Understanding Crohn’s Disease & Ulcerative Colitis

Written & reviewed by the BestDoctors.sg editorial team · Last updated 29 Aug 2026 · General information, not medical advice

Ulcerative colitis

Ulcerative colitis affects the large bowel – the colon – and the rectum, and it does not affect other parts of the digestive system. Its inflammation is continuous, meaning it spreads without gaps from the rectum upwards, and it affects the inner lining of the bowel wall (the mucosa) rather than going deeper. How much of the colon is involved varies from person to person, from just the rectum to the whole colon. Because ulcerative colitis is confined to the colon and rectum, surgery to remove the colon can, in some cases, effectively cure the colitis itself, though it is a major decision made with the specialist team.

Crohn’s disease

Crohn’s disease is different in important ways. It can affect any part of the gut, from the mouth all the way to the anus, though it most often involves the end of the small bowel and the start of the large bowel. Its inflammation occurs in patches, sometimes called skip lesions, with healthy stretches of gut in between, and it goes deeper, affecting the full thickness of the bowel wall rather than just the lining. This deeper, patchy pattern is why Crohn’s can cause complications such as narrowing (strictures) and fistulas, and why, unlike ulcerative colitis, surgery does not cure it – the disease can recur elsewhere.

Symptoms of both

Crohn’s disease and ulcerative colitis share many symptoms. Both commonly cause ongoing or recurrent diarrhoea, urgency to open the bowels, abdominal pain and cramping, fatigue, a reduced appetite and unintended weight loss. Blood or mucus in the stools is common, and tends to be more prominent in ulcerative colitis, while Crohn’s more often causes weight loss, mouth ulcers and problems around the anus. Recognising these symptoms, and reporting them to a doctor, is important, because ongoing changes in bowel habit, bleeding or weight loss should always be assessed rather than dismissed.

Symptoms beyond the gut

IBD can also cause symptoms beyond the digestive system, known as extra-intestinal manifestations, because it is an immune-driven condition affecting the whole body, not just the bowel. These can include joint pain and inflammation, eye problems, and skin conditions, and they may sometimes flare along with the bowel disease or independently of it. Understanding that IBD can affect the joints, eyes and skin helps you recognise when a new symptom might be related to your condition, and it is worth mentioning such symptoms to your IBD team, who can assess them and coordinate care with other specialists if needed.

What causes IBD

The exact cause of IBD is not fully understood, but it is clear that it develops when the immune system attacks the bowel, causing inflammation. This involves a combination of genetic and environmental factors, and IBD can run in families. Certain environmental factors, such as smoking, are recognised as relevant. It is important to be clear that IBD is not caused by diet or stress alone, even though food and stress can affect symptoms. This means there is nothing you ate or did to bring it on, which can relieve the guilt some people feel, and it directs attention to managing the immune-driven inflammation.

How IBD is diagnosed

There is no single test for IBD; diagnosis combines several. Blood tests check for signs of inflammation and for anaemia. Stool tests are important: faecal calprotectin measures inflammation in the bowel and helps distinguish IBD from IBS and decide whether a colonoscopy is needed, while other stool tests rule out infection. The key confirmatory test is usually a colonoscopy, sometimes a sigmoidoscopy, in which a camera examines the bowel lining and small tissue samples (biopsies) are taken to confirm inflammation and its type. Together, these build a clear picture that guides treatment, and the process can take some time.

The role of scans

Imaging scans play an important part too, especially in Crohn’s disease, where inflammation can affect parts of the small bowel that a colonoscopy cannot reach. Scans such as MRI or CT enterography, and sometimes ultrasound, help assess how much of the gut is involved and look for complications such as narrowing or fistulas. This information helps the team understand the extent and pattern of the disease, which shapes treatment decisions. Because Crohn’s and ulcerative colitis behave differently, getting the diagnosis and the extent right matters, and the combination of endoscopy and imaging is how the specialist builds that full picture.

Getting assessed

If you have persistent symptoms suggestive of IBD – ongoing diarrhoea, blood in the stools, abdominal pain, or unexplained weight loss and fatigue – it is important to see a doctor, who can arrange the right tests and, if needed, refer you to a gastroenterologist. In Singapore, dedicated IBD services at hospitals including the Singapore General Hospital, the National University Hospital and Tan Tock Seng Hospital provide specialist diagnosis and care. An accurate diagnosis, including whether you have Crohn’s or ulcerative colitis and how extensive it is, is the foundation for effective treatment, so proper assessment is well worth pursuing.

Related IBD topics

• Living with inflammatory bowel disease
• Managing flares & staying in remission
• IBD medication & treatment
• Diet & nutrition with IBD
• Fatigue, anaemia & IBD
• IBD surgery: what to expect
• Mental health & emotional wellbeing
• Pregnancy, fertility & IBD
• Living well: work, travel & daily life
• Crohn’s disease: an overview
• Ulcerative colitis: an overview
• Gastroenterologists in Singapore

Sources

• NHS – Crohn’s disease: nhs.uk
• NHS – Ulcerative colitis: nhs.uk
• Crohn’s and Colitis UK – Crohn’s Disease: crohnsandcolitis.org.uk
• Crohn’s and Colitis UK – All about Crohn’s and Colitis: crohnsandcolitis.org.uk