The differential diagnoses for paediatric abdominal pain are broad and encompass almost every organ system. In addition, distinguishing acute from chronic abdominal pain may be particularly difficult in children. Although the most common aetiologies are not immediately life-threatening, the ability to diagnose urgent pathology remains paramount. A thorough history and physical examination are required to narrow the differential diagnosis, as well as an understanding of the more common diseases affecting the age group of the child in question.
GI sources are the most common aetiology, encompassing infectious, congenital, and mechanical causes.
Childhood constipation is typically characterised by infrequent bowel evacuations, large stools, and difficult or painful defecation.
Symptoms usually result from low-fibre, poor-nutrient intake, and too little water, which leads to high levels of colonic reabsorption of water and hardening of the stool. Additional risk factors include genetic predisposition, infection, stress, obesity, low birth weight, cerebral palsy, spina bifida, and developmental delay. Constipation starts as an acute problem but can progress to faecal impaction and chronic constipation.
It tends to develop during 3 stages of childhood: weaning (infants), toilet training (toddlers), starting school (older children).
Develops when the appendiceal lumen becomes obstructed by stool, barium, food, or parasites.
Can occur in all age groups, but is rare in infants.
If left untreated, acute appendicitis progresses to ischaemia, necrosis, and eventually perforation. The overall rate of perforation varies from 15.5% to 47%. 
May be due to acute or chronic viral (especially rotavirus), bacterial, or parasitic GI infection.
Eosinophilic gastroenteritis, defined as a condition affecting the GI tract with eosinophil-rich inflammation without a known cause for the eosinophilia, can result in significant abdominal pain. 
Haemolytic uraemic syndrome, characterised by microangiopathic haemolytic anaemia, thrombocytopenia, and nephropathy, can occur as a complication of gastroenteritis caused by verotoxin-producing Escherichia coli . Abdominal pain is a common presenting symptom. 
Occurs when a proximal segment of the intestine telescopes into the lumen of an immediately distal segment.
Most commonly seen in infants between 3 and 18 months of age; reported by some to be the most common cause of intestinal obstruction in infants and children between 3 months and 6 years of age, and the second most common cause of acute abdomen in this age group. 
In most cases, the intussusception is in the ileocaecal area.
In infants <2 years of age, episodes of intussusception are most likely caused by mesenteric lymphadenopathy secondary to an associated illness (e.g., viral gastroenteritis). In older children, mesenteric lymphadenopathy is still the most likely cause, but other aetiologies should be considered (e.g., intestinal lymphomas, Meckel's diverticulum). Therefore, children ≥6 years or with jejuno-jejunal or ileo-ileal intussusception should be assessed for a malignant lead point.
Ileo-ileal intussusception may also be indicative of Henoch-Schonlein purpura (HSP). HSP is a vasculitis that affects small veins and primarily occurs in children <11 years of age.
A finger-like projection located in the distal ileum arising from the anti-mesenteric border; usually 40 to 60 cm from the ileocaecal valve, measuring 1 to 10 cm long and 2 cm wide.
The majority of symptomatic patients present before the age of 2 years.
The prevalence is estimated to be 2% to 3%. 
Intestinal obstruction is a known complication and may be observed in as many as 40% of all symptomatic Meckel's diverticula (according to some series). 
Refers to inflammation of the mesenteric lymph nodes. This process may be acute or chronic. It is often mistaken for other diagnoses, such as appendicitis; up to 20% of patients undergoing negative appendectomy have been found to have non-specific mesenteric adenitis. 
Most commonly diagnosed in the first year of life, but can present later in childhood; slightly higher male preponderance.
Congenital condition characterised by partial or complete colonic obstruction associated with the absence of intramural ganglion cells. Because of the aganglionosis, the lumen is tonically contracted, causing a functional obstruction. The aganglionic portion of the colon is always located distally, but the length of the segment varies.
May be associated with Down's syndrome and multiple endocrine neoplasia type IIA.
Small or large bowel obstruction may be the result of various aetiologies and can occur at any age. Abdominal pain may not occur until the obstruction has progressed to include extensive abdominal distension or intestinal ischaemia. Intestinal obstruction may mimic intestinal ileus, which usually does not require surgical intervention.
The aetiology of intestinal obstruction can be congenital or acquired. Congenital causes include atresias or stenosis, which present in the newborn period.
