Paediatric Respiratory Reviews 16 (2015) 11–17

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Paediatric Respiratory Reviews

Mini-symposium: Chest Wall Disease

Pulmonary Complications of Abdominal Wall Defects Howard B. Panitch * Professor of Pediatrics, Perelman School of Medicine at The University of Pennsylvania, Division of Pulmonary Medicine, The Children’s Hospital of Philadelphia

EDUCATIONAL AIMS 1) To understand the contribution of the ventral abdominal wall to inspiration and exhalation 2) To recognize the pulmonary complications of giant abdominal wall defects in neonates 3) To recognize the influence of the abdominal contents and the ventral abdominal wall on formation of the thoracic cage

A R T I C L E I N F O

S U M M A R Y

Keywords: Abdominal wall defects pulmonary function omphalocele gastroschisis prune belly syndrome

The abdominal wall is an integral component of the chest wall. Defects in the ventral abdominal wall alter respiratory mechanics and can impair diaphragm function. Congenital abdominal wall defects also are associated with abnormalities in lung growth and development that lead to pulmonary hypoplasia, pulmonary hypertension, and alterations in thoracic cage formation. Although infants with ventral abdominal wall defects can experience life-threatening pulmonary complications, older children typically experience a more benign respiratory course. Studies of lung and chest wall function in older children and adolescents with congenital abdominal wall defects are few; such investigations could provide strategies for improved respiratory performance, avoidance of respiratory morbidity, and enhanced exercise ability for these children. ß 2014 Elsevier Ltd. All rights reserved.

INTRODUCTION The abdominal wall, rib cage and intercostal muscles comprise the chest wall. The back, lower rib cage, pelvis and iliac crests constrain movement of the lateral and posterior abdomen, so that the ventral abdominal wall is the only freely moving part of the abdominal compartment. Its motion in relationship to that of the thorax lends important insight into the mechanics of the chest wall and respiratory pump. Alterations in the characteristics of the abdominal wall affect overall respiratory pump function. In addition, congenital malformations of the ventral abdominal wall can be associated with pulmonary hypoplasia and neonatal respiratory distress. This review will focus on select congenital

* 3501 Civic Center Boulevard, Philadelphia, PA 19104. Tel.: +215 590 3749; fax: +215 590 3500. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.prrv.2014.10.004 1526-0542/ß 2014 Elsevier Ltd. All rights reserved.

lesions that alter the properties of the ventral abdominal wall and thus respiratory pump function. EMBRYOLOGY OF THE VENTRAL ABDOMINAL WALL During the third and fourth weeks of gestation, the embryo undergoes gastrulation, becoming a trilaminar structure including ectoderm, mesoderm and endoderm. At the same time, the head and tail of the embryo bend ventrally towards each other and cause the embryo to assume the fetal position [1]. A second body folding occurs from the sides towards the middle, involving structures called the lateral body folds. The lateral body folds form from an area of the mesoderm called the lateral plate mesoderm and include the overlying ectoderm [1]. They move ventrally and fuse in the midline by the end of the fourth week of gestation. The processes that lead to the ventral movement of the lateral body folds and their fusion with each other in the midline are poorly understood, but likely involve some combination of cell proliferation, cell migration, production of extracellular matrix materials,

