Chorioamnionitis

Chorioamnionitis

Introduction

Chorioamnionitis is a condition that can affect pregnant women in which the chorion and amnion (the membranes that surround the fetus) and the amniotic fluid (in which the fetus floats) are infected by bacteria, virus, fungal, parasite. This can lead to infection in both the mother and fetus, and in most cases mean the fetus has to be delivered as soon as possible ^1,2.

           

Prenatal infections are important aspects of placental pathology. They are common and varied. Their pathogenesis and related circumstances must be understood if the pathologic lesions are to be interpreted correctly. Many types of infection cause placental changes, but in some types the infection may be difficult to prove from placental examination.

Ultrastructural studies are especially lacking in this area and might be helpful, particularly when virus infection is suspected. Infections may ascend from the endocervical canal, or they may reach the placenta hematogenously through the maternal blood. Rarely are they acquired by amniocentesis, chorionic villus sampling, amnioscopy (Horky & Amon, 1967), percutaneous umbilical blood sampling (PUBS; Wilkins et al., 1989), or intrauterine fetal transfusion (Goodlin, 1965; Scott & Henderson, 1972).

Many infections cause gross and microscopic changes of the placenta, but others, for example, the Coxsackie virus infection, leave few characteristic or specifically recognizable traces. This is also the case with Parvovirus B19 infection, which often leads to fetal hydrops but has no specific placental alteration other than perhaps intranuclear inclusions in nucleated red blood cell precursors and endothelium, as a report by Hartwick et al. (1989) showed. Samra et al. (1989) described villous necrosis and calcification in the placenta from a 20 weeks’ gestation with hydrops due to this infection ³.

Structure and Histology of the Fetal Membranes

The membranes are the “bag of waters” in which the fetus is enclosed. Their structure and function include turnover of water and enzymatic activity during the initiation of labor. The structure of the membranes remains constant from the fourth month until term. The mean thickness after separation from the uterine wall is about 200 to 300 mm, but because of local edema of the amnionic mesoderm, considerably thicker membranes are sometimes observed. After birth, the following layers can be seen histologically (Figure 1).

Macroscopically, the amnion is a translucent structure, easily separated from the underlying chorion. It never truly fuses with the chorion, cellularly speaking, as it is only passively attached by the internal pressure of the amniotic fluid. It is important to recognize that the amnion does not possess its own blood vessels and obtains its nutrition and oxygen from the surrounding chorionic fluid, amniotic fluid, fetal surface vessels, and, during early gestation, from the magma reticulare. The amnionic epithelium is derived from the fetal ectoderm and is thus contiguous over the umbilical cord and fetal skin. The epithelium is composed of a single layer of flat, cuboidal to columnar cells. Taller, columnar cells are usually present near the insertion of the membranes at the placental margin, whereas flatter cells are generally present in the periphery. Defects in the epithelial layer may be due to expulsion of dying cells into the amniotic fluid. The amnionic epithelium rests on a basement membrane, which is connected to a thin connective tissue layer by filamentous strands (Figure 1). The latter consists of a compact stromal layer and a fibroblast layer. The compact stromal layer contains bundles of collagen and scattered elastic fibers. The fibroblast layer consists of a network of fibroblasts and a few macrophages (Figure 1). Although the amnionic mesoderm is divided into a compact stromal layer and fibroblast layer, these are difficult to appreciate on histologic section. The amnion has multiple functional roles in the placenta. It is essential for the structural integrity and junctional permeability of the membranes, serving as a permeability barrier to anionic macromolecules. It is thought to have a role in the onset of labor including the initiation and maintenance of uterine contractions. The amnion is also involved in the turnover of the amniotic fluid including resportion and secretory activity. It also appears to be responsible for regulating the pH of the amniotic fluid and thus is involved in fetal pH regulation because the fetus delivers considerable amounts of proteins and bicarbonate via its kidneys into the amniotic fluid.

