Human Reproduction, Vol. 14, No. 7, 1890-1893,
July 1999
© 1999 European Society of Human Reproduction and Embryology
Prenatal assessment of the Hyrtl anastomosis and evaluation of its function: Case report
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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The presence of a communicating vessel, the Hyrtl anastomosis, between the umbilical arteries is well described in pathological studies. Using different injection techniques, it has been speculated that this vessel acts as a pressure-equalizing mechanism between the different lobes of the placenta. However, its detection during fetal life has never been reported. We report on two cases of ultrasonographic detection and Doppler assessment of the Hyrtl anastomosis during pregnancy. A pulsatile blood flow from the umbilical artery with higher resistance to that with lower resistance has been demonstrated at the level of the Hyrtl anastomosis, which was confirmed after delivery. In addition, this report supports the hypothesis that the anastomosis plays an important role in regulating the blood pressure in the placental lobes and in equalizing the blood pressure between umbilical arteries.
Key words: Hyrtl anastomosis/ultrasound/umbilical artery Doppler/umbilical cord
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Introduction |
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A macroscopic peculiarity of the umbilical cord is the presence of a vessel, the Hyrtl anastomosis, connecting the umbilical arteries (Hyrtl, 1870
; Benirschke and Kaufmann, 1995). In >95% of umbilical cords some sort of anastomosis can be observed between the arteries (Young, 1972). In 90% of the cases the anastomosis is located within 1–3 cm of the placental end of the umbilical cord and varies in length from 1.5 to 2 cm and in calibre from 0.33 to 1.5 times that of the umbilical artery (Priman, 1959; Szpakowski, 1974).
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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A 27 year old woman, gravida 3, para 1, underwent a routine ultrasound examination at 36.4 weeks gestation. Her obstetric history was significant for a previous Caesarean section which was performed for cephalo-pelvic disproportion. The ultrasound examination was performed with a Toshiba SSH-140A unit (Toshiba Corporation, Medical Systems Division, Tokyo, Japan) equipped with a 3.75 MHz transducer. The fetal biometry and the amniotic fluid volume were appropriate for gestational age. No fetal structural abnormalities were noted. The placenta was situated on the posterior uterine wall. A targeted ultrasound revealed a three-vessel umbilical cord inserted marginally with a cross-sectional area of 202 mm2 which is within the normal range for gestational age (Raio et al., 1999
). The diameters of the arteries were 4.2 and 4.3 mm respectively, while the diameter of the vein was 8.4 mm. The Hyrtl anastomosis connecting the stems of the umbilical arteries was identified close to the placental surface (Figure 1). The diameter of the anastomosis was 3.2 mm. The characteristic of the umbilical arteries blood flow was assessed by pulsed Doppler before (fetal side) and after (placental side) the interarterial anastomosis. The resistance indices before and after the Hyrtl anastomosis were 0.59 and 0.67 for one artery and 0.56 and 0.57 for the other artery. The umbilical artery resistance indices differed, being lower on the fetal than on the placental side (0.03 versus 0.10 respectively). The resistance index of the Hyrtl anastomosis was 0.62 (Figure 2). Colour Doppler analysis revealed that the direction of the blood flow in the anastomosis was from the umbilical artery with higher resistance to that with lower resistance. The patient delivered a 3230 g male infant by elective Caesarean section at 38.7 weeks gestation. The placental weight was 750 g and a macroscopically normal umbilical cord, 53 cm long, was inserted marginally. The presence of an oblique interarterial anastomosis crossing over the umbilical vein was confirmed macroscopically after delivery within 2 cm of the placental surface. The length of the Hyrtl anastomosis was ~16 mm.
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Case 2 |
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A 35 year old woman, gravida 2, para 1, was referred for an ultrasound examination at 34.3 weeks gestation because of an increased amniotic fluid volume. The patient did not have risk factors for diabetes. The ultrasound examination was performed with an Aloka Prosound 5500 unit (Aloka, Tokyo, Japan) equipped with a 3.5 MHz transducer. The fetal biometry was appropriate for gestational age and no fetal anatomical abnormalities were observed. The amniotic fluid index was 27 cm. A detailed examination of the placenta and the umbilical cord was carried out. The placenta was inserted on the left uterine wall and the umbilical cord had a central insertion. The umbilical cord had three vessels and its cross-sectional area was 189 mm2. The umbilical artery diameter was 2.9 mm in both vessels and the vein diameter was 8.1 mm. A cross-sectional scan of the umbilical cord close to the placental surface revealed the presence of a fourth vessel 1.6 mm diameter which represented the Hyrtl anastomosis (Figure 3
). Doppler examination of the umbilical arteries was performed before and after the interarterial communicating vessel. The pulsatility index of the two arteries was 0.78 and 0.80 respectively on the fetal side and 0.80 and 0.85 on the placental side. Also in this case, the umbilical artery pulsatility indices were higher after than before the anastomosis (0.05 versus 0.02). Doppler flow velocimetry of the Hyrtl anastomosis showed a pulsatility index of 0.86. Colour Doppler assessment of the anastomosis revealed a unidirectional blood flow. The patients delivered spontaneously at 40.5 weeks of gestation a 3680 g male infant. The placental weight was 680 g while the umbilical cord length was 63 cm. Macroscopic examination of the umbilical cord revealed the presence of a communicating vessel 12 mm long at about 1 cm from the placental surface (Figure 4). In this case, the Hyrtl anastomosis emerged at right angles from the umbilical arteries.
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Discussion |
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A Medline search was conducted on studies published between 1966 and February 1999 to identify whether the anastomosis between the umbilical arteries has been investigated during fetal life. Although a case of fusion between umbilical arteries before entering the placenta has been previously diagnosed at ultrasound (Rosenak and Meizner, 1994
), the cases reported here are the first two of prenatal identification and Doppler assessment of the Hyrtl anastomosis.
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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4 To whom correspondence should be addressed
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References |
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Submitted on December 31, 1998; accepted on April 8, 1999.
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