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.

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.

Even though anatomically part of the GI tract, diseases of the mouth are often not considered alongside other gastrointestinal diseases. By far the most common oral conditions are plaque-induced diseases (e.g. gingivitis, periodontitis, dental caries). Some diseases which involve other parts of the GI tract can manifest in the mouth, alone or in combination, including:

- Gastroesophageal reflux disease can cause acid erosion of the teeth and halitosis.

- Gardner's syndrome can be associated with failure of tooth eruption, supernumerary teeth, and dentigerous cysts.

- Peutz–Jeghers syndrome can cause dark spots on the oral mucosa or on the lips or the skin around the mouth.

- Several GI diseases, especially those associated with malabsorption, can cause recurrent mouth ulcers, atrophic glossitis, and angular cheilitis (e.g. Crohn's disease is sometimes termed orofacial granulomatosis when it involves the mouth alone).

- Sideropenic dysphagia can cause glossitis, angular cheilitis.

The small intestine consists of the duodenum, jejunum and ileum. Inflammation of the small intestine is called enteritis, which if localised to just part is called duodenitis, jejunitis and ileitis, respectively. Peptic ulcers are also common in the duodenum.

Chronic diseases of malabsorption may affect the small intestine, including the autoimmune coeliac disease, infective Tropical sprue, and congenital or surgical short bowel syndrome. Other rarer diseases affecting the small intestine include Curling's ulcer, blind loop syndrome, Milroy disease and Whipple's disease. Tumours of the small intestine include gastrointestinal stromal tumours, lipomas, hamartomas and carcinoid syndromes.

Diseases of the small intestine may present with symptoms such as diarrhoea, malnutrition, fatigue and weight loss. Investigations pursued may include blood tests to monitor nutrition, such as iron levels, folate and calcium, endoscopy and biopsy of the duodenum, and barium swallow. Treatments may include renutrition, and antibiotics for infections.

In addition to the extent of involvement, people may also be characterized by the severity of their disease.

- "Mild disease" correlates with fewer than four stools daily, with or without blood, no systemic signs of toxicity, and a normal erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP). Mild abdominal pain or cramping may occur. Patients may believe they are constipated when in fact they are experiencing tenesmus, which is a constant feeling of the need to empty the bowel accompanied by involuntary straining efforts, pain, and cramping with little or no fecal output. Rectal pain is uncommon.

- "Moderate disease" correlates with more than four stools daily, but with minimal signs of toxicity. Patients may display anemia (not requiring transfusions), moderate abdominal pain, and low grade fever, .

- "Severe disease", correlates with more than six bloody stools a day or observable massive and significant bloody bowel movement, and evidence of toxicity as demonstrated by fever, tachycardia, anemia or an elevated ESR or CRP.

- "Fulminant disease" correlates with more than ten bowel movements daily, continuous bleeding, toxicity, abdominal tenderness and distension, blood transfusion requirement and colonic dilation (expansion). Patients in this category may have inflammation extending beyond just the mucosal layer, causing impaired colonic motility and leading to toxic megacolon. If the serous membrane is involved, a colonic perforation may ensue. Unless treated, the fulminant disease will soon lead to death.

As ulcerative colitis is believed to have a systemic (i.e., autoimmune) origin, patients may present with comorbidities leading to symptoms and complications outside the colon. The frequency of such extraintestinal manifestations has been reported as anywhere between 6 and 47 percent, and include the following:

- Aphthous ulcer of the mouth

- Ophthalmic

- Iritis or uveitis, which is inflammation of the eye's iris

- Episcleritis

- Musculoskeletal:

- Seronegative arthritis, which can be a large-joint oligoarthritis (affecting one or two joints), or may affect many small joints of the hands and feet

- Ankylosing spondylitis, arthritis of the spine

- Sacroiliitis, arthritis of the lower spine

- Cutaneous (related to the skin):

- Erythema nodosum, which is a panniculitis, or inflammation of subcutaneous tissue involving the lower extremities

- Pyoderma gangrenosum, which is a painful ulcerating lesion involving the skin

- Deep venous thrombosis and pulmonary embolism

- Autoimmune hemolytic anemia

- Clubbing, a deformity of the ends of the fingers.

