Placental hypoxia: The lesions of maternal malperfusion

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Available online at www.sciencedirect.com

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Placental hypoxia: The lesions of maternal malperfusion W. Tony Parks, MD Department of Pathology, Magee-Women's Hospital, University of Pittsburgh School of Medicine, 300 Halket St, Pittsburgh, PA 15213

article info

abstra ct

Keywords:

The placental lesions classically ascribed to placental hypoxia, here denoted maternal

hypoxia

malperfusion (MMP), are among the more significant that a placental pathologist may

preeclampsia

encounter. Yet the appearance of these lesions may be subtle, and the clinical implication

trophoblast

of their diagnosis is frequently unclear. The aim of this review is to provide a more

vasculopathy

nuanced perspective on the clinical utility of placental pathology for the detection of MMP. The review will first detail MMP lesions in the placenta and discuss their associations with pregnancy complications. The review will then delve into the diagnostic and interpretive difficulties of these lesions. Finally, recent research findings that may aid in the development of better diagnostic tools will be briefly discussed. & 2014 Elsevier Inc. All rights reserved.

Clinical syndromes associated with MMP The various topics that can be subsumed under the rubric of placental hypoxia have spawned considerable research, but despite these efforts, surprising gaps still remain. This confusion can be readily appreciated simply by tabulating some of the many terms that have been used to encompass this entity. These include placental hypoxia, placental insufficiency, maternal vascular underperfusion, shallow implantation, disorders of deep placentation, placental ischemic disease, and many others. These designations may suggest a mechanism (hypoxia and underperfusion), an underlying etiology (shallow implantation), or a more generalized abnormality (placental insufficiency). In order to utilize a more general term while minimizing imputations of etiology, this review will refer to the overall process as maternal malperfusion (MMP). Individual component lesions (e.g., placental infarcts or decidual vasculopathy) will be designated as MMP-related lesions. Maternal malperfusion is of tremendous clinical significance, as aspects of this entity have been associated with

E-mail address: [email protected] http://dx.doi.org/10.1053/j.semperi.2014.10.003 0146-0005/& 2014 Elsevier Inc. All rights reserved.

multiple severe pregnancy-related complications.1 In fact, it is difficult to discuss the topic of MMP without reference to the highly intertwined issue of preeclampsia. Many MMPrelated lesions were originally described in the context of preeclampsia, including acute atherosis2 and Tenney–Parker change,3 and these lesions were initially considered relatively specific for preeclampsia. Subsequent research has demonstrated that these lesions are not specific to preeclampsia but can be found in such diverse entities as fetal growth restriction, systemic lupus erythematosus, antiphospholipid antibody syndrome, preterm labor, preterm premature rupture of membranes, abruptio placentae, and stillbirth.1 These lesions may even be detected in otherwise normal pregnancies. Research has been active in the field of preeclampsia, and our understanding of the disease has improved substantially over time. Preeclampsia appears to develop in two stages.4 The first is a subclinical stage in the late first and early second trimesters during which inadequate spiral artery remodeling leads to perturbed, likely diminished (at least transiently), maternal blood flow to the placenta. The second stage is

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clinical manifestation of the disease later in pregnancy, with associated endothelial dysfunction, angiogenic imbalance, and activation of inflammatory pathways. A modification of this scenario proposes inadequate development of maternal tolerance to the allogeneic fetus as an early initial etiologic step.5,6 However, this relatively simple two (or three)-step process fails to account for the remarkable variability seen in preeclampsia. It now seems clear that there are at least two forms of preeclampsia, including an early-onset form and a late-onset form.7 Early-onset preeclampsia includes those cases that occur prior to 34 weeks' gestation. Occurrence may be defined as time of onset or time of delivery, although time of delivery is preferred since it is more readily objectively verified.5 The early-onset form is typically more severe than the late-onset form, with higher incidences of fetal growth restriction and fetal death.8 Maternal morbidities, such as HELLP syndrome, also occur at higher frequencies in early-onset preeclampsia. Unsurprisingly, the placentas from women with early-onset preeclampsia often show significant damage, with MMP-related lesions occurring at a much higher rate than is seen in the placentas from women with late-onset preeclampsia.9 In fact, the placentas from women with late-onset preeclampsia are generally similar to the placentas from uncomplicated pregnancies. MMP-related lesions are also frequently found in disorders other than preeclampsia. They are particularly common in cases of fetal growth restriction in the absence of preeclampsia.10–12 The findings in fetal growth restriction parallel those of preeclampsia in that cases with early-onset fetal growth restriction have more evidence of MMP-related placental damage than cases with late-onset fetal growth restriction.13,14

Normal vascular remodeling in the placenta While the underlying etiology for MMP is unknown, a frequent characteristic is inadequate remodeling of maternal spiral arteries. Normal vascular remodeling begins early in the first trimester of pregnancy, when extravillous trophoblast cells (EVT) invade from the base of the developing placenta into the underlying decidua.15 Subsets of these EVT perform different functions in the decidua. One particular subset of EVT transforms the spiral arteries by helping to degrade the existing vascular smooth muscle walls and to replace them with fibrinoid.16 This process yields dilated vessels with terminal diameters 5–10 times larger than prior to remodeling.17 These remodeled vessels can no longer vasoconstrict in response to maternal or fetal signals. Interestingly, early in the first trimester, these vessels are also occluded by plugs of EVT.18,19 These trophoblastic plugs restrict the flow of red blood cells (and therefore oxygen) to the developing placenta. The oxygen concentration within the developing early first trimester placenta has been measured at o20 mmHg (3–5% oxygen),20,21 while the comparable value for the maternal decidua is approximately 60 mmHg (8–10% oxygen). This apparently low oxygen level is actually physiologic for the gestation. Secretions from endometrial glands and the slow flow of plasma provide nourishment to the developing gestation,22 while the low oxygen tension minimizes oxidative damage from free radicals.