Duodenal atresia or stenosis may cause complete or partial obstruction of the duodenum as a result of failed re-canalisation during development. This results in either stenosis with incomplete obstruction of the duodenal lumen (allowing some but not all gas and liquid to pass) or an atresia where the duodenum ends blindly causing a true complete obstruction.
Jejuno-ileal atresia or stenosis is a complete or partial obstruction of any part of the jejunum or ileum. Although uncertain, it is believed to result from a vascular accident during development. Jejunal stenosis may still have bowel lumen continuity with a narrowed lumen and thickened muscular layer. There are 4 types of atretic bowel, and all result in a complete obstruction due to a blind-ending lumen.
Colonic atresia is an extremely rare complete obstruction of any part of the colon, although it usually occurs near the splenic flexure. Like jejuno-ileal atresia, it is thought to occur as a result of a vascular event.
Acquired causes of intestinal obstruction include small bowel adhesions, incarcerated or strangulated hernias, or tumours. These can occur at any age. Tumours may be intraluminal or extra-intestinal. Hernias may be internal or external and congenital or acquired. A history of previous intra-abdominal surgery or inflammation (such as necrotising enterocolitis should prompt concern for adhesive small bowel obstruction.
Omental cysts, although rare, can present with intestinal obstruction; may be confused with ovarian cysts on ultrasound.
Meconium ileus is an important cause of intestinal obstruction in the neonatal period; cystic fibrosis should be suspected as an associated disease. There may also be associated pancreatic abnormalities.
Duplication cysts occur most commonly in the small intestine; they may serve as a lead point for volvulus and intussusception and can also result in obstruction. With duodenal duplication cysts, peptic ulcer disease, haemorrhage, or perforation may result secondary to ectopic gastric mucosa.
In patients with cystic fibrosis, partial bowel obstruction may sometimes be referred to as distal intestinal obstruction syndrome (DIOS) or meconium ileus-equivalent syndrome. This entity is not related to meconium. This refers to a distal small bowel obstruction caused by impacted bowel contents; it typically occurs in adolescents and adults with cystic fibrosis.
This can occur in any age group, but is most common in children <1 year old; 60% of children present before 1 month of age.   Mid-gut volvulus is the most common type. Sigmoid volvulus can also occur.
Intestinal malrotation is a term used to encompass the entire spectrum of anatomical arrangements that result from incomplete rotation of the gut during embryonic development. Volvulus of the entire small bowel and part of the colon is only possible when malrotation exists.
In malrotation, the most significant pathological concerns are a lack of gut fixation to the retroperitoneum and narrow mid-gut mesenteric base that predisposes patients to mid-gut volvulus, which occurs when the duodenum or colon twist around this mesenteric base.
A disease primarily of premature infants, particularly those weighing less than 1500 g. The pathogenesis is multi-factorial and not well understood, although ischaemia, reperfusion injury, and infectious pathogens may play a role.
Peptic ulcer disease
Gastric and duodenal ulcers are uncommon among the paediatric population (5/2500 hospitalisations).  When they occur, they are classified as primary or secondary peptic ulcers.
Primary ulcers occur without predisposing factors and are most commonly located in the duodenum or pyloric channel. They manifest most often in older children and adolescents with a positive family history. Rarely, primary peptic ulcers can occur in the first month of life, presenting with bleeding and possible perforation. Most are located in the stomach. Primary ulcers may be associated with Helicobacter pylori .
Secondary ulcers are usually associated with stress, burns, trauma, infection, neonatal hypoxia, chronic illness, and ulcerogenic medications or lifestyle habits (e.g., NSAIDs, salicylates, corticosteroids, smoking, intake of caffeine, nicotine, or alcohol). It is important to treat the predisposing condition. Exacerbations and remissions can last for weeks to months.
Inflammatory bowel disease
This category includes ulcerative colitis (UC) and Crohn's disease (CD).
UC affects the rectum and extends proximally, and is characterised by diffuse inflammation of the colonic mucosa and a relapsing, remitting course. UC is uncommon in people younger than 10 years old.
CD may involve any or all parts of the entire GI tract from mouth to perianal area. Unlike UC, CD is characterised by skip lesions. The transmural inflammation often leads to fibrosis, causing intestinal obstruction. The inflammation can also result in sinus tracts that burrow through and penetrate the serosa, thereafter giving rise to perforations and fistulas. The peak age of onset is between 15 and 40 years.
