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An overview of Crohn’s disease

By Eamon Brady, MPSI, Owner of Whelehans Pharmacies, Mullingar - 08th Feb 2021

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A detailed overview of the presentation, complications, and treatment of Crohn’s disease

Crohn’s disease is one of the two main forms
of inflammatory bowel disease (IBD), the second being ulcerative colitis. Crohn’s disease causes inflammation of the digestive system, and while it is due to an immune response, it does not appear to be an autoimmune disease.

Crohn’s is a chronic condition, with many experiencing it as an ongoing and life-long disease, often with periods of remission as well as relapses or flare-ups. There is currently no cure for Crohn’s but medication and sometimes surgery can give long periods of relief.

Ulcerative colitis only affects the inner lining of the colon; the large intestine. In Crohn’s disease, inflammation can appear anywhere in the digestive tract, from the mouth to the anus. Crohn’s generally affects all the layers of the bowel walls, not just the inner lining, so tends to have more serious symptoms.


The exact cause is not known, but it is commonly accepted that Crohn’s is caused by a combination of factors, including:

  • Genetic factors.
  • An abnormal reaction of the digestive system to bacteria in the intestine.
  • An unknown trigger or perhaps set of triggers that may include viruses, other bacteria, diet, stress, or other environmental factors.

How Crohn’s disease affects the intestinal tract

The most common area of the intestinal tract to be affected by Crohn’s is the last part of the small intestine (terminal ileum) and the first part of the large intestine (colon), near the appendix. For some, only the colon is affected, in a pattern like ulcerative colitis. In others, multiple parts of the intestinal tract are affected. Rarely, the mouth, throat, oesophagus, or stomach may be affected.

A patch of inflammation may be as small as a few centimetres or extend most of the distance along the intestinal tract. As well as affecting the lining of the bowel, Crohn’s may also go deeper into the bowel wall. In some cases, the inflammation in the intestinal tract triggers inflammation outside the intestine, leading to other inflammatory complaints affecting the joints, eye, skin, and endocrine system, to name the most common.


Crohn’s disease is a very individual condition, ranging from very few symptoms, to frequent flare-ups or constant disease. The part of the intestinal tract most affected by Crohn’s influences the symptoms.
In general, the most common symptoms during a flare-up are:

  • Abdominal pain and diarrhoea. Sometimes mucus, pus or blood is mixed with the diarrhoea.
  • Tiredness and fatigue. This can be due to the illness itself, from the weight loss associated with flare-ups or surgery, anaemia from blood loss, or simply due to a lack of sleep due to pain, diarrhoea and other symptoms which inevitably affect sleep
  • Feeling generally unwell. Some people may have a raised temperature and feel feverish.
  • Mouth ulcers.
  • Loss of appetite and weight loss.
  • Anaemia. Crohn’s makes anaemia more likely due to blood loss, not eating enough because of symptoms like pain and diarrhoea, and because the body is not fully absorbing nutrients from food. Anaemia is a major factor in tiredness.


It is estimated that Crohn’s disease affects one in every 650
people in the UK. The figures in Ireland are not thought to be significantly different. Crohn’s appears to be slightly more common in women than in men. The Irish Society for Colitis and Crohn’s Disease indicated there were 5.9 new cases of Crohn’s disease in Ireland per 100,000 population in 2011, compared to 14.9 new cases of ulcerative colitis per 100,000 the same
year. The incidence of Crohn’s disease is higher than ulcerative colitis in children.

The peak age of incidence of Crohn’s is between the ages of 15 and 35 years, with a second (smaller) peak from the 50s to 70s. IBD diagnosed in children can behave differently and can be treated differently to that diagnosed in adults.

It is more common in urban rather than rural areas and in northern developed countries, although the numbers are beginning to increase in developing nations. Crohn’s is also more common in smokers.

Main types of Crohn’s disease

Crohn’s is often categorised according to which part or parts of the intestinal tract are most affected. The main types are:

Terminal ileal and ileocecal
Crohn’s in the ileum (the last part of the small intestine) is known as ileal or sometimes ‘terminal ileal’ Crohn’s because it affects the terminus or end of the ileum. If it affects the beginning of the large bowel, it is known as ileocecal Crohn’s. In this type of Crohn’s, pain is often experienced in the lower right side of the abdomen, especially after eating. There is often weight loss, and diarrhoea may occur.