The chorionic membrane is a tough fibrous layer carrying the fetal bloodvessels.  The amnion and chorion are easily separated and will readily slide along one another. This is due to the existence of the spongy layer, which is the result of incomplete fusion of the amnionic and chorionic mesoderm and is thus between the two layers. It is composed of loosely arranged bundles of collagen fibers with a few scattered fibroblasts, separated by a communicating system of clefts (Figure 1). Fibroblasts, macrophages, or remainders of amnionic and chorionic mesothelium occasionally line the clefts. The collagen composition of both amnion and chorion contributes to the mechanical stability and tensile properties of the membranes. The spongy layer continues without sharp demarcation into the next layer, the chorionic mesoderm. Its composition is similar to the fibroblast layer of the amnion, as it consists of a coarse network of collagen bundles intermingled with finer argyrophilic fibrils. Fibroblasts, myofibroblasts, and macrophages are also regular findings (Figure 1). Chorionic vessels are present in the mesoderm of the chorionic plate. Occasional remnants of atrophied chorionic villi may also be found. These former villi appear as round “balls” of loose connective tissue, without a trophoblastic cover (Figure 1). Finally, there is a basal lamina that is highly variable in thickness and structure ⁴.

Pathogenesis

The pathogenesis of prenatal infections and related circumstances must be understood if the associated pathologic lesions are to be interpreted correctly. Infections may reach the placenta and fetus in several ways:

•       By ascension from through endocervical canal

•       By hematogenous transmission from maternal blood

•       By direct introduction via amniocentesis, chorionic villus sampling, amnioscopy, percutaneous umbilical blood sampling or intrauterine fetal transfusion, and

•       By direct extension from infection in the endometrium.

The majority of infections arise by one of the first two routes, and these are the focus of this script ^1,2,4,5,6.

Etiology

Chorioamnionitis usually develops when bacteria that are part of the normal vaginal flora "ascend" into the uterine cavity. The amniotic fluid and placenta, as well as the baby, become infected. E. coli, group B streptococci, and anaerobic bacteria are the most common causes of chorioamnionitis, though, E. coli and group B streptococci are also the two most common causes of infection in newborns. The most common risk factors for chorioamnionitis include: young age (e.g., less than 21 years old); low socioeconomic status; first pregnancy;long labor; prolonged rupture of the membranes (bag of waters); rupture of membranes at an early gestational age; multiple vaginal examinations during labor (only in women with ruptured membranes); pre-existing infections of the lower genital tract (bacterial vaginosis and group B streptococcal infection); and internal fetal and uterine monitoring ^1,7.

Frequency

United States

Incidence of maternal chorioamnionitis in the US population cannot be stated with accuracy, but the occurrence declines as pregnancy advances toward term gestation. The risk of chorioamnionitis increases based on health conditions and behaviors. Furthermore, factors such as gestational age, economic conditions, and ethnic differences influence the incidence. Histopathology of the placenta suggests inflammation may occur in the normal course of parturition at term gestation, thus complicating the definition of chorioamnionitis. An increase in histopathologic chorioamnionitis is noted in preterm birth compared with delivery of the healthy term infant. Signs of placental inflammation are present in 42% of extremely low birth weight infants. Most agree that infection is directly or indirectly associated with 40-60% of all preterm births.

  

International

Developed countries (eg, Canada, western European countries, Australia) probably have an incidence equal to, or perhaps even less than, the rate of chorioamnionitis observed in the United States. In underdeveloped countries, premature rupture of membranes has a strong association with chorioamnionitis, and chorioamnionitis in this setting results in preterm birth with a high mortality rate. Classic studies by Naeye demonstrated that malnourished pregnant women in Africa had a higher risk of ascending urogenital infection with subsequent amniotic fluid infection. The pathophysiology increased the risk of fetal infection and perinatal death. Infection in these malnourished women was attributed to a decrease in host defense factors in amniotic fluid that regularly prevents disease in this liquor. In developed countries where women receive suboptimal care and have poor nutrition during pregnancy, a higher incidence of infection can be expected because of altered immune defenses ⁸.

