Crohn’s disease:The second

This is the second instalment of the seventh article in a series of short biographies of persons whose names are directly used for diseases, conditions or syndromes familiar to those in clinical pathology laboratories.

In the previous article on Crohn’s disease (CD), published in the January issue of the Biomedical Scientist, the main focus was to briefl y review advances in knowledge of CD from around the 16th century to the 1980s. Here we review more recent developments in our knowledge of clinical features, aetiology, investigations and treatment options.

IMAGE: SUPPLIED BY NATIONAL INSTITUTES OF HEALTH

Current perspectives

CD is an idiopathic chronic regional enteritis commonly aff ecting the terminal ileum in 40–50% of cases, with 30–40% aff ecting small intestine and colon. There is transmural thickening and infl ammation of discontinuous sections of intestine and in some cases non-necrotising granulomas with epithelioid histiocytes.

Patients may present with a history of cramping abdominal pain, diarrhoea with intestinal fi stula and intramural abscesses. A pattern of periods of remission and relapses when symptoms fl are may become apparent. A diff erential diagnosis between CD and ulcerative colitis is essential due to the choice of treatment options available. This involves an understanding of clinical, radiological, endoscopic, and pathological features of CD.

There is a wide range of prevalence of around 26-199 cases/100,000 persons. CD is more common in white people living in Western industrialised countries. There is a steady increase in cases of unknown cause and although CD may occur at any age it is more common in two age groups – between 13 and 20 years and 60 and 70 years, with a slight increase in females.

Even 90 years after Crohn’s landmark publication, the exact cause of CD is not known but a number of risk factors have been identifi ed, such as family history, smoking, diet and ethnicity. It has been proposed that there are multifactorial causes, notably that the immune response of the intestinal mucosa becomes defective and there are possible changes in the balance of bacteria in the gut fl ora. Professor John Hunter at Addenbrooke’s Hospital suggests that there is a reduction in lactobacilli and bifi dobacteria with an increase in facultative anaerobes. In addition, abnormal fermentation of food digests may damage the intestinal

Left. Dr Burill Crohn Below. X-ray of the intestines of a patient with Crohn’s disease.

mucosa causing infl ammation, ulceration and bleeding and immune system factors may attack and destroy healthy gut microbiota in genetically susceptible individuals.

It has been established that fi rst-degree relatives have a 13-18 % increased risk of CD and consequently there is a nonMendelian polygenic mode of inheritance. Molecular linkage studies of aff ected families identifi ed NOD2 (nucleotide binding domain 2) as an aff ected gene in CD and the protein product may prevent excessive immune activation and may also have antibacterial properties. However, it is important to note that only around 10% of CD cases are due to a mutation of NOD2 and genomic screening has shown linkage to at least four chromosomes – 3, 7, 12 and 16. A number of other genes such as autophagy-related 16 (ATG16LI) and immunity related GTPaseM (IRGM) may be involved whilst interleukin receptor 23 has a role in immune activation in the intestinal mucosa.

This is a rapidly developing fi eld of research and the interested reader will fi nd helpful the excellent 2014 review by Jimmy Liu and Carl Anderson from the Wellcome Trust Sanger Institute, Cambridge.

Several defects in epithelial function have been identifi ed, notably aff ecting junction barrier transport and associated proteins. The polarisation of T cells to T-helper cell type 1 is a well-known feature of CD and TH17 cells may also be involved. Some studies have shown that neutrophil dysfunction may alter innate immunity in the development of CD. It has also been proposed that the infl ammation may be due to an unrestrained immune response to luminal bacteria. As shown later, immunosuppression can be an important treatment option and infers those changes in mucosal immunity and immunoregulation are signifi cant factors in CD development and progression.

This may include fi ssures, fi stulas and abscesses, post-prandial “bloating”, constipation and bowel obstruction. Chronic mucosal infl ammation may lead to fi ssures with resultant abscesses and fi stulas and the latter are more common post-surgery. Non-intestinal complications may aff ect the mouth (e.g. ulcers), eyes, skin and joints. Patients with Crohn’s colitis are at increased risk for high-grade dysplasia and colonic adenocarcinoma. Those with extensive colonic disease from a young age require endoscopic surveillance for eight years after the onset of symptoms then every one to two years.