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Near the end of the first trimester, the endovascular trophoblastic plugs dissolve, and a true circulation to the placenta is established. The oxygen level in the intervillous space rises at this time, measuring between 40 and 80 mmHg.23 A second wave of EVT-mediated vascular remodeling then occurs.15,24 During this wave, the remodeling extends through the decidua and into the myometrium, eventually involving the inner third of the myometrium (a region sometimes termed the junctional zone). Since a possible vascular smooth muscle sphincter is located at the junction of the decidua and the myometrium,25 remodeling through this region may be particularly crucial. The final oxygen level in the intervillous space is generally around 60 mmHg after the second wave of EVT-mediated vascular remodeling.20,21,23,26 While this value is substantially higher than the first trimester oxygen level, it is still well below the normal maternal arterial value of 90 mmHg and would be considered at the borderline for hypoxemia postnatally.27

MMP-related lesions A number of placental abnormalities have been associated with MMP. Among the most characteristic are the lesions indicative of failed or inadequate vascular remodeling. These alterations to the maternal vasculature can collectively be termed decidual vasculopathy, with decidual arteriopathy and decidual arteriolopathy as synonyms. Included under this umbrella term are three related but distinct lesions: persistence of muscularized basal plate arteries (lack of physiologic remodeling), mural hypertrophy of membrane arterioles, and acute atherosis.28 Persistence of muscularized basal plate arteries is, exactly as its name suggests, the focal absence of EVT invasion with retention of the vascular smooth muscle wall involving at least one spiral artery of the placental basal plate (Fig. 1A).28 Mural hypertrophy of membrane arterioles specifically involves the decidual vessels of the extraplacental membranes (decidua parietalis) (Fig. 1B). Since these vessels are not situated beneath the placenta, they experience a lesser degree of vascular remodeling. Retention of vascular smooth muscle in this location thus does not represent pathologic alteration. However, hypertrophy of the smooth muscle wall (possibly in response to maternal hypertension) is pathologic. Diagnosis of this lesion requires that the mean vascular wall diameter be greater than 30% of the total vessel diameter.28 A quick first approximation to this criterion is that the luminal diameter of the vessel must be less than one-third the total vascular crosssectional diameter for diagnosis. In contrast to the previous relatively simple lesions, acute atherosis is more variable and complex in its appearance. The most frequent and possibly earliest form of acute atherosis is fibrinoid necrosis of the vessel wall.29 This lesion presents as a dense, glassy or waxy, deep pink to red alteration in the vascular smooth muscle wall (Fig. 1C).28 Normal spiral artery remodeling can mimic this appearance, but the vascular wall in fibrinoid necrosis will typically be a deeper shade of red. More classic acute atherosis presents with foamy macrophages embedded within fibrinoid necrosis (Fig. 1D). Fibrinoid necrosis need not be present, with replacement of the

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Fig 1 – Lesions of MMP: decidual vasculopathy. (A) Persistence of muscularized basal plate arteries. This image shows a spiral artery segment from the basal plate that has been only incompletely remodeled. Note the retention of the smooth muscle wall. A mild chronic inflammatory infiltrate is also present. (B) Mural hypertrophy of a membrane arteriole. This image shows a small vessel within the extraplacental membranes. The unremodeled smooth muscle wall shows prominent muscular hypertrophy with a minimal luminal diameter. (C) Atherosis. The multiple adjacent lumens of this spiral artery show replacement of the vascular wall by dense fibrinoid necrosis. (D) Atherosis with foamy macrophages. The adjacent lumens of this spiral artery again show fibrinoid replacement of the vessel wall, but this time also including the presence of foamy macrophages.

vascular wall solely by foamy macrophages. A tight perivascular cuff of lymphocytes may also be noted around these lesions. A recent hypothesis suggests that acute atherosis may represent the end result of a final common pathway for multiple underlying etiologies, with increased decidual inflammation as a significant factor in the genesis of this lesion.30 Several issues hamper detection of these lesions. The most significant clinically is that these lesions (particularly absence of vascular remodeling) are found at a much higher frequency in the deeper decidual or myometrial segments of the spiral arteries than in the superficial segments. These pathologic entities were initially well characterized in studies begun in the 1950s using placental bed biopsies,31,1 which permit examination of the vessels in the deeper decidua and the inner myometrium. Based on studies of placental bed biopsies, on average only a minority (approximately 25%) of

myometrial spiral artery segments are remodeled in cases of preeclampsia compared to nearly 90% from normal pregnancies.32 Delivered placentas, on the other hand, are usually the only tissues available for pathologic examination, and they retain only a thin layer of decidua. The more superficial segments of the spiral arteries that are present on the delivered placenta are more likely to have been transformed by invading trophoblast than the underlying deep decidual and myometrial segments.33 Also important is that spiral artery remodeling is not uniform across the placental bed. Trophoblast invasion is less prominent at the placental periphery, where conversion of spiral arterioles is less efficient and less complete.1,24,34 These incompletely remodeled peripheral vessels can thus be misinterpreted as evidence for decidual vasculopathy. Finally, the lesions of decidual vasculopathy are only focal, so the likelihood of finding these lesions varies with the intensity of the search. The typically

