The Abdominal Wall Normal Anatomy of the Abdominal Gastroschisis/ 224 Wall/ 209 Body Stalk Anomaly/ 226 Diaphragmatic Hernia/ 211 Bladder Exstrophy and Cloacal Omphalocele/ 220 Exstrophy/ 228 Normal Anatomy of the Abdominal Wall The superior wall of the abdominal cavity is formed by the diaphragm. This muscle can be seen as a hipoechogenic line separating the lung and fetal liver (Fig. 6-1). In cases of severe hydrops, the structure appears as a hyperechogenic sheet between the pleural effusion and the ascites. When a massive Pleural effusion is present, the muscle can be inverted. The floor of the abdominal cavity is formed by the pelvic diaphragm. The pelvic bones (iliac crests, ischial ossification centers) and pelvic muscles can be imaged with ultrasound but have limited diagnostic interest (Fig. 6-2). The anterior abdominal wall is formed by the skin, subcutaneous tissue, and muscles. The muscles are visible as hypoechogenic structures (Fig. 6-3). In the past, the image of the muscles has been confused with ascites and referred to as "pseudoascites."1,2 The entrance of the umbilical cord into the fetal abdomen should always be imaged to screen for the presence of an omphalocele (Fig. 6-4). Examination of the anterior abdominal wall should include visualization of its infraumbilical portion in order to rule out caudal fold defects (e.g., bladder exstrophy). Figure 6-5 shows the Figure 6-1. Coronal section of a third trimester fetus. The normal contours of the lower portion of the anterior diaphragm appears as a hypoechogenic line between the thorax and the abdomen. L, lung; H, heart. abdominal wall. The posterior wall of the abdomen can be easily REFERENCES imaged. Structures that can be identified include the spine and the paraspinal muscles. 1. Rosenthal SJ, Filly RA, Callen PW, et al.: Fetal pseudoascites. Radiology 131:195, 1979. 2. Hashimoto BE, Filly RA, Callen PW: Fetal pseudoascites: Further anatomic observations. J Ultrasound Med 5:151, 1986. 209 ©1987-2002 Romero-Pilu-Jeanty-Ghidini-Hobbins 210 THE ABDOMINAL WALL Figure 6-2. Coronal scan of a male fetal pelvis. A curved Figure 6-4. Longitudinal scan showing the insertion of the arrow points to the right ileopsoas muscle. IW, iliac wings; IT, umbilical cord in the abdomen of a first trimester fetus. ischial tuberosities. Figure 6-3. Transverse section of the abdomen of a third Figure 6-5. Longitudinal scan of the fetal abdomen between trimester fetus. The abdominal muscles appear as the insertion of the umbilical cord (UC) and the gender. B, hypoechogenic structures (arrowheads). bladder; P, penis. ©1987-2002 Romero-Pilu-Jeanty-Ghidini-Hobbins DIAPHRAGMATIC HERNIA 211 Diaphragmatic Hernia Definition septum tranversum is a mesodermal structure that Diaphragmatic hernia consists of protrusion of the migrates from the cranial portion of the embryo to the abdominal organs into the thoracic cavity through a definitive location of the diaphragm. It gives origin to diaphragmatic defect. It includes posterolateral the central tendon of the diaphragm. The dorsal diaphragmatic hernia, Bochdalek hernia, retrosternal esophageal mesentery contributes to the median portion diaphragmatic hernia, Morgagni hernia, and eventration of the organ. The pleuroperitoneal membranes are of the diaphragm. structures that close the pleuroperitoneal cavity. Although they form a large segment of the embryonal Incidence diaphragm, their contribution to the final and fully The incidence of diaphragmatic hernia varies developed diaphragm is relatively small. The depending on the type of study. The best estimates participation of the body wall is limited to a narrow vary from 0.033 to 0. 05 percent of births. 11, 19,31,47 peripheral segment corresponding to the insertion of the Neonatal surgical series report a frequency of 0.012 muscle to the ribs and sternum (Fig. 6-6). The percent.67 This difference is due to an underestimation diaphragm is completely formed by the end of the 8th of the incidence of the disease because of its week of conceptional age. However, modeling association with stillbirths and early neonatal deaths.36 continues throughout gestation. Expansion of the lungs The male to female ratio in the two largest series to results in the formation of the costodiaphragmatic date varies between 0.67 and 0.77.11,19 recesses, which have a dome-shaped configuration. The normal diaphragm allows the passage of organs, Etiology and Risk of Recurrence vessels, and nerves from the thoracic to the abdominal cavity. This is accomplished by three main orifices, Congenital diaphragmatic hernia can be both a sporadic allowing passage for the aorta, esophagus, and inferior and a familial disorder. The pattern of inheritance for vena cava (Fig. 6-7). The thoracic duct and azygos familial cases is unknown, but a multifactorial type of vein cross through the aortic foramen, and the vagal inheritance has been suggested,17,57,58 indicating a nerves use the esophageal foramen. The right phrenic recurrence risk for siblings of 2 percent.57 Familial nerves cross through the inferior vena cava orifice. cases have