Gastrointestinal

Microscopic colitis: diagnosis and treatment

Clinical Overview and When to Suspect Microscopic Colitis

— Lymphocytic colitis (LC): ≥20 intraepithelial lymphocytes per 100 surface epithelial cells

— Collagenous colitis (CC): subepithelial collagen band >10 μm thick

— Middle-aged to older women (peak 60–70s), F:M ~3:1 for CC, less skewed for LC

— Incidence rising; now accounts for 10–20% of chronic watery diarrhea workups in adults >65

— Strong association with autoimmune disease: celiac (up to 5–10% comorbidity), thyroid disease, RA, type 1 DM, psoriasis

— Older woman with >4 weeks of watery, non-bloody diarrhea, often nocturnal, with urgency and fecal incontinence

— Weight loss is mild; significant weight loss should prompt alternative diagnosis

— Triggered or worsened by medications: PPIs, NSAIDs, SSRIs (especially sertraline), ranitidine, ticlopidine, acarbose, statins

— Smoking (current and former) is a strong independent risk factor

Board pearl: In an elderly woman on a PPI and an SSRI presenting with chronic watery nocturnal diarrhea and a normal-appearing colonoscopy, the diagnosis is microscopic colitis only if random biopsies were taken — without biopsies of normal-appearing mucosa, the diagnosis will be missed. Always biopsy in chronic watery diarrhea workup, even when the colon looks pristine.

Microscopic colitis (MC) is a chronic inflammatory diarrheal disorder characterized by chronic, non-bloody, watery diarrhea with a macroscopically normal colon on endoscopy but diagnostic histologic abnormalities on biopsy.

Two main histologic subtypes (clinically managed almost identically):

Epidemiology — classic Step 3 demographic:

When to suspect:

Pathophysiology snapshot: dysregulated mucosal immune response to luminal antigens (bile acids, drugs, infection) on a genetically susceptible background; bile acid malabsorption coexists in up to 40%.

Presentation Patterns and Key History

— Nocturnal diarrhea (wakes patient from sleep) — strongly favors organic disease, present in ~50% of MC

— Fecal urgency and incontinence — disproportionately severe

— Onset often abrupt, sometimes after a GI infection or new medication

— Symptoms do not reliably correlate with meals

— Crampy abdominal pain (~50%), bloating, fatigue

— Mild weight loss (typically <5 kg); substantial weight loss → reconsider diagnosis (celiac, malignancy, IBD)

— Arthralgias in ~20%

— PPIs (lansoprazole most implicated), NSAIDs, SSRIs (sertraline), statins, ranitidine, acarbose, ticlopidine, clozapine, carbamazepine, mycophenolate, checkpoint inhibitors

— Ask about timing: symptoms within weeks-to-months of drug initiation suggest drug-induced MC

— Tobacco use — current smokers develop MC ~10 years earlier and have worse response to therapy

— Celiac disease — overlapping symptoms; up to 1 in 3 celiac patients with persistent diarrhea despite gluten-free diet has MC

— Autoimmune thyroid disease, type 1 DM, RA, Sjögren

— Hematochezia, melena, fevers, palpable mass, family history of colon cancer at young age, iron-deficiency anemia

— Travel, antibiotic exposure (rule out C. difficile, parasites)

Key distinction: IBS-D vs MC — both can give chronic non-bloody diarrhea in middle-aged women, but nocturnal symptoms, abrupt onset, age >50, and weight loss push you toward MC and mandate colonoscopy with biopsies. Rome criteria do not exclude MC; biopsy does.

Cardinal symptom: chronic or recurrent watery, non-bloody diarrhea, typically 5–10 stools/day, often persisting >4 weeks before diagnosis.

Distinguishing features from IBS-D:

Associated symptoms:

Critical medication history (Step 3 loves this):

Lifestyle and comorbidities:

Red flags that argue against MC and demand alternative workup:

Physical Exam Findings and Volume Assessment

— Patient usually well-appearing; chronic mild fatigue

— Significant cachexia, pallor, or lymphadenopathy suggests an alternative diagnosis (malignancy, IBD, celiac with complication)

— Watch for orthostatic hypotension from chronic diarrheal losses, particularly if patient is on diuretics or antihypertensives

— Resting tachycardia or postural drop ≥20 mmHg systolic → admit for IV rehydration consideration

— Dry mucous membranes, decreased skin turgor, reduced JVP

— Soft, non-distended; mild diffuse tenderness possible

— No rebound, guarding, palpable mass, or organomegaly

— Hyperactive bowel sounds may be present

— Rectal exam: stool brown and guaiac-negative; any heme positivity should prompt evaluation for IBD, malignancy, or ischemia

— Goiter, dry eyes/mouth (Sjögren), synovitis (RA), dermatitis herpetiformis or angular cheilitis (celiac)

— Check for thyroid enlargement and joint exam in every suspected MC patient

— Lower-extremity edema → think hypoalbuminemia from protein-losing enteropathy or alternative dx

— Hepatomegaly, jaundice → carcinoid, hyperthyroidism, infiltrative disease

— Lymphadenopathy → lymphoma, TB, HIV-related diarrhea

Step 3 management: In an elderly patient with MC presenting with >10 stools/day, orthostatic hypotension, or AKI on labs, the outpatient pathway is insufficient — admit for IV fluids, electrolyte repletion, and expedited budesonide initiation, then arrange GI follow-up. Do not simply send home with loperamide.

Physical exam in microscopic colitis is typically unremarkable — this is part of the diagnostic challenge and a frequent Step 3 trap.

General appearance:

Vital signs and volume status — especially important in elderly:

Abdominal exam:

Extraintestinal clues (autoimmune overlap):

Findings that should redirect workup:

Diagnostic Workup — Initial Labs, Stool Studies, and Imaging

— CBC: usually normal; anemia or leukocytosis suggests alternative dx (IBD, malignancy, infection)

— CMP: assess electrolytes (hypokalemia, non-anion-gap metabolic acidosis from bicarb loss), renal function, albumin

— TSH: rule out hyperthyroidism as cause of diarrhea and screen for autoimmune thyroid overlap

— CRP/ESR: typically normal in MC — elevation suggests IBD

— Celiac serology: tissue transglutaminase IgA + total IgA; coexists in 5–10%

— HIV if risk factors; vitamin B12, iron studies if anemia

— C. difficile toxin/PCR (especially if recent antibiotics, hospitalization)

— Stool culture, ova and parasites (Giardia in particular), Cryptosporidium if immunocompromised

— Fecal calprotectin or lactoferrin: often mildly elevated in MC (typically <200 µg/g) but markedly elevated in IBD — useful triage tool

— Stool electrolytes/osmotic gap: MC is a secretory diarrhea (osmotic gap <50), persists with fasting

— Consider fecal fat if steatorrhea suspected (think celiac, pancreatic insufficiency, bile acid issues)

— Not routinely needed; CT/MR enterography reserved for ruling out IBD or malignancy when red flags present

— Abdominal radiograph only if obstruction or toxic megacolon concern (rare in MC)

Board pearl: A normal CRP and normal-to-mildly-elevated fecal calprotectin in a patient with chronic watery diarrhea makes IBD unlikely and shifts the pretest probability toward microscopic colitis, IBS, bile acid diarrhea, or celