Distended Abdomen
John H. Seashore
Newborn infants normally have protuberant abdomens owing to the relatively large size of their viscera, so judging distention may be more challenging than in older patients. Abdominal distention is a fairly common presenting sign of a variety of congenital anomalies and acquired diseases, however, and its presence or absence should be evaluated during all physical examinations of babies. Abdominal distention may result from abnormal accumulations of air, fluid, or solid tissue. History and physical examination may provide clues as to the likely etiology. For example, an infant who is vomiting bile probably has an intestinal obstruction, one who has severe congenital heart disease and hydrops fetalis most likely has ascites as the cause of distention, and a palpable mass is usually an enlarged organ or a tumor. Plain radiography of the abdomen is most useful for diagnosing conditions resulting from air, whereas ultrasound is the best imaging method for fluid and solid tissue. Many of the conditions that present with distention are covered in other sections of this book. This chapter presents a differential diagnosis and detailed discussion of conditions that are not covered elsewhere.
DISTENTION CAUSED BY AIR
Excessive air is the most common cause of abdominal distention in the neonate (Box 58.1). The distention is uniform throughout the abdomen, which is tympanitic on examination. Air may be within the lumen of the bowel or free in the peritoneal cavity. Intraluminal air is most common and is usually an indication of intestinal obstruction. Adynamic ileus may also cause distention by air-filled loops of bowel. Common causes include necrotizing enterocolitis and sepsis. Plain radiography of the abdomen is the primary means to diagnose these
conditions. Intestinal obstruction is manifested by a few or multiple loops of very dilated bowel with air–fluid levels seen on lateral decubitus views. Air is absent in the region of the distal bowel. Ileus presents a pattern of more diffusely dilated bowel, sometimes very irregular, but not as enlarged as in obstruction. The clinical context often is helpful to distinguish these conditions as well. Infants with an ileus are usually acutely ill and have signs and symptoms of sepsis. Infants with obstruction typically appear well and present with distention, bilious vomiting, and frequently a failure to pass meconium.
conditions. Intestinal obstruction is manifested by a few or multiple loops of very dilated bowel with air–fluid levels seen on lateral decubitus views. Air is absent in the region of the distal bowel. Ileus presents a pattern of more diffusely dilated bowel, sometimes very irregular, but not as enlarged as in obstruction. The clinical context often is helpful to distinguish these conditions as well. Infants with an ileus are usually acutely ill and have signs and symptoms of sepsis. Infants with obstruction typically appear well and present with distention, bilious vomiting, and frequently a failure to pass meconium.
BOX 58.1 Abdominal Distention Caused by Excess Air
Air within bowel
Adynamic ileus
Intestinal obstruction
Atresia
Meconium ileus
Hirschsprung disease
Midgut volvulus
Air in the peritoneal cavity
Perforation of the bowel
Necrotizing enterocolitis
Intestinal obstruction
Spontaneous perforation of the bowel
Dissection from a pneumomediastinum (pulmonary barotrauma)
Free air in the peritoneal cavity is almost always indicative of perforation of the bowel and is an ominous sign. Infants who have perforated bowel are usually acutely ill with signs of peritonitis and sepsis, but for unknown reasons some babies have a rather bland response to perforation, presenting only with distention. In either case, free air is identified on plain radiography of the abdomen. A round or oblong lucency in the mid- to upper abdomen with a central vertical density (the football sign) seen on a supine film represents free air outlining the falciform ligament. However, the right lateral decubitus view is the most sensitive for seeing free air as a triangular or crescent-shaped radiolucency at the uppermost part of the abdomen or outlining the liver edge.
The most common cause of free air in the neonate is perforation from necrotizing enterocolitis. This is primarily a disease of premature infants but may occur in full-term infants, even after discharge from the hospital. It is discussed in Chapter 59.
Spontaneous perforation of the bowel is an uncommon disorder of uncertain cause. It has been recognized as an entity for many years and occurs in both full-term and premature infants. It has been associated with administration of indomethacin. The onset is sudden, with distention as the primary and sometimes only sign. Free air is present on the initial radiographs, often in massive amounts. At operation, an isolated perforation is found, most commonly in the stomach, colon, or terminal ileum. The hole may be pinpoint or large, but signs of ischemia are lacking in the surrounding bowel, and relatively little evidence of peritonitis exists if the condition is diagnosed and treated promptly. The hole is oversewn, resected, or exteriorized depending on individual findings. Although some of these cases, particularly in infants with known risk factors, may be variants of necrotizing enterocolitis, spontaneous perforation is clearly a discrete entity. The prognosis depends on age, associated conditions, and timeliness of treatment.
