) dichorionic twins through-out pregnancy.

Monochorionic twins have a 6-fold loss rate before 24 weeks gestation1 and 2–3 fold increased risk of stillbirth and early neo-natal deaths when compared with dichorionic twins. The excess loss is largely the result of specific complications. These can arbi-trarily be classified into those arising due to the pathophysiol-ogy of the shared monochorionic placenta and those arising from increasingly late division of an embryo.

The monochorionic placenta: specific complications

In virtually all monochorionic placentae there is a shared central portion in which there exists a number of arterial to venous anas-tomoses, venous to venous anastomoses and arterial to arterial anastomoses between the two foetal-placental circulations. The nature of these anastomoses and the relative number of each type predispose monochorionic twins to specific complications: twin-twin transfusion syndrome, twin reversed arterial perfusion syndrome and adverse outcome of a co-twin in the event of a single intrauterine death (Figure 1).

Twin-twin transfusion syndrome (TTTS)TTTS complicates 10–15% of monochorionic twins. TTTS occurs as the result of a net transfer of blood from one ‘donor’ twin to the other ‘recipient’ twin through a predominance of unidirectional arterial to venous anastomoses within the placenta. The lack of a bi-directional arterial-to-arterial anastomosis – a potential com-pensatory redistribution channel – is also believed to contribute to the pathological state.2–4 The unbalanced sharing of blood results in a volume overloaded ‘recipient’ twin. This results in foetal polyuria and polyhydramnios and may progress to conges-tive cardiac failure, hydrops and death (Figure 2). The ‘donor’ struggles to meet the demands of both circulations, demonstrates reduced renal perfusion, oligohydramnios, growth retardation. This twin is also at risk of cardiac (high output) failure and death. Preterm labour, triggered by severe recipient polyhydramnios is

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OccasiOnal Review

Classification of monozygotic twins

Embryonic division Number of chorions Number of

amniotic sacs

Percentage of

monozygotic twins (%)

Foetal loss

(<24 weeks) (%)

Foetal loss

(>24 weeks) (%)

<4 days Dichorionic Diamniotic 30 2 1.2

4–8 days Monochorionic Diamniotic 67 16 2.8

8–14 days Monochorionic Monoamniotic 2 Up to 40

>14 days (conjoint twins) Monochorionic Monoamniotic <1 >50

The risk of adverse foetal outcome increases with the duration between ovum fertilisation and embryonic division.

Table 1

a common confounding pathology, and may precipitate delivery and loss as early as 16–24 weeks gestation. Without intervention there is an 80–95% overall foetal loss rate. TTTS may present in various degrees of severity and poor prognostic features can be identified using ultrasound (see Table 2).5

To confound the poor survival rate of TTTS, this condition is also responsible for a high morbidity rate in any live born infants. Even in the face of obstetric intervention, up to 15% of survivors have serious neurological morbidity, such as cere-bral palsy. Heart valve damage is identified in 12% of ‘recipi-ent’ twins at birth.6 Furthermore, there is growing evidence that in-utero vascular stress (of either twin) can be associated with long-term cardiovascular morbidity in adult life.7 Renal and gas-trointestinal complications may also occur.

Obstetric intervention for TTTSIf diagnosed before foetal death or preterm labour two inter-ventions have proven useful to improve the outcome of this


condition: serial amniodrainage or laser ablation therapy of the causative placental blood vessels.

Amniodrainage is performed essentially like an amniocente-sis, but with a larger gauge needle (18 G) to remove large vol-umes (2–3 litres) of amniotic fluid from the recipient’s sac. It acts to reduce the distention of the uterus and hence reduce the risk of spontaneous preterm labour or abortion. It typically requires to be repeated at weekly intervals as the underlying pathological process continues.

