The main manifestations of peritonitis are acute abdominal pain, abdominal tenderness and abdominal guarding, which are exacerbated by moving the peritoneum, e.g., coughing (forced cough may be used as a test), flexing one's hips, or eliciting the Blumberg sign (a.k.a. rebound tenderness, meaning that pressing a hand on the abdomen elicits less pain than releasing the hand abruptly, which will aggravate the pain, as the peritoneum snaps back into place). Rigidity (involuntary contraction of the abdominal muscles) is the most specific exam finding for diagnosing peritonitis (+ likelihood ratio: 3.9). The presence of these signs in a patient is sometimes referred to as peritonism. The localization of these manifestations depends on whether peritonitis is localized (e.g., appendicitis or diverticulitis before perforation), or generalized to the whole abdomen. In either case, pain typically starts as a generalized abdominal pain (with involvement of poorly localizing innervation of the visceral peritoneal layer), and may become localized later (with the involvement of the somatically innervated parietal peritoneal layer). Peritonitis is an example of an acute abdomen.

Twenty percent of infants born with meconium peritonitis will have vomiting and dilated bowels on x-rays which necessitates surgery.

Meconium peritonitis is sometimes diagnosed on prenatal ultrasound where it appears as calcifications within the peritoneum.

Peritonitis is inflammation of the peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One part or the entire abdomen may be tender. Complications may include shock and acute respiratory distress syndrome.

Causes include perforation of the intestinal tract, pancreatitis, pelvic inflammatory disease, stomach ulcer, cirrhosis, or a ruptured appendix. Risk factors include ascites and peritoneal dialysis. Diagnosis is generally based on examination, blood tests, and medical imaging.

Treatment often includes antibiotics, intravenous fluids, pain medication, and surgery. Other measures may include a nasogastric tube or blood transfusion. Without treatment death may occurs within a few days. Approximately 7.5% of people have appendicitis at some point in time. About 20% of people with cirrhosis who are in hospital have peritonitis.

Meconium peritonitis refers to rupture of the bowel prior to birth, resulting in fetal stool (meconium) escaping into the surrounding space (peritoneum) leading to inflammation (peritonitis). Despite the bowel rupture, many infants born after meconium peritonitis "in utero" have normal bowels and have no further issues.

Infants with cystic fibrosis are at increased risk for meconium peritonitis.

The presentation of acute appendicitis includes abdominal pain, nausea, vomiting, and fever. As the appendix becomes more swollen and inflamed, it begins to irritate the adjoining abdominal wall. This leads to the localization of the pain to the right lower quadrant. This classic migration of pain may not be seen in children under three years. This pain can be elicited through signs and can be severe. Signs include localized findings in the right iliac fossa. The abdominal wall becomes very sensitive to gentle pressure (palpation). There is severe pain on sudden release of deep pressure in the lower abdomen (rebound tenderness). If the appendix is retrocecal (localized behind the cecum), even deep pressure in the right lower quadrant may fail to elicit tenderness (silent appendix). This is because the cecum, distended with gas, protects the inflamed appendix from pressure. Similarly, if the appendix lies entirely within the pelvis, there is usually complete absence of abdominal rigidity. In such cases, a digital rectal examination elicits tenderness in the rectovesical pouch. Coughing causes point tenderness in this area (McBurney's point), historically called Dunphy's sign.

Signs and symptoms may include a sudden pain in the epigastrium to the right of the midline indicating the perforation of a duodenal ulcer. In a gastric ulcer perforation creates a history of burning pain in epigastrium, with flatulence and dyspepsia.

In intestinal perforation, pain starts from the site of perforation and spreads across the abdomen.

Gastrointestinal perforation results in severe abdominal pain intensified by movement, nausea, vomiting and hematemesis. Later symptoms include fever and or chills. In any case, the abdomen becomes rigid with tenderness and rebound tenderness. After some time the abdomen becomes silent and heart sounds can be heard all over. Patient stops passing flatus and motion, abdomen is distended.

The symptoms of esophageal rupture may include sudden onset of chest pain.

Acute abdomen is occasionally used synonymously with peritonitis. While this is not entirely incorrect, peritonitis is the more specific term, referring to inflammation of the peritoneum. It manifests on physical examination as rebound tenderness, or pain upon "removal" of pressure more than on "application" of pressure to the abdomen. Peritonitis may result from several of the above diseases, notably appendicitis and pancreatitis. While rebound tenderness is commonly associated with peritonitis, the most specific finding is rigidity.

Emergency action may be required if severe abdominal pain develops, particularly if it is accompanied by fever, rapid heart rate, tenderness when the abdomen is pressed, bloody diarrhea, frequent diarrhea, or painful bowel movements.

Colonoscopy is contraindicated, as it may rupture the dilated colon resulting in peritonitis and septic shock.

