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

Acute appendicitis seems to be the end result of a primary obstruction of the appendix. Once this obstruction occurs, the appendix becomes filled with mucus and swells. This continued production of mucus leads to increased pressures within the lumen and the walls of the appendix. The increased pressure results in thrombosis and occlusion of the small vessels, and stasis of lymphatic flow. At this point spontaneous recovery rarely occurs. As the occlusion of blood vessels progresses, the appendix becomes ischemic and then necrotic. As bacteria begin to leak out through the dying walls, pus forms within and around the appendix (suppuration). The end result is appendiceal rupture (a 'burst appendix') causing peritonitis, which may lead to sepsis and eventually death. These events are responsible for the slowly evolving abdominal pain and other commonly associated symptoms.

The causative agents include bezoars, foreign bodies, trauma, intestinal worms, lymphadenitis and, most commonly, calcified fecal deposits that are known as appendicoliths or fecoliths. The occurrence of obstructing fecaliths has attracted attention since their presence in people with appendicitis is higher in developed than in developing countries. In addition an appendiceal fecalith is commonly associated with complicated appendicitis. Fecal stasis and arrest may play a role, as demonstrated by people with acute appendicitis having fewer bowel movements per week compared with healthy controls.

The occurrence of a fecalith in the appendix was thought to be attributed to a right-sided fecal retention reservoir in the colon and a prolonged transit time. However, a prolonged transit time was not observed in subsequent studies. From epidemiological data, it has been stated that diverticular disease and adenomatous polyps were unknown and colon cancer exceedingly rare in communities exempt from appendicitis. And acute appendicitis has been shown to occur antecedent to cancer in the colon and rectum. Several studies offer evidence that a low fiber intake is involved in the pathogenesis of appendicitis. This low intake of dietary fiber is in accordance with the occurrence of a right-sided fecal reservoir and the fact that dietary fiber reduces transit time.

Most people with appendicitis recover easily after surgical treatment, but complications can occur if treatment is delayed or if peritonitis occurs. Recovery time depends on age, condition, complications, and other circumstances, including the amount of alcohol consumption, but usually is between 10 and 28 days. For young children (around 10 years old), the recovery takes three weeks.

The possibility of peritonitis is the reason why acute appendicitis warrants speedy evaluation and treatment. People with suspected appendicitis may have to undergo a medical evacuation. Appendectomies have occasionally been performed in emergency conditions (i.e., not in a proper hospital), when a timely medical evacuation was impossible.

Typical acute appendicitis responds quickly to appendectomy and occasionally will resolve spontaneously. If appendicitis resolves spontaneously, it remains controversial whether an elective interval appendectomy should be performed to prevent a recurrent episode of appendicitis. Atypical appendicitis (associated with suppurative appendicitis) is more difficult to diagnose and is more apt to be complicated even when operated early. In either condition, prompt diagnosis and appendectomy yield the best results with full recovery in two to four weeks usually. Mortality and severe complications are unusual but do occur, especially if peritonitis persists and is untreated.

Another entity known as appendicular lump is talked about. It happens when the appendix is not removed early during infection and omentum and intestine adhere to it, forming a palpable lump. During this period, surgery is risky unless there is pus formation evident by fever and toxicity or by USG. Medical management treats the condition.

An unusual complication of an appendectomy is "stump appendicitis": inflammation occurs in the remnant appendiceal stump left after a prior incomplete appendectomy. Stump appendicitis can occur months to years after initial appendectomy and can be identified with imaging modalities like ultrasound.

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.

An acute abdomen refers to a sudden, severe abdominal pain. It is in many cases a medical emergency, requiring urgent and specific diagnosis. Several causes need surgical treatment.

The causes of diverticulitis are poorly understood, with approximately 40 percent due to genes and 60 percent due to environmental factors. Conditions that increase the risk of developing diverticulitis include arterial hypertension and immunosuppression. Obesity is another risk factor.