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submitted sections for pathology—two sections of placental parenchyma and one or two membrane rolls—provide only limited material to detect these lesions. Infarction of the placental villi is one of the more readily identifiable MMP lesions. Villous infarction occurs when maternal blood flow becomes insufficient to sustain the viability of the villi within a localized region. Five different varieties of infarction have been defined,35 but only two will be included in this section. The most common type of villous infarction is that due to occlusion of a maternal spiral artery. The local cessation of maternal blood first leads to shrinkage of the intervillous space with collapse of the villi in a wedgeshaped or rectangular region overlying the occluded spiral artery. Villous congestion also develops at this time (Fig. 2A). Over time, this recent infarct will evolve into an older (“remote”) infarct. The trophoblast layer typically dies first, with early preservation of the stromal and vascular core of the villi. The dying trophoblast often has a smudgy and faded appearance. With time, the villous vessels and stromal cells die also, leaving faded ghost villi (Fig. 2B). Infarctions due to occlusion of maternal spiral arteries are common at term,35,36 particularly in the placental periphery, and are generally of little clinical significance. Such infarctions in a preterm placenta or occupying more than 5% of the placental volume are both abnormal and clinically significant.35,36 The second type of placental infarction includes those central lesions not centered on the placental basal plate (and not due to occlusion of a specific spiral artery). These tend to be small, round infarcts scattered throughout the placental parenchyma. They are thought to arise in the watershed zones on the edges of the flows from adjacent spiral arteries. These watershed zones are poorly perfused, relatively hypoxic, and therefore particularly susceptible to hypoxic–ischemic insults.35 Other evidence of MMP frequently accompanies these watershed infarcts. An entity closely related to villous infarction is villous agglutination. Foci of villous agglutination consist of aggregates of adjacent villi (more than 2 but less than 20 villi) that have adhered to each other, typically due to small amounts of intervening fibrin28 (Fig. 2C). The surface trophoblast of the agglutinated villi may be degenerated and may be focally replaced by fibrin. The villous stroma may be degenerated, with stromal fibrosis or nuclear karyorrhexis.28 The etiology for this lesion is thought to be localized ischemic necrosis of the villous syncytiotrophoblast.28,37 Estimating the appropriateness of the villous maturation for the clinical gestational age is one of the more commonly used tools to identify MMP.38 Villous appearance varies on an almost week-by-week basis throughout gestation, and finding villous maturation more advanced than expected for gestational age (sometimes termed accelerated villous maturation) is evidence for MMP. While it is beyond the scope of this review to detail the changes that occur with gestation, several generalities apply. One readily observable transformation is the shift in the proportions of villous populations over time. While immature intermediate villi predominate in the second-trimester placenta, mature intermediate villi and terminal villi largely replace the immature intermediate villi in the third trimester. This shift in villous proportions is paralleled by alterations in villous morphology. With increasing gestational age, villi

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become smaller, syncytial knots increase, villous vessel crosssectional area increases, vessels concentrate around the periphery of the villous and become more sinusoidal, vasculosyncytial membranes form, and the villous stroma becomes denser. Importantly, villous morphology is not uniform across the placental lobule. The central zone immediately overlying the opening of a spiral artery will generally have less mature villi, while villous maturation will be more advanced in the peripheral regions. This heterogeneity is a normal feature of an appropriately mature placenta, and its absence is a clue to the presence of aberrant villous maturation. An increased frequency of syncytial knots populating the surface of chorionic villi is one of the most widely acknowledged components of MMP.28 Active syncytiotrophoblast nuclei are normally distributed widely throughout this true syncytium. Syncytial knots represent clusters of syncytiotrophoblast nuclei that protrude from the villous surface and have been considered to represent senescent or degenerating syncytiotrophoblast nuclei. At least three different types of syncytial knots have been identified, however, and the difficulty in distinguishing among these types has complicated the assessment of syncytial knotting.39,40 Early in gestation, syncytial sprouts predominate (Fig. 2D). These are rounded protrusions extending from the syncytium on a long neck with lightly staining nuclear chromatin.39 A prominent nucleolus is common. Syncytial sprouts represent one of the earliest stages in villous branching and villous formation. A background level of syncytial sprouts, varying with the gestational age, will be found throughout the pregnancy. False knots derive from tangential sections of villous surfaces.39 As villous branching increases, both normally throughout gestation and yet further in complicated pregnancies, false knots become more prevalent. False knots can be identified by their transcriptional activity.39 True knots are the syncytial knot variants considered most relevant to MMP. True knots have highly condensed nuclear chromatin, and nucleoli are generally absent. True knots show no evidence of recent proliferative or transcriptional activity. Instead, oxidative damage can be detected within their nuclei.39 Diagnosis of increased syncytial knots (Fig. 2E) requires the presence of syncytial knots on more villi than anticipated over a total of more than 30% of the villi examined in the lower 75% of the section.39 Recently, normal values for the percentages of villi containing at least one syncytial knot have been developed for a broad range of gestational ages.41 These normal values may assist in identifying cases of increased syncytial knots. One additional pattern of abnormal syncytial knot has also been described, the wave-like syncytial knot.42 These knots arise early in gestation (typically second trimester) and present as regularly spaced tight clusters of hyperchromatic syncytial nuclei along the surfaces of larger villi (Fig. 2F). It has been proposed that wave-like syncytial knots develop in cases of extensive senescence of syncytial nuclei. These senescent nuclei cluster along preexisting lines of syncytial nuclear organization (linear nuclear patterning), with the linear organization generating the regular spacing observed microscopically.43 Wave-like syncytial knots are almost inevitably found with other evidence for MMP, such as distal villous hypoplasia, increased syncytial knots, or accelerated villous maturation.

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Fig 2 – Lesions of MMP. (A) Recent villous infarction. This image shows a recent villous infarct filling the panel. The villous space is collapsed, the villous vessels are congested, and the trophoblast is beginning to degenerate. (B) Old (remote) villous infarction. This image shows a remote infarct filling the panel. All villous structures show degeneration with loss of color intensity. (C) Villous agglutination. The center of this panel contains a cluster of adherent villi. The villous stroma shows early degeneration with loss of the normal vasculature. (D) Syncytial sprouts. The two first trimester villi in this image are surrounded by paddle-shaped extensions. These trophoblastic sprouts are one of the earliest stages of new villous formation. Numerous free-floating syncytial sprouts are also present. (E) Increased syncytial knots. This image contains numerous small terminal villi, the majority of which harbor at least one densely basophilic syncytial knot. (F) Wave-like syncytial knots. The linear cluster of protuberant hyperchromatic syncytial knots along the undersurface of the stem villous in this image is characteristic of wave-like syncytial knots.