Underlying causes include gastric ulcers, duodenal ulcers, appendicitis, gastrointestinal cancer, diverticulitis, inflammatory bowel disease, superior mesenteric artery syndrome, trauma and ascariasis. Typhoid fever, non-steroidal anti-inflammatory drugs, ingestion of corrosives may also be responsible.

Children: Gastroenteritis, mesenteric adenitis, Meckel's diverticulitis, intussusception, Henoch–Schönlein purpura, lobar pneumonia, urinary tract infection (abdominal pain in the absence of other symptoms can occur in children with UTI), new-onset Crohn's disease or ulcerative colitis, pancreatitis, and abdominal trauma from child abuse; distal intestinal obstruction syndrome in children with cystic fibrosis; typhlitis in children with leukemia.

Women: A pregnancy test is important for all women of childbearing age since an ectopic pregnancy can have signs and symptoms similar to those of appendicitis. Other obstetrical/gynecological causes of similar abdominal pain in women include pelvic inflammatory disease, ovarian torsion, menarche, dysmenorrhea, endometriosis, and Mittelschmerz (the passing of an egg in the ovaries approximately two weeks before menstruation).

Men: testicular torsion

Adults: new-onset Crohn's disease, ulcerative colitis, regional enteritis, cholecystitis, renal colic, perforated peptic ulcer, pancreatitis, rectus sheath hematoma and epiploic appendagitis.

Elderly: diverticulitis, intestinal obstruction, colonic carcinoma, mesenteric ischemia, leaking aortic aneurysm.

The term "" is used to describe a condition mimicking appendicitis. It can be associated with "Yersinia enterocolitica".

The differential diagnoses of acute abdomen include but are not limited to:

1. Acute appendicitis

2. Acute peptic ulcer and its complications

3. Acute cholecystitis

4. Acute pancreatitis

5. Acute intestinal ischemia (see section below)

6. Acute diverticulitis

7. Ectopic pregnancy with tubal rupture

8. Ovarian torsion

9. Acute peritonitis (including hollow viscus perforation)

10. Acute ureteric colic

11. Bowel volvulus

12. Bowel obstruction

13. Acute pyelonephritis

14. Adrenal crisis

15. Biliary colic

16. Abdominal aortic aneurysm

17. Familial Mediterranean fever

18. Hemoperitoneum

19. Ruptured spleen

20. Kidney stone

21. Sickle cell anaemia

Signs and symptoms of spontaneous bacterial peritonitis include fevers, chills, nausea, vomiting, abdominal tenderness, and general malaise. Affected individuals may complain of abdominal pain and worsening ascites. Thirteen percent of patients have no signs or symptoms. Hepatic encephalopathy may be the only manifestation of SBP; in the absence of a clear precipitant for the encephalopathy, all patients should undergo paracentesis, or sampling of the ascites fluid, in order to assess for SBP.

Signs and symptoms of typhlitis may include diarrhea, a distended abdomen, fever, chills, nausea, vomiting, and abdominal pain or tenderness.

Toxic megacolon ("megacolon toxicum") is an acute form of colonic distension. It is characterized by a very dilated colon (megacolon), accompanied by abdominal distension (bloating), and sometimes fever, abdominal pain, or shock.

Toxic megacolon is usually a complication of inflammatory bowel disease, such as ulcerative colitis and, more rarely, Crohn's disease, and of some infections of the colon, including "Clostridium difficile" infections, which have led to pseudomembranous colitis. Other forms of megacolon exist and can be congenital (present since birth, such as Hirschsprung's disease). Also, it can be caused by "Entamoeba histolytica" and "Shigella".

Diverticulitis typically presents with left lower quadrant abdominal pain of sudden onset. There may also be fever, nausea, diarrhea or constipation, and blood in the stool.

The condition is typically seen in premature infants, and the timing of its onset is generally inversely proportional to the gestational age of the baby at birth (i.e. the earlier a baby is born, the later signs of NEC are typically seen). Initial symptoms include feeding intolerance, increased gastric residuals, abdominal distension and bloody stools. Symptoms may progress rapidly to abdominal discoloration with intestinal perforation and peritonitis and systemic hypotension requiring intensive medical support.

Intraabdominal infection (IAI) is a group of infections that occur within the abdominal cavity. They vary from appendicitis to fecal peritonitis. Risk of death despite treatment is often high.

In complicated diverticulitis, an inflamed diverticulum can rupture, allowing bacteria to subsequently infect externally from the colon. If the infection spreads to the lining of the abdominal cavity (the peritoneum), peritonitis results. Sometimes, inflamed diverticula can cause narrowing of the bowel, leading to an obstruction. In some cases, the affected part of the colon adheres to the bladder or other organs in the pelvic cavity, causing a , or creating an abnormal connection between an organ and adjacent structure or other organ (in the case of diverticulitis, the colon and an adjacent organ).