This is based on clinical examination and assessment, essential imaging, histology and laboratory evidence.

A variety of imaging modes are available. Contrast imaging, such as barium meal or enema, can be used to assess the extent of disease, severity, potential complications and treatment strategy. A plain X-ray of the abdomen can reveal gut infl ammation, swelling of the bowel and complications such as perforations. More complex techniques such as magnetic resonance enterography can detect ulceration, abscesses or the presence of a fi stula. Colonoscopy can be used to allow visual examination of the colon using a fi bre-optic telescope to assess disease activity and to obtain biopsy tissue.

Whilst other conditions need to be considered, such as drug-induced colitis and diverticular disease, once eliminated the main focus is to diff erentiate CD from ulcerative colitis as surgery is better tolerated in the latter. CD is characterised by skip lesions, granulomas, transmural lymphocyte aggregates, fi ssures and occurrence in any part of the gastrointestinal tract. Ulcerative colitis is limited to the colon, patchy areas may mimic skip lesions and only the mucosal layer is aff ected. Diff erentiation remains challenging and it is estimated that around 30% remain undetermined. The quality of the biopsies taken at colonoscopy is crucial to diagnosis.

Biopsy microscopy results in CD can be highly variable but classic features may include areas of chronic infl ammation with

increased lymphocytes and patchy neutrophilic infl ammation. Transmural infl ammation with plasma cells, neutrophils and aggregates of lymphocytes are also typical. Granulocytes may be only present in around 50% of cases and are a non-specifi c fi nding and may be seen in ulcerative colitis. However, microscopic skipping of lesions can be diagnostic. With the increased risks of small bowel cancer, colorectal cancer and lymphoma the presence of intraepithelial neoplasia is signifi cant.

Immunohistochemistry has a limited role in CD, but stains for cytomegalovirus may be helpful if there is a relapse of severe symptoms when on immunosuppressive therapy to exclude a new or reactivated viral infection.

Objectives for adult patients include the best possible control of infl ammation, achieving stable and best possible life-style with minimal adverse side eff ects of treatment. For children, the promotion of growth and adequate nutrition are essential. There are two main lines of treatment medication, but the mode of use can vary between a step-up approach in mild CD commencing with less aggressive agents, such as antibiotics, to more potent agents, notably corticosteroid immunosuppressants, typically methotrexate or azathioprine. In more severe CD a step-down approach including biological agents, such as infl iximab and steroids or biological agents combined with immunosuppressants, may be eff ective. The more recent introduction of biological agents is to target and inhibit tumour necrosis factor (TNF) activity as TNF is a key infl ammatory cytokine and moderator of intestinal infl ammation that is overexpressed in infl ammatory bowel disease. Their use is not without risk, with some severe adverse reactions reported, most notably diff erent forms of leukaemia. There is also some evidence that alteration to the immune system over a long period can increase the risk of carcinogenesis and longer exposure to immunosuppressant therapy may promote the risk.

Steroids are not indicated for maintenance therapy due to the increased risks of osteoporosis, cataracts and diabetes.

If medical therapy is not eff ective and severe symptoms continue, with supporting imaging evidence, resection of infl amed bowel is required, but due to risks and consequences is only undertaken when strictly necessary.

A number of anti-diarrhoea agents can be eff ective in CD, such as loperamide and bile acid binders, but there is a risk of toxic megacolon with an expanded colon, which may be life-threatening.

Left. Small intestine in Crohn’s disease. Centre. Immune response to infl ammatory bowel disease, illustration.

Even with successful surgery it has been reported that careful endoscopic followup has shown that around 90% of cases develop further disease and probably during the next decade will require further surgery. Unlike ulcerative colitis, if a specifi c area of disease only is resected then a stoma is not always required. Poor prognostic indicators include diagnosis at a young age, perianal disease and perforating disease.