Ascending Infection and Acute Chorioamnionitis

Acute chorioamnionitis is common, with an incidence of 20% to 24% of live births and up to 67% of preterm deliveries. It is an important cause of preterm labor and preterm deliveries, which are significant causes of perinatal morbidity and mortality ^1,4,6,8. Acute chorioamnionitis has also recently been implicated in the development of poor long-term neurologic outcome and cerebral palsy ^1,4,9.

Ascending infection, also called “amniotic sac infection syndrome,” develops from an infection that commences in the vagina and endocervix and then ascends to the uterine cavity (Figure2). Acute chorioamnionitis is always due to infection, and the ascending nature of this infection is supported by three pathologic findings:

•       It is usually associated with severe acute deciduitis.

•       In twin gestations, it is invariably the lower twin that has chorioamnionitis or whose membranes are more severely inflamed.

•       The point of spontaneous membrane rupture, which is in proximity to the cervical os, has the most severe inflammation.

It is often stated that infection occurs secondary to membrane rupture, but in actuality it is the loss of membrane integrity resulting from inflammation that makes rupture a probability. In fact, ascending infection occurs most often in the presence of intact fetal membranes. Furthermore, it is now the predominant opinion that amniotic sac infection is the primary cause of premature labor and delivery, at least in those pregnancies that terminate spontaneously before 30 weeks gestation, and that antimicrobial therapy usually fails to prolong pregnancy when chorioamnionitis is already extant ⁴.

Causes or associations of preterm labor include acute chorioamnionitis, chronic endocervicitis, maternal smoking, parity, and prior cervical surgery ^4,6. It has also been suggested that intercourse during late pregnancy may initiate premature delivery, but this is still controversial. There is now a very large body of investigation incriminating inflammatory mediators in the initiation of labor during the process of ascending infection. Interleukin-1 (IL-1), IL-6, and IL-8 are elevated in the amniotic fluid and cord blood in the presence of an ascending infection. Tumor necrosis factor (TNF) activates the cytokine machinery and may well be at the starting point of labor initiation by stimulating prostaglandin production from the decidua. Deciduitis, decidual macrophage activation, and neutrophils exudation, in particular, play an important role in the initiation of premature labor. However, the precise chemical cascade that ultimately leads to myometrial contractions is not yet elucidated. Suffice it to say that the cytokine system is intimately involved in premature labor when it is caused by infection ^1,4.

Clinical Acute Chorioamnionitis

Making the diagnosis of clinical chorioamnionitis is problematic as only some gravidas with amniotic sac infection experience fever, leukocytosis, uterine tenderness, or maternal or fetal tachycardia. Indeed, close to 75% of women with histologic acute ascending infection and acute chorioamnionitis do not fulfill the criteria for clinical chorioamnionitis, and many have no symptoms at all. On the other hand, only a minority

of patients with a diagnosis of clinical chorioamnionitis ultimately show histologic chorioamnionitis. Other tests such as culture and Gram stain of the amniotic fluid, assays of the amniotic fluid for cytokines, esterase, or endotoxin, and fetal fibronectin have provided better correlation with histologic chorioamnionitis than clinical evaluation ^2,4,8,10.

Macroscopic Appearance

The pathologic features of an ascending infection are those of acute chorioamnionitis. Inflammatory lesions are present only when microbacterial contamination exists in the amniotic cavity. Typically, the fetal surface of the placenta lacks the blue sheen of the normal organ, and both the membranes and fetal surface are white and opaque, being obscured by the inflammatory exudate of polymorphonuclear leukocytes (PMNs) (Figure 3). The surface may become yellow or green when much leukocytic exudate has accumulated or the process has been of long duration ^3,4,6. The placenta is frequently malodorous, and the very astute observer may sometimes identify the prevailing organism by the odor. The fecal odor of Fusobacterium and Bacteroides infections and the sweet odor of Clostridium and Listeria infections are useful identifiers for the adept examiner. The membranes are typically more friable, and the decidua capsularis is frequently detached and hemorrhagic. In many cases, the membranes are incomplete and consist of mere fragments of amnion without chorion or decidua capsularis. Particularly in preterm placentas, the inflammation is often accompanied by acute marginal hemorrhage that undermines the edge of the placenta. Although this mimics abruptio placentae clinically, this process (Figure 4) markedly differs from the typical abruptio placentae as it originates from the associated acute deciduitis ^3,4.

