Section 7 -

Pathology of the Fetal Membranes Phyllis C. Huettner, M.D.

Case 8: Gastroschisis Gastroschisis is a defect in the abdominal wall through which abdominal organs may protrude. The defect is usually located lateral to the umbilicus and is not covered by a membrane. Usually gastroschisis is an isolated defect and is associated with young maternal age. The fetal membranes in cases of gastroschisis undergo a characteristic change. Microscopically, the amniotic epithelium displays striking vacuolization. The vacuoles are relatively uniform, optically clear and round with focal confluence. The nuclei of the amniotic epithelium are centrally placed with an equal distribution of vacuoles on all sides. Occasional fibroblasts and macrophages in the chorion exhibit similar vacuolization. The gross appearance is not specific but may show meconium staining as meconium passage usually complicates the delivery of infants with gastroschisis. Ultrastructural studies demonstrate amorphous, electron-lucent material not bound by membranes distributed throughout the cytoplasm without displacement of cytoplasmic organelles. These ultrastructural features are consistent with lipid. The source of this lipid possibly includes adipose tissue from the fetal abdomen, lipids in the amniotic fluid and lipids transported through the amniotic epithelium. Although only a small number of cases with gastroschisis and this peculiar vacuolization of amniotic epithelium have been reported, it appears specific for this condition and was not noted in cases of omphalocele in which the membranes were specifically examined for it. Meconium Fetal passage of meconium can often be suspected on gross examination of the membranes by obvious green or brown staining. The gross appearance may help to differentiate the timing of meconium passage relative to delivery and thereby give some indication of fetal risk. Benirschke has suggested that in acute meconium staining, the placenta and membranes are blue-green and glistening and may contain residual, slimy meconium. Fetal outcome in this situation is typically good. With subacute meconium staining the membranes are dark, edematous and slippery. These features correlate with an increased risk of meconium aspiration syndrome. In chronic meconium staining the membranes and often the cord are dull and muddy brown. Some of these infants exhibit features of prenatal asphyxia. Microscopically, one of the characteristic changes seen with meconium is heaped up, pseudostratified amniotic epithelium. The nuclei may become pyknotic and eventually the epithelial cells undergo necrosis. Frequently, there is marked edema of the subamniotic connective tissue. Meconium is picked up by macrophages in the subamniotic tissue. Usually inconspicuous, these macrophages now become stuffed with granular, greenish-yellow pigment. The pigment can also be found in macrophages in the chorion or even the decidua. Pathologists should be aware that exposure of histologic sections to fluorescent light in the laboratory causes a significant reduction in both the number of pigmented macrophages identified and the intensity of the pigment staining. We have very little information correlating the timing of meconium passage with histologic localization. There is a single in vitro study in the literature in which placentas were incubated in a solution of meconium for varying times and then processed histologically. But how closely the results of this study approximate the in vivo situation is completely unknown. The histologic differential diagnosis includes hemosiderin and non-meconium, non-hemosiderin pigments. In hemosiderin staining the membranes are often brown but may also be green. Microscopically, hemosiderin is yellowish rather than green, slightly refractile and more granular and less waxy than meconium. An iron stain such as Prussian blue will stain hemosiderin bright blue but meconium will be negative. Benirschke describes cases with microscopic membrane pigment that do not have the gross appearance of meconium, are not associated with clinical evidence of meconium passage, and are negative for stainable iron. He has seen this pigment in cases of amniotic bands, hydrops fetalis secondary to α-thalassemia, unexplained hydramnios and other diverse conditions and speculates that it may be a metabolite remaining after remote meconium passage. The implications of meconium stained amniotic fluid are unclear. About 12% of infants are born through meconium-stained amniotic fluid. While 20 to 30% of these infants are depressed at birth, only about 5% are severely affected (meconium aspiration syndrome). The meconium aspiration syndrome (MAS) is defined as respiratory distress in an infant born through meconium-stained amniotic fluid whose symptoms cannot otherwise be explained. Meconium passage in utero has been significantly correlated with various parameters of perinatal distress such as low Apgar scores (<3 at 1 and 5 minutes), umbilical artery pH <7.0, respiratory distress, seizures in the first 24 hours and the need for delivery room resuscitation, particularly when accompanied by abnormalities in fetal heart rate. What is unclear is whether the passage of meconium is the cause of this distress or the result of it. Meconium passage is best thought of as a continuum. At one end are the majority of infants in whom meconium passage is the result of physiologic maturity. These infants generally do not have respiratory distress. In the middle are infants in whom meconium passage is associated with adverse stimuli. Some of these infants will exhibit respiratory distress and require oxygen or, occasionally, short term mechanical ventilation. At the severe end of the spectrum are infants with MAS who respond poorly to mechanical ventilation and require high oxygen settings. Infants at this end have likely suffered acute or chronic in utero hypoxemia leading to in utero meconium aspiration and are at risk for severe pulmonary complications including pulmonary hypertension. The presence of meconium in the amniotic fluid or in histologic sections of fetal membranes does not predict where on this continuum an infant will fit. Meconium may also play a