Because Crohn’s in the ileum can make it difficult for the body to absorb bile salts, this can build up, leading to irritation in the bowel lining; diarrhoea often occurs and is most likely to be watery. The diarrhoea is unlikely to be bloody, as any blood lost will be digested by the time it reaches the rectum. About four-in-10 people with Crohn’s have ileal or ileocecal disease.

Small bowel

Abdominal pain and diarrhoea are also common symptoms if Crohn’s occurs further up the small bowel. Again, the diarrhoea is unlikely to be blood-stained, but weight loss and anaemia may be experienced. Nearly a third of people with Crohn’s have it in the small bowel.


Crohn’s disease in the colon (large intestine or large bowel) is known as ‘Crohn’s colitis’. This is a common form of Crohn’s disease. The main symptom tends to be blood-stained diarrhoea. Because of the inflammation, the colon cannot hold as much waste as normal, so very frequent bowel movements occur (six or more a day), especially if the rectum is inflamed.


Crohn’s in the upper intestinal tract (the oesophagus, stomach, or duodenum) is much less common. Symptoms that indicate Crohn’s in the upper intestinal tract include indigestion-like pain, nausea, loss of appetite, and weight loss.


Crohn’s in the area around the anus can occur on its own or at the same time as inflammation in other parts of the body. It can cause symptoms such as:

  • Fissures: Tears in the lining of the anal canal, which can cause pain and bleeding, especially during bowel movements.
  • Skin tags: Small fleshy growths around the anus.
  • Haemorrhoids: Swollen areas in the anal canal.
  • Abscesses: Collections of pus that can become swollen and painful. Most often found in the area around the anus and can cause a fever or lead to a fistula.
  • Fistulas: Narrow tunnels or passageways between the intestinal tract and the skin or another organ. In perianal Crohn’s, fistulas often run from the anal canal to the skin around the anus. They appear as tiny openings in the skin that leak pus or sometimes faecal matter. They can irritate the skin and are painful, but can usually be treated with medication and/or surgery.

Oral Crohn’s

Crohn’s can occasionally affect the mouth. True oral Crohn’s, which typically causes swollen lips and mouth fissures, is rare. However, about one-in-five people with Crohn’s tend to develop mouth ulcers.


Complications of Crohn’s may occur in the intestinal tract or other areas of the body and can include strictures, perforations, and fistulas.

Complications of Crohn’s disease affecting the intestinal tract


Ongoing inflammation and then healing in the bowel may cause scar tissue to form. This can create a narrow section of the bowel, known as a stricture. A stricture can make it difficult for food to pass, leading to a blockage. Symptoms include severe cramping abdominal pain, nausea, vomiting and constipation. The abdomen may become bloated and distended and the intestinal tract may make loud noises. Strictures are usually treated surgically, (mainly stricturoplasty).


Very occasionally, a severe blockage caused by a stricture may lead to a perforation or rupture of the bowel, making a hole. The contents of the bowel can leak through the hole and form an abscess. This causes pain and a fever. An abscess may also develop into a fistula.

Fistulas (fistulae)

A fistula can form when inflammation in Crohn’s spreads through the whole thickness of the bowel wall and continues to tunnel through the layers of other tissues. These tunnels or passageways can connect the bowel to other loops of bowel, to the surrounding organs, such as the bladder and vagina, or to the outside skin, including the skin around the anus (discussed earlier). Fistulas may be treated medically or with surgery.

Complications of Crohn’s disease on other parts of the body

Crohn’s disease can also cause problems outside the intestinal tract. Some people with Crohn’s develop conditions affecting the joints, eyes, or skin. These often occur during active disease, but they can develop before any signs of bowel disease or during times of remission.