- Primary sclerosing cholangitis, a distinct disease that causes inflammation of the bile ducts

In spite of Crohn's and UC being very different diseases, both may present with any of the following symptoms: abdominal pain, vomiting, diarrhea, rectal bleeding, severe internal cramps/muscle spasms in the region of the pelvis and weight loss. Anemia is the most prevalent extraintestinal complication of inflammatory bowel disease. Associated complaints or diseases include arthritis, pyoderma gangrenosum, primary sclerosing cholangitis, and non-thyroidal illness syndrome (NTIS). Associations with deep vein thrombosis (DVT) and bronchiolitis obliterans organizing pneumonia (BOOP) have also been reported. Diagnosis is generally by assessment of inflammatory markers in stool followed by colonoscopy with biopsy of pathological lesions.

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.

Many people with gastritis experience no symptoms at all. However, upper central abdominal pain is the most common symptom; the pain may be dull, vague, burning, aching, gnawing, sore, or sharp. Pain is usually located in the upper central portion of the abdomen, but it may occur anywhere from the upper left portion of the abdomen around to the back.

Other signs and symptoms may include the following:

- Nausea

- Vomiting (if present, may be clear, green or yellow, blood-streaked, or completely bloody, depending on the severity of the stomach inflammation)

- Belching (if present, usually does not relieve the pain much)

- Bloating

- Early satiety

- Loss of appetite

- Unexplained weight loss

Intestinal failure is decreased intestinal function such that nutrients, water, and electrolytes are not sufficiently absorbed. Short bowel syndrome is when there is less than of working bowel and is the most common cause of intestinal failure.

The symptoms of short bowel syndrome can include:

- Abdominal pain

- Diarrhea and steatorrhea (oily, bulky stool, which can be malodorous)

- Fluid depletion

- Weight loss and malnutrition

- Fatigue

Persons with short bowel syndrome may have complications caused by malabsorption of vitamins and minerals, such as deficiencies in vitamins A, D, E, K, B (folic acid), and B, calcium, magnesium, iron, and zinc. These may appear as anemia, hyperkeratosis (scaling of the skin), easy bruising, muscle spasms, poor blood clotting, and bone pain.

Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine. Crohn's disease and ulcerative colitis are the principal types of inflammatory bowel disease. It is important to note that not only does Crohn's disease affect the small intestine and large intestine, it can also affect the mouth, esophagus, stomach and the anus whereas ulcerative colitis primarily affects the colon and the rectum.

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.

Achlorhydria or hypochlorhydria refers to states where the production of hydrochloric acid in gastric secretions of the stomach and other digestive organs is absent or low, respectively. It is associated with various other medical problems.

Common causes include "Helicobacter pylori" and NSAIDs. Less common causes include alcohol, cocaine, severe illness and Crohn disease, among others.

Most of the symptoms of BLS are non specific but nevertheless warrant the utmost attention. These include:

- Loss of appetite

- Nausea

- Flatulence

- Diarrhea

- Fullness after a meal

- Fatty stools (steatorrhea)

- Unintentional weight loss

- Generalised weakness

As a result of the concomitant vitamin and mineral deficiencies that occur as a result of the malabsorption associated with BLS patients with advanced cases should be investigated for:

- Vitamin B12 deficiency

- Folate deficiency

- Iron deficiency

- Vitamin E deficiency

Intestinal Connective tissue abnormality may cause Intestinal Desmosis The absence of the tendinous plexus layer was first described in 1998 by Meier-Ruge. Desmosis is implicated in disturbed gut motility.

Normal peristalsis depends upon the interaction between muscles, nerve cells and tendinous connective tissue. A malfunction of any of these leads to intestinal motility disorders.

Desmosis may be congenital (aplastic form) or acquired (atrophic form).

The "aplastic" form is rare. Typical clinical findings are hypoperistalsis, and pseudo-obstruction. These are found in premature infants, associated with low birth weight. The "atrophic" form is more frequent. Inflammation of the muscularis propria releases enzymes including collagenases which destroy the connective tissue of the bowel wall. Primarily newborns and small children are affected, although this manifestation can also be found in adults. The most common location is the colon with a necrotizing enterocolitis as well as Crohn Disease and diverticulitis. If the taenia are also affected, the disease is defined as complete atrophic desmosis, all other forms without involvement of the taenia are referred to as incomplete. Clinically, patients demonstrate chronic constipation.