Laser therapy is performed under regional or local anaesthe-sia. Under ultrasound guidance, a cannula and introducer are inserted through the maternal abdominal wall, uterine wall and into the recipient’s amniotic sac. The introducer is replaced with a 2–3 mm fetoscope and a laser fibre. This allows direct visualisa-tion of the anastomosing vessels on the foetal surface of the pla-centa. The placental vessels are then coagulated using the laser energy. Excess amniotic fluid is removed from the recipient’s sac to achieve a normal volume before the cannula is removed. After

Donor twin

Arterial to venous anastomosis

aRecipient twin

’Acardiac twin’

’Pump’ twin

Ductus venosus

Arterial to arterial anastomosis

b

Figure 1 schematic diagrams of the dominant inter-fetal vascular anastomoses within the monochorionic placenta which are implicated in the

etiology of a Twin-Twin Transfusion syndrome (TTTs) and b Twin Reversed arterial Perfusion syndrome (TRaP). Umbilical and placental arteries

are demonstrated in red. These carry deoxygenated blood from the fetus to the placenta. The corresponding veins are shown in blue. These carry

oxygenated blood from the placenta to the fetus. (a) in TTTs there is a net transfer of blood from a small donor twin to a larger recipient twin

via a number of arterial (donor) to venous (recipient) anastomoses. (b) in TRaP the acardiac twin is perfused in a ‘reversed’ fashion. it receives

deoxygenated blood from the ‘pump’ twin via an arterial-to-arterial anastomosis.


OccasiOnal Review

PaeDi

Fetal armMonochorionicplacenta

Umbilical cordof recipient twin

Fetal skull

Recipient twin:cross section of fetal head

Polyhydramnias in recipital amniotic sac

Fetal rib

Donor twin:cross section at level of fetal chest

a b

Figure 2 an ultrasound image of Twin-Twin Transfusion syndrome demonstrating massive polyhydramnios in the recipient twin’s amniotic

sac. within this sac the recipient is freely mobile. Ultrasound imaging of this fetus may be difficult due to the large distance (depth) it

may occupy relative to the ultrasound probe. The donor twin appears to lie within the recipients amniotic scan however it is in fact ‘stuck’

to the side of its portion of the monochorionic placenta by its own amniotic sac. in this image this sac contains no liquor and cannot be

imaged separately from its chest wall. imaging of this twin can also be difficult due to its fixed position.

completion of the therapy the twins will be vascularly discon-nected from each other.8,9 This therapy, therefore, seeks to cor-rect the underlying pathophysiology of this condition.

There are a number of non-randomised comparable series of laser therapy versus amnioreduction. These studies have sug-gested improved foetal outcomes especially in advanced stage disease with laser therapy.10–12 The first randomised control

Quintero staging twin-twin transfusion

Quintero stage Ultrasound findings

i Polyhydramnios (>8 cm MvP) on one

sideof dividing amniotic membranes and

oligohydramnios (<2 cm MvP) on the other side*

ii Bladder of the donor twin not visible (empty)

iii abnormal arterial or venous Doppler in either

twin†iv Hydrops foetalis (cardiac failure) of either twin

v intrauterine demise of one or both twins

This is an ultrasound-derived assessment of disease severity.MvP, maximal vertical pocket.*These are the minimal diagnostic criteria for twin-twin transfusion syn-drome and will be present at each stage.†The donor twin typically can develop absent end diastolic flow in the umbilical artery, a sign of increased vascular resistance/reduced placental perfusion. The recipient can develop a reversed a-wave in the ductus veno-sus, a sign of cardiac overload.

Table 2

aTRics anD cHilD HealTH 18:12 570

trial of laser ablation versus amnioreduction also suggested that laser therapy was the most effective intervention (70% survival vs 50% survival with amnioreduction). Laser therapy was also associated with a lower rate of significant neurological morbid-ity in surviving twins than amnioreduction (5% vs 15%).13 A further randomised control trial was halted when laser therapy performed poorly compared with amnioreduction in stage III/IV disease.14 The finding of this study has been questioned9 and laser therapy is still generally considered the optimal therapy for advanced stage disease. It should be provided in a specialised centre for foetal therapy.15