Inflammation of the joints affects up to one-in-three people with IBD. In people with Crohn’s, arthritis is more commonly associated with Crohn’s colitis (Crohn’s disease in the colon). The inflammation usually affects the large joints of the arms and legs, including the elbows, wrists, knees, and ankles. Fluid collects in the joint space, causing painful swelling, although there can be pain without obvious swelling.

Symptoms generally improve with treatment for intestinal symptoms and there is mainly no lasting joint damage. A small percentage develop swelling and pain in the smaller joints of the hands or feet. This may be longer-lasting and persist while the inflammatory bowel disease is in remission. More rarely, ankylosing spondylitis can develop, in which the joints in the spine and pelvis become inflamed. This can flare up independently of the Crohn’s. Medication and physiotherapy can be helpful in treating arthritic symptoms.


Crohn’s can cause skin problems. The most common skin problem is erythema nodosum, which affects about one-in-seven people with Crohn’s. Painful red swellings appear, usually on the legs, and then fade, leaving a bruise-like mark. This condition tends to occur during flare-ups and generally improves with treatment for the Crohn’s.

More rarely, a skin condition called pyoderma gangrenosum affects people with Crohn’s disease. This starts as small tender blisters, which become painful, deep ulcers. These can occur anywhere on the skin, but most commonly appear on the shins or near stomas. It is usually treated with steroids or immune suppressants, but may need biological therapy.


Eye problems affect approximately one-in-20 people with Crohn’s. The most common condition is episcleritis, which affects the layer of tissue covering the sclera (the white outer coating of the eye) making it red, sore, and inflamed. Two other eye conditions associated with Crohn’s are scleritis (inflammation of the sclera itself) and uveitis (inflammation of the iris).

These conditions can usually be treated with local steroid drops, although uveitis and scleritis may need treatment with immune suppressants or biologic drugs. Patients with Crohn’s should be aware to mention any eye condition promptly to their doctor.


Crohn’s increases the risk of bone-thinning; this is mainly due to poor absorption of calcium needed for bone formation. Sometimes, low calcium levels are because the diet does not contain enough dairy foods or the use of steroid medication. Calcium supplementation and for some, drug treatment with the likes of alendronates, is needed.


About one-in-four people with Crohn’s develop gallstones. These are small ‘stones’ made of cholesterol, which can get trapped in the gallbladder and can be very painful. Several factors linked with Crohn’s can make gallstones more likely, including poor absorption of bile salts often caused by inflammation. Bile salts help to digest fats during digestion. Some of the drugs used to treat Crohn’s, such as azathioprine and methotrexate, may increase liver problems.

A rare condition called primary sclerosing cholangitis (PSC) affects up to one-in-25 people with Crohn’s, usually those with the disease in the colon. PSC causes inflammation of the bile ducts and can eventually damage the liver. Symptoms include fatigue, pain, itching, jaundice, and weight loss. Treatment is usually with ursodeoxycholic acid.

Blood circulation

Crohn’s disease doubles the risk of blood clots in the veins, including DVT (deep-vein thrombosis) in the legs. Risk is highest during a flare-up or if confined to bed, ie, during a hospital visit. Warning symptoms that patients need to be aware of include pain, swelling and tenderness in the leg, or chest pains and shortness of breath.


People with inflammatory bowel disease are most likely to develop iron deficiency anaemia. This is caused by a lack of iron in the diet or poor absorption of iron from food and can be made worse by ongoing intestinal blood loss due to inflammation. Vitamin deficiency anaemia, caused by a low intake or poor absorption of certain vitamins, such as vitamin B12 or folic acid particularly affects people with Crohn’s who have had sections of the small intestine removed. Some of the drugs used for Crohn’s, such as sulfasalazine and azathioprine, can also cause anaemia.

Crohn’s disease and the link to cancer

Severe or extensive Crohn’s disease affecting all or most of the colon for many years can mean a slightly increased risk than normal of developing colon cancer.


Diarrhoea, abdominal pain, and weight loss lasting for several weeks or longer indicate that Crohn’s is a possibility, particularly if there is a family history of IBD. Tests and physical examinations can confirm a diagnosis.