Related pathologies may include:

- Bowel obstruction

- Peritonitis

- Abscess

- Bleeding

- Strictures

Typhlitis is diagnosed with a radiograph CT scan showing thickening of the cecum and "fat stranding".

Spontaneous bacterial peritonitis (SBP) is the development of a bacterial infection in the peritoneum causing peritonitis, despite the absence of an obvious source for the infection. It occurs almost exclusively in people with portal hypertension (increased pressure over the portal vein), usually as a result of cirrhosis of the liver. It can also occur in patients with nephrotic syndrome.

The diagnosis of SBP requires paracentesis (aspiration of fluid with a needle) from the abdominal cavity. If the fluid contains bacteria or large numbers of neutrophil granulocytes (>250 cells/µL) (a type of white blood cells), infection is confirmed and antibiotics are required to avoid complications. In addition to antibiotics, infusions of albumin are usually administered.

Signs and symptoms of enteritis are highly variable and vary based on the specific cause and other factors such as individual variance and stage of disease.

Symptoms may include abdominal pain, cramping, diarrhoea, dehydration, fever, nausea, vomiting and weight loss.

Colic can be divided broadly into several categories:

1. excessive gas accumulation in the intestine (gas colic)

2. simple obstruction

3. strangulating obstruction

4. non-strangulating infarction

5. inflammation of the gastrointestinal tract (enteritis, colitis) or the peritoneum (peritonitis)

6. ulceration of the gastrointestinal mucosa

These categories can be further differentiated based on location of the lesion and underlying cause (See Types of colic).

Regardless of cause, volvulus causes symptoms by two mechanisms:

- Bowel obstruction manifested as abdominal distension and bilious vomiting.

- Ischemia (loss of blood flow) to the affected portion of intestine.

Depending on the location of the volvulus, symptoms may vary. For example, in patients with a cecal volvulus, the predominant symptoms may be those of a small bowel obstruction (nausea, vomiting and lack of stool or flatus), because the obstructing point is close to the ileocecal valve and small intestine. In patients with a sigmoid volvulus, although abdominal pain may be present, symptoms of constipation may be more prominent.

Volvulus causes severe pain and progressive injury to the intestinal wall, with accumulation of gas and fluid in the portion of the bowel obstructed. Ultimately, this can result in necrosis of the affected intestinal wall, acidosis, and death. This is known as a closed loop obstruction because there exists an isolated ("closed") loop of bowel. Acute volvulus often requires immediate surgical intervention to untwist the affected segment of bowel and possibly resect any unsalvageable portion.

Volvulus occurs most frequently in middle-aged and elderly men. Volvulus can also arise as a rare complication in persons with redundant colon, a normal anatomic variation resulting in extra colonic loops.

Sigmoid volvulus is the most-common form of volvulus of the gastrointestinal tract. and is responsible for 8% of all intestinal obstructions. Sigmoid volvulus is particularly common in elderly persons and constipated patients. Patients experience abdominal pain, distension, and absolute constipation.

Cecal volvulus is slightly less common than sigmoid volvulus and is associated with symptoms of abdominal pain and small bowel obstruction.

Volvulus can also occur in patients with Duchenne muscular dystrophy due to the smooth muscle dysfunction.

The diagnosis is suspected based on polyhydramnios in uteru, bilious vomiting, failure to pass meconium in the first day of life, and abdominal distension. The presentations of NBO may vary. It may be subtle and easily overlooked on physical examination or can involve massive abdominal distension, respiratory distress and cardiovascular collapse. Unlike older children, neonates with unrecognized intestinal obstruction deteriorate rapidly.

Necrotizing enterocolitis (NEC) is a medical condition where a portion of the bowel dies. It typically occurs in newborns that are either premature or otherwise unwell. Symptoms may include poor feeding, bloating, decreased activity, blood in the stool, or vomiting of bili.

The exact cause is unclear. Risk factors include congenital heart disease, birth asphyxia, exchange transfusion, and prolonged rupture of membranes. The underlying mechanism is believed to involve a combination of poor blood flow and infection of the inestines. Diagnosis is based on symptoms and confirmed with medical imaging.

Prevention includes the use of breast milk and probiotics. Treatment includes bowel rest, oralgastric tube, intravenous fluids, and intravenous antibiotics. Surgery is required in those who have free air in the abdomen. A number of other supportive measures may also be required. Complications may include short-gut syndrome, intestinal strictures, or developmental delay.

About 7% of those that are born premature develop necrotizing enterocolitis. Onset is typically in the first four weeks of life. Among those affected about 25% die. The sexes are affected equally frequently. The condition was first described in between 1888 and 1891.