Symptoms of ileus include, but are not limited to:

- moderate, diffuse abdominal discomfort

- constipation

- abdominal distension

- nausea/vomiting, especially after meals

- vomiting of bilious fluid

- lack of bowel movement and/or flatulence

- excessive belching

Clinical features of intestinal pseudo-obstruction can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. In addition, in the moments in which abdominal colic occurs, an abdominal x-ray shows intestinal air fluid level. All of these features are also similar in true mechanical obstruction of the bowel.

Decreased propulsive ability may be broadly classified as caused either by bowel obstruction or intestinal atony or paralysis. However, instances with symptoms and signs of a bowel obstruction occur, but with the absence of a mechanical obstruction, mainly in acute colonic pseudo-obstruction, Ogilvie's syndrome.

Diverticular disease can present with painless rectal bleeding as bright red blood per rectum. Diverticular bleeding is the most common cause of acute lower gastrointestinal bleeding. However, it is estimated that 80% of these cases are self-limiting and require no specific therapy.

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.

Intestinal pseudo-obstruction is a clinical syndrome caused by severe impairment in the ability of the intestines to push food through. It is characterized by the signs and symptoms of intestinal obstruction without any lesion in the intestinal lumen. Clinical features can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. The condition can begin at any age and it can be a primary condition (idiopathic or inherited) or caused by another disease (secondary).

It can be chronic or acute.

Some people with diverticulosis complain of symptoms such as cramping, bloating, flatulence, and irregular defecation. However, it is unclear if these symptoms are attributable to the underlying diverticulosis or to coexistent irritable bowel syndrome.

Diverticular disease was found associated with a higher risk of left sided colon cancer.

People who develop microscopic colitis are characteristically, though not exclusively, middle-aged females. The average age of diagnosis is 65 but 25% of cases are diagnosed below the age of 45. Patients have a history of non-bloody watery diarrhoea, which may be profuse. Patients may also experience abdominal pain, fecal incontinence, and weight loss. Microscopic colitis is the diagnosis in around 10% of cases investigated for chronic non-bloody diarrhea.

Colonoscopic appearances are normal or near normal. As the changes are often patchy, an examination limited to the rectum may miss cases of microscopic colitis, and so a full colonoscopy is necessary. Multiple colonic biopsies are taken in order to make the diagnosis. Histological features of colonic biopsies indicating microscopic colitis are: greater than 20 intraepithelial lymphocytes per 100 epithelial cells and, additionally, 10-20 μm of a thickened subepithelial collagen band in collagenous colitis. Inflammation of the lamina propria, with mainly mononuclear cells, may be observed in collagenous colitis.

Differential diagnoses, which should be ruled out, include celiac disease, Crohn's disease, ulcerative colitis and infectious colitis.

External signs and symptoms are constipation of very long duration, abdominal bloating, abdominal tenderness and tympany, abdominal pain, palpation of hard fecal masses and, in toxic megacolon, fever, low blood potassium, tachycardia and shock. Stercoral ulcers are sometimes observed in chronic megacolon, which may lead to perforation of the intestinal wall in approximately 3% of the cases, leading to sepsis and risk of death.

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.

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).

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

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.

Microscopic colitis causes chronic watery diarrhea with greater than 10 bowel movements per day. Some patients report nocturnal diarrhea, abdominal pain, urgency, fecal incontinence, fatigue, dehydration and weight loss. Patients report a significantly diminished quality of life.

Diseases that affect the large intestine may affect it in whole or in part. Appendicitis is one such disease, caused by inflammation of the appendix. Generalised inflammation of the large intestine is referred to as colitis, which when caused be the bacteria "Clostridium difficile" is referred to as pseudomembranous colitis. Diverticulitis is a common cause of abdominal pain resulting from outpouchings that particularly affects the colon. Functional colonic diseases refer to disorders without a known cause, and include irritable bowel syndrome and intestinal pseudoobstruction. Constipation may result from lifestyle factors, impaction of a rigid stool in the rectum, or in neonates, Hirschprung's disease.