role in ischemic, hypoxemic brain injury. Substances in meconium, possibly components of bile, may induce vasoconstriction providing a mechanism for tissue damage. Altshuler reported meconium-associated vascular necrosis involving the umbilical cord vessels and vessels of the chorionic plate. This is uncommon, seen in less than 1% of meconium-stained placentas, and only in cases of chronic meconium staining. This finding is associated with very poor neonatal outcomes. In a recent study of meconium-associated vascular necrosis, King et al have determined that rather than necrosis, the smooth muscle cells of the chorionic plate vessels that are oriented toward the amniotic cavity undergo increased karyorrhexis that appears to be specific for meconium. Diffuse Chorioamniotic Hemosiderosis In some cases the fetal membranes are deeply pigmented, brown or sometimes green, secondary to extensive hemosiderin deposition. Microscopically, numerous pigmented macrophages are seen in the free membranes and in the chorion and amnion of the fetal plate. These will stain with an iron stain such as Prussian blue. Redline has termed this finding diffuse chorioamniotic hemosiderosis (DCH). Redline found that DCH is often associated with old blood clot in the placenta and with circumvallation. He believes that DCH is an indication of chronic peripheral separation, which may lead to oligohydramnios in the absence of membrane rupture and preterm delivery (chronic abruption oligohydramnios sequence). In a large retrospective, case-control study of DCH, Ohyama et al confirmed the clinical and pathologic findings highlighted by Redline. DCH involved about 4% of cases and was associated with dry lung syndrome/persistent pulmonary hypertension of the newborn and chronic lung disease. They speculate that in utero aspiration of bloody fluid may harm the developing lung. Squamous metaplasia Squamous metaplasia of the amnion is a very common finding present in nearly all placentas. Grossly, it forms small grey or white, granular plaques that are difficult to remove from the amnion. They are most commonly located on the fetal surface of the placenta clustered around the umbilical cord insertion site. In microscopic sections these plaques appear as stratified squamous epithelium often with a granular cell layer and hyperkeratotic scale. As would be suspected from the gross impression, areas of squamous metaplasia are sharply demarcated from the adjacent columnar epithelium of the normal amnion. Squamous metaplasia has no known clinical or pathologic significance. Amnion nodosum Amnion nodosum has many features in common with squamous metaplasia but there are important gross, microscopic and clinical differences. Grossly, amnion nodosum is characterized by small, discrete, shiny, slightly raised gray-yellow nodules or plaques usually a few millimeters in diameter that can easily be removed from the amnion. These can be found anywhere on the membranes and surface of the umbilical cord but are most commonly seen on the fetal plate in the area of umbilical cord insertion. Microscopically, nodules of amnion nodosum are composed of granular, amorphous, eosinophilic material that may contain cell fragments or bits of lanugo hairs. The amniotic epithelium beneath these nodules may be intact or may exhibit degeneration of epithelium and/or basement membrane. Sometimes the surface of nodules demonstrates re-epithelialization by epithelium contiguous with adjacent amniotic epithelium. The importance of recognizing amnion nodosum is that it is almost always seen in cases of oligohydramnios and therefore may be a useful marker of those infants who are at risk for harboring anomalies that cause oligohydramnios, such as renal or urinary tract anomalies and those that are at risk for developing complications of oligohydramnios such as respiratory distress secondary to pulmonary hypoplasia. Examination of the fetus will also usually provide evidence of oligohydramnios. These fetuses exhibit the features of Potter's syndrome - flattened nose, limb positioning defects and breech presentation. Some have noted that amnion nodosum is seen more frequently in conditions where the oligohydramnios is due to decreased production, such as renal agenesis, rather than cases of longstanding amniorrhea where there is prolonged leakage of amniotic fluid. Presumably in the latter situation shed fetal debris is excreted through the vagina along with amniotic fluid whereas in the former it accumulates in the amniotic cavity and can be deposited on membranes. Chorion nodosum is a related, recently described process in which flat, vernix-containing nodules are embedded in the chorionic mesenchyme of the free membranes or the chorionic plate. Because they are flat, this process is not visible on gross examination. By definition, amnionic epithelium and mesenchyme should be absent. Chorion nodosum appears to be much less common than amnion nodosum and appears to be associated with some cases of limb body wall complex, expecially those associated with early vascular disruption and the development of amnionic bands, as well as extraamniotic pregnancies. Amniotic bands Amniotic bands, adhesions and strings may be associated with a spectrum of variable, major structural defects involving the limbs, trunk and craniofacial structures. The defects are often a combination of malformations (interference with the normal sequence of development), deformations (alterations in form or structure) and disruptions (tearing apart of previously normally formed structures). No two cases are alike. Various names have been applied to this spectrum of abnormalities including amniotic band syndrome, amniotic band disruption complex, the early amnion rupture spectrum, limb-body wall complex and the amnion adhesion malformation syndrome. Estimates of incidence range from 1 in 2500 to 1 in 8620 liveborns although the incidence for spontaneously aborted previable fetuses is much higher (1 in 55). One epidemiologic study has found that the highest rate occurs in black multigravidas who are under 20 years of age. There are important epidemiologic differences among cases with amniotic bands and no associated body wall