Enteropathy-associated T-cell lymphoma (EATL), also enteropathy-type T-cell lymphoma (ETTL), is a type of T-cell lymphoma that affects the small intestine. It is the most common primary gastrointestinal T-cell lymphoma, arising from the T cells that are found between the cells that line the small intestinal (brush border cells or small intestinal epithelial cells). These cancerous T-cells are a possible consequence of refractory cases of coeliac disease or in chronic, untreated cases in genetically susceptible individuals.

EATL can be classified as an extranodal peripheral T cell lymphoma, a category it shares with hepatosplenic T cell lymphoma and panniculitic T cell lymphoma.

It can be further classified in type I and II EATL.

Heavy chain disease is a form of paraproteinemia and plasma cell dyscrasia that involves the proliferation of cells producing immunoglobulin heavy chains.

This disease is characterized by an excessive production of heavy chains that are short and truncated. These heavy chain disease proteins have various deletions, mainly in their amino-terminal part, which causes the heavy chains to lose the ability to form disulfide bonds with the light chains. The defect in the immunoglobulins presumably arises during somatic hypermutation. Deletion of the N-terminal part of the heavy chain disease protein leads to aggregation and signaling of the B cell receptor, presumably due to the loss of the anti-aggregating properties of the light chain.

There are four forms:

- alpha chain disease (Seligmann's disease)

- gamma chain disease (Franklin's disease)

- mu chain disease

- delta chain disease

Microvillus inclusion disease, also known as Davidson's disease, congenital microvillus atrophy and, less specifically, microvillus atrophy (note: microvillus is often misspelled as microvillous), is a rare genetic disorder of the small intestine that is inherited in an autosomal recessive pattern.

Signs and symptoms of the disease include diarrhea, nausea, swelling of the legs, protein-losing enteropathy, immunodeficiency and loss of lymphatic fluid into the intestines. It is usually diagnosed before the patient is 3 years old, but it is sometimes diagnosed in adults.

It is characterized by chronic, intractable diarrhea in new-born infants, starting in the first few days of life.

This results in metabolic acidosis and severe dehydration. Pregnancy and birth are usually normal.

The illness is usually caused by lymphatic vessels that were misshaped at birth, causing obstruction and subsequent enlargement. The condition can also be a result of other illnesses such as constrictive pericarditis and pancreatitis. The disease is diagnosed by doing a biopsy of the affected area. Severity of the disease is then determined by measuring alpha-antitrypsin proteins in a stool sample.

The most common symptoms are diarrhea, abdominal pain, weight loss, and joint pains. The joint pains may be due to migratory non-deforming arthritis, which may occur many years before any digestive tract symptoms develop; they tend to involve the large joints but can occur in any pattern and tend not to damage the joint surface to the point that the joint becomes deformed. Fever and chills occur in a small proportion of people.

In its more advanced form, malabsorption (insufficient absorption of nutrients from the diet) leads to wasting and the enlargement of lymph nodes in the abdomen. Neurological symptoms (discussed below) are more common in those with the severe form of the abdominal disease. Chronic malabsorptive diarrhea leads to the poor absorption of fat, causing steatorrhea (fatty, offensive stool), flatulence, and abdominal distension. Protein-losing enteropathy may also occur, causing depletion of albumin, a blood protein, which may lead to peripheral edema caused by the lowered oncotic pressures.

Hyperpigmentation of the skin occurs in almost half; some also have skin nodules. Various eye problems, such as uveitis, may occur; this is typically associated with deteriorating vision and pain in the affected eye. Endocarditis (infection of the heart valve) has been reported in a small number of cases, sometimes in people with no other symptoms of Whipple's disease; this is typically noticed as breathlessness and leg swelling due to fluid accumulation as the heart is unable to pump fluid through the body.

Of those affected by Whipple's disease, 10–40% of people have problems related to the involvement of the brain; the symptoms relate to the part of the brain that is affected. The most common problems are dementia, memory loss, confusion, and decreased level of consciousness. Eye movement disturbances and myorhythmia (rapidly repetitive movements of the muscles) of the face, together referred to as "oculomasticatory myorhythmia", are highly characteristic for Whipple's disease. Weakness and poor coordination of part of the body, headaches, seizures, as well as a number of more uncommon neurological features, are present in some cases.

CNE is a necrotizing inflammation of the small bowel (especially the jejunum but also the ileum). Clinical results may vary from mild diarrhea to a life-threatening sequence of severe abdominal pain, vomiting, bloody stool, ulceration of the small intestine with leakage (perforation) into the peritoneal cavity and possible death within a single day due to peritonitis. Many patients exhibit meteorism. Treatment involves suppressing the toxin-producing organisms with antibiotics such as penicillin G or metronidazole. About half of seriously ill patients require surgery for perforation, persistent intestinal obstruction, or failure to respond to the antibiotics. An investigational toxoid vaccine has been used successfully in some developing countries but is not available outside of research.

Causes of paraproteinemia include the following:

- Leukemias and lymphomas of various types, but usually B-cell Non-Hodgkin lymphomas with a plasma cell component.

