1. Intestinal obstruction. The more distal the obstruction, the greater is the distention.
a. Causes in neonates. Intestinal atresia, meconium ileus, meconium plug.
b. Causes in older children. Incarcerated hernias, intussusception, midgut volvulus, Meckel diverticulum, adhesions from previous surgery.
2. Adynamic (paralytic) ileus. Intestine dilates from accumulated gas and fluid when there is diminished peristaltic activity. Seen with infectious or inflammatory conditions of the abdomen (eg, necrotizing enterocolitis), systemic infections (eg, UTIs, pneumonia), hypokalemia, and following surgical manipulation of intra-abdominal organs.
3. Pneumoperitoneum. Abdomen can distend from free air or sequestered fluid accompanying peritonitis.
a. Causes in neonates. Necrotizing enterocolitis, spontaneous gastric perforation.
b. Causes in older children. Perforated peptic ulcer, Meckel diverticulum.
4. Excessive air in GI tract. Infants and children may swallow large quantities of air with feeding or crying.
C. Ascites. Nonpurulent fluid in peritoneal cavity. May be serous, chylous, urinary, biliary, or bloody.
1. Serous. From increased splanchnic capillary hydrostatic pressure, and decreased plasma oncotic pressure. Causes: Hepatobiliary (cirrhosis, biliary atresia, other causes of portal hypertension), cardiac (congestive heart failure, constrictive pericarditis), or peritoneal (infectious, from tuberculosis or bacteria producing peritonitis; intra-abdominal tumors).
2. Chylous. From congenital malformation of lymphatic channels, lymphatic obstruction due to abdominal or mediastinal masses, or traumatic disruption.
3. Urinary. From fluid retention in renal failure, decreased plasma protein with nephrotic syndrome, and direct extravasation of urine with obstructive uropathy.
4. Biliary. From perforated gallbladder or common bile duct.
D. Constipation. Most often functional; may also be mechanical (from an anteriorly displaced anus), neurologic (with defects of the spinal cord), metabolic (associated with hypokalemia and hypothyroidism), or a result of Hirschsprung disease.
E. Masses. May represent enlarged organ (liver, spleen, kidney, bladder), tumor (neuroblastoma, Wilms tumor, hepatoblastoma, ovarian tumors), or cyst (ovarian, omphalomesenteric, mesenteric).
F. Pregnancy. In adolescent girls.
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