Diseases affecting the large intestine may cause blood to be passed with stool, may cause constipation, or may result in abdominal pain or a fever. Tests that specifically examine the function of the large intestine include barium swallows, abdominal x-rays, and colonoscopy.

Megacolon is an abnormal dilation of the colon (also called the large intestine). The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas (literally, "fecal tumor"), which can require surgery to be removed.

A human colon is considered abnormally enlarged if it has a diameter greater than 12 cm in the cecum (it is usually less than 9 cm), greater than 6.5 cm in the rectosigmoid region and greater than 8 cm for the ascending colon. The transverse colon is usually less than 6 cm in diameter.

A megacolon can be either acute or chronic. It can also be classified according to cause.

Regardless of cause, volvulus causes symptoms by two mechanisms:

- Bowel obstruction manifested as abdominal distension and bilious vomiting.

- Ischemia (loss of blood flow) to the affected portion of intestine.

Depending on the location of the volvulus, symptoms may vary. For example, in patients with a cecal volvulus, the predominant symptoms may be those of a small bowel obstruction (nausea, vomiting and lack of stool or flatus), because the obstructing point is close to the ileocecal valve and small intestine. In patients with a sigmoid volvulus, although abdominal pain may be present, symptoms of constipation may be more prominent.

Volvulus causes severe pain and progressive injury to the intestinal wall, with accumulation of gas and fluid in the portion of the bowel obstructed. Ultimately, this can result in necrosis of the affected intestinal wall, acidosis, and death. This is known as a closed loop obstruction because there exists an isolated ("closed") loop of bowel. Acute volvulus often requires immediate surgical intervention to untwist the affected segment of bowel and possibly resect any unsalvageable portion.

Volvulus occurs most frequently in middle-aged and elderly men. Volvulus can also arise as a rare complication in persons with redundant colon, a normal anatomic variation resulting in extra colonic loops.

Sigmoid volvulus is the most-common form of volvulus of the gastrointestinal tract. and is responsible for 8% of all intestinal obstructions. Sigmoid volvulus is particularly common in elderly persons and constipated patients. Patients experience abdominal pain, distension, and absolute constipation.

Cecal volvulus is slightly less common than sigmoid volvulus and is associated with symptoms of abdominal pain and small bowel obstruction.

Volvulus can also occur in patients with Duchenne muscular dystrophy due to the smooth muscle dysfunction.

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.

On colonoscopy, the mucosa of the colon typically looks normal, but biopsies of affected tissue usually show deposition of collagen in the lamina propria, which is the area of connective tissue between colonic glands. Radiological tests, such as a barium enema are also typically normal.

Microscopic colitis refers to two related medical conditions which cause diarrhea: collagenous colitis and lymphocytic colitis. Both conditions are characterized by the presence of chronic non-bloody watery diarrhea, normal appearances on colonoscopy and characteristic histopathology findings of inflammatory cells.

Usually the patient has abdominal distention, pain and altered bowel movements. There may also be nausea and vomiting.

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".

Neonatal bowel obstruction (NBO) or neonatal intestinal obstruction is the most common surgical emergency in the neonatal period. It may occur due to a variety of conditions and has an excellent outcome based on timely diagnosis and appropriate intervention.

Midgut volvulus occurs in people (usually babies) that are predisposed because of congenital intestinal malrotation. Segmental volvulus occurs in people of any age, usually with a predisposition because of abnormal intestinal contents (e.g. meconium ileus) or adhesions. Volvulus of the cecum, transverse colon, or sigmoid colon occurs, usually in adults, with only minor predisposing factors such as redundant (excess, inadequately supported) intestinal tissue and constipation.

The diagnosis is suspected based on polyhydramnios in uteru, bilious vomiting, failure to pass meconium in the first day of life, and abdominal distension. The presentations of NBO may vary. It may be subtle and easily overlooked on physical examination or can involve massive abdominal distension, respiratory distress and cardiovascular collapse. Unlike older children, neonates with unrecognized intestinal obstruction deteriorate rapidly.