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Fibrin (also called fibrinoid) is comprised of varying proportions of fibrin-type fibrinoid (deposition products from the coagulation cascade) and extracellular matrix-type fibrinoid (deposition of extracellular matrix material primarily by extravillous trophoblast cells). Several different patterns of deposition have been defined. Any pattern of fibrin deposition has the potential to obstruct blood flow within the placenta and therefore lead to MMP. The most obviously disruptive pattern is massive perivillous fibrin deposition, in which huge amounts of fibrin are deposited throughout the intervillous space, encasing and suffocating placental villi and creating innumerable impediments to maternal blood flow (Fig. 3A). Massive perivillous fibrin deposition is a rare entity that does not appear to arise solely in the context of MMP, however, and it is often not included among MMP lesions.28 Finding increased intervillous fibrin is considered evidence for MMP.28 This

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increased fibrin can take several forms. These include deposition of fibrin on the surfaces of larger stem villi, small foci of fibrin deposition either adjacent to or within distal villi (Fig. 3B), or rounded accumulations of fibrin containing increased numbers of extravillous trophoblast (Fig. 3C).28 Distal villous hypoplasia is a characteristic lesion for late MMP (Fig. 3D). It commonly presents in pregnancies that have already progressed to absent or reversed end-diastolic umbilical artery blood flow. The villi in distal villous hypoplasia are commonly elongated and thin, with narrow diameters. Vascular branching is decreased. The slim villous diameters with minimal branching lead to a much more open intervillous space. For the diagnosis of distal villous hypoplasia, this process must involve more than 30% of the visualized placental parenchyma (excluding the 25% of the placental parenchyma immediately beneath the chorionic plate).

Fig 3 – Lesions of MMP. (A) Perivillous fibrin deposition. This image shows large swathes of perivillous fibrin. The fibrin encases numerous villi. Hugely increased amounts of perivillous fibrin characterize the entity massive perivillous fibrin deposition (not normally considered within the spectrum of MMP). Smaller but increased amounts, particularly along stem villi, are abnormal and represent features of MMP. (B) Increased intervillous fibrin. This image shows a second variant of increased intervillous fibrin, in which small amounts of fibrin deposit eccentrically along distal villi. (C) Increased intervillous fibrin with intermediate trophoblast. This image shows prominent deposition of fibrinoid material containing large numbers of intermediate trophoblast cells. These large cells are generally solitary or form loosely aggregated groups within the fibrin. (D) Distal villous hypoplasia. The villi in this image are narrow and elongated with minimal branching. This architecture leaves the villi widely spaced and easily recognized.

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Two abnormalities of decidual trophoblast, termed increased placental site giant cells and increased immature intermediate trophoblast, may also represent MMP.28 During the first and early second trimesters, large numbers of trophoblast emerge from the base of the placenta and stream through the underlying decidua. Many of these cells remain in the decidua. Some cohere into large aggregates, while others appear to fuse into multinucleated giant cells. Increased immature intermediate trophoblast (Fig. 4A), defined as sheets of at least 10–20 cells within the superficial decidua, has been found in preeclamptic placentas.44 Immature intermediate trophoblast, with their eosinophilic or vacuolated cytoplasm, must be differentiated from mature intermediate trophoblast, whose cytoplasm is generally a deeper purple. Similarly, increased placental site giant cells (Fig. 4B), defined as trophoblastic cells with three or more nuclei in a cluster of three or more cells, have also been detected in the decidua from preeclamptic placentas.44,45

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Two membrane lesions laminar necrosis (Fig. 4C) and microscopic chorionic pseudocysts (Fig. 4D), have recently received attention as possible markers for MMP.38,45–49 Laminar necrosis is defined as linear bands of coagulation necrosis involving the choriodecidual interface of the membrane roll.45 In practice, this lesion typically involves the more superficial decidua while sometimes also involving the overlying trophoblast, chorion, and/or amnion. To ensure that the diagnosis represents a relatively diffuse lesion, at least 10% of the membrane roll must show involvement by laminar necrosis. Variants of laminar necrosis have long been noted and have been associated with preeclampsia,50 while more recent work has confirmed an association with preeclampsia and further suggested association with other pregnancy complications.45 Microscopic chorionic pseudocysts are defined as microscopically identified cystic spaces within the trophoblast layer of the membranes.38,46 These

Fig 4 – Lesions of MMP. (A) Increased immature intermediate trophoblast. The image contains a large central cluster of immature intermediate trophoblast within the basal plate decidua. The individual trophoblast cells have prominently vacuolated cytoplasm like that seen in the trophoblast of the extraplacental membranes. They are embedded within an abundant eosinophilic fibrin. (B) Increased placental site giant cells. Numerous trophoblast giant cells are apparent in this image of placental decidua, with at least one cluster containing three or more adjacent giant cells. (C) Laminar necrosis. A large band of coagulation necrosis involving the decidua of the membrane roll occupies the center of this panel. Possible focal degeneration of the overlying trophoblast may also be seen. (D) Microscopic chorionic pseudocysts. Three small cysts filled with amorphous eosinophilic material are present in the center of this image.

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microcysts are lined by extravillous trophoblast and contain amorphous eosinophilic material. Diagnosis of microscopic chorionic pseudocysts requires at least three such microcysts in one membrane roll. Microscopic chorionic pseudocysts have been associated with a number of MMP-related lesions and with clinical disorders such as preeclampsia and diabetes mellitus.46 The significance of these diagnoses has recently been called into question, and their utility as a marker for MMP is disputed.49 Though not specifically a lesion, one aspect of placental pathology of considerable significance is the placental weight.35,51 A small placental weight is associated with a range of pathologies including fetal growth restriction and preeclampsia,52 and these small placentas often show other evidence of MMP. When determining the appropriateness of a placental weight, important considerations include the handling of the placenta and the choice of weight chart. The weight of a placenta placed rapidly in formalin immediately after delivery will be relatively heavy due to the fixation of most of the blood within the placenta and to the added weight from the formalin itself. By contrast, much of the blood will have leaked out from a placenta stored in a refrigerator over the weekend, for example, and the fresh weight will lack the added formalin component. Published weight charts vary substantially in their normal ranges, possibly due to differences in handling, to alternative weighing schemes (trimmed or including membranes and the umbilical cord), to ethnic or racial differences, or to the generally increasing body weights of those living in the Western world. Whatever the reasons for the differences, choosing a placental weight chart that “fits” one's population is valuable. A narrow umbilical cord, defined as a well-sampled umbilical cord with no cross-sectional diameter greater than 0.8 cm, also may suggest MMP. With prolonged poor maternal perfusion of the intervillous space, fetal peripheral vascular resistance increases, and the fetus may become volume depleted. This process may manifest clinically in oligohydramnios, particularly later in gestation. Fetal volume depletion also leads to depletion of the fluid in the umbilical cord, resulting in a narrow cord.53 Reference curves for umbilical cord diameter have recently been published and may aid in the diagnosis of a narrow cord.54

