The exact cause of Crohn's disease is not well understood, but it is thought to result from a combination of genetic, environmental, and immune factors:

• Genetic factors:
  • Family history: People with a family history of Crohn's disease or other forms of IBD (ulcerative colitis) are at a higher risk. About 15-20% of people with Crohn’s disease have a first-degree relative (parent or sibling) with the disease.
  • Specific genes, like those involved in the immune system (e.g., NOD2 gene), have been linked to an increased risk of developing Crohn's disease.
• Immune system dysfunction: Crohn's disease is considered an autoimmune disease. The immune system attacks the body’s own tissues in the GI tract, mistaking normal gut bacteria for harmful invaders. This results in chronic inflammation. The immune system’s response is thought to be triggered by an environmental factor, such as an infection, but it ultimately results in an overactive immune response.
• Environmental factors:
  • Diet: Though not directly causing Crohn’s disease, certain diets high in fat and sugar may contribute to flare-ups. However, no specific diet has been definitively linked to Crohn's onset.
  • Smoking: Smoking increases the risk of developing Crohn's disease and can worsen the course of the disease.
  • Geography: Crohn's disease is more common in developed countries, particularly in North America, Europe, and parts of Australia. It is less common in rural areas and in developing countries, though its incidence is rising in these regions.
• Infections: Some studies suggest infections might trigger the onset or exacerbate the disease, although they are not a direct cause.
• Age: Crohn's disease is often diagnosed in young adults, typically between the ages of 15 and 35. However, it can occur at any age.

Symptoms can vary greatly depending on the severity of inflammation and the specific part of the GI tract involved. The most common symptoms include:

• Abdominal pain: Often felt in the lower right quadrant of the abdomen, especially if the ileum is affected. Pain may be crampy and comes and goes.
• Diarrhoea: Chronic, watery diarrhoea is a hallmark symptom. This may be accompanied by blood or mucus, particularly during flare-ups.
• Weight loss: Due to malabsorption of nutrients and a reduced appetite during flare-ups, weight loss is common, even if food intake is normal.
• Fatigue: People with Crohn's disease often experience chronic fatigue, which may be related to inflammation, anaemia, or malnutrition.
• Blood in stools: Rectal bleeding or passing blood in the stool is more common when the colon is involved, although it can occur if other areas are affected.
• Fever: During active flare-ups, fever may be present, especially if there is significant inflammation or infection.
• Nausea and vomiting: This can occur, particularly if there is a blockage or stricture in the intestines.
• Extra-intestinal symptoms: Joint pain, skin rashes, and eye inflammation (such as iritis or uveitis) may occur due to the systemic nature of Crohn’s disease.
• Liver and bile duct problems: (e.g., primary sclerosing cholangitis) are also associated with Crohn's in some cases.

Diagnosing Crohn's disease involves a combination of clinical evaluation, laboratory tests, imaging, and endoscopic procedures:

• Clinical history and physical examination: A thorough medical history is taken, including symptom duration, pattern of symptoms (diarrhoea, pain, weight loss), and family history of IBD. Physical examination may reveal tenderness in the abdomen, particularly in the right lower quadrant.
• Laboratory tests:
  • Blood tests: These can show signs of inflammation (e.g., elevated C-reactive protein [CRP] or erythrocyte sedimentation rate [ESR]), anaemia, or nutritional deficiencies.
  • Stool tests: These help exclude infections that may mimic Crohn’s disease, such as bacterial or parasitic infections.
• Endoscopy:
  • Colonoscopy with biopsy: is a key diagnostic tool. It allows visualisation of the colon and terminal ileum, which are most commonly affected in Crohn’s disease. Biopsy samples may be taken to confirm the diagnosis and rule out other conditions.
  • Upper endoscopy (esophagogastroduodenoscopy, or EGD) may be performed if the upper GI tract is involved.
• Imaging:
  • CT or MRI: These imaging studies are useful in evaluating the extent of the disease, detecting complications such as fistulas, abscesses, and strictures, and for assessing the bowel wall.
  • MRI enterography and CT enterography: are particularly helpful in imaging the small intestine and detecting inflammation or strictures.
  • Capsule endoscopy: A small pill with a camera that you swallow to capture images of the small intestine. It can be particularly useful when the disease is suspected to affect areas of the small intestine that are difficult to reach with traditional endoscopy.
• Radiological signs: On imaging, signs of Crohn's disease may include thickening of the bowel wall, strictures, fistulas, or abscesses.

There is currently no cure for Crohn’s disease, but the goal of treatment is to control inflammation, induce remission, and maintain long-term remission. Treatment options include medications, lifestyle changes, and sometimes surgery.

• Medications:
  • Anti-inflammatory medications: Aminosalicylates (e.g., sulfasalazine, mesalamine) may be used for mild cases to reduce inflammation in the bowel.
  • Corticosteroids: Prednisone or other corticosteroids are used for flare-ups to quickly reduce inflammation. However, these are typically used for short-term management due to long-term side effects.
  • Immunosuppressive drugs: Thiopurines (e.g., azathioprine, mercaptopurine) and methotrexate suppress the immune response to reduce inflammation. Cyclosporine may be used in severe cases.
  • Biologics: TNF inhibitors (e.g., infliximab, adalimumab), integrin inhibitors (e.g., vedolizumab), and IL-12/23 inhibitors (e.g., ustekinumab) are used to target specific immune pathways involved in inflammation and are typically used for moderate-to-severe disease or when other treatments fail. These biologics can be administered as injections or infusions.
  • Antibiotics: Antibiotics (e.g., metronidazole, ciprofloxacin) may be used to treat or prevent infections such as abscesses or fistulas.
  • Probiotics and nutritional support: Probiotics may help with managing symptoms of Crohn’s, though evidence is mixed. Nutritional support, including enteral nutrition (liquid diets), may be used to help heal the intestines and improve symptoms, especially in children.
• Surgery: Surgery is not a cure but may be necessary for complications or when medications fail. Surgical options include:
  • Resection: Removal of the diseased part of the bowel. This is most common for strictures or fistulas.
  • Bowel resection and anastomosis: Involves cutting out damaged parts and rejoining the healthy sections.
  • Stoma creation: In severe cases, a colostomy or ileostomy is needed.