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apoptosis and specialized cell to cell contacts [1–3]. Several genes have been implicated in the process of ventral abdominal wall formation [4,5], but it is likely that additional genes are responsible for proper formation of the ventral body wall. It has been hypothesized that defects in the movement of one or both of the lateral body folds to meet their contralateral counterpart result in such defects as ectopia cordis, gastroschisis, and bladder exstrophy [3,6]. The midgut begins to grow rapidly during the 6th week of gestation, resulting in a physiologic herniation of the intestines through the umbilical ring. The midgut then rotates and returns to the abdominal compartment by the 10th week of gestation. If the intestine fails to return to the abdominal compartment, a small hernia into the umbilicus can occur, resulting in a small omphalocele, with minimal widening of the umbilical ring [7]. The embryological events that cause large omphaloceles are unclear: some authors propose that an abnormality in body folding in the cephalic region results in an epigastric omphalocele as seen in pentalogy of Cantrell, while a caudal folding abnormality causes a low or hypogastric omphalocele often in association with bladder or cloacal exstrophy, and a lateral body wall folding problem results in a mid-abdominal defect [5,7]. Others, however, point to the fact that the organs within an omphalocele are covered by a sac that includes amnion along with Wharton’s jelly and peritoneum, so that an omphalocele does not represent a body closure defect and some other mechanism for its creation must exist [3]. Even when no defect in body wall closure exists, additional abnormalities in ventral abdominal wall formation can take place. Between 6 and 10 weeks of gestation, muscles of the ventral abdominal wall form. Absence or marked diminution of abdominal wall musculature can occur, often in association with urinary tract abnormalities that lead to obstruction of the bladder and cryptorchidism in males. This triad of abnormalities, known as prune belly syndrome, triad syndrome or Eagle Barrett syndrome [8], has been ascribed to two possible mechanisms: the first involves a defect in intermediate and lateral plate mesoderm resulting in maldevelopment of both abdominal wall and urinary tract musculature [9]. The second mechanism invokes blockage of egress of urine from the bladder in utero, which then results in marked distension of the bladder, ureter and kidneys. The distended bladder compresses the abdominal wall and causes atrophy of the muscles, either by direct pressure or by interruption of blood flow [8]. PHYSIOLOGICAL CONTRIBUTION OF THE ABDOMINAL WALL TO BREATHING The abdominal wall plays important roles during inspiration, exhalation and airway clearance via coughing. During quiet

breathing in an adult, the diaphragm runs parallel to the rib cage in a zone of apposition that includes approximately ¼ to 1/3 of the rib cage [10]. Thus, abdominal contents that are not compressible occupy a substantial portion of the lower rib cage. By virtue of this arrangement, contraction of the diaphragm not only lowers intrathoracic pressure, but it also increases intraabdominal pressure through the area of apposition (Figure 1). The incompressible abdominal viscera act as a fulcrum, resulting in expansion of the lower rib cage. Notably, experimental removal of abdominal viscera from the abdominal cavity causes the costal fibers of the rib cage to contract rather than expand the lower rib cage [11]. Lower rib cage motion in infants with lesions like giant omphalocele, in which the liver and other abdominal organs are displaced from the upper abdomen, has not been quantitatively studied to determine if a similar paradoxical motion of the lower rib cage exists during unassisted breathing and before reduction of viscera back into the abdominal cavity. Because the ventral abdominal wall has freedom to move, it is normally displaced outwards along with the lower rib cage. In contrast, in several patients with giant omphalocele, diaphragmatic contraction resulted in a cephalad rotation of the omphalocele rather than an outward movement of the abdominal wall [12], perhaps reflecting abnormal lower rib cage movement. Compliance of the abdominal wall is an important factor in the determination of diaphragmatic motion during inspiration. For a given degree of neural activation of the diaphragm, the distance its dome will descend will depend upon the resistance to fiber shortening imposed by the stiffness of the abdominal wall and rib cage [10]. A poorly compliant abdominal compartment will limit diaphragm descent, and a non-compliant abdominal wall will restrict lower rib cage expansion (much like what happens after a large meal). On the other hand, a highly compliant abdominal wall, as seen in patients with prune belly syndrome, can lead to alterations in rib cage-abdominal wall relationships and diaphragmatic function in the upright position that resolve when subjects are supine [13]. Here, Ewig and coworkers postulated that the highly compliant abdominal wall led to loss of the fulcrum effect of abdominal contents on the lower rib cage, thereby limiting lower rib cage expansion and allowing the diaphragm muscle fibers to shorten excessively. These mechanical disadvantages in turn resulted in functional diaphragmatic weakness, a need for recruitment of accessory muscles of inspiration, and ultimately abdominal paradox (inward motion of the abdominal wall during inspiration) [13]. These findings all disappeared when the same subjects were studied in a supine position, and gravity caused abdominal viscera to exert cephalad pressure on the diaphragm thereby improving its lengthtension relationships and increasing the area of apposition. Under normal circumstances, the abdominal muscles are typically considered to be accessory muscles of exhalation.