Twin-twin transfusion can occur as early as 16 weeks ges-tation. Intervention is required typically by 21–22 weeks gesta-tion. Therefore, timely recognition of this condition is required to influence outcome. All monochorionic twin pregnancies should be surveyed at fortnightly intervals for this condition. Optimal surveillance can be performed by abdominal circumference esti-mations and measurement of the maximal depth of liquor on each side of the fused amniotic membranes. Referral should be made to the regional foetal medicine unit for more detailed assessment if the condition is suspected. If the diagnosis is confirmed and this unit does not perform laser therapy, onward referral to a specialised foetal medicine unit may then be required. Pathways of referral should be established to allow quick access to treat-ment once diagnosed.

Single intrauterine foetal death (sIUFD)The death of one of a monochorionic twin in utero carries with it severe risks to the co-twin as a direct result of the vascular anastomoses between the two feto-placental circulations. Upon death, the vascular tone of a foetus is lost and its foetal-placental

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OccasiOnal Review

vascular volume, therefore, expands and acts as a siphon, drain-ing the feto-placental volume of the co-twin. The co-twin is, therefore, at risk of severe acute hypotension, predisposing it to organ hypoxia. Toxic thromboplastins may also be released from the dead twin tissue and pass through the joint circulation into the co-twin confounding organ damage. It is estimated that there is a 30% risk of co-twin death or if this twin survives, a 20% risk of severe neurological damage.16

Obstetric intervention for sIUFDTypically diagnosis of sIUFD occurs at an unknown time period after the event. Early delivery of the survivor is not indicated as any pathology will have occurred, and premature delivery may confound the risk of adverse outcome. The foetus can be monitored by ultrasound for overt neurological damage, such as hydrocephalus. Magnetic resonance imaging of the brain of the surviving twin, and the demonstration of leukomalacia or brain atrophy 3 weeks after sIUFD, may provide earlier evidence of neurological damage.17 In these circumstances termination of pregnancy is an option for the parents. In the absence of these signs, the neurological outcome of the survivor is difficult to pre-dict and some parents may still understandably request termina-tion of the pregnancy.

If the diagnosis of sIUFD occurs within 48 hours of the event, immediate intrauterine transfusion to ‘restore’ the circulating volume of the survivor and hopefully reduce the risk of neu-rological morbidity has been attempted. However, the success of such intervention is not proven and ongoing assessment is continuing in specialised centres.

The best intervention for this condition is obviously avoidance. However, prediction of imminent foetal death is difficult, even in predisposing pathological conditions such as severe intrauterine growth restriction (IUGR) or potentially lethal foetal anomaly of one monochorionic twin. The clinical dilemma, balancing the risk of one intrauterine death versus iatrogenic preterm delivery, is therefore weighted more towards significant preterm delivery in monochorionic compared with dichorionic twins when simi-lar risks of sIUFD are diagnosed. If sIUFD is anticipated before a viable gestation in monochorionic twins, then selective feti-cide by cord occlusion with ligature, laser or diathermy of the compromised foetus may be considered by specialised centres. To date, this has not been proven to be beneficial with regards to the co-twin’s outcome. A randomised clinical trial compar-ing conservative management versus laser ablation therapy of placental anastomosing vessels is currently being conducted in monochorionic twin pregnancies complicated by severe IUGR of one foetus. This study will assess whether laser therapy to ‘un-link’ the two foetal circulations, and hence allow each foetus to ‘independently’ take its chance with advancing gestation, is beneficial to the neonatal outcome of both twins.9

Twin reversed arterial perfusion sequence (TRAP)TRAP is a rare complication affecting approximately 1% of mono-chorionic twins. If the heart of one monochorionic twin stops, it may continue to be partially perfused by the surviving twin if large foetal arterial-to-arterial anastomoses exist in the shared pla-centa. Normally, oxygenated blood originating from the placenta enters a foetus through its umbilical artery, most of which is then shunted through the ductus venosus to preferentially oxygenate


the upper foetal structures: brain and heart. In TRAP, the dead twin is perfused in a reverse manner via its artery. The blood is poorly oxygenated having originated from the other ‘pump’ twin. Furthermore, this poorly oxygenated blood perfuses the lower part of the foetus first. The upper foetal structures including the heart undergo secondary atrophy and the resultant ‘twin’ has been described as an ‘acardiac monster’.