Complications with the pathologic diagnosis of MMP One issue particularly related to MMP is the problem of interobserver variability. Identification of these many lesions unfortunately shows only fair reproducibility, even among experienced placental pathologists. While kappa values for various diagnoses within the acute chorioamnionitis spectrum generally show good to excellent agreement between pathologists,55,56 (although contrasting views can be found57,58) the agreement is much poorer for lesions of MMP.28,56,59,60 One of the most vexing problems of interobserver agreement involves the assessment of villous maturity. Even for expert subspecialists, correctly assessing gestational age based on villous maturation is surprisingly difficult. A blinded reliability study involving six perinatal pathologists

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found that estimates of gestational age were within 2 weeks of the correct (clinical) gestation for only 54% of the study slides.61 Recognizing these problems of poor interobserver reliability, and concerned about variations in diagnostic criteria, the Perinatal Section of the Society for Pediatric Pathology developed a diagnostic framework for MMP.28 In an attempt to improve reproducibility, diagnostic criteria in this framework were established, evaluated, refined, and then evaluated again. The final set of diagnostic criteria decreased interobserver variability, but even after this refinement of the diagnostic criteria, kappa values for these lesions were generally only moderate (8 of the 11 kappa values were in the range of 0.42–0.57). This study highlights the difficulty of consistently and accurately diagnosing MMP-related lesions. Even ignoring the difficulties with interobserver variability, there remains the additional issue of how to interpret the placental pathologic findings. Clinical pathologic diagnosis of MMP is still somewhat imprecisely defined. It seems likely that essentially all experienced placental pathologists have signed out cases with overwhelming, unequivocal evidence for MMP. Conversely, these same pathologists have almost certainly reviewed cases in which MMP was expected based on clinical parameters but which revealed no such lesions. However, only a minority of cases fall into these two extremes. In between lies the frequent case where microscopic examination of the standard H&E sections detects only one to several small MMP-related lesions. The demarcation between those cases qualifying as MMP and those not qualifying has not been well established for such clinical cases. Complicating the interpretation of these cases is that these same lesions are found in the normal population (albeit at a lower frequency than in clinically affected populations). The end result is that the positive predictive value for many, if not most, MMP-related lesions is low.13 More so than almost any other field in pathology, placental pathology is dependent on the quantitation of lesions. A single tiny malignant gland is often sufficient for diagnosis of a carcinoma. A single small placental lesion will rarely be so definitive. While the diagnostic identity of the placental lesion will likely be readily determined, the importance of the lesion for the health of the mother or baby will be less clear. This issue is particularly problematic for the diagnosis of MMP. The question is not “do I see syncytial knots?” but rather “are there increased syncytial knots?” and, if so, “over what percentage of the villi?” Even for placental infarcts, the important questions are quantitative—determining not just the locations of the infarcts but also the percentage of the placental volume they involve. For clinical practice, the intertwined problems of the low positive predictive value for individual placental lesions and the quantitations necessary for accurate diagnosis of MMP have not yet been fully addressed. It has been found that the overall impression of a placental pathologist is superior to the detection of lesions for the diagnosis of MMP, and the addition of clinical data improves the diagnostic accuracy further.28 However, a more rigorously quantitative approach has not yet been developed for clinical practice. The research community, on the other hand, has developed tools to address some of the deficiencies in the diagnosis of MMP. The most common method has been to group lesions

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into physiologically or pathologically logical clusters, sometimes including grades of severity.62 These latent constructs have been utilized for multiple types of placental pathologies, including maternal vascular lesions, fetal vascular lesions, and inflammatory lesions.63,64 To generate these latent constructs, the placental lesions are first clustered into constructs. Scores can then be derived within each construct using regression models and the scores related to clinical outcomes.63 This technique has typically been applied to datasets from well-controlled individual studies with relatively large numbers of subjects, a circumstance not readily replicable clinically. Although difficult to introduce into routine practice, these models may provide the foundation for a more quantitative approach to the clinical diagnosis of MMP.

space. Remodeling, with its characteristic distal funnel-shaped vascular dilatation, slows the inflow of blood to 10 cm/s. This slow flow rate permits rapid mixing of the blood in the intervillous space (avoiding hypoxia/reperfusion damage due to large swings in the oxygen concentration) and matches the predicted transit time of the blood through the intervillous space. Unremodeled vessels, with their high-speed jets, are predicted to damage placental structures. The force of the jets may push apart and potentially injure chorionic villi (including the large anchoring villi), promoting fibrin deposition. The high flow rate may also cause a turbulent flow, leading to the formation of intervillous lakes with surrounding thrombus. These new findings may help explain the alterations in placentas with MMP, and they provide the opportunity for more investigation into this complicated problem.

Subtleties in the etiology of MMP-related placental damage

Future directions

The damage to the placenta represented by the lesions described above has traditionally been ascribed to vascular underperfusion with resulting hypoxia–ischemia. In localized instances, such as occlusion of a single maternal spiral arteriole with infarction of the overlying placental parenchyma, this simple hypothesis is almost certainly the entirety of the explanation. Confirmation of hypoxia as the underlying etiology for more global alterations, such as distal villous hypoplasia, increased syncytial knots, or fetal growth restriction, has been more difficult to obtain, however, and the etiology for the placental damage in these cases is likely complex. In support of hypoxia as an etiology for these lesions, the preeclamptic placenta has been shown to have a gene expression profile consistent with hypoxia.65 Similarly, up-regulation of the HIF-1alpha pathway has been noted in preeclamptic placentas.66 On the other hand, oxygen levels measured in the intervillous space of placentas from complicated pregnancies have never been found to be low.67 Instead, surrogate measures of oxygen levels in growthrestricted fetuses have only been elevated.68,69 Along the same lines, placentas from cases of preeclampsia are notable for substantial oxidative damage, as might be expected from hypoxia/reperfusion injury.5 These data, if they hold up under further testing, suggest that the standard model of spiral artery vascular lesions uniformly reducing blood flow to the placenta, leading to chronic hypoxia with resulting additional MMP-related lesions, is likely too simple. To begin to address these counterintuitive data, a group has reanalyzed blood flow from spiral arteries.70 It had been hypothesized that the primary result of spiral artery remodeling is an increase in blood flow to the placenta—specifically, the newly dilated vessels increase the amount of blood entering the intervillous space. A recent model examining the effects of vascular remodeling on blood flow to the placental intervillous space suggests, however, that the rise in the amount of blood flowing into the placenta is proportionally relatively small—only a two-fold increase. Of greater significance is the alteration in the flow rate.70 Based on the model's assumptions, an unremodeled, undilated artery would generate a high-speed jet of blood, 1–2 m/s, entering the intervillous