Figure 1. A) Diaphragm and chest wall at end-expiration. The zone of apposition is demarcated by the bracket; B) At end-inspiration, the diaphragm has descended, increasing pressure in the abdominal compartment. In turn, the abdominal contents act as a fulcrum to help elevate the lower rib cage.

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Because their contraction can enhance expiratory airflow, reduce end expiratory lung volume, and allow the respiratory system to operate at a lower lung volume, they can also be considered accessory muscles of inspiration. Here, abdominal muscle contraction during exhalation displaces the diaphragm into the thorax, placing its fibers on a more advantageous portion of their lengthtension curve. This allows for a larger tidal volume during the subsequent breath [10]. The main action of abdominal muscle contraction, however, is to compress the abdominal contents and cause an inward motion of the ventral abdominal wall along with an increase in intraabdominal pressure. These actions are crucial for forced exhalation and coughing. Their absence or compromise could therefore interfere with exercise ability and airway clearance. Children with prune belly syndrome, for instance, are at increased risk for recurrent atelectasis or pneumonia [13,14]. Not all ventral abdominal wall defects lead to respiratory compromise. Isolated lesions below the umbilicus like bladder exstrophy are not associated with pulmonary complications. SPECIFIC LESIONS: GASTROSCHISIS, OMPHALOCELE, PRUNE BELLY SYNDROME The two most common abdominal wall defects include gastroschisis and omphalocele. Gastroschisis refers to extrusion of bowel not covered by any membrane through a ventral abdominal wall defect usually 50% and as often as high as 80% of cases [16]. Recent assessments of large patient registries, however, have disclosed that other lesions occur in up to 30% of patients with gastroschisis, including urogenital, musculoskeletal, cardiac and central nervous system defects [15,19,20]. Omphalocele is more commonly associated with syndromes like Beckwith-Wiedemann

Figure 2. Gastroschisis, involving stomach and small bowel. Note the abdominal wall defect just to the right of the umbilicus. Photograph courtesy of Holly Hedrick, M.D.

Figure 3. Small omphalocele. Note umbilicus attached to membrane covering abdominal contents. Photograph courtesy of Holly Hedrick, M.D.

syndrome and chromosomal abnormalities including trisomy 13, 18 and 21 [15,19]. The risk of chromosomal abnormalities is greater in a fetus with a central rather than an epigastric omphalocele [21] or in a small (5 cm abdominal wall defects and contain most or all of the liver (Figure 4), are associated with pulmonary compromise even before the reduction of viscera back into the abdominal compartment occurs [12,23,29–32]. Prenatal ultrasound identification of extrusion of the liver through the omphalocele (giant omphalocele) was found to be the single factor that predicted adverse neonatal outcome, including death, prolonged hospital stay (>70 days), or need for more than

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Figure 4. Giant omphalocele containing liver and bowel. Photograph courtesy of Holly Hedrick, M.D.

2 surgical procedures among 33 infants with omphalocele at a single center [33]. Griscom and Driscoll noted that fetuses and infants with omphalocele who were stillborn all had radiographic evidence of ‘‘markedly reduced chest capacities’’ [34]. To determine if such radiographic abnormalities were unique to those infants with giant omphalocele, Hershenson and coworkers obtained detailed measurements of the chest from radiographs taken in newborns with four types of abdominal wall defects: cloacal exstrophy, gastroschisis, small omphalocele and giant omphalocele (GO)[12]. The chest wall configuration of those infants with GO was different from all other groups: the chests were narrow and the ribs exhibited marked caudal declination rather than the usual horizontal position seen in newborns (Figure 5). Measures of chest width and lung area

Figure 5. Chest radiograph of an infant with giant omphalocele. Note the narrow elongated chest and markedly downslanting ribs.