The removal of any part of the main intestinal tract by surgery may have consequences, notably malabsorption and resection of ileum can lead to B12 and folate malabsorption, which may cause chronic fatigue, anaemia and potential damage to the nervous system, in addition the failure to reabsorb bile salts may cause their leakage into the colon and cause diarrhoea.

Appetite may be aff ected and dietary advice should be sought, when necessary, the remaining bowel may adapt to recover absorptive capacity and allow a relatively normal diet. However, there is a reduced capacity for fat absorption and a reduction in dietary fat may help. If fat malabsorption continues, the fat-soluble vitamins A and D, along with essential

minerals, notably calcium and magnesium, become depleted with clinical consequences.

Treatment with immunosuppressants may lead to abscess formation, which may require drainage, and fi stulae, which may connect with other organs such as the bladder and require corrective surgery.

Anaemia may occur in CD by a gradual loss of blood into the gastrointestinal tract, chronic infl ammation and malabsorption. This is usually iron defi ciency anaemia and can be treated with oral iron, but if the haemoglobin is <10 g/dL, parenteral administration is required.

Mean cell haemoglobin and mean cell volume are reduced with hypochromia and microcytosis. Serum ferritin is low and is a sensitive marker of iron defi ciency in the absence of infl ammation. Other fi ndings include low transferrin saturation, a low serum iron and low iron-binding capacity. Other causes of anaemia should be considered – eff ective treatment and management is necessary as anaemia in CD is a common cause of hospitalisation.

White cells and platelets may be raised in the presence of active infl ammation and infection.

A range of assays are required to monitor CD, which refl ects the impact of reduced intestinal function and response to infl ammatory bowel disease. Serum urea and electrolytes are measured to assess salt imbalance and water loss in periods of severe diarrhoea. Serum proteins, notably albumin, are lowered in CD in severe ulceration and it has been proposed from an admittedly relatively small study of 71 patients that the reduction is associated with the response to oxidative stress. Liver function tests may be raised transiently due to infl ammation or chronically in sclerosing cholangitis. There is an increased risk of gallstones and obstructive jaundice in CD.

Other abnormalities include defi ciencies in iron, folate and B12 and serum calcium and magnesium.

Acute infl ammatory markers, such as high-sensitivity serum C-reactive protein or erythrocyte sedimentation, can be helpful during relapse phases and the risk of recurrence.

Faecal calprotectin is a relatively new sensitive marker of infl ammation. Calprotectin is present in high concentrations in neutrophils and in active disease neutrophils migrate into the gastrointestinal tract. Faecal levels exceeding 150 μg/g indicate signifi cant bowel infl ammation and correlate well with endoscopic examination.

Faecal samples should be tested for the presence of white blood cells, occult blood, parasites and Clostridioides diffi cile toxin, which may be responsible for relapse and certainly before commencing immunosuppressant therapy.

Anti-Saccharomyces cerevisiae antibodies (ASCA) are most often found in CD and it has been observed that positive results have an increased risk of early surgical intervention. Other markers, such as perinuclear anti-neutrophil cytoplasmic antibody and Pseudomonas fl uorescens (ant-12), are less specifi c.

To detect and minimise adverse side eff ects of this more aggressive therapy, a full blood count and chemistry panel for renal and liver function are required. It is essential to measure the activity of thiopurine methyl transferase in red blood cells as low levels, such as <1 mg/kg have an increased risk of severe myelosuppression and haematopoietic toxicity. Monitoring of haematology and chemistry should be performed monthly and then every three months.

This short review describes some of the progress made in understanding Crohn’s disease, however, the exact cause still remains unknown and there is concern about the increasing incidence and prevalence worldwide and for individual patients the often serious impact on their lifestyle. Treatment hopes may rely on the use of genetic markers to predict risk and novel drug targets.

Stephen Clarke is a retired IBMS fellow. To read this article with full references, visit

The fi rst instalment of this article was be published in January’s Biomedical Scientist and can be read online at bit.ly/3JMjNyc