Twin gestations account for 1-2% of all pregnancies. Dyzygotic twins originate from 2 eggs and are associatedwith dichorionic, diamniotic pregnancies whereas monozygotic twins originate from mitotic division of one egg and may have monochorionic or dichorionic placentation(1).Population probabilities are that 30% of twinpregnancies are monozygotic. The type of chorionicity can be determined accurately by ultrasound (2) while prediction of zygosity is less certain(3).The outcome of multiple pregnancies is greatly affected by chorionicity whereas the effect of zygosity is less clear. Having a shared placenta increases the risk of adverse perinatal outcome thus, efforts should be made to determine chorionicity(1).
Multiple pregnancies and congenital heart disease
In a population based study, twins of the same sex appeared to be at greater risk of congenital anomalies (4)but it was less clear if this was related to zygosity or chorionicity. More recently, Dube et al showed higher-but no statistical difference in the incidence of major anomalies when comparing monozygotic twins (mono or dichorionic) with dyzygotic pairs (1).Congenital heart defects occur more commonly in monozygotic twins who are monochorionic than in monozygotic dichorionic twins.5In another study a higher incidence ofcongenital heart disease was shown in twins who are monochorionic diamniotic (~3.8%) compared to that ofthe general population. In addition, the risk appeared to be higher in those with (6.9%) than in those without(2.3%) associated twin-to-twin transfusion syndrome (TTTS)(6).Thus, there is a place to perform detailed fetalechocardiography in monochorionic pregnancies.
Following the diagnosis of a major structural abnormality in a singleton, subsequent pregnancy managementmay be relatively straightforward. If, however, the pregnancy involves one affected and one unaffected twin,management issues such as termination are more complex(7).Counselling is best performed by a combinedapproach between the fetal cardiologist and most importantly, the obstetric team. Copel et al reviewed their experience of fetal echo in multiple gestations in order to evaluate how the presence of a cardiac defect in one twin affected decision management for the pregnancy(8).There were 11 defects in 10 set of twins out of 36 pregnancies studied, 7 of whom were referred because of suspected heart defect. Three fetuses were aneuploid including one set of monozygotic twins with trisomy 18. Three of 8 pregnancies were monochorionicand there were no cases of selective fetocide. The information provided by fetal echocardiography allowed comprehensive family counselling leading to informed decisions regarding intrapartum and neonatal management in a similar fashion to management of a singleton pregnancy given that no termination was performed.
Of growing interest in twins pregnancies however, is the assessment of twin-to-twin transfusion syndromewhich is associated with important morbidity and mortality(9;10).In addition to characterising structural cardiacabnormalities, there also appears to be an important role for fetal echocardiography in assessing cardiac function in these high-risk pregnancies.
Fetal echocardiography and twin-to-twin transfusion syndrome
TTS is a common, severe complication of monochorionic pregnancies with a quoted incidence that variesfrom 5-35%(9;11;12)Its pathophysiology is not fully understood but thought to occur due to vascular connectionsin the placenta leading to an umbalanced circulation between the twins with shift of blood from one (the donor)to the other (the recipient)(13;14).
Most dramatic changes occur in the recipient twin and characteristically affect the right ventricle more than theleft. Progression of TTTS leads to cardiomegaly, impaired cardiac function and ultimately hydrops. Variable degree of right and left ventricular hypertrophy and dilatation, tricuspid regurgitation and some left ventriculardysfunction have been reported (1) Ventricular dilatation and hypertrophy is thought to be due to increased preload. Some fetuses may show disproportionate degree of hypertrophy. Possible explanations for this may involve the effect of hormones originating in the donor twin that cross the placenta further to affect the recipient twin by increasing its afterload. Examples are the renin-angiotensin system (16;17) and endothelin-118 Angiotensin may also be linked to hypertension seen in the recipient twin (19) which may explain ventricularhypertrophy. Right ventricular outflow tract obstruction is also well documented in the recipient. It canprogress during fetal life leading to pulmonary atresia in the most severe cases and may progress further post-natally. The obstruction may be seen at the level of the pulmonary valve and / or muscular subvalvar area (20-22).
Characteristically and contrary to the recipient, the donor twin shows no overt cardiac disease (15;21).More recentlyhowever. left ventricular shortening fraction has been shown to be higher in the donor than in the recipienttwin, possibly reflecting a response to anemia (23;24).No intertwin differences in haemodynamic parameters areexpected to occur in normal twin pregnancies(25).Raboisson and colleagues have recently shown abnormalities of diastolic function early in the course of TTTS. Myocardial performance indices (MPI, ‘Tei index’) were shown to be systematically higher in the recipient when compared to the donor twin. The higher MPI values encountered were due to prolongation of the isovolumic relaxation time, in keeping with diastolic abnormalities. At the same time, in the earlier phases of TTTS, indices of systolic function remained normal(24).Progressive biventricular hypertrophy (cardiomyopathy) with predominant right ventricular systolic and biventricular diastolic dysfunction have also been shown in the course of TTTS(26).These recent studies may further substantiate the theory of increased afterload being important in the pathophysiology of TTTS.
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