While placental pathologists are generally quite good at detecting significant MMP using standard light microscopy, this limited skill set may not be sufficient to maintain relevance in the future. In particular, the high interobserver variability and the insensitivity to milder manifestations of MMP are problematic. Recent research may offer options for identifying immunohistochemical markers that more clearly determine the severity and significance of MMP-related placental damage. As mentioned previously, oxidative damage is commonly identified in these placentas. Multiple different methods have been utilized to measure oxidative damage experimentally, and at least one well-characterized possible immunohistochemical marker 8-oxo-deoxyguanosine is currently available. Immunopositivity for 8-oxo-deoxyguanosine has been recently reported as a marker for true syncytial knots.39 The presence of nitrotyrosine has also been used as a measure of MMP-related damage in the placenta. Nitrotyrosine, a product of protein nitration, develops in the presence of peroxynitrite, a product of superoxide anion and nitric oxide.71 Increased protein nitration is seen both in cases of increased inflammation and in cases of increased oxidative stress, however, potentially limiting its utility. Numerous other physiologic pathways have been explored only minimally in the clinically oriented pathology literature, including autophagy,72,73 the unfolded protein response,74 miRNA expression,75,76 and cellular senescence,77,78 and research in these and other areas may provide useful markers for MMP on clinical placental samples.

Summary MMP is one of the major processes associated with several of the more pressing problems in obstetrical practice, including fetal growth restriction, preterm birth, and stillbirth. Numerous MMP-related lesions have been identified and characterized. While the clinical relevance of these lesions has been evaluated (extensively for some), their significance for clinical practice remains to be fully delineated. Explorations of the many pathways regulating placental biology may provide new options for identifying MMP.

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r e f e r e n c e s 23. 1. Brosens I, Pijnenborg R, Vercruysse L, Romero R. The “Great Obstetrical Syndromes” are associated with disorders of deep placentation. Am J Obstet Gynecol. 2011;204(3):193–201. 2. Hertig AT. Vascular pathology in hypertensive albuminuric toxemias of pregnancy. Clinics. 1945;4:602–614. 3. Tenney B, Parker F. The placenta in toxemia of pregnancy. Am J Obstet Gynecol. 1940;39:1000–1005. 4. Roberts JM, Bell MJ. If we know so much about preeclampsia, why haven't we cured the disease? J Reprod Immunol. 2013;99(12):1–9. 5. Staff AC, Benton SJ, von Dadelszen P, et al. Refining preeclampsia using placenta-derived biomarkers. Hypertension. 2013;61(5):932–942. 6. Laresgoiti-Servitje E, Gomez-Lopez N, Olson DM. An immunological insight into the origins of pre-eclampsia. Hum Reprod Update. 2010;16(5):510–524. 7. Von Dadelszen P, Magee LA, Roberts JM. Subclassification of preeclampsia. Hypertens Pregnancy. 2003;22(2):143–148. 8. Lisonkova S, Joseph KS. Incidence of preeclampsia: risk factors and outcomes associated with early- versus lateonset disease. Am J Obstet Gynecol. 2013;209(6):544.e1–544.e12. 9. Moldenhauer JS, Stanek J, Warshak C, Khoury J, Sibai B. The frequency and severity of placental findings in women with preeclampsia are gestational age dependent. Am J Obstet Gynecol. 2003;189(4):1173–1177. 10. Brosens I, Dixon HG, Robertson WB. Fetal growth retardation and the arteries of the placental bed. Br J Obstet Gynaecol. 1977;84(9):656–663. 11. Khong TY, De Wolf F, Robertson WB, Brosens I. Inadequate maternal vascular response to placentation in pregnancies complicated by pre-eclampsia and by small-for-gestational age infants. Br J Obstet Gynaecol. 1986;93(10):1049–1059. 12. Salafia CM, Charles AK, Maas EM. Placental and fetal growth restriction. Clin Obstet Gynecol. 2006;49(2):236–256. 13. Mifsud W, Sebire NJ. Placental pathology in early-onset and late-onset fetal growth restriction. Fetal Diagn Ther. 2014;36(2): 117–128. 14. Kovo M, Schreiber L, Ben-Haroush A, et al. The placental factor in early- and late-onset normotensive fetal growth restriction. Placenta. 2013;34(4):320–324. 15. Pijnenborg R, Dixon G, Robertson WB, Brosens I. Trophoblastic invasion of human decidua from 8 to 18 weeks of pregnancy. Placenta. 1980;1(1):3–19. 16. Pijnenborg R, Vercruysse L, Hanssens M. The uterine spiral arteries in human pregnancy: facts and controversies. Placenta. 2009;30(6):473–482. 17. Brosens I. A study of the spiral arteries of the decidua basalis in normotensive and hypertensive pregnancies. J Obstet Gynaecol Br Commonw. 1964;71(2):222–230. 18. Hustin J, Schaaps JP. Echogenic and anatomic studies of the maternotrophoblastic border during the first trimester of pregnancy. Am J Obstet Gynecol. 1987;157(1):162–168. 19. Hustin J, Schaaps JP, Lambotte R. Anatomical studies of the utero-placental vascularization in the first trimester of pregnancy. Trophoblast Res. 1988;3:49–60. 20. Jauniaux E, Watson AL, Hempstock J, Bao YP, Skepper JN, Burton GJ. Onset of maternal arterial blood flow and placental oxidative stress. A possible factor in human early pregnancy failure. Am J Pathol. 2000;157(6):2111–2122. 21. Rodesch F, Simon P, Donner C, Jauniaux E. Oxygen measurements in endometrial and trophoblastic tissues during early pregnancy. Obstet Gynecol. 1992;80(2):283–285. 22. Burton GJ, Watson AL, Hempstock J, Skepper JN, Jauniaux E. Uterine glands provide histiotrophic nutrition for the human

24.

