Hirschsprung's disease or congenital aganglionic megacolon is the most common cause of lower intestinal obstruction in neonates accounting for 15-20% of cases. It is characterized histologically by the total absence of ganglion cells in the myenteric (Auerbach's) and submucosal (Meissner's) plexuses with hypertrophy of the nerve fibers in the submucosa, muscularis mucosa and lamina propria. These plexuses supply parasympathetic innervation to the bowel. This absence of parasympathetic innervation results in the result in lack of peristalsis in the involved segment and failure of the internal sphincter to relax resulting in obstruction. The aganglionic segment can be variable in length ranging from 1-2 cm to the entire colon and distal ileum, however, 75% of cases are confined to the rectosigmoid
   Hirschsprung's is caused by a failure of neural crest cells to migrated from the neural crest down the GI tract early in embryonic development, usually completed by 12 weeks. The reason for this failure in migration is not entirely known, however are thought to have an immunologic origin.
   The incidence of Hirschsprung's is approximately 1 in 5000 live births, with about 700 new cases diagnosed in the U.S. each year. The male-to-female ratio is 4:1 for patients with rectosigmoid a more proximal neurogenic process disease and 2:1 in those with pancolonic involvement. There is no race predilection. A family history is reported in 7% of cases with about a 3.5% incidence in siblings. Additional congenital abnormalities are found in 11-30% of patients and this risk appears to increase with more colonic involvement.
   Hirschsprung's is diagnosed in within the 1 month of life in 40-60% of patients following the failure of the neonate to pass meconium within the first 24 hrs which occurs in 90% of normal neonates. Associated symptoms include abdominal distention, bilious vomiting and refusal to feed.
   Diagnosis is best made by rectal biopsy, which shows the absence of ganglion cells. This can be performed by atraumatic suction to obtain a submucosal biopsy or by a full thickness rectal biopsy. Plain radiographs may show gaseous distention of the bowel, air fluid levels and lots of stool all characteristics of obstruction. Air contrast barium enema often demonstrates a classic "transition zone" from the narrow aganglionic segment to the proximal dilated bowel and delayed evacuation on plain film take 24 hours later. Rectal manometry is a very sensitive and specific test, which would indicate the absence of the normal rectosphinteric reflex usually present by 12 days of life. This reflex is elicited by dilation of the rectum with a bulb and observation of the subsequent tone of the internal anal sphincter, which normally relaxes aiding defecation.
   In neonates the differential diagnosis should include other causes of bowel obstruction: ileal atresia or stenosis, meconium ileus, intestinal duplication, colonic atresia, incarcerated inguinal hernia, meconium plug syndrome and various other neuronal dysplasias.
   Surgery is the only safe and effective treatment of Hirschsprung's disease, which consists of resection of the aganglionic bowel and re-anastomosis of the proximal normal bowel to the anal canal. Traditionally this is done in multiple stages with initial surgery consisting of colostomy and a Hartman's pouch formation. Definitive surgery and re-anastomosis is then performed when the child is 6-12 months and weighs 15-20 lbs. The three operations most commonly performed are the Swenson, Soave, and Duhamel.