Touch preparations made from the fetal surface of placentas from women with prenatal infection may be used to identify the inflammatory exudate and the  bacteria quickly, especially when the infection is due to Listeria monocytogenes. When chorioamnionitis is found in twin placentas, it is nearly invariably twin A whose cavity has inflammation or who has the more severely inflamed portion (Benirschke, 1960) (Figure 5). We have considered this to mean that the amnionic sac infection is always ascending through the cervical canal. This correlation was questioned by Thiery et al. (1970), however, who found umbilical phlebitis in twin B as often as in twin A. These authors did not state how their twins were delivered, though, or how the twins were positively identified as to their intrauterine location. These specifications are crucial to the interpretation of such data ³.

Microscopic Appearance

Acute chorioamnionitis, by definition, is the presence of acute inflammatory cells, PMNs in particular, within the fetal membranes (Figure 6) ^3,4,6. Eosinophils are found at times but usually only in protracted infections. Macrophages may participate to a variable extent. Chronic inflammatory cells, such as lymphocytes and plasma cells, are generally not present in acute chorioamnionitis. When present and admixed with acute inflammatory cells, the diagnosis of subacute chorioamnionitis may be made. There is a grading system for acute chorioamnionitis; the idea being that grade correlates with neonatal outcome. However, grading is not reliable for this purpose. We believe that the most important features of ascending infection are the infectious agent and whether there is a fetal response. For example, Trichomonas can be enormously leukotactic, yet have little effect on neonatal morbidity and mortality. Conversely, group B Streptococcus may be associated with minimal leukocyte infiltration but have a devastating effect upon fetal and neonatal life ^3,4.

The severity of chorioamnionitis was graded histologically by a single pathologist as mild, moderate, or severe according to a modification of Blanc’s classification. With mild chorioamnionitis, neutrophils were present in the subchorionic space and infiltrated the adjacent amnion; the total neutrophil count was less than ten per high power field. In cases of moderate chorioamnionitis, neutrophil infiltrated the lower half of chorion from the subchorionic space and there were 11-30 neutrophils per high power field. Chorioamnionitis was classified as severe when dense infiltration of neuthrophils (more than 30 per high power field) extended from the subchorionic space throughout the chorion ¹¹.

Maternal Response, Fetal Response and Fetal Infection

In acute chorioamnionitis, the inflammatory infiltrate may be maternal or fetal in origin. The maternal component of the leukocytic reaction originates in the intervillous space and in the maternal vessels of the decidua in the free membranes. The emigration of leukocytes is always directional, toward the amniotic cavity, presumably toward an antigenic source in the amniotic fluid. Initially, when organisms first gain access to the uterine cavity, the fetal membranes covering the internal cervical os may show an inflammatory infiltrate (Figure 7). This is often accompanied by acute deciduitis (Figure 8). This local phenomenon does not indicate an intraamniotic infection is present. The first evidence that true intraamniotic infection has occurred is the presence of   marginate from beneath the fibrin under the chorionic plate. As the infection progresses, the leukocytes then infiltrate the chorion and eventually the amnion of the chorionic plate. Abscess formation underneath the chorionic plate and dissemination of exudate between villous trunks are rare events.

After the organism gains access to the amniotic cavity, a fetal response to the infection may occur. Fetal inflammatory cells migrate from the umbilical vessels and the superficial fetal vessels in the chorionic plate, constituting a fetal inflammatory response. Fetal response is, however, rare before the 20th week of gestation due to immaturity of the fetal immune system. In general, acute funisitis occurs first.