In rare cases, free air is not from a bowel perforation but has dissected down from the mediastinum. This can occur in infants who are on ventilators with very high inflation pressure. The resulting barotrauma may cause pneumothorax or pulmonary interstitial emphysema. The key finding, however, is pneumomediastinum, from whence the air may dissect through the various hiatuses of the diaphragm, then into the mesentery, where it ruptures into the free peritoneal cavity. The babies typically have sudden onset of abdominal distention and often have massive amounts of free air in the abdomen. These are obviously very sick infants who are at risk for necrotizing enterocolitis. Therefore, there must be a low threshold to explore these infants unless the air clearly has come from the chest. Measuring the partial pressure of oxygen in the gas aspirated from the abdomen has been suggested as a method to make this distinction.
NEONATAL ASCITES
Serous Fluid
Ascites is an abnormal accumulation of fluid in the peritoneal cavity. The fluid may be serous, urine, an exudate, chyle, blood, or bile (Box 58.2). A fluid wave on physical examination and central displacement of bowel gas on plain radiography are suggestive of ascites, but the key to diagnosis is ultrasound examination. The type of fluid is determined by paracentesis with visual and laboratory analysis of the fluid.
BOX 58.2 Fluids that Cause Abdominal Distention
Serous fluid
Hydrops fetalis
Hematologic disorders
Rh isoimmunization
Thalassemia
Other congenital hemolytic anemias
Cardiac failure
Congenital heart disease
Congenital arrhythmias
Congenital infection
Cytomegalic inclusion disease
Syphilis
Toxoplasmosis
Rubella
Metabolic disorders
Lysosomal storage diseases
Tyrosinemia
Galactosemia
Urine
Obstructive uropathy
Posterior urethral valves
Ureterovesical junction obstruction
Ureteropelvic junction obstruction
Idiopathic
Inflammatory and infected fluid
Perforation of the bowel
Necrotizing enterocolitis
Spontaneous perforation
Appendicitis
Hirschsprung disease
Meconium peritonitis
Chyle
Idiopathic
Mesenteric cyst
Lymphangiectasia
Midgut volvulus
Blood
Ruptured liver or spleen
Birth trauma
Other injury
Child abuse (rare in neonate)
Ruptured liver mass
Hemangioma
Mesenchymal hamartoma
Bile
Spontaneous perforation of the bile duct
The peritoneum, in common with all serous surfaces, constantly secretes and absorbs fluid that is in equilibrium with extracellular fluid and therefore has a similar chemical makeup. Conditions that alter the rate of secretion or absorption may lead to an accumulation of serous fluid in the peritoneal cavity.
Hydrops fetalis is characterized by generalized edema and accumulation of fluid in all the serous cavities and is easily diagnosed by physical examination and ultrasound. Mortality is high in both fetuses and neonates. Rh isoimmunization, which
causes profound fetal anemia, is still a common cause of hydrops worldwide, but pretreatment with anti-D antibodies (RhoGAM) has made this condition rare in developed countries.
causes profound fetal anemia, is still a common cause of hydrops worldwide, but pretreatment with anti-D antibodies (RhoGAM) has made this condition rare in developed countries.
Nonimmune hydrops may result from a variety of conditions. Congenital heart disease or arrhythmia causing severe heart failure is most common. Thalassemia and other hematologic disorders may cause hemolysis, anemia, and hydrops. Congenital infection, particularly cytomegalovirus and neonatal syphilis, may cause hydrops or isolated ascites. Genetic metabolic disorders that are associated with ascites and occasionally hydrops include lysosomal storage diseases such as Gaucher disease and inborn errors that may cause early liver disease such as galactosemia or tyrosinemia.
Urgent diagnosis of the cause of hydrops to allow treatment where possible is essential to reduce the substantial risk of mortality. Supportive therapy, transfusion, and treatment of heart failure or arrhythmia are provided as appropriate. If the amount of abdominal fluid is massive enough to cause respiratory distress, removal by paracentesis offers temporary relief, but fluid reaccumulation is rapid unless the underlying disorder has been treated adequately.
Urine
Urinary ascites is the next most common cause of increased abdominal fluid. Paracentesis yields a yellow fluid with high levels of urea and other constituents of urine. Most of these patients have obstructive uropathy leading to distention of various portions of the urinary tract and subsequent leakage of urine into the peritoneal cavity. A careful evaluation for posterior urethral valves, ureterovesical junction obstruction, and ureteropelvic junction obstruction is necessary. The actual site of leakage is usually small and hard to find. Hyponatremia is present in approximately 70% of patients, probably due to movement of sodium down its concentration gradient from the higher sodium concentration in extracellular fluid to the lower sodium concentration of urine (autodialysis). Treatment is directed toward the underlying condition, either by definitive repair or diversion of the urinary flow proximal to the obstruction.
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