If properly treated, typical cases of surgically correctable peritonitis (e.g., perforated peptic ulcer, appendicitis, and diverticulitis) have a mortality rate of about <10% in otherwise healthy patients. The mortality rate rises to about 40% in the elderly, or in those with significant underlying illness, as well as cases that present late (after 48 hours).

Without being treated, generalised peritonitis almost always causes death. The stage magician Harry Houdini died this way, having contracted streptococcus peritonitis after his appendix ruptured and was removed too late to prevent spread of the infection.

It is unclear what role dietary fibre plays in diverticulitis. It is often stated that a diet low in fibre is a risk factor; however, the evidence to support this is unclear. There is no evidence to suggest that the avoidance of nuts and seeds prevents the progression of diverticulosis to an acute case of diverticulitis. It appears in fact that a higher intake of nuts and corn could help to avoid diverticulitis in adult males.

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.

If the condition does not improve, the risk of death is significant. In case of poor response to conservative therapy, a colectomy is usually required.

In the United States, the annual incidence is 18 cases of acute pancreatitis per 100,000 population, and it accounts for 220,000 hospitalizations in the US. In a European cross-sectional study, incidence of acute pancreatitis increased from 12.4 to 15.9 per 100,000 annually from 1985 to 1995; however, mortality remained stable as a result of better outcomes. Another study showed a lower incidence of 9.8 per 100,000 but a similar worsening trend (increasing from 4.9 in 1963-74) over time.

In Western countries, the most common cause is alcohol, accounting for 65 percent of acute pancreatitis cases in the US, 20 percent of cases in Sweden, and 5 percent of those in the United Kingdom. In Eastern countries, gallstones are the most common cause of acute pancreatitis. The causes of acute pancreatitis also varies across age groups, with trauma and systemic disease (such as infection) being more common in children. Mumps is a more common cause in adolescents and young adults than in other age groups.

Locoregional complications include pancreatic pseudocyst (Most common, occurring in up to 25% of all cases) and phlegmon / abscess formation, splenic artery pseudoaneurysms, hemorrhage from erosions into splenic artery and vein, thrombosis of the splenic vein, superior mesenteric vein and portal veins (in descending order of frequency), duodenal obstruction, common bile duct obstruction, progression to chronic pancreatitis, pancreatic ascites, pleural effusion, sterile/infected pancreatic necrosis.

Specific causes of colic are best managed with certain drugs. These include:

- Spasmolytic agents, most commonly Buscopan, especially in the case of gas colic.

- Pro-motility agents: metoclopramide, lidocaine, bethanechol, and erythromycin are used in cases of ileus.

- Anti-inflammatories are often used in the case of enteritis or colitis.

- Anti-microbials may be administered if an infectious agent is suspected to be the underlying cause of colic.

- Phenylephrine: used in cases of nephrosplenic entrapment to contract the spleen, and is followed by light exercise to try to shift the displaced colon back into its normal position.

- Psyllium may be given via nasogastric tube to treat sand colic.

- Anthelminthics for parasitic causes of colic.

Cholecystitis occurs when the gallbladder becomes inflamed. Gallstones are the most common cause of gallbladder inflammation but it can also occur due to blockage from a tumor or scarring of the bile duct. The greatest risk factor for cholecystitis is gallstones. Risk factors for gallstones include female sex, increasing age, pregnancy, oral contraceptives, obesity, diabetes mellitus, ethnicity (Native North American), rapid weight loss.

In the case of colics requiring surgery, survival rates are best improved by quick recognition of colic and immediate surgical referral, rather than waiting to see if the horse improves, which only increases the extent of intestinal compromise. Survival rates are higher in surgical cases that do not require resection and anastomosis. 90% of large intestinal colic surgeries that are not due to volvulus, and 20–80% of large colon volvuluses, are discharged; while 85–90% of non strangulating small intestinal lesions, and 65–75% of strangulating intestinal lesions are discharged. 10–20% of small intestinal surgical cases require a second surgery, while only 5% of large intestinal cases do so. Horses that survive colic surgery have a high rate of return to athletic function. According to one study, approximately 86% of horses discharged returned to work, and 83.5% returned to same or better performance.