25.

26.

27. 28.

29.

30.

31.

32.

33.

34.

35.

36. 37.

38. 39.

40.

41.

42.

43.

39 (2015) 9–19

fetus during the first trimester of pregnancy. J Clin Endocrinol Metab. 2002;87(6):2954–2959. Soothill PW, Nicolaides KH, Rodeck CH, Campbell S. Effect of gestational age on fetal and intervillous blood gas and acid– base values in human pregnancy. Fetal Ther. 1986;1(4):168–175. Pijnenborg R, Bland JM, Robertson WB, Brosens I. Uteroplacental arterial changes related to interstitial trophoblast migration in early human pregnancy. Placenta. 1983;4(4): 397–413. Brosens JJ, Pijnenborg R, Brosens IA. The myometrial junctional zone spiral arteries in normal and abnormal pregnancies: a review of the literature. Am J Obstet Gynecol. 2002;187(5): 1416–1423. Jauniaux E, Watson AI, Ozturk O, Quick D, Burton GJ. In-vivo measurements of intrauterine gases and acid–base values early in human pregnancy. Hum Reprod. 1999;14(11):409–419. Schneider H. Oxygenation of the placental-fetal unit in humans. Respir Physiol Neurobiol. 2011;178(1):51–58. Redline RW, Boyd T, Campbell V, et al. Maternal vascular underperfusion: nosology and reproducibility of placental reaction patterns. Pediatr Dev Pathol. 2004;7(3):237–249. Khong TY. Acute atherosis in pregnancies complicated by hypertension, small-for-gestational-age infants, and diabetes mellitus. Arch Pathol Lab Med. 1991;115(7):722–725. Staff AC, Dechend R, Redman CWG. Review: preeclampsia, acute atherosis of spiral arteries and future cardiovascular disease: two new hypotheses. Placenta. 2013;34(suppl A):S73–S78. Dixon HG, Robertson WB. A study of the vessels of the placental bed in normotensive and hypertensive women. J Obstet Gynaecol Br Emp. 1958;65(5):803–809. Brosens I, Khong TY. Defective spiral artery remodeling. In: Pijnenborg R, Brosens I, Romero R, et al., eds. Placental Bed Disorders. Cambridge, UK: Cambridge University Press; 2010. 11–21. Meekins JW, Pijnenborg R, Hanssens M, McFadyen IR, van Asshe A. A study of placental bed spiral arteries and trophoblast invasion in normal and severe pre-eclamptic pregnancies. Br J Obstet Gynaecol. 1994;101(8):669–674. Pijnenborg R, Bland JM, Robertson WB. The pattern of interstitial trophoblastic invasion of the myometrium in early human pregnancy. Placenta. 1981;2(4):303–316. Roberts DJ, Post MD. The placenta in pre-eclampsia and intrauterine growth restriction. J Clin Pathol. 2008;61(12): 1254–1260. Kaplan CG. Fetal and maternal vascular lesions. Semin Diagn Pathol. 2007;24(1):14–22. Huppertz B, Kingdom J, Caniggia I, et al. Hypoxia favors necrotic versus apoptotic shedding of placental syncytiotrophoblast into the maternal circulation. Placenta. 2003;24(2-3):181–190. Stanek J. Hypoxic patterns of placenta injury. Arch Pathol Lab Med. 2013;137(5):706–720. Fogarty NME, Ferguson-Smith AC, Burton GJ. Syncytial knots (Tenney–Parker changes) in the human placenta. Am J Pathol. 2013;183(1):144–152. Cantle SJ, Kaufmann P, Luckhardt M, Schweikhart G. Interpretation of syncytial sprouts and bridges in the human placenta. Placenta. 1987;8(3):221–234. Loukeris K, Sela R, Baergen RN. Syncytial knots as a reflection of placental maturity: reference values for 20 to 40 weeks' gestational age. Pediatr Dev Pathol. 2010;13(4):305–309. Kaufmann P, Huppertz B. Tenney–Parker changes and apoptotic versus necrotic shedding of trophoblast in normal pregnancy and pre-eclampsia. In: Lyall F, Belfort M, et al., eds. Pre-eclampsia: Etiology and Clinical Practice. New York: Cambridge University Press; 2007. 152–163. Fitzgerald B, Levytska K, Kingdom J, Walker M, Baczyk D, Keating S. Villous trophoblast abnormalities in extremely

S

44.

45.

46.

47. 48.

49.

50.

