Crohn's disease is transmural, meaning complications extend through all layers of the bowel wall. Intra-abdominal abscesses develop in up to 25% of patients and may present with subtle signs — low-grade fever, RIF tenderness, or a palpable mass. A perianal abscess requires surgical incision and drainage — antibiotics alone are inadequate and delay creates fistula-in-ano. Patients on biologics (anti-TNF agents) cannot mount a normal febrile or inflammatory response, making sepsis assessment difficult.

The Montreal classification (location A + behaviour B) drives treatment decisions — a stricturing Crohn's flare presenting with obstructive symptoms needs surgical assessment, not just steroids. Giving steroids to a patient with an intra-abdominal abscess is dangerous — steroids mask the septic response and allow the abscess to enlarge. The IBD nurse specialist knows the patient's phenotype, baseline, and rescue protocol — a 5-minute phone call can prevent unnecessary GP visits or A&E admissions.

The Harvey-Bradshaw Index is simpler than the Crohn's Disease Activity Index (CDAI) and can be calculated in primary care without laboratory data. Faecal calprotectin is particularly valuable in Crohn's patients because they frequently develop IBS-like functional symptoms during and after active disease — calprotectin prevents unnecessary steroid exposure in those with functional rather than inflammatory symptoms. Over-treating functional symptoms with steroids contributes to steroid dependency and osteoporosis.

Perianal examination is essential in Crohn's — perianal disease occurs in 25–50% of patients and can be the presenting feature or a major source of morbidity. A palpable RIF mass in Crohn's should not be assumed to be a flare — it may be an abscess or rarely a cancer (IBD patients have higher colorectal cancer risk). Mouth ulcers parallel gut disease activity and are a useful quick marker of flare severity. Nutritional assessment is critical — Crohn's causes malabsorption and many patients are significantly malnourished.

Terminal ileal Crohn's causes B12 malabsorption — B12 deficiency causes irreversible neurological damage if untreated. Vitamin D deficiency is present in up to 70% of Crohn's patients and is associated with more severe disease activity and increased fracture risk. MRI enterography (MRE) is now the preferred imaging modality for Crohn's disease assessment — it provides bowel wall characterisation without radiation and should be arranged by gastroenterology specialists. Biologic drug levels guide whether a flare is due to loss of response (low levels) or antibody formation — this drives the step-up decision.

Perianal Crohn's disease management requires a combined surgical and gastroenterological approach — fistulae are assessed by MRI pelvis and EUA (examination under anaesthesia), and treatment involves seton placement, biologics (infliximab reduces fistula healing time), or combined therapy. Exclusive enteral nutrition induces remission in 80% of paediatric Crohn's and avoids steroid exposure in growing children — dietitian referral is essential. Loss of response to biologics (secondary failure) is managed by gastroenterologists with drug level testing and either dose escalation or switch.

Budesonide 9 mg is preferred over prednisolone for mild–moderate ileocolonic Crohn's because its high first-pass metabolism limits systemic steroid side effects (growth retardation in children, adrenal suppression, osteoporosis). However, it is not effective for severe disease. Metronidazole for perianal disease is a bridge — definitive management requires surgical assessment and usually biologic therapy. TPMT enzyme deficiency (0.3% of population) causes life-threatening myelosuppression with azathioprine — testing is mandatory before prescribing.

Smoking is the single most important modifiable lifestyle factor in Crohn's — smokers have twice the relapse rate, higher surgical rates, and worse fistula outcomes. Stopping smoking is as important as medication. Exclusive enteral nutrition is underutilised in adults but has equivalent efficacy to steroids for induction of remission without the side-effect profile — particularly valuable in children, teenagers, and malnourished adults. Methotrexate teratogenicity is absolute — it causes fetal death and major malformations and must be stopped 3–6 months before pregnancy planning.

Unlike UC, Crohn's disease is not cured by surgery and flares are often more complex due to transmural disease and complications. The 48–72 hour review is essential — a patient with an unrecognised abscess will worsen on steroids (which mask the inflammatory response). Cancer surveillance in Crohn's colitis follows the same timeline as UC (every 1–5 years depending on extent and duration). Primary sclerosing cholangitis (PSC), though predominantly associated with UC, can occur in Crohn's colitis — jaundice or abnormal LFTs should prompt hepatology referral.