Cholelithiasis describes the entity of stones in the gallbladder (usually asymptomatic or an incidental finding). Biliary colic refers to the classic description of intermittent, recurrent, right upper quadrant (RUQ) pain that resolves without intervention. This is usually caused by intermittent obstruction of the cystic duct due to cholelithiasis and contraction of a distended gallbladder.
Cholecystitis refers to inflammation of the gallbladder precipitated by obstruction of bile through the cystic duct. Symptoms do not usually resolve spontaneously, and there are specific findings on diagnostic imaging. Cholecystitis may be acalculous (without stones) or calculous (with stones). Choledocholithiasis is the term describing a gallstone(s) in the common bile duct.
Budd-Chiari syndrome (obstruction of hepatic veins) may present very similarly to acute cholecystitis with RUQ pain, and when it occurs it is generally related to hypercoaguable states, prolonged total parenteral nutrition, tumour, infection (e.g., tuberculosis, aspergillosis, filariasis, and echinococcus), or other mechanical causes. It is therefore important to keep in mind as a rare cause of RUQ pain in the correct clinical scenario.
The viral hepatitides include A, B, C, D, and E.
Hepatitis A virus remains a significant aetiology of acute viral hepatitis and jaundice, particularly in developing countries, in travellers to those countries, and in sporadic food-borne outbreaks in the Western world.
Hepatitis B virus (HBV) frequently causes acute hepatitis and is the most common cause of chronic hepatitis in Africa and the Far East.
Hepatitis C virus (HCV) represents the leading cause of chronic viral hepatitis in the Western world.
Hepatitis D virus is a defective virus that needs the presence of hepatitis B to cause clinically recognisable disease.
Hepatitis E virus represents a major cause of mortality in developing countries, especially among pregnant females.
Characterised by symptoms of biliary colic (intermittent, recurrent RUQ pain that resolves without intervention) in the absence of documented stones in the gallbladder; the diagnosis should be considered in those with symptoms suggestive of biliary colic but with negative laboratory tests and ultrasound in their work-up for symptomatic cholelithiasis.
Caused by abnormal or altered contraction of the gallbladder resulting in biliary colic. Patients frequently have gone through a comprehensive work-up prior to being diagnosed with this entity; increasing recognition and testing for the disease has led to more frequent diagnosis in children.
Refers to inflammation of the pancreas; it does not necessarily imply that infection is present.
Excessive alcohol and gallstones are the most common causes of pancreatitis in adults; these causes are relatively less common in children, although they may still occur. There are numerous other aetiologies of pancreatitis in children, which can be categorised as follows:
Choledochal cyst causing abnormal pancreas and bile drainage; pancreas divisum (congenital)
Parasites ( Ascaris lumbricoides )
Tumours (intrinsic or external).
Trauma (blunt or sharp)
Drug induced (e.g., corticosteroids, adrenocorticotropic hormones, oestrogens including contraceptives, azathioprine, asparaginase, tetracycline, chlorothiazides, valproic acid)
End-stage renal disease
Metabolic disorders (e.g., aminoaciduria, hyperlipoproteinaemia types 1 and 5, hypercalcaemia, porphyria).
Collagen vascular diseases
Systemic lupus erythematosus
Paediatric pancreatitis is rare, but the growing population of children with gallstones will likely increase future incidence.
Splenic infarction and cysts
Cysts are classified as either primary or secondary (acquired). Primary cysts are usually congenital and have a true epithelial lining. Eighty percent of splenic cysts are pseudocysts related to infection, infarction, or trauma.  Most cysts are incidental diagnoses, although some patients may present with dull, left-sided abdominal pain. In paediatric patients, the most common splenic masses are congenital and/or acquired cysts. 
Splenic infarction occurs when there is occlusion of the splenic blood supply. It may affect the whole organ or only a portion of the spleen, depending on the blood vessels involved. The incidence of splenic infarction is difficult to assess.
The third leading cause of death in paediatric trauma patients.  It is generally classified as penetrating or blunt. Occult blunt abdominal trauma should always be considered in the setting of vague or inconsistent history. Additionally, it is important to consider child abuse/non-accidental trauma in this patient population (e.g., a kick to the abdomen). The liver, spleen, and kidneys are the most commonly injured intra-abdominal organs in blunt trauma. Most cases of blunt injury to the liver and spleen are managed non-operatively.
