First described by Smith (1953), and elaborated upon by Cameron et al. (1976), internal pancreatic fistulas can result in pancreatic ascites, mediastinital pseudocysts, enzymatic mediastinitis, or pancreatic pleural effusions, depending on the flow of pancreatic secretions from a disrupted pancreatic duct or leakage from a pseudocyst.

Internal pancreatic fistulas are most commonly caused by disruption of the pancreatic duct due to chronic pancreatitis. The chronic pancreatitis is usually alcoholic in origin in adults, and traumatic in origin in children. They may also be caused by leakage from a pancreatic pseudocyst.

Anterior disruption of a pseudocyst or a pancreatic duct leads to leakage of pancreatic secretions into the free peritoneal cavity, leading to pancreatic ascites. If the duct is disrupted posteriorly, the secretions leak through the retroperitoneum into the mediastinum via the aortic or esophageal hiatus. Once in the mediastinum, the secretions can either be contained in a mediastinal pseudocyst, lead to enzymatic mediastinitis, or, more commonly, leak through the pleura to enter the chest and form a chronic pancreatic pleural effusion.

Causes of high serum-ascites albumin gradient (SAAG or transudate) are:

- Cirrhosis – 81% (alcoholic in 65%, viral in 10%, cryptogenic in 6%)

- Heart failure – 3%

- Hepatic venous occlusion: Budd–Chiari syndrome or veno-occlusive disease

- Constrictive pericarditis

- Kwashiorkor (childhood protein-energy malnutrition)

Causes of low SAAG ("exudate") are:

- Cancer (metastasis and primary peritoneal carcinomatosis) – 10%

- Infection: Tuberculosis – 2% or spontaneous bacterial peritonitis

- Pancreatitis – 1%

- Serositis

- Nephrotic syndrome

- Hereditary angioedema

Other rare causes:

- Meigs syndrome

- Vasculitis

- Hypothyroidism

- Renal dialysis

- Peritoneum mesothelioma

- Abdominal tuberculosis

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.

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.

Ascites is the abnormal buildup of fluid in the abdomen. Technically it is more than 25 mL of fluid in the peritoneal cavity. Symptoms may include increased abdominal size, increased weight, abdominal discomfort, and shortness of breath. Complications can include spontaneous bacterial peritonitis.

In the developed world the most common cause is liver cirrhosis. Other causes include cancer, heart failure, tuberculosis, pancreatitis, and blockage of the hepatic vein. In cirrhosis the underlying mechanism involves high blood pressure in the portal system and dysfunction of blood vessels. Diagnosis is typically based on a examination together with ultrasound or a CT scan. Testing the fluid can help in determining the underlying cause.

Treatment often involves a low salt diet, medication such as diuretics, and draining the fluid. A transjugular intrahepatic portosystemic shunt (TIPS) may be placed but is associated with complications. Effects to treat the underlying cause, such as by a liver transplant may be considered. Of those with cirrhosis, more than half develop ascites in the ten years following diagnosis. Once ascites has developed in this group, average life expectancy is less than three years. The term is from the Greek "askítes" meaning "baglike".

The overall incidence is 0.5 to 1 cases per 100,000 people per year. It is slightly more common in women than men (male:female ratio = 9:11). The median age at presentation is typically about 50 years with a range of 20–25 years.

A peritoneal inclusion cyst is a cyst-like structure that appears in the pelvis due to non neoplastic reactive mesothelial proliferation, often as a consequence of prior episodes of pelvic inflammation, as can occur in pelvic inflammatory disease. It has the potential to mimic ovarian cysts, hydrosalpinx or even malignancy, due to its nonspecific anechoic appearance.

Asbestos is a known cause of peritoneal mesothelioma in humans.

A 1975 study of three small villages in central Cappadocia, Turkey—Tuzköy, Karain and Sarıhıdır—found that peritoneal mesothelioma was causing 50% of all deaths. Initially, this was attributed to erionite, a zeolite mineral with similar properties to asbestos, but detailed epidemiological investigation demonstrated that the substance causes the disease mostly in families with a genetic predisposition to mineral fiber carcinogenesis. The studies are being extended to other parts of the region.

