Natural History: Epidemiology
After gastroenteritis, appendicitis is the most common acute abdominal inflammatory process in childhood (1). The true incidence of appendicitis is not known, but the annual rate of appendectomy is about 4 in every 1000 children under the age of 14 years (2). An active pediatric emergency department may encounter 3 - 4 cases each week. Appendicitis becomes progressively more common after infancy (1).
Peak incidence occurs in teens and young adults; frequency of occurrence is greatest in the spring and fall. There is a familial tendency to develop appendicitis.
Diagnosis in children can be difficult, with atypical clinical symptoms mimicking a number of other disease entities, such as small bowel obstruction, inflammatory bowel disease, or gynecologic disorders. Laboratory studies are frequently non-specific or confusing.
Natural History: Pathophysiology
Appendiceal inflammation occurs when the appendiceal lumen is obstructed by fecaliths ,lymphoid hyperplasia, or fibrous stenosis associated with prior infection. Other less common causes of appendicitis include carcinoid tumor, foreign body, and parasitic infections. Conditions that cause increased colonic pressure and decreased motility, such as Hirschsprung's disease or meconium ileus, can also produce obstruction of the appendix (5). The mucosa continues to secrete fluid resulting in increased intraluminal pressure within the obstructed appendix. This may result in ischemia, necrosis, and ulceration of the mucosa. Bacterial invasion and invasive infection of the appendix follow. Perforation accompanies complete transmural necrosis, either at the tip or base of the appendix.
In older children, the omentum and adjacent ileum adhere to the inflamed appendix prior to perforation, which results in a localized abscess and prevents widespread intraperitoneal fecal contamination. The omentum is smaller in younger children, resulting in a greater incidence of generalized peritonitis .
Non-obstructive appendicitis rarely occurs and usually resolves without perforation.
Bacterial invasion and infection of the obstructed appendix result in an inflammatory infiltrate, which involves all layers of the appendiceal wall, and a fibrinous serosal exudate. Bacteriologic studies in appendicitis usually show mixed intestinal flora. Enteric bacteria are the most common organisms associated with appendicitis. In patients with perforation, E. coli, Enterococcus sp., Bacteroides sp., and Pseudomonas sp. are most commonly isolated. Anaerobes are commonly found in intraperitoneal abscesses which occur following perforation or surgery.
A possible role of viral illness has been implicated. Viral infections lead to mesenteric adenitis, lymphoid hyperplasia, and, occasionally, fecalith formation due to excessive dehydration (5).
Ascaris has also been reported in association with appendicitis (5).
In the early stages of acute appendicitis, the appendix is grossly thick and turgid, the distal portion is often distended, and the mucosa is ulcerated. Serosal vascularity is increased. Microscopically, inflammatory infiltration of the mucosa is identified, followed by inflammation and necrosis of all layers of the appendiceal wall.
With perforation, cloudy, foul-smelling peritoneal fluid is found from which polymicrobial flora can be cultured. Microscopic examination reveals dense sheets of polymorphonuclear leukocytes; erythrocytes are present within the lumen, muscularis, and the mesoappendix. The site of perforation may be difficult to identify pathologically, particularly if the appendix has been surrounded by the omentum (5).
Histopathologic examination of the resected appendix may reveal both acute and chronic inflammatory changes as well as fibrosis. These findings imply prior episodes of acute appendicitis (5).
Natural History: Clinical Course
In most cases of acute appendicitis, obstruction of the appendiceal lumen persists leading to gangrene and perforation. Rarely, appendicitis may spontaneously resolve(7). It may also recur. It is doubtful that chronic inflammation of the appendix occurs (5).
The risk of perforation is greatest in infants, 80 - 90%, and decreases with age: 74% risk at 1 -4 years, 66% risk at 5-8 years and 30 - 40% risk in adolescents (2,8).
The mean time between the onset of symptoms and diagnosis of nonperforated appendicitis is 36 hours; it is 67 hours for perforated appendicitis. If diagnosis is delayed for more than 36 hours, the perforation rate is 65% (2). This time frame is less useful in children under five years of age, as their disease tends to be more virulent and an accurate history is more difficult to obtain. It is also rare and fortuitous to make a diagnosis of acute appendicitis in infants prior to perforation. The mortality rate in infants with perforated appendicitis is 10% (5).