It is important to exclude duodenal and/or pancreatic injuries with bicycle handlebar injuries and/or direct blows to the abdomen. Hollow viscous injuries (e.g., stomach and intestines) are more common with penetrating trauma.
Urinary tract infection (UTI)
Infection may arise along any part of the urinary tract including the urethra, bladder, ureter, and kidney. Diagnosis and treatment is paramount to prevent potential long-term side effects, including renal or urinary tract scarring and hypertension.
Bacterial infections are the most common cause, particularly Escherichia coli infection.
Dysmenorrhoea, or painful menstruation, is one of the most common gynaecological conditions affecting females of reproductive age. 
Primary dysmenorrhoea is characterised by menstrual pain in the absence of pelvic pathology.
Refers to stones that may be located anywhere in the genitourinary tract; the majority of stones are noted in the kidneys, followed by the bladder and ureter.
Most patients have a predisposing factor, such as a family history of nephrolithiasis, high-risk diet (e.g., high oxalate intake), chronic disease (e.g., renal tubular acidosis), to name but a few.
Stones less than 5 mm in diameter will generally pass spontaneously.
Considered a urological emergency caused by the twisting of the testicle on the spermatic cord, leading to constriction of the vascular supply and time-sensitive ischaemia and/or necrosis of testicular tissue. 
Has a bimodal distribution, with extravaginal torsion affecting neonates in the perinatal period, and intravaginal torsion affecting males of any age but most commonly adolescent boys. 
Ruptured ovarian cyst
Ovarian cyst rupture is rare and may occur in conjunction with torsion.
Although it can affect females of any age, it most commonly occurs in the early reproductive years.
In children, torsion of the ovary is often associated with the presence of an ovarian tumour, most commonly a teratoma.
Twisting or torsion of the ovary compromises the arterial inflow and venous outflow, producing ischaemia, which, if not relieved promptly, can affect the viability of the ovary.
Pelvic inflammatory disease (PID)
Represents a spectrum of upper genital tract infections that includes any combination of endometritis, salpingitis, pyosalpinx, tubo-ovarian abscess, and pelvic peritonitis; usually caused by Neisseria gonorrhoeae or Chlamydia trachomatis and less commonly by normal vaginal flora including streptococci, anaerobes, and enteric gram-negative rods.
Adolescents are at higher risk of developing PID compared with older women.  STDs are a key risk factor.
PID in a young child should prompt work-up for possible sexual abuse, because it is extremely rare for PID to occur in the absence of sexual activity.
Miscarriage and ectopic pregnancy should be a concern in any female of reproductive age presenting with lower abdominal pain, amenorrhoea, and vaginal bleeding.
Miscarriage is defined as an involuntary, spontaneous loss of a pregnancy before 22 completed weeks.  The majority of spontaneous miscarriages occur in the first trimester with less than 3% occurring in the second trimester.  The aetiology can be divided into embryonic and/or maternal factors, although it is more likely to be multi-factorial.
Ectopic pregnancy is defined as a fertilised ovum implanting and maturing outside of the uterine endometrial cavity, with the most common site being the fallopian tube (97%), followed by the ovary (3.2%), and the abdomen (1.3%).  Age <18 years at first sexual encounter is associated with higher rates of ectopic pregnancy.  Mortality from ectopic pregnancy has declined, but still remains a danger.
Primary respiratory illnesses such as pneumonia or empyema may present as abdominal pain in the paediatric population. Recurrent pneumonia in children is usually the result of a particular susceptibility, such as disorders of immunity and leukocyte function, ciliary function, anatomical abnormalities, or specific genetic disorders such as cystic fibrosis. 
Functional abdominal pain
Also referred to as non-specific abdominal pain; pain is usually chronic or recurrent. Functional abdominal pain disorders are classified according to Rome IV criteria, which describe functional dyspepsia, irritable bowel syndrome, abdominal migraine, and functional abdominal pain-not otherwise specified.  
Typically affects children between 5 and 14 years of age.
Family history of functional disorder common (irritable bowel syndrome, mental illness, migraine, anxiety).
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