Pneumoperitoneum is (abnormal presence of air or other gas) in the peritoneal cavity, a potential space within the abdominal cavity. The most common cause is a perforated abdominal viscus, generally a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma. A perforated appendix seldom causes a pneumoperitoneum.

In the mid-twentieth century, an "artificial" pneumoperitoneum was sometimes intentionally administered as a treatment for a hiatal hernia. This was achieved by insufflating the abdomen with carbon dioxide. The practice is currently used by surgical teams in order to perform laparoscopic surgery.

Omental infarction, or omental torsion, is an acute vascular disorder which compromises tissue of the greater omentum—the largest peritoneal fold in the abdomen.

As differential diagnoses, a subphrenic abscess, bowel interposed between diaphragm and liver (Chilaiditi syndrome), and linear atelectasis at the base of the lungs can simulate free air under the diaphragm on a chest X-ray.

Lipodermatosclerosis (also known as "chronic panniculitis with lipomembranous changes", "hypodermitis sclerodermiformis", "sclerosing panniculitis", and "stasis panniculitis") is a skin and connective tissue disease. It is a form of lower extremity panniculitis, an inflammation of the layer of fat under the epidermis.

A hematosalpinx from a tubal pregnancy may be associated with pelvic pain and uterine bleeding. A gynecologic ultrasound will show the hematosalpinx. A hematosalpinx from other conditions may be painless but could lead to uterine bleeding.

The condition is rare but serious, and appears in all mammals. It results from leakage of lymph fluid from the thoracic duct (or one of its tributaries). This can result from direct laceration (e.g., from surgery) or from nontraumatic causes. The most common nontraumatic cause is malignancy, especially lymphoma. Less common is left-heart failure, infections, and developmental abnormalities such as Down syndrome and Noonan syndrome.

Given its rarity, there are no established guidelines for the treatment of peritoneal mesothelioma. The modern approach to malignant peritoneal mesothelioma includes cytoreductive surgery, hyperthermic intraperitoneal chemotherapy (HIPEC), intraperitoneal chemotherapy, and intravenous chemotherapy. These are often used in conjunction and in a complementary fashion, and this multifaceted approach has significantly improved outcomes when compared to intravenous chemotherapy alone. For instance, the reported median survival time for patients with stage IV mesothelioma as reported by the American Cancer Society is 12 months; however, with adequate cytoreduction, intraperitoneal, and intravenous chemotherapy combined, some authors report 10-year survival rates projected at nearly 75%.

Multiple factors have been shown to be significant in predicting the outcome and overall survival. Age greater than 60 at surgery, more overall disease burden (defined as a PCI greater than 15), complete cytoreduction (no visible disease), and epitheliod subtype pathology have all been shown to be predictors of both mortality and disease progression. These known predictors notwithstanding, many patients with advanced peritoneal mesothelioma are still surgical candidates, and even patients with the highest possible score on the peritoneal carcinomatosis index (39) can be completely reduced to a PCI of 0 with adequate surgery.

Neutrophilic lobular panniculitis is a cutaneous condition characterized by inflammation of the subcutaneous fat.

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.

A number of causes may account for a hematosalpinx, by far the most common being a tubal pregnancy. Blood may also escape into the peritoneal cavity leading to a hemoperitoneum.

A hematosalpinx can also be associated with endometriosis or tubal carcinoma. Further, if menstrual blood flow is obstructed (cryptomenorrhea), caused for instance by a transverse vaginal septum, and gets backed up it may lead to a hematosalpinx.

EN is associated with a wide variety of conditions, including:

- Idiopathic

In about 30–50% of cases, the cause of EN is unknown.

- Infection

- Streptococcal infection which, in children, is by far the most common precipitant,

- Primary infection of Tuberculosis

- "Mycoplasma pneumoniae"

- "Histoplasma capsulatum"

- "Yersinia"

- Epstein-Barr virus

- "Coccidioides immitis" (Valley fever)

- Cat scratch disease

- Autoimmune disorders, including

- Inflammatory bowel disease (IBD)

- Behçet's disease

- Sarcoidosis

- Pregnancy

- Medications, including

- Sulfonamides

- Penicillins

- Oral contraceptives

- Bromides

- Hepatitis B vaccination

- Cancer, including

- Non-Hodgkins lymphoma (NHL)

- Carcinoid tumours

- Pancreatic cancer

EN may also be due to excessive antibody production in lepromatous leprosy leading to deposition of immune complexes.