Treatment for Crohn’s may be medical, surgical (not covered in this article) or a combination of both. For mild Crohn’s, no drug treatment may be needed. Dietary therapy may be another option for some. Treatment will depend on the type of Crohn’s.

Drug treatment for Crohn’s aims to reduce symptoms and control flare-ups, and then to prevent a relapse once the disease is under control. This can mean taking medication on an ongoing basis, sometimes for many years.

Anti-inflammatory drugs

These help to reduce inflammation and include:

  • 5-ASAs or aminosalicylates, such as mesalazine and sulfasalazine.
  • Corticosteroids such as prednisolone, hydrocortisone, and budesonide.
  • Immunosuppressants such as azathioprine, methotrexate, and tacrolimus.
  • Biological, anti-TNF, drugs such as infliximab and adalimumab.

Biological therapies are generally reserved for people in poor general health with severe symptoms of Crohn’s disease, especially if corticosteroids and immunosuppressants are unsuitable or ineffective. Biological treatment usually lasts at least 12 months unless these drugs stop being effective sooner or the patient cannot tolerate. After this time, the condition will be assessed to determine if further treatment is necessary.

Symptomatic drugs

These help control and reduce common symptoms such as pain, diarrhoea, and constipation. They include:

  • Anti-diarrhoeal, such as loperamide and cholestyramine.
  • Bulking agents, such as ispaghula husk.
  • Painkillers, such as paracetamol and aspirin.

Dietary treatment for Crohn’s disease

Enteral nutrition is widely used for children with Crohn’s disease, because it helps their growth and avoids the use of steroids. There is less evidence for the effectiveness of enteral nutrition in adults, particularly for active Crohn’s disease. Research has shown it to be less effective than steroids. However, enteral nutrition may be recommended for adults who prefer not to use drug therapy and it can be useful as a supplement for people who need extra nutrition.

Does a change in diet help?

No clear evidence indicates that any food or food additive directly causes or improves Crohn’s. Generally, the most important thing is to try to eat a nutritious and balanced diet to help maintain weight and strength, and to drink sufficient fluids to prevent dehydration. Some people with Crohn’s find that certain foods trigger symptoms or make them worse and that reducing or adjusting the amount of fibre they eat or cutting out wheat or dairy products may help.

To ensure the diet remains healthy and well balanced, it is important the patient gets advice from a doctor or from a qualified dietitian before making significant changes. If the patient has a stricture, avoiding ‘hard to digest’ or ‘lumpy’ foods that might cause a blockage is advised. Such foods might include nuts and seeds, fruit and vegetable skins, and tough meat or gristle. It may also help to have small, frequent meals or snacks, and to chew food thoroughly.

If the bowel is not absorbing nutrients properly, perhaps because of extensive inflammation or a shortened bowel after surgery, some people find a low-fat diet reduces diarrhoea. Avoiding carbonated drinks or other foods containing benzoates or cinnamon can help prevent symptoms. Many Crohn’s patients lack certain vitamins and minerals, such as iron, calcium, vitamin D or vitamin B12, especially if they have a poor appetite or active diarrhoea or blood loss.

Some of the drugs used for Crohn’s can also lead to deficiencies; for example, sulfasalazine can affect the body’s ability to absorb folates, and steroids can cause calcium loss.

In these cases, a supplement may be useful, but should be discussed with the doctor, pharmacist, or dietitian. If tests show a serious deficiency, a course of supplements or enteral nutrition may be advised. Vitamin B12 supplements are sometimes given by injection and iron supplements intravenously, as this can make them easier to absorb.

Are complementary and alternative approaches helpful?

Some people with Crohn’s disease have found complementary and alternative medicines helpful for controlling symptoms such as abdominal pain and bloating. However, there are few reliable scientific studies to show the effectiveness of such therapies and it is possible that their symptoms may have gone into remission coincidentally, given the unpredictable course of Crohn’s, or there may be a placebo effect.

One area where there has been some scientific research is the use of omega 3 fish oils. However, a recent review concluded that fish oils were probably not effective at keeping people with Crohn’s in remission, as although some studies found symptoms improved, two larger studies showed no benefit.

References on request

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