The inflammatory cells migrate toward the amnionic surface, marginate first at the vascular intima, and then begin to dissect among the muscle bundles of the umbilical vein and arteries, finally infiltrating Wharton’s jelly (Figure 9). They also reach the cord’s surface and may accumulate there in substantial numbers. Funisitis does not signify the existence of fetal sepsis. Fetal sepsis is a relatively late event in the course of an ascending bacterial infection and often results from invasion of organisms into the fetal lung, intestinal tract, and even the middle ear. If fetal infection occurs, PMNs can be found in the lung and stomach of the neonate intermixed with squames. Initially, this pus is likely aspirated from the amniotic fluid and not produced in the fetal lung, as only later in the infectious process can one find an inflammatory accumulation within the alveolar tissue ^1,4,6.

Figure 9. Acute funisitis involving umbilical vein. Leukocytes have penetrated between muscle fibers toward the cord surface ⁴.

In longer-standing infections, necrotizing funisitis may occur. Compared to the fetal membranes, the umbilical cord is not able to efficiently remove the chronic accumulation of inflammatory debris. The connective tissue cells of the umbilical cord often then degenerate completely. This results in exudate being deposited in successive waves, which accumulate in concentric perivascular rings giving the appearance of Ouchterlony immunodiffusion plates (Figure 10). This exudate is more prone to develop mineralization than the exudate of the fetal membranes and thus may become calcified. The calcification may reach extraordinary proportions at times, so that the cord cannot be readily clamped at delivery. Mural thrombosis may also be present. Necrotizing funisitis is classically associated with congenital syphilis infection but is not specific as other organisms, such as Candida, streptococci, and other bacteria, have been isolated. Mural thrombosis in chorionic veins is also frequently present when the infection has been of longer duration and there is some destruction of the vessel walls through which the neutrophils migrate. The thrombi may be grossly apparent as yellow-white streaks and are usually attached to the intima of the veins, toward the amnionic surface (Figure 11) ^4,6.

Effects of Ascending Infection

Chorioamnionitis caused by ascending infection can evoke effects on the fetus, mother and placenta. Effects on the fetus are preterm labor and preterm rupture of the membrane, fetal hypoxia, cerebral plasy, bronchopulmonary dysplasia, fetal growth restriction, fetal and neonatal death, fetal sepsis. Effects on the mother are abruption placenta, intrapartum pyrexia, endotoxic shock. Effect on the placenta is impared of placental circulation ¹.

Subacute Chorioamnionitis

In the amniotic infection syndrome, the amnionic epithelium may be degenerated or necrotic, especially in areas of severe inflammation. If the infection progresses, the PMNs die and the dead inflammatory cells may accumulate in large quantities underneath the amnion in the potential space that exists between amnion and chorion. There may be accumulation of mononuclear cells as well. This has been referred to as subacute chorioamnionitis (Figure 12). It occurs in situations where the causative organism is of low pathogenicity and does not result in immediate delivery or when there are repetitive bouts of infection. Thus, similar to necrotizing funisitis and thrombosis, it is indicative of a longer-standing infection. Like the exudate in the umbilical cord, some of the necrotic exudate underneath the amnion may ultimately calcify, but the fetus usually delivers before this happens. Subacute chorioamnionitis should be distinguished from chronic chorioamnionitis  in which the infiltrate is predominantly mononuclear ⁴.

Chronic Chorioamnionitis

Chronic chorioamnionitis is a recently described lesion in which the inflammatory infiltrate in the fetal membranes consists predominantly of a mononuclear cells, lymphocytes and histiocytes rather than acute inflammatory cells (Figure 13). The diagnosis should not be made if scattered chronic inflammatory cells in the membranes or greater numbers in the decidua are present. Chronic chorioamnionitis is found in association with VUE in approximately 79% of cases. Thus, its identification should alert the examiner to search for foci of chronic villitis. Clinically, chronic chorioamnionitis has been associated with maternal hypertension, diabetes, fetal hydrops, growth restriction, and oligohydramnios. If a mixed acute and chronic inflammatory infiltrate is present, subacute chorioamnionitis is likely present 4.

References

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Epidemiology: Histologic Chorioamnionitis and Preterm Delivery. Jhons Hopkins Bloomberg School of Public Health. East Lansing. 2007: 786-794.

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