51. 52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

E M I N A R S I N

P

E R I N A T O L O G Y

preterm deliveries with elevated second trimester maternal serum hCG or inhibin-A. Placenta. 2011;32(4):339–345. Redline RW, Patterson P. Pre-eclampsia is associated with an excess of proliferative immature intermediate trophoblast. Hum Pathol. 1995;26(6):594–600. Stanek J, Al-Ahmadie HA. Laminar necrosis of placental membranes, a histologic sign of uteroplacental hypoxia. Pediatr Dev Pathol. 2005;8(1):34–42. Stanek J, Weng E. Microscopic chorionic pseudocysts in placental membranes: a histologic lesion of in utero hypoxia. Pediatr Dev Pathol. 2007;10(3):192–198. Stanek J. Acute and chronic placental membrane hypoxic lesions. Virchows Arch. 2009;455(4):315–322. Stanek J. Diagnosing placental membrane hypoxic lesions increases the sensitivity of placental examination. Arch Pathol Lab Med. 2010;134(7):989–995. Bendon RW, Coventry SC, Reed RC. Reassessing the clinical significance of chorionic membrane microcysts and linear necrosis. Pediatr Dev Pathol. 2012;15(3):213–216. Salafia CM, Pezzullo JC, Lopez-Zeno JA, Simmens S, Minior VK, Vintzileos AM. Placental pathologic features of preterm preeclampsia. Am J Obstet Gynecol. 1995;173(4):1097–1105. Naeye RL. Do placental weights have clinical significance? Hum Pathol. 1987;18(4):387–391. Heinonen S, Taipale P, Saarikoski S. Weights of placentae from small-for-gestational age infants revisited. Placenta. 2001;22(5):399–404. Bruch JF, Sibony O, Benali K, Challier JC, Blot P, Nessmann C. Computerized microscope morphometry of umbilical vessels form pregnancies with intrauterine growth retardation and abnormal umbilical artery Doppler. Hum Pathol. 1997;28(10): 1139–1145. Proctor LK, Fitzgerald B, Whittle WL, et al. Umbilical cord diameter percentile curves and their correlation to birth weight and placental pathology. Placenta. 2013;34(1):62–66. Simmonds M, Jeffrey H, Watson G, Russell P. Intraobserver and interobserver variability for the histologic diagnosis of chorioamnionitis. Am J Obstet Gynecol. 2004;190(1):152–155. Beebe LA, Cowan LD, Hyde SR, Altshuler G. Methods to improve the reliability of histopathological diagnoses in the placenta. Pediatr Perinat Epidemiol. 2000;14(2):172–178. Holzman C, Lin X, Senagore P, Chung H. Histologic chorioamnionitis and preterm delivery. Am J Epidemiol. 2007;166(7): 786–794. Salafia CM, Misra D, Miles JNV. Methodologic issues in the study of the relationship between histologic indicators of intraamniotic infection and clinical outcomes. Placenta. 2009;30(11):988–993. Sun CC, Revell VO, Belli AJ, Viscardi RM. Discrepancy in pathologic diagnosis of placental lesions. Arch Pathol Lab Med. 2002;126(6):706–709. Kramer MS, Chen MF, Roy I, et al. Intra- and interobserver agreement and statistical clustering of placental histopathologic features relevant to preterm birth. Am J Obstet Gynecol. 2006;195(6):1674–1679. Khong TY, Staples A, Bendon RW, et al. Observer reliability in assessing placental maturity by histology. J Clin Pathol. 1995;48 (5):420–423. Mestan KK, Check J, Minturn L, et al. Placental pathologic changes of maternal vascular underperfusion in

63.

64.

65.

66.

67.

68.

69.

70.

71. 72.

73.

74.

75.

76.

77.

78.

39 (2015) 9–19

19

bronchopulmonary dysplasia and pulmonary hypertension. Placenta. 2014;35(8):570–574. Kelly R, Holzman C, Senagore P, et al. Placental vascular pathology findings and pathways to preterm delivery. Am J Epidemiol. 2009;170(2):148–158. Holzman C, Bullen B, Fisher R, Paneth N, Reuss L, Prematurity Study Group. Pregnancy outcomes and community health: the POUCH study of preterm delivery. Paediatr Perinat Epidemiol. 2001;15(suppl 2):136–158. Soleymanlou N, Jurisica I, Nevo O, et al. Molecular evidence of placental hypoxia in preeclampsia. J Clin Endocrinol Metab. 2005;90(7):4299–4308. Caniggia I, Winter JL. Adriana and Luisa Castellucci Award lecture 2001. Hypoxia inducible factor-1: oxygen regulation of trophoblast differentiation in normal and pre-eclamptic pregnancies—a review. Placenta. 2002;23(suppl A):S47–S57. Huppertz B, Weiss G, Moser G. Trophoblast invasion and oxygenation of the placenta: measurements versus presumptions. J Reprod Immunol. 2014;101–102:74–79. Kakogawa J, Sumimoto K, Kawamura T, Minoura S, Kanayama N. Noninvasive monitoring of placental oxygenation by near-infrared spectroscopy. Am J Perinatol. 2010;27(6): 463–468. Sibley CP, Pardi G, Cetin I, et al. Pathogenesis of intrauterine growth restriction (IUGR)—conclusions derived from a European Biomed 2 concerted action project “importance of oxygen supply in intrauterine growth restricted pregnancies”—a workshop report. Placenta. 2002;23(suppl A):S75–S79. Burton GJ, Woods AW, Jauniaux E, Kingdom JC. Rheological and physiological consequences of conversion of the maternal spiral arteries for uteroplacental blood flow during human pregnancy. Placenta. 2009;30(6):473–482. Webster RP, Roberts VH, Myatt L. Protein nitration in placenta —functional significance. Placenta. 2008;29(12):985–994. Goldman-Wohl D, Cesla T, Smith Y, et al. Expression profiling of autophagy associated genes in placentas from preeclampsia. Placenta. 2013;34(10):958–962. Saito S, Nakashima A. A review of the mechanisms for poor placentation in early-onset preeclampsia: the role of autophagy in trophoblast invasion and vascular remodeling. J Reprod Immunol. 2014;101–102:80–88. Yung HW, Atkinson D, Campion-Smith T, Olovsson M, Charnock-Jones DS, Burton GJ. Differential activation of placental unfolded protein response pathways implies heterogeneity in causation in early- and late-onset pre-eclampsia. J Pathol. 2014;234(2):262–276. Rudov A, Balduini W, Carloni S, Perrone S, Buonocore G, Albertini MC. Involvement of miRNAs in placental alterations mediated by oxidative stress. Oxidative Med Cell Longevity. 2014 [article ID 103068] Pubmed 2014:103068. http://dx.doi.org/10. 1155/2014/103068. [Epub 2014 Mar 18]. Morales-Prieto DM, Ospina-Prieto S, Chaiwangyen W, Schoenleben M, Markert UR. Pregnancy-associated miRNA-clusters. J Reprod Immunol. 2013;97(1):51–61. Chuprin A, Gal H, Biron-Shental T, et al. Cell fusion induced by ERVWE1 or measles virus causes cellular senescence. Genes Dev. 2013;27(21):2356–2366. Goldman-Wohl D, Yagel S. United we stand not dividing: the syncytiotrophoblast and cell senescence. Placenta. 2014;35(6): 341–344.