1 Department of Obstetrics and Gynaecology, Kantonsspital, CH 8596 Münsterlingen, Switzerland, 2 Department of Obstetrics and Gynaecology, University of Insubria, Varese, and 3 Department of Obstetrics and Gynaecology, University of Bari, Bari, Italy
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Abstract |
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Key words: Hyrtl anastomosis/ultrasound/umbilical artery Doppler/umbilical cord
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Introduction |
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Although several types of interarterial anastomosis have been described (i.e. fusion of the arteries, anastomosis by two branches), the most frequent condition, accounting for >80% of cases, is the presence of an intermediate communicating vessel between the stems of the umbilical arteries (Priman, 1959; Szpakowski, 1974
).
The presence of an interarterial anastomosis in the umbilical cord seems to be a recent evolutionary development. Indeed, it has not been found in lower primates (e.g. Lemurus) (Young, 1972), while the presence of an anastomosis has been described in 30% of New World Primates (Platyrrhine) and in about 80% of Old World Primates (Catarrhine).
The functional significance of the interarterial anastomosis of the umbilical cord was first described in 1870 (Hyrtl, 1870). Hyrtl speculated that its function is to equalize the blood pressure between the two arteries and to allow a uniform distribution of blood in the different regions of the placenta. It has been suggested that the Hyrtl anastomosis acts as a safety valve for the placenta (comparable to the circle of Willis for the brain) in case of compression or occlusion of one of the umbilical arteries (Priman, 1959
; Benirschke and Kaufmann, 1995
). In addition, it was proposed that the anastomosis plays the role of a `buffer' system during uterine contractions when the blood pressure in the corresponding part of the intervillous space and cotyledons of the placenta is increased (Bacsich and Smout, 1938
). The presence of a pressure-equalizing system can also explain the similar calibre of the umbilical arteries even when the territories supplied by them are of different size (Szpakowski, 1974
).
In this paper, we report the identification of the Hyrtl anastomosis and its functional evaluation during fetal life.
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Case 1 |
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Case 2 |
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Discussion |
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Several authors have described the presence of this vessel and speculated on its function during pregnancy (Benirschke and Kaufmann, 1995). However, these investigations were conducted after delivery studying the placenta and the umbilical cord and relating their morphology to that of the Hyrtl anastomosis. It has been demonstrated that in case of transverse anastomosis, the areas of the placenta supplied by the umbilical arteries is nearly equal (Priman, 1959
). In this type of anastomosis blood can flow, if needed, in both directions. On the contrary, in oblique anastomosis there is a distinct difference in the size of the areas of the placenta supplied by each umbilical artery (Szpakowski, 1974
). The artery into which the anastomosis empties supplies a larger area of the placenta, the size of which is proportional to the diameter of the Hyrtl anastomosis, and the angle of the junction is more acute. Using corrosion techniques, it has been demonstrated that there are no signs of peripheral anastomosis between the branches of the umbilical arteries except at the level of the Hyrtl anastomosis. (Bacsich and Smout, 1938
). Moreover, it is possible to fill the entire arterial system of the placenta through one umbilical artery in all specimens in which an anastomosis is present (Shordania, 1929
).
With regard to the function of the Hyrtl anastomosis during pregnancy, it has been postulated that its absence could be in part responsible for the occurrence of discordant umbilical arteries, a condition often associated with placental anomalies (e.g. velamentous and marginal insertion, infarcts, chorioangiosis) (Dolkart et al. 1992; Raio et al. 1998
). Indeed, when the anastomosis is present, the umbilical arteries are generally of equal size even in cases of great discrepancy between the territories supplied by them (Benirschke and Kaufmann, 1995
). Recently, we have reported a case of discordant umbilical arteries associated with the absence of the Hyrtl anastomosis and the presence of an abnormal Doppler waveform only in the hypoplastic artery (Raio et al., 1998
). The presence of discordant flow velocity waveform between umbilical arteries has been described in a case in which one umbilical artery was hypoplastic, the Hyrtl anastomosis was missing and the lobes supplied by the umbilical arteries were of vastly different size (Hitschold et al., 1992
).
Recently, some investigators (Jorn et al., 1994; Predanic et al., 1998
) have shown that the resistance to blood flow in one umbilical artery often differs considerably from that in the other. Since the difference decreases as pregnancy advances, it has been suggested that the equalization of blood flow in the umbilical arteries is the result of functional maturation of the Hyrtl anastomosis (Predanic et al., 1998
). In addition, these authors postulated that failure of the anastomosis to develop anatomically or functionally may be responsible for differences in umbilical artery flow patterns. This is supported by the present study in which, in both cases, the difference in the impedance to blood flow between umbilical arteries was more pronounced on the placental than on the fetal side. Moreover, pulsatile blood flow from the artery with higher resistance to that with lower resistance was demonstrated in both cases.
In conclusion, this report has shown the sonographic finding of the Hyrtl anastomosis and its functional assessment by Doppler analysis. Further studies are required to investigate the detection rate of the anastomosis throughout gestation and whether its Doppler assessment could have clinical value in evaluating fetal status in the case of abnormal umbilical artery resistance to blood flow.
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Notes |
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References |
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Submitted on December 31, 1998; accepted on April 8, 1999.