The outlook is generally based on the severity of the infection. It is however a severe complication which may result in the death of the patient if the appropriate treatment is not administered. Patients are at risk of sepsis and multiple organ failure and in cases in which the infected abscess is not removed through surgery, the mortality rate can reach 100%.

Untreated cholecystitis can lead to worsened inflammation and infected bile that can lead to a collection of pus surrounding the gallbladder, also known as empyema. The symptoms of empyema are similar to uncomplicated choleystitis but greater severity: high fever, severe abdominal pain, more severely elevated white blood count.

Cholesterol gallstone formation risk factors include age, female sex, family history, race, pregnancy, parity, obesity, birth control, diabetes mellitus, cirrhosis, prolonged fasting, rapid weight loss, total parenteral nutrition, ileal disease and impaired gallbladder emptying.

Patients that have gallstones and biliary colic are at increased risk for complications, including cholecystitis. Complications from gallstone disease is 0.3% per year and therefore prophylactic cholecystectomy are rarely indicated unless part of a special population that includes porcelain gallbladder, individuals eligible for organ transplant, diabetics and those with sickle cell anemia.

Ileus is a cause of colic in horses due to functional obstruction of the intestines. It most commonly seen in horses postoperatively, especially following colic surgery. Horses experiencing ileus are at risk for gastric rupture due to rapid reflux build-up, and require intense medical management with frequent nasogastric intubation. Ileus may increase adhesion formation, because intestinal segments have more prolonged contact and intestinal distention causes serosal injury and ischemia. It is usually treated with aggressive fluid support, prokinetics, and anti-inflammatories.

The presence of gallstones can lead to inflammation of the gall bladder (cholecystitis) or the biliary tree (cholangitis) or acute inflammation of the pancreas (pancreatitis). Rarely, a gallstone can become impacted in the ileocecal valve that joins the caecum and the ileum, causing gallstone ileus (mechanical ileus).

Complications from delayed surgery include pancreatitis, empyema, and perforation of the gallbladder, cholecystitis, cholangitis, and obstructive jaundice.

Biliary pain in the absence of gallstones, known as postcholecystectomy syndrome, may severely impact the patient's quality of life, even in the absence of disease progression.

In some cases, abscesses may be prevented by draining an existing pseudocyst which is likely to become inflamed. However, in most cases the developing of abscesses cannot be prevented.

The condition is diagnosed most often in infancy and early childhood. It strikes about 2,000 infants (one in every 1,900) in the United States in the first year of life. Its incidence begins to rise at about one to five months of life, peaks at four to nine months of age, and then gradually declines at around 18 months.

Intussusception occurs more frequently in boys than in girls, with a ratio of approximately 3:1.

In adults, intussusception represents the cause of approximately 1% of bowel obstructions and is frequently associated with neoplasm, malignant or otherwise.

Causes of intussusception are not clearly established or understood. About 90% of cases of intussusception in children arise from an unknown cause. They can include infections, anatomical factors, and altered motility.

- Meckel's diverticulum

- Polyp

- Duplication

- Appendix

- Hyperplasia of Peyer's patches

- Idiopathic

An earlier version of the rotavirus vaccine that is no longer used was linked to intussusception, but the current versions are not clearly linked. Due to a potential risk, they are thus not recommended in babies who have had intussusception.

In adults in the United Kingdom, the estimated average total direct and indirect costs of chronic pancreatitis is roughly £79,000 per person on an annual basis. Acute recurrent pancreatitis and chronic pancreatitis occur infrequently in children, but are associated with high healthcare costs due to substantial disease burden. Globally, the estimated average total cost of treatment for children with these conditions is approximately $40,500 annually.

Surgical decompression can be achieved by opening the abdominal wall and abdominal fascia anterior in order to physically create more space for the abdominal viscera. Once opened, the fascia can be bridged for support and to prevent loss of domain by a variety of medical devices (Bogota bag, artificial bur, and vacuum devices using negative pressure wound therapy ).