- Myeloma

- Plasmacytoma

- Lymphoplasmacytic lymphoma

- Idiopathic (no discernible cause): some of these will be revealed as leukemias or lymphomas over the years.

- Monoclonal gammopathy of undetermined significance

- Primary AL amyloidosis (light chains only)

Hypergammaglobulinemia is a medical condition with elevated levels of gamma globulin.

It is a type of immunoproliferative disorder.

Clostridial necrotizing enteritis (CNE), also called enteritis necroticans and pigbel, is an often fatal type of food poisoning caused by a β-toxin of "Clostridium perfringens", Type C. It occurs in some developing countries, but was also documented in Germany following World War II. The toxin is normally inactivated by certain proteolytic enzymes and by normal cooking, but when these protections are impeded, the disease emerges.

Hypergammaglobulinemia is a condition that is characterized by the increased levels of a certain immunoglobulin in the blood serum. The name of the disorder refers to an excess of proteins after serum protein electrophoresis (found in the gammaglobulin region).

Most hypergammaglobulinemias are caused by an excess of immunoglobulin M (IgM), because this is the default immunoglobulin type prior to class switching. Some types of hypergammaglobulinemia are actually caused by a deficiency in the other major types of immunoglobulins, which are IgA, IgE and IgG.

There are 5 types of hypergammaglobulinemias associated with hyper IgM.

MeSH considers hyper IgM syndrome to be a form of dysgammaglobulinemia, not a form of hypergammaglobulinemia .

People who have been treated with radiotherapy for pelvic and other abdominal cancers frequently develop gastrointestinal symptoms. These include:

- rectal bleeding

- diarrhea and steatorrhea

- other defecation disorders including fecal urgency and incontinence.

- nutritional deficiencies and weight loss

- abdominal pain and bloating

- nausea, vomiting and fatigue

Gastrointestinal symptoms are often found together with those in other systems including genitourinary disorders and sexual dysfunction. The burden of symptoms substantially impairs the patients' quality of life.

Nausea, vomiting, fatigue and diarrhea may happen early during the course of radiotherapy. Radiation enteropathy represents the longer-term, chronic effects which may be found after a latent period most commonly of 6 months to 3 years after the end of treatment. In some cases, it does not become a problem for 20-30 years after successful curative therapy.

Radiation enteropathy or radiation enteritis is a syndrome that may develop following abdominal or pelvic radiation therapy for cancer. Many affected people are cancer survivors who had treatment for cervical cancer or prostate cancer; it has also been termed pelvic radiation disease with radiation proctitis being one of the principal features.

Whipple's disease is a rare, systemic infectious disease caused by the bacterium "Tropheryma whipplei". First described by George Hoyt Whipple in 1907 and commonly considered a gastrointestinal disorder, Whipple's disease primarily causes malabsorption but may affect any part of the body including the heart, brain, joints, skin, lungs and the eyes. Weight loss, diarrhea, joint pain, and arthritis are common presenting symptoms, but the presentation can be highly variable and approximately 15% of patients do not have these classic signs and symptoms.

Whipple's disease is significantly more common in men, with 87% of the patients being male. When recognized and treated, Whipple's disease can usually be cured with long-term antibiotic therapy; if the disease is left untreated, it is ultimately fatal.

Eosinophilic gastroenteritis (EG) is a rare and heterogeneous condition characterized by patchy or diffuse eosinophilic infiltration of gastrointestinal (GI) tissue, first described by Kaijser in 1937. Presentation may vary depending on location as well as depth and extent of bowel wall involvement and usually runs a chronic relapsing course. It can be classified into mucosal, muscular and serosal types based on the depth of involvement. Any part of the GI tract can be affected, and isolated biliary tract involvement has also been reported.

The stomach is the organ most commonly affected, followed by the small intestine and the colon.

EG typically presents with a combination of chronic nonspecific GI symptoms which include abdominal pain, diarrhea, occasional nausea and vomiting, weight loss and abdominal distension. Approximately 80% have symptoms for several years; a high degree of clinical suspicion is often required to establish the diagnosis, as the disease is extremely rare. It doesn't come all of a sudden but takes about 3–4 years to develop depending upon the age of the patient. Occasionally, the disease may manifest itself as an acute abdomen or bowel obstruction.

- Mucosal EG (25–100%) is the most common variety, which presents with features of malabsorption and protein losing enteropathy. Failure to thrive and anaemia may also be present. Lower gastrointestinal bleeding may imply colonic involvement.

- Muscular EG (13–70%) present with obstruction of gastric outlet or small intestine; sometimes as an obstructing caecal mass or intussusception.

- Subserosal EG (4.5% to 9% in Japan and 13% in the US) presents with ascites which is usually exudative in nature, abundant peripheral eosinophilia, and has favourable responses to corticosteroids.

- Other documented features are cholangitis, pancreatitis, eosinophilic splenitis, acute appendicitis and giant refractory duodenal ulcer.

Paraproteinemia, also known as monoclonal gammopathy, is the presence of excessive amounts of paraprotein or single monoclonal gammaglobulin in the blood. It is usually due to an underlying immunoproliferative disorder or hematologic neoplasms, especially multiple myeloma. It is sometimes considered equivalent to plasma cell dyscrasia.

