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.

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.

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.

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.

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

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.

In normal conditions, the peritoneum appears greyish and glistening; it becomes dull 2–4 hours after the onset of peritonitis, initially with scarce serous or slightly turbid fluid. Later on, the exudate becomes creamy and evidently suppurative; in dehydrated patients, it also becomes very inspissated. The quantity of accumulated exudate varies widely. It may be spread to the whole peritoneum, or be walled off by the omentum and viscera. Inflammation features infiltration by neutrophils with fibrino-purulent exudation.

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.

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.

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.

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

The condition is usually caused by Gram-positive enteric commensal bacteria of the gut (gut flora). "Clostridium difficile" is a species of Gram-positive bacteria that commonly causes severe diarrhea and other intestinal diseases when competing bacteria are wiped out by antibiotics, causing pseudomembranous colitis, whereas Clostridium septicum is responsible for most cases of neutropenic enterocolitis.

Typhlitis most commonly occurs in immunocompromised patients, such as those undergoing chemotherapy, patients with AIDS, kidney transplant patients, or the elderly.

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.

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

Stercoral perforation is the perforation or rupture of the intestine's walls by its internal contents, such as foreign objects, or, more commonly, by hardened feces (fecalomas) which may form in long constipations or other diseases which cause obstruction of transit, such as Chagas disease, Hirschprung's disease, toxic colitis and megacolon.

Stercoral perforation is a very dangerous, life-threatening situation, as well as a surgical emergency, because the spillage of contaminated intestinal contents into the abdominal cavity leads to peritonitis, a rapid bacteremia (bacterial infection of the blood), with many complications.

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".

Subphrenic abscess is a disease characterized by an accumulation of infected fluid between the diaphragm, liver, and spleen. This abscess develops after surgical operations like splenectomy.

Presents with cough, increased respiratory rate with shallow respiration, diminished or absent breath sounds, hiccups, dullness in percussion, tenderness over the 8th–11th ribs, fever, chills, anorexia and shoulder tip pain on the affected side. Lack of treatment or misdiagnosis could quickly lead to sepsis, septic shock, and death. It is also associated with peritonitis.

Depending on the level of obstruction, bowel obstruction can present with abdominal pain, swollen abdomen, abdominal distension, vomiting, fecal vomiting, and constipation.

Bowel obstruction may be complicated by dehydration and electrolyte abnormalities due to vomiting; respiratory compromise from pressure on the diaphragm by a distended abdomen, or aspiration of vomitus; bowel ischemia or perforation from prolonged distension or pressure from a foreign body.

In small bowel obstruction, the pain tends to be colicky (cramping and intermittent) in nature, with spasms lasting a few minutes. The pain tends to be central and mid-abdominal. Vomiting may occur before constipation.

In large bowel obstruction, the pain is felt lower in the abdomen and the spasms last longer. Constipation occurs earlier and vomiting may be less prominent. Proximal obstruction of the large bowel may present as small bowel obstruction.

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.

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.

In medicine, Valentino's syndrome is pain presenting in the right lower quadrant of the abdomen caused by a duodenal ulcer with perforation through the retroperitoneum.

It is named after Rudolph Valentino who presented with right lower quadrant pain which turned out to be perforated peptic ulcer. He subsequently died from an infection inspite of surgery to repair the perforation. The pain is caused by gastric and duodenal fluids that tend to settle in the right paracolic gutter causing peritonitis and RLQ pain.

Patients with perforated Valentino's syndrome usually present with a sudden onset of severe, sharp abdominal pain which is reminiscent of appendicitis. Most patients describe generalized pain; a few present with severe epigastric pain. As even slight movement can tremendously worsen their pain, these patients assume a fetal position. Abdominal examination usually discloses generalized tenderness, rebound tenderness, guarding, and rigidity. However, the degree of peritoneal findings is strongly influenced by a number of factors, including the size of perforation, amount of bacterial and gastric contents contaminating the abdominal cavity, time between perforation and presentation, and spontaneous sealing of perforation.

These patients may also demonstrate signs and symptoms of septic shock, such as tachycardia, hypotension, and anuria. Not surprisingly, these indicators of shock may be absent in elderly or immunocompromised patients or in those with diabetes. Patients should be asked if retching and vomiting occurred before the onset of pain.

Pylephlebitis (also called pyelophlebitis and infective suppurative thrombosis of the portal vein) is an uncommon thrombophlebitis of the portal vein or any of its branches (i.e. a portal vein thrombosis) that is caused by infection. It is usually a complication of intraabdominal sepsis, most often following diverticulitis, perforated appendicitis, or peritonitis. Considered uniformly lethal in the pre-antibiotic era, it still carries a mortality of 10-30%.

It typically presents with fever, rigors, and right upper quadrant abdominal pain, but sometimes abdominal pain may be absent. Liver function test abnormalities are usually present but frank jaundice is uncommon. In the modern era, it is usually diagnosed by CT scans of the abdomen and pelvis. Bacteriology is often polymicrobial and blood cultures are positive in some cases. A significant fraction of people presenting with this condition have an underlying hypercoagulable state.

Treatment is with a prolonged course of broad-spectrum antibiotics, with the addition of anticoagulants if other clots are present outside the portal vein or if fever persists on antibiotic therapy.

It is a cause of portal hypertension and can cause bowel ischemia sometimes leading to bowel infarction.

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.

Sclerosing mesenteritis may present with no or nearly no signs or symptoms, but many people have chronic and severe pain in the abdomen as the most common chief complaint. Other people have chronic problems with bowel movements, resulting in diarrhea, bloating, gas, and cramping which can range from severe to mild.

The disorder is identified by histopathology showing fat necrosis, fibrosis and chronic inflammation of the small intestine. Examination of the mesentery may indicate a solitary mass, but diffuse mesentery thickening is common.

It often mimics other abdominal diseases such as pancreatic or disseminated cancer. CT scanning is important for making the initial diagnosis.

Causes of small bowel obstruction include:

- Adhesions from previous abdominal surgery (most common cause)

- Barbed sutures.

- Pseudoobstruction

- Hernias containing bowel

- Crohn's disease causing adhesions or inflammatory strictures

- Neoplasms, benign or malignant

- Intussusception

- Volvulus

- Superior mesenteric artery syndrome, a compression of the duodenum by the superior mesenteric artery and the abdominal aorta

- Ischemic strictures

- Foreign bodies (e.g. gallstones in gallstone ileus, swallowed objects)

- Intestinal atresia

After abdominal surgery, the incidence of small bowel obstruction from any cause is 9%. In those where the cause of the obstruction was clear, adhesions are the single most common cause (more than half).