The condition is very rare and there is a 50–75% incidence of mortality in the pump twin due to intrauterine cardiac failure and polyhydramnios-induced preterm delivery. The risk of loss of the pump twin increases if the relative size of the acardiac twin is greater than 50% of the pump twin.

Obstetric intervention in TRAPVenous Doppler ultrasound can be used to survey the pump twin for signs of cardiac failure and facilitate delivery timing if a viable gestation has been reached. Prior to 24 weeks gestation, cord ligation of the acardiac twin has been used with success (up to 80% survival of the pump twin) in specialised centers.18

Complications of monochorionic twins arising from late embryonic division

In addition to the specific risks to monochorionic twins arising from the shared placenta circulation, monochorionic twins are also at risk of specific complications that result from late division of the embryo – foetal structural anomalies, monoamniotic twins and conjoint twins.

Foetal structural anomalyMonochorionic twins have an increased risk of structural abnor-mality than dichorionic twins. It is believed that the process of embryo division is intrinsically teratogenic. This may occur either because the embryo divided into two uneven cell masses, or because embryo division occurs after certain cells have dif-ferentiated their polarity, that is right-sided or left-sided cells. Anomalies of the heart – a non-symmetrical organ – are com-mon, supporting the later mechanism. Neural tube abnormali-ties are also prevalent. Anomalies of the cerebral cortex, kidneys and gut can also occur. The aetiology of the latter may reflect a destructive or deformation influence, that is acute hypoxia due to acute fluxes in inter-foetal cardiovascular perfusion. Mono-chorionic twins should, therefore, be offered detailed ultrasound assessment at 18–20 weeks gestation. A detailed cardiac ultra-sound may also be indicated at 20–22 weeks gestation when this organ is more readily imaged.

The incidence of structural anomalies increases the later embryo division occurs. The congenital anomaly rate is approxi-mately 3–4 fold higher in monochorionic diamniotic pregnancies than in dichorionic or singleton pregnancies. Structural anomalies are seen in 20–25% of monochorionic monoamniotic twins and obviously 100% of conjoint twins. In conjoint twins, structural anomalies of unshared organs occurs in up to 80% of cases.

In monochorionic diamniotic twins, the structural abnormal-ity usually affects only one twin (80%). In this situation, man-agement options and parental counselling are difficult. Selective termination of the pregnancy by intra-cardiac potassium chlo-ride (as with dichorionic twin pregnancies) cannot be used due to the vascular anastomoses between the two feto-placental


OccasiOnal Review

circulations. Cord occlusive techniques are the only ‘selective’ option for termination in monochorionic twin pregnancies. This technique can only be attempted in a small number of highly spe-cialised foetal medicine units, and due to its relatively invasive nature, carries a significant risk of ruptured membranes (5–10%) and loss of the structurally normal twin.

The parents may alternatively adopt a conservative manage-ment strategy and prepare themselves for the birth of one child with a structural abnormality. Paradoxically, foetal anomalies that are likely to be non-viable may not be best managed by such a conservative approach. Severe structural anomalies place a foetus at increased risk of sIUFD, with its resultant risks to the co-twin. Additionally, anomalies such as anencephaly or dia-phragmatic hernia are typically complicated by polyhydramnios and place the co-twin at risk of severe preterm delivery due to uterine distension.

Monoamniotic twinsMonoamniotic twins occur in less than 1–2% of monochori-onic twins. These twins share a single amniotic sac and are, therefore, at risk of cord entanglement in utero. The twins also have an increased incidence of structural abnormalities. Due to these pathological states, the twins have a risk of mortality of approximately 20–40%.