There is an association with the HLA-B27 histocompatibility antigen, which is present in 65% of patients with erythema nodosum.

A useful mnemonic for causes is SORE SHINS (Streptococci, OCP, Rickettsia, Eponymous (Behçet), Sulfonamides, Hansen's Disease (Leprosy), IBD, NHL, Sarcoidosis.

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 ).

Pseudomyxoma peritonei (PMP) is a clinical condition caused by cancerous cells (mucinous adenocarcinoma) that produce abundant mucin or gelatinous ascites. The tumors cause fibrosis of tissues and impede digestion or organ function, and if left untreated, the tumors and mucin they produce will fill the abdominal cavity. This will result in compression of organs and will destroy the function of colon, small intestine, stomach, or other organs. Prognosis with treatment in many cases is optimistic, but the disease is lethal if untreated, with death by cachexia, bowel obstruction, or other types of complications.

This disease is most commonly caused by an appendiceal primary cancer (cancer of the appendix); mucinous tumors of the ovary have also been implicated, although in most cases ovarian involvement is favored to be a metastasis from an appendiceal or other gastrointestinal source. Disease is typically classified as low- or high-grade (with signet ring cells). When disease presents with low-grade histologic features the cancer rarely spreads through the lymphatic system or through the bloodstream.

Meigs syndrome may mimic other conditions, since it is tumor arising from ovaries, pathology of any organs present in the abdomen may show a similar set of symptoms. Various gynecological disorders of the uterus such as endometrial tumor, sarcoma, leiomyoma (pseudo-Meigs syndrome); fallopian tube disorders such as hydrosalpinx, granulomatous salpingitis, fallopian tube malignancy; ovarian disorders such as serous, mucinous, endometrioid, or clear cell carcinoma, Brenner tumor, granulosa cell tumor, stromal tumor, dysgerminoma, fibroma, or metastatic tumor to the ovary.

Meigs syndrome is characterized by the presence of a benign solid ovarian tumor associated with ascites and right hydrothorax that disappear after tumor removal. Non-gynecological manifestations include:

ascites, portal vein obstruction, inferior vena cava obstruction, hypoproteinaemia, thoracic duct obstruction, tuberculosis, amyloidosis, pancreatitis, ovarian hyperstimulation, pleural effusion transudative, congestive heart failure, metastatic tumors to the peritoneal surfaces, collagen-vascular disease, and cirrhosis of the liver.

These entities must be clinically excluded.

Clinical condition characterized by ovarian mass, ascites, and right-sided pleural effusion.Ovarian malignancy and the

other causes (see “Differential Diagnosis”) of pelvic mass, ascites, and pleural effusion to be considered, History of early satiety,

weight loss with increased abdominal girth, bloating, intermittent abdominal pain, dyspnea, nonproductive cough may help in differentiating potential local factor causing such symptoms.

Patients will present with a sudden onset of cramps/abdominal pain or a 'stitch'. The pain localises in the area of the umbilicus and can radiate to the lumbar and surrounding regions.

The exact cause of lipodermatosclerosis is unknown. Venous disease, such as venous incompetence, venous hypertension, and body mass ("obesity") may be relevant to the underlying pathogenesis.

Increased blood pressure in the veins (venous hypertension) can cause diffusion of substances, including fibrin, out of capillaries. Fibrotic tissue may predispose the tissue to ulceration. Recurrent ulceration and fat necrosis is associated with lipodermatosclerosis. In advanced lipodermatosclerosis the proximal leg swells from chronic venous obstruction and the lower leg shrinks from chronic ulceration and fat necrosis resulting in the inverted coke bottle appearance of the lower leg.

Lipodermatosclerosis is most commonly diagnosed in middle-aged women.

The origin of lipodermatosclerosis is probably multifactorial, involving tissue hypoxia, leakage of proteins into the interstitium, and leukocyte activation. Studies of patients with lipodermatosclerosis have demonstrated significantly decreased concentrations of cutaneous oxygen associated with decreased capillary density. Capillaries are virtually absent in areas of fibrotic scars, leading to a condition known as atrophie blanche or livedoid vasculopathy.