Lower Gastrointestinal Disorders: IBS, IBD, Diverticulitis & Colorectal Screening
Key Takeaways
- Irritable Bowel Syndrome (IBS) is diagnosed using Rome IV criteria: recurrent abdominal pain at least 1 day per week over the last 3 months associated with 2 or more defecation or stool features.
- Crohn's Disease causes transmural inflammation, skip lesions from mouth to anus, and non-caseating granulomas, whereas Ulcerative Colitis is limited to mucosal continuous rectal and colonic inflammation.
- Fecal calprotectin and lactoferrin are biomarker assays used to differentiate organic inflammatory bowel disease (IBD) from functional gut disorders like IBS.
- Abdominal/pelvic CT with contrast is the imaging study of choice for acute diverticulitis; colonoscopy is strictly contraindicated during an acute flare due to perforation risk.
- USPSTF guidelines recommend colorectal cancer screening for average-risk adults aged 45 to 75 years using colonoscopy every 10 years, annual FIT, or FIT-fecal DNA every 3 years.
Lower Gastrointestinal Disorders: IBS, IBD, Diverticulitis & Colorectal Screening
Irritable Bowel Syndrome (IBS)
Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits in the absence of structural, metabolic, or infectious abnormalities. Pathophysiology involves visceral hypersensitivity, altered intestinal motility, brain-gut axis dysregulation, low-grade immune activation, and intestinal dysbiosis.
Rome IV Diagnostic Criteria
According to Rome IV criteria, IBS is diagnosed when patients experience recurrent abdominal pain, on average at least 1 day per week in the last 3 months, with symptom onset at least 6 months prior, associated with 2 or more of the following:
- Related to defecation (pain relieved or exacerbated by bowel movements).
- Associated with a change in stool frequency.
- Associated with a change in stool form (appearance) (e.g., Bristol Stool Form Scale Types 1-2 vs Types 6-7).
Subtypes based on predominant stool consistency:
- IBS-C (Constipation predominant): >25% hard/lumpy stools (Bristol 1-2).
- IBS-D (Diarrhea predominant): >25% loose/watery stools (Bristol 6-7).
- IBS-M (Mixed habits): >25% hard AND >25% loose stools.
- IBS-U (Unclassified).
Diagnostic evaluation must confirm the absence of red flags: Age >= 50 years, nocturnal diarrhea, hematochezia, unintentional weight loss, fever, or family history of colorectal cancer, IBD, or celiac disease.
Evidence-Based Management
- Non-Pharmacologic: Low-FODMAP diet restricting fermentable oligosaccharides, disaccharides, monosaccharides, and polyols implemented in a 3-phase approach (elimination for 2-6 weeks, reintroduction, and personalization); soluble fiber (psyllium) supplementation.
- IBS-C Pharmacotherapy: Osmotic laxatives (polyethylene glycol [PEG]); Guanylate Cyclase-C agonists (linaclotide, plecanatide which stimulate cGMP to increase intestinal fluid secretion); Chloride channel activators (lubiprostone).
- IBS-D Pharmacotherapy: Opioid agonists (loperamide); Eluxadoline (mu-opioid agonist); Non-absorbable antibiotic rifaximin (550 mg TID for 14 days); 5-HT3 antagonists (alosetron).
- Visceral Hypersensitivity & Cramping: Antispasmodics (dicyclomine, hyoscyamine) PRN before meals; low-dose Tricyclic Antidepressants (TCAs like desipramine) to modulate central pain pathways.
Inflammatory Bowel Disease (IBD): Crohn's vs. Ulcerative Colitis
IBD includes Crohn's Disease (CD) and Ulcerative Colitis (UC), chronic idiopathic autoimmune inflammatory disorders of the gastrointestinal tract.
Clinical and Histopathologic Comparison
| Feature | Crohn's Disease | Ulcerative Colitis |
|---|---|---|
| Location | Any segment from mouth to anus (ileocolonic most common) | Restricted to colon and rectum; continuous from rectum proximally |
| Inflammation Depth | Transmural (full thickness through mucosa to serosa) | Mucosal and submucosal only |
| Pattern | Skip lesions interspersed with normal tissue | Continuous mucosal involvement |
| Histology | Non-caseating granulomas, cobblestoning | Crypt abscesses, pseudopolyps |
| Presentation | Crampy RLQ pain, diarrhea, weight loss, fever | Relapsing bloody diarrhea with mucus, tenesmus, LLQ pain |
| Complications | Strictures, fistulas, perianal abscesses, vitamin malabsorption | Toxic megacolon (>6 cm distension + toxicity), severe anemia, PSC |
Extraintestinal Manifestations & Biomarkers
Extraintestinal features include enteropathic arthritis, erythema nodosum, pyoderma gangrenosum, uveitis, and Primary Sclerosing Cholangitis (PSC, overwhelmingly associated with UC).