defects and those without body wall defects suggesting that perhaps the etiology and pathogenesis of certain groups of defects associated with amniotic bands is different. Incidence figures need to be interpreted with caution, however, since this constellation of findings may be misdiagnosed in from one-half to two-thirds of patients. Placental examination in many of these cases reveals shredded amnion with thin strands connecting various fetal parts to the placental surface. Often amniotic bands cause constriction of the umbilical cord. This is responsible for death in a high proportion of the previable fetuses with this syndrome. Microscopically, these bands may consist of amnion with recognizable epithelium or the epithelium may have undergone degenerative changes and contain only fibrous tissue. Examination of the fetus will often reveal small bands tightly wrapped around the distal digits with associated amputations or constricting rings around other parts of limbs or trunk. Other limb defects include syndactyly, polydactyly, hypoplasia, distal lymphedema and club foot. Craniofacial abnormalities include encephalocele, unusual asymmetric facial clefts, and ear, eye and nose defects. Other defects include abdominal or thoracic wall defects, gastroschisis, omphalocele and scoliosis. Cases with craniofacial defects may show amnion right at the edge of the skin at the site of the defect. These cases may show broad amniotic adhesions at the site of the defect without evidence of amnion rupture. Cases with severe defects often contain a short umbilical cord and it is thought that tethering of the cord may lead to additional postural and other defects. A high percentage of severely affected cases show internal anomalies such as renal agenesis. Several theories have been proposed to explain the pathogenesis of this intriguing syndrome. One or more may explain various types of defects in a given case. One theory, brought to the fore by Torpin, postulates that amniotic bands are the direct cause of the defects. The amnion is thought to rupture early in gestation and the small, highly mobile fetus becomes entangled in shreds of amnion. Because the distal digits are the most mobile part of the fetus they have the greatest likelihood of becoming entangled explaining the high proportion of digital abnormalities in these cases. The craniofacial and abdominal wall defects are thought to be due to tethering of the fetus in a way that disrupts these structures. Swallowing of bands with tethering is thought to account for the body wall defects and internal anomalies. A recent study detailing the topography of the defects and the topography of the amniotic bands found that adhesions were significantly associated with adhesions in the same anatomic area. The cause of amnion rupture is not known. Perhaps there is trauma to the amnion by the fetus before the amnion and chorion are fused. In some cases, such as those associated with osteogenesis imperfecta and Ehlers-Danlos syndrome type IV, there may be an abnormality in amniotic collagen which predisposes to rupture. Maternal abdominal trauma is another possibility. Therapeutic amniocentesis has rarely been implicated but the digital abnormalities do occur more frequently in infants who were exposed to chorionic villous sampling. It is difficult to explain some of the complex anomalies and especially the internal anomalies by this theory alone, however. Also, in some cases, no amnion rupture or amniotic bands can be identified. Another theory, proposed by Streeter, postulates that an inherent developmental abnormality early in embryonic development causes defects in the embryonic disc as well as the amniotic cavity. In this theory, necrosis and degeneration of tissues such as the distal digits are thought to result secondarily in the formation of adhesions. This theory explains the high incidence of internal anomalies and features such as single umbilical artery. This theory would also explain the rare cases with a very similar spectrum of anomalies. Difficult to explain are why so many of the defects are disruptions and deformations rather than malformations, and the variety and asymmetry of the defects. A third theory postulates that early embryonic vascular disruption is the underlying cause of these defects. An animal model using amniocentesis between 14 and 16 days gestational age (equivalent to about 4 to 6 weeks in humans) can reproduce a wide variety of the external defects. Pathologically, the affected tissues show progressive rupture of vessels, hemorrhage into soft tissue, necrosis and disruption lesions with increasing time after damage. Compression from oligohydramnios may contribute to deformation lesions. The formation of amniotic bands is not seen and internal anomalies have not been evaluated in detail in this animal model, however. This theory may explain the increased incidence in infants exposed to chorionic villous sampling. Loss of villous tissue could lead to hypoxia or hemorrhage causing tissue damage and eventual loss. It is important to have a high index of suspicion for this syndrome. A clue to the diagnosis is the variety and asymmetry of fetal defects that don't fit easily into an established hereditary syndrome. The fetal digits and the placenta and cord should be examined carefully for amniotic bands which may be subtle. Nearly all cases so studied have had normal karyotypes. Familial cases are very rare and there is nearly no risk of recurrence in subsequent pregnancies in a given patient, although a recent report of a small number of cases suggests that limb deformities of amniotic band syndrome are more likely in infants that have an inherited thrombophilia. Correct identification of this spectrum of diseases allows the transmission of this reassuring information to the parents. References Meconium: Ahanya SN et al. Meconium passage in utero: mechanisms, consequences and management. Obstet Gynecol Survey 2004;60:45-56. Altshuler G. Meconium-induced vasoconstriction: a potential cause of cerebral and other fetal hypoperfusion and of poor pregnancy outcome. J Child Neurol 1987; 4:137-42. Altshuler G, et al. 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