Many people with Crohn's disease have symptoms for years before the diagnosis. The usual onset is between 15 and 30 years of age, but can occur at any age. Because of the 'patchy' nature of the gastrointestinal disease and the depth of tissue involvement, initial symptoms can be more subtle than those of ulcerative colitis. People with Crohn's disease experience chronic recurring periods of flare-ups and remission.

Abdominal pain may be the initial symptom of Crohn's disease usually in the lower right area. It is often accompanied by diarrhea, especially in those who have had surgery. The diarrhea may or may not be bloody. The nature of the diarrhea in Crohn's disease depends on the part of the small intestine or colon involved. Ileitis typically results in large-volume, watery feces. Colitis may result in a smaller volume of feces of higher frequency. Fecal consistency may range from solid to watery. In severe cases, an individual may have more than 20 bowel movements per day and may need to awaken at night to defecate. Visible bleeding in the feces is less common in Crohn's disease than in ulcerative colitis, but may be seen in the setting of Crohn's colitis. Bloody bowel movements typically come and go, and may be bright or dark red in color. In the setting of severe Crohn's colitis, bleeding may be copious. Flatulence and bloating may also add to the intestinal discomfort.

Symptoms caused by intestinal stenosis are also common in Crohn's disease. Abdominal pain is often most severe in areas of the bowel with stenoses. Persistent vomiting and nausea may indicate stenosis from small bowel obstruction or disease involving the stomach, pylorus, or duodenum. Although the association is greater in the context of ulcerative colitis, Crohn's disease may also be associated with primary sclerosing cholangitis, a type of inflammation of the bile ducts.

Perianal discomfort may also be prominent in Crohn's disease. Itchiness or pain around the anus may be suggestive of inflammation, fistulization or abscess around the anal area or anal fissure. Perianal skin tags are also common in Crohn's disease and may appear with or without the presence of colorectal polyps. Fecal incontinence may accompany perianal Crohn's disease. At the opposite end of the gastrointestinal tract, the mouth may be affected by recurrent sores (aphthous ulcers). Rarely, the esophagus, and stomach may be involved in Crohn's disease. These can cause symptoms including difficulty swallowing (dysphagia), upper abdominal pain, and vomiting.

Crohn's disease, like many other chronic, inflammatory diseases, can cause a variety of systemic symptoms. Among children, growth failure is common. Many children are first diagnosed with Crohn's disease based on inability to maintain growth. As it may manifest at the time of the growth spurt in puberty, up to 30% of children with Crohn's disease may have retardation of growth. Fever may also be present, though fevers greater than 38.5 °C (101.3 °F) are uncommon unless there is a complication such as an abscess. Among older individuals, Crohn's disease may manifest as weight loss, usually related to decreased food intake, since individuals with intestinal symptoms from Crohn's disease often feel better when they do not eat and might lose their appetite. People with extensive small intestine disease may also have malabsorption of carbohydrates or lipids, which can further exacerbate weight loss.

Intestinal neuronal dysplasia (or neuronal intestinal dysplasia or NID) is an inherited disease of the intestine that affects one in 3000 children and adults. The intestine uses peristalsis to push its contents toward the anus; IND sufferers have a problem with the motor neurons that lead to the intestine, inhibiting this process and thus preventing digestion.

It can often be confused for Hirschsprung's disease, as both have similar symptoms.

The illness usually starts with an attack of acute diarrhoea, fever and malaise following which, after a variable period, the patient settles into the chronic phase of diarrhoea, steatorrhoea, weight loss, anorexia, malaise, and nutritional deficiencies.

The symptoms of tropical sprue are:

- Diarrhoea

- Steatorrhoea or fatty stool (often foul-smelling and whitish in colour)

- Indigestion

- Cramps

- Weight loss and malnutrition

- Fatigue

Left untreated, nutrient and vitamin deficiencies may develop in patients with tropical sprue. These deficiencies may have these symptoms:

- Vitamin A deficiency: hyperkeratosis or skin scales

- Vitamin B and folic acid deficiencies: anaemia

- Vitamin D and calcium deficiencies: spasm, bone pain, numbness, and tingling sensation

- Vitamin K deficiency: bruises

Berdon syndrome, also called Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIH syndrome), is an autosomal recessive fatal genetic disorder affecting newborns. In a 2011 study of 227 children with the syndrome, "the oldest survivor [was] 24 years old." The Ann Arbor News reported a five year old survivor at the end of 2015.

It is more prevalent in females, 7 females to 3 males, and is characterized by constipation and urinary retention, microcolon, giant bladder (megacystis), intestinal hypoperistalis, hydronephrosis, and dilated small bowel. The pathological findings consist of an abundance of ganglion cells in both dilated and narrow areas of the intestine. It is a familial disturbance of unknown cause.

Walter Berdon "et al." in 1976 first described the condition in five female infants, two of whom were sisters. All had marked dilatation of the bladder and some had hydronephrosis and the external appearance of prune belly. The infants also had microcolon and dilated small intestines.