a higher male to female ratio (M:F=2.10 Congenital diaphragmatic hernias occur when a versus 0.67), a higher incidence of bilateral defects (20 diaphragmatic defect allows the protrusion of percent versus 3 percent), and a lower incidence of life- abdominal visceral content into the thoracic cavity. threatening malformations than sporadic cases.17 The spectrum of embryologic defects is wide, ranging However, no specific features allow identification of a from complete absence of the diaphragm through familial case. Diaphragmatic hernia has been pathologic orifices (Bochdaleck foramen) to congenital associated with Fryns syndrome,49 Beckwith- hiatal hernia, in which the viscera protrude through a Wiedemann syndrome (see p. 221),68 Pierre Robin physiologic orifice. syndrome, and congenital choanal atresia.22 The defect Congenital diaphragmatic hernias are classified has been associated with chromosomal defects (see according to the location of the diaphragmatic defect: section on associated anomalies). The etiology of sporadic cases is largely unknown. 1 Posterolateral defect, or Bochdaleck hernia, Maternal ingestion of bendectin,10 thalidomide,43 occurring through the primitive communication of the quinine,46 and antiepileptic drugs40 has been reported in pleuroperitoneal canal or foramen of Bochdaleck association with diaphragmatic hernia. In animal 2 Parasternal defect, or Morgagni hernia, located in models, hypovitaminosis A was able to induce the anterior portion of the diaphragm between the diaphragmatic hernia in rats.1,72 The etiologic role of costal and the sternal origins of the muscle (foramen of diabetes is considered weak.45 Morgagni or sternocostal hiatus) 3 Septum transversum defects, occurring because of Embryology and Anatomy of the Diaphragm a defect of the central tendon The diaphragm is a dome-shaped septum dividing the 4 Hiatal hernias, occurring through a congenitally thoracic and abdominal cavities. It consists of a central large esophageal orifice or aponeurotic segment and a peripheral or muscular one. It is formed by the fusion of four different Another pathologic entity, eventration of the structures: (1) the septum transversum, (2) the dorsal diaphragm, is frequently considered with congenital esophageal mesentery, (3) the pleuroperitoneal 25 ,55 ,65 diaphragmatic hernias because they have a similar membrane, and (4) the body wall. The pathophysiologic sequence. However, eventration of ©1987-2002 Romero-Pilu-Jeanty-Ghidini-Hobbins 212 THE ABDOMINAL WALL abdominal viscera would prevent complete closure of the diaphragm. An alternative hypothesis suggests that a primary defect occurs in the formation of the diaphragm, and this creates the condition for a subsequent migration of the abdominal organs into the thoracic cavity. The negative pressure created by fetal thoracic wall movements (fetal breathing) could be responsible for migration of the viscera into the thorax. Two observations support the first hypothesis. (1) In some cases, organs firmly attached to the abdomen in late fetal life, such as the pancreas, can be found in the thorax. This implies a precocious migration of abdominal organs into the thoracic cavity. (2) The discrepancy between the size of the defect and the dimensions of the herniated organs also suggests an early migration of the viscera and secondary partial closure of the defect. The main cause of death of infants with congenital Figure 6-6. Development of the diaphragm (viewed from below). diaphragmatic hernia without associated anomalies is A. sketch of a lateral view of a 5-week embryo (actual size), indicating the level of the section. B. Transverse section showing respiratory failure due to pulmonary hypoplasia. This can the pleuroperitoneal membranes not yet fused. C. Same section be easily understood if it is considered that development of at the end of the 6th week; the pleuroperitoneal membranes the normal lung is an active process from the 5th week of have fused with the other two diaphragmatic components. D. gestation.44,61 The fetal lung goes through four different Same section at 12 weeks; the fourth diaphragmatic component has formed from the body wall. E. view in the newborn, stages (Figs. 6-8, 6-9): (1) embryonic period, from indicating the probable embryologic origin of the different conception to the 5th week, (2) pseudoglandular period, components. (Reproduced from Moore: The Developing Human, from the 5th to the 17th week, (3) canalicular period, from 3d ed. Philadelphia, Saunders, 1982.) the 17th to the 24th week, and (4) terminal sac period, from the 24th week until term. By the 16th week of gestation, the development of the conductive part of the airways is the diaphragm consists of an upward displacement of the completed (from trachea to terminal bronchioles). From abdominal content into the thoracic cavity because of a this time until birth, respiratory airways are fully developed. congenitally weak diaphragm, which is virtually reduced This includes respiratory bronchioles and saccules. Most to an aponeurotic sheet. alveolar development is a postnatal event.