Vascular Distribution Patterns in Monochorionic Twin Placentas
Introduction
Approximately 10–20% of monochorionic twin gestations are complicated by severe chronic twin-to-twin transfusion syndrome (TTTS), characterized by a gradual shift of blood volume from the donor twin to the recipient twin through inter-twin vascular anastomoses. The pathogenesis of TTTS remains incompletely understood. However, the risk for development of TTTS in diamniotic-monochorionic twin gestations has been linked to a number of anatomic characteristics of the placenta, specifically, unequal sharing of the single-disc placenta, velamentous or marginal cord insertion, fewer anastomoses overall, and relative paucity of superficial artery-to-artery anastomoses in particular [1], [2], [3], [4], [5].
In view of its putative critical role in the pathogenesis of TTTS, the architecture of the inter-twin vascular communications has been the subject of numerous studies [1], [5], [6], [7], [8]. Placental vascular communications occur in virtually all monochorionic placentas [7], [8], [9] and are either superficial (artery-to-artery or vein-to-vein) or deep (artery-to-vein). Whereas superficial artery-to-artery anastomoses are believed to protect against the development of TTTS, deep artery-to-vein anastomoses are unidirectional and have been implicated in the unequal distribution of flow between the two twins.
In contrast to the inter-twin vascular communications, no attention has been given thus far to the characteristics of the vascular distribution patterns of the individual diamniotic-monochorionic twins. Chorionic vascular patterns are traditionally described as disperse, magistral or mixed [6]. The “disperse” pattern is characterized by a fine network of vessels that branch out from the cord insertion to the various placental cotyledons. The “magistral” pattern has arteries of relatively uniform size that course across the placental surface nearly to the edge without diminishing in diameter. In singleton placentas, the disperse type of vessel distribution is more common (62%), while the magistral type occurs in 38% [6], [10].
Placental vascular diameter, vascular resistance and flow patterns are widely believed to be determinants of TTTS in monochorionic placentas. In their mathematical model of monochorionic placental circulation, Umur et al. [11] have shown that vascular diameter and resistance are important determinants of hemodynamic imbalance in diamniotic-monochorionic placentas, especially in blood vessels with a high pressure differential, such as artery-to-vein anastomoses. We therefore speculated that the particular vascular distribution types, each characterized by specific vascular branching patterns and peripheral vascular diameters, may play an important role in the development of TTTS in monochorionic twins.
The aim of this study was (1) to determine the relative prevalence of disperse and magistral vascular distribution patterns in monochorionic twin placentas, and (2) to correlate these patterns with the presence of TTTS and other anatomic placental characteristics linked to TTTS. Increased knowledge of the placental anatomy in diamniotic-monochorionic gestations may lead to a better understanding of the pathophysiology of TTTS, and may even be of prognostic value in this often lethal condition.
Section snippets
Materials and methods
The vascular distribution patterns were determined in a consecutive series of diamniotic-monochorionic twin placentas submitted to the Department of Pathology at Women and Infants Hospital (2001–2003). The accompanying charts were reviewed to determine whether the pregnancy was complicated by severe chronic TTTS, as defined by ultrasonographic evidence of severe polyhydramnios in one twin and concomitant oligohydramnios in the other, as well as additional signs (absence of bladder in the donor
Clinical data
The vascular distribution patterns were studied in a consecutive series of 89 diamniotic-monochorionic twin placentas. Fifteen cases (17%) showed clinical and ultrasound evidence of severe chronic TTTS. Nine of these 15 twins had undergone fetoscopic laser coagulation of the communicating vessels. Seventy-four twins (83%) showed no evidence of TTTS and are designated as ‘non-TTTS’ cases. The gestational age at delivery ranged from 20 to 37 weeks for TTTS twins (mean: 26 ± 6 weeks) and from 24 to
Discussion
This study describes the relative frequency of disperse, magistral, or mixed vascular distribution patterns in diamniotic-monochorionic twin placentas. We found that the prevalence of magistral or mixed vascular types in monochorionic twin placentas was higher than that previously reported for singletons (47% versus 38%) [10]. In addition, we determined that the prevalence of magistral/mixed vascular types was significantly higher in placentas of twins with TTTS compared with those without TTTS
Acknowledgements
The authors wish to thank Francois I. Luks, M.D., Ph.D., for the artwork.
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