Obstetric intervention in monoamniotic twinsThe outcome can be improved by intensive cardiotocography monitoring once the twins reach viability. The recommended frequency of foetal heart rate monitoring varies between once weekly to 3 times daily in reported series. Antenatal corticoste-roids are usually prescribed to attain foetal lung maturity between 24 and 28 weeks gestation. Delivery is indicated if cord compres-sion is subsequently diagnosed. Otherwise, as foetal death can occur abruptly in spite of recent reassuring foetal monitoring, delivery is electively planned for 32 weeks gestation when rea-sonable foetal maturity has been reached. Delivery should be performed by Caesarean section, as the risk of a cord accident is particularly high during labour.

Oligohydramnios induced by non-steroidal anti-inflammatory drugs (NSAIDs) has been suggested as a management strategy to ‘splint’ and reduce foetal movement to reduce the risks of cord accident and foetal death. A study of the NSAID, sulindac, in addition to intensive monitoring and delivery, at 32 weeks gesta-tion demonstrated excellent outcomes.19 However, this treatment has not been proven to independently improve foetal outcome. Cord entanglement is present in virtually all cases by an early gestation and in theory oligohydramnios may increase the risk of cord accident by compression forces.

Conjoint twinsConjoint twins are also a rare complication of monochorionic twins. Their incidence is approximately 1 in 1000 twin pregnan-cies. There is no known risk factors identified for their causation and the embryology of their formation is not understood. Incom-plete division of the embryonic disk or fusions of embryos at symmetrical developmental axes are aetiological theories. There is a predominance of female foetuses (60%).

Conjoint twins can be and indeed are most easily diagnosed by ultrasound in the first trimester. Monoamniotic monochorionic


twins should be carefully assessed by ultrasound for this compli-cation. With this aim transvaginal ultrasound is useful.

The most common sub-types of conjoint twins consist of those joined at the chest or chest and abdomen. Those joined only at the abdomen are less common. Rare presentations include those joined at the pelvis and cranium. Structural abnormalities of internal organs occur in virtually all cases. Shared cardiac struc-tures, a shared common bile duct and posterior cerebral struc-tures have extremely poor prognoses. Up to 50% of conjoint twins may die in utero. Of those that survive birth, 30% will not be suitable for surgery and will die in the early neonatal period. In cases selected for surgery, there is a 50% infant survival rate in the short term.20

Parents require accurate counselling regarding foetal outcomes and chance of separation. Termination of pregnancy should be considered and is commonly requested. If the pregnancy contin-ues, detailed structural survey of the twins is required to allow forward planning of any appropriate surgery. Delivery typically occurs by Caesarean section to avoid mechanical difficulties. The uterine incision is likely to be significant in size and may require vertical extension(s). This will have adverse implications for the delivery and outcome of future pregnancies.

Summary

Monochorionic twin pregnancies are at high risk of adverse foe-tal and neonatal outcomes. The chorionicity of a twin pregnancy can be diagnosed with a high degree of accuracy by ultrasound in the first and early second trimesters of pregnancy.21 Failure to diagnose monochorionic twin pregnancy is substandard care, as identification is required to plan intensive surveillance for com-plications throughout pregnancy. Care should be provided at a specialist-led multiple pregnancy clinic. Monochorionic twins discordant for foetal anomalies, affected by TTTS, severe IUGR of one twin or single twin demise should be referred to a regional foetal medicine centre with recourse to specialist expertise. ◆

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Practice points

• Monochorionic twins have a significantly increased foetal and

neonatal loss rate than dichorionic twins

• Monochorionic twins should be diagnosed in the first or

early second trimester of pregnancy to allow appropriate

surveillance for complications

• Obstetric interventions for complications are highly

specialised, invasive and carry a risk of loss of both twins

• Once reasonable foetal viability is reached planned preterm

delivery in complicated pregnancies especially those at risk

of siUFD is often the most successful and ethical option


http://www.nice.org.uk/IPG198

complications of monochorionic twins

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