Fecal Calprotectin and Fecal Lactoferrin are non-invasive neutrophil biomarkers excreted in stool during active inflammation that reliably distinguish organic IBD from functional IBS.
Medical Therapy & Surgical Considerations
- 5-ASAs: Oral/topical mesalamine is first-line for mild-to-moderate UC induction and maintenance.
- Corticosteroids: Oral budesonide or prednisone for acute flare induction, not long-term maintenance.
- Immunomodulators & Biologics: Azathioprine, methotrexate, anti-TNF agents (infliximab, adalimumab), anti-integrins (vedolizumab), IL-12/23 inhibitors (ustekinumab), and oral JAK inhibitors (tofacitinib).
- Surgical Nuance: Total proctocolectomy is curative for Ulcerative Colitis, whereas bowel resection in Crohn's Disease is non-curative due to high risk of postoperative recurrence.
Diverticular Disease & Acute Diverticulitis
Diverticulosis involves mucosal outpouchings herniating through the colonic muscular layer, predominantly in the sigmoid colon.
Presentation, Imaging & Management
Acute diverticulitis occurs when microperforation triggers focal pericolic inflammation, presenting with constant LLQ abdominal pain, fever, leukocytosis, and altered bowel habits.
- Imaging Modality of Choice: Abdominal/pelvic CT with IV and oral contrast, showing colonic wall thickening (>4 mm) and pericolic fat stranding.
- CONTRAINDICATION: Colonoscopy is strictly contraindicated during acute diverticulitis due to high colonic perforation risk. Colonoscopy should be delayed 6 to 8 weeks post-resolution to exclude underlying malignancy.
- Outpatient Management: Mild uncomplicated cases in reliable patients are treated with oral broad-spectrum antibiotics for 7-10 days: Ciprofloxacin (500 mg BID) + Metronidazole (500 mg TID), OR Amoxicillin-clavulanate (875/125 mg BID). Clear liquid diet initially, advancing as tolerated.
- Post-Resolution: Transition to a high-fiber diet (25-35 g daily). Restricting nuts, seeds, or popcorn is no longer recommended.
Colorectal Cancer (CRC) Screening Guidelines
USPSTF (2021/2026) screening recommendations for average-risk adults:
- Ages 45 to 75 Years: Screening is strongly recommended (Grade A/B) starting at age 45.
- Ages 76 to 85 Years: Screening decisions should be individualized (Grade C) based on health status and life expectancy.
- Age > 85 Years: Discontinue screening.
Screening Options & Intervals
- Colonoscopy: Every 10 years (Gold standard).
- Stool DNA-FIT (Cologuard): Every 3 years (combines NDRG4/BMP3 gene hypermethylation with stool hemoglobin).
- Annual FIT or annual gFOBT: Every 1 year.
- CT Colonography: Every 5 years. Note: Any positive non-colonoscopy screening test requires follow-up diagnostic colonoscopy.
High-Risk Screening
Individuals with a first-degree relative diagnosed with CRC before age 60 should initiate colonoscopy at age 40 or 10 years earlier than the youngest affected relative, repeated every 5 years.
A 28-year-old male undergoes a diagnostic colonoscopy for chronic bloody diarrhea, tenesmus, and crampy left lower quadrant abdominal pain. Biopsies reveal continuous inflammation extending from the rectum into the descending colon, confined to the mucosal and submucosal layers, with crypt abscesses. Non-caseating granulomas are absent. Which diagnosis is indicated?
An average-risk 45-year-old male presents for a routine annual physical examination. He has no personal or family history of colorectal polyps, cancer, or inflammatory bowel disease. According to the U.S. Preventive Services Task Force (USPSTF) guidelines, what is the recommended colorectal cancer screening recommendation for this patient?
A 34-year-old female presents with a 9-month history of crampy lower abdominal pain occurring 2 to 3 days per week. She reports that the pain is typically relieved following a bowel movement and is accompanied by alternating episodes of hard, lumpy stools and loose, watery stools. Physical exam, complete blood count, and C-reactive protein are normal. Which set of diagnostic criteria supports the diagnosis of Irritable Bowel Syndrome (IBS)?
A 66-year-old female presents to the urgent care clinic with severe left lower quadrant abdominal pain, low-grade fever (100.8°F), and leukocytosis (14,500/mcL). Physical exam reveals localized left lower quadrant tenderness without rebound or guarding. Computed tomography (CT) of the abdomen and pelvis confirms acute uncomplicated sigmoid diverticulitis. Which of the following management plans is most appropriate?