were significantly smaller in the GO infants than in infants with other abdominal wall defects. The respiratory course of the infants with GO was also worse than of any other group: 5 deaths resulting from respiratory failure occurred only among GO infants. Additionally, as a group the GO infants required significantly longer courses of positive pressure ventilation and supplemental oxygen than the other groups, with 9/22 (41%) requiring respiratory support for >2 weeks. The authors speculated that the respiratory distress exhibited by infants with GO was the result of a narrow chest deformity and/or some degree of pulmonary hypoplasia [12]. In a larger series, assessment of radiographic chest wall configuration was coupled with medical record review and, in a subset of infants, autopsy findings in subjects with gastroschisis, isolated small or giant omphalocele, omphalocele associated with cloacal exstrophy (lower midline syndrome) and omphalocele associated with ectopia cordis (upper midline syndrome) [29]. A narrow chest wall with downslanting ribs was found in 14 of 33 infants with giant abdominal wall defects, 12 of whom died with respiratory distress. Various post-mortem studies including lung weight to body weight ratios and radial alveolar counts confirmed varying degrees of pulmonary hypoplasia in these infants. In another series, pulmonary hypoplasia was described in 14 of 20 (70%) infants with GO [35]. Danzer and coworkers measured spirometry by the raised volume rapid thoracic compression technique and fractional lung volumes by body plethysmography, as well as tidal mechanics by the single breath occlusion technique in 14 subjects with GO between 1 and 58 months of age [30]. Five still required supplemental oxygen at the time of study, and 2 were receiving mechanical ventilation for part of the day. Spirometry showed a proportional reduction in FVC and FEV0.5, with values >2 SD below published normal values in 11 (79%) and with a normal FEV0.5/FVC ratio in 13 (93%). Consistent with a restrictive process, all but 1 subject had a total lung capacity (TLC) below the published mean, with 8 (57%) having values below the 95% confidence interval. As in other studies in neonates [25,26], Crs, whether normalized to body weight or FRC, was abnormally low. The authors noted that the children who underwent lung function testing had more severe disease than those who were followed in their program but who did not undergo testing. Nevertheless, 8 of the subjects were >12 months of age at time of study, demonstrating that lung function abnormalities persisted beyond the first year of life. The authors speculated that the abnormal lung functions noted were the result of disturbed lung growth and development that began in utero. Such abnormalities of prenatal lung growth in infants with giant abdominal wall defects have been proposed to be the result of a deformation sequence in utero, with several potential contributing factors [12,29]. The liver is displaced and so does not mold the lower thoracic cage. In addition, there is decreased intraabdominal pressure because of absence of the viscera, with a tendency of the lower rib cage to collapse inward rather than move outward with breathing movements. Rather than meeting in the midline superiorly, the rectus abdominus muscles attach laterally to the costal margins of the ribs in many infants with GO [35,36]; the laterally displaced abdominal muscles then would exert a downward force on the rib cage, causing a more caudal declination of the ribs and narrowing of the chest wall. Together, these factors would cause the developing chest to assume a narrow configuration with down-slanting ribs. Prenatal ultrasound evaluation of fetuses with abdominal wall defects has been able to identify pulmonary hypoplasia [32,37]. A 2-D ultrasonographic measurement of the transverse lung to thorax ratio was able to predict the presence of pulmonary hypoplasia in fetuses with abdominal wall defects [32]. The same investigators also showed that the sonographic measurement of chest/trunk length ratio could describe the narrow, elongated

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chest seen in some infants with abdominal wall defects [32]. More recently, ultrafast fetal MRI images were used to calculate total lung volumes in fetuses with GO [38]. An observed/expected (O/E) total lung volume ratio was calculated using gestational-aged matched reference values. As a group, the 17 fetuses studied had a mean O/E total lung volume ratio that was lower (52.3  16.8%) than age-matched norms. When the investigators evaluated the effect of severity of pulmonary hypoplasia on their subjects’ short-term neonatal courses, the 11 who had an O/E total lung volume ratio 50%. Two of the subjects required tracheostomy placement for prolonged mechanical ventilation, and both had an O/E total lung volume ratio

Pulmonary complications of abdominal wall defects.

The abdominal wall is an integral component of the chest wall. Defects in the ventral abdominal wall alter respiratory mechanics and can impair diaphr...
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