As proposed by Giuseppe Martucciello, microscopic diagnosis requires laparoscopic intestinal full-thickness biopsies from colon. Histological findings are absence of the tendinous plexus layer and connective tissue fibers in longitudinal and circular muscle layer.

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.

Irrespective of the cause, achlorhydria can result as known complications of bacterial overgrowth and intestinal metaplasia and symptoms are often consistent with those diseases:

- gastroesophageal reflux disease (source needed)

- abdominal discomfort

- early satiety

- weight loss

- diarrhea

- constipation

- abdominal bloating

- anemia

- stomach infection

- malabsorption of food.

- carcinoma of stomach.

Since acidic pH facilitates the absorption of iron, achlorhydric patients often develop iron deficiency anemia.

Acidic environment of stomach helps conversion of pepsinogen into pepsin which is most important to digest the protein into smaller component like complex protein into simple peptides and amino acids inside stomach which is later absorbs by gastro intestinal tract.

Bacterial overgrowth and B12 deficiency (pernicious anemia) can cause micronutrient deficiencies that result in various clinical neurological manifestations, including visual changes, paresthesias, ataxia, limb weakness, gait disturbance, memory defects, hallucinations and personality and mood changes.

Risk of particular infections, such as "Vibrio vulnificus" (commonly from seafood) is increased. Even without bacterial overgrowth, low stomach acid (high pH) can lead to nutritional deficiencies through decreased absorption of basic electrolytes (magnesium, zinc, etc.) and vitamins (including vitamin C, vitamin K, and the B complex of vitamins). Such deficiencies may be involved in the development of a wide range of pathologies, from fairly benign neuromuscular issues to life-threatening diseases.

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.

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.

Blind loop syndrome (BLS), commonly referred to in the literature as small intestinal bacterial overgrowth (SIBO) or bacterial overgrowth syndrome (BOS), is a state that occurs when the normal bacterial flora of the small intestine proliferates to numbers that cause significant derangement to the normal physiological processes of digestion and absorption. In some cases of blind loop syndrome, overgrowth of pathogenic non-commensal bacteria has also been noted. It has long been understood that from birth, and throughout life, large amounts of bacteria reside symbiotically within animal gastrointestinal tracts such as the human gastrointestinal tract. The understanding of this gut flora has even led to novel treatments for bowel irregularity that utilize so called "probiotics" or good bacteria that aid in normal digestion.

The problem of BLS arises when the bacterial colonies residing in the upper gastrointestinal tract begin to grow out of control or are altered in their makeup thereby creating a burden on the normal physiological processes occurring in the small intestine. This results in problems inclusive of but not restricted to vitamin B12 deficiency, fat malabsorption and steatorrhea, fat-soluble vitamin deficiencies and intestinal wall injury.

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.

Enteritis is inflammation of the small intestine. It is most commonly caused by food or drink contaminated with pathogenic microbes. but may have other causes such as NSAIDs, cocaine, radiation therapy as well as autoimmune conditions like Crohn's disease and coeliac disease. Symptoms include abdominal pain, cramping, diarrhoea, dehydration, and fever. Related diseases include inflammation of the stomach (gastritis) and large intestine (colitis).

Duodenitis, jejunitis and ileitis are subtypes of enteritis which are only localised to a specific part of the small intestine. Inflammation of both the stomach and small intestine is referred to as gastroenteritis. Inflammation of related organs of the gastrointestinal system are:

- gastritis

- gastroenteritis

- colitis

- enterocolitis

Risk factors for small intestine cancer include:

- Crohn's disease

- Celiac disease

- Radiation exposure

- Hereditary gastrointestinal cancer syndromes: familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, Peutz-Jeghers syndrome

- Males are 25% more likely to develop the disease

Benign tumours and conditions that may be mistaken for cancer of the small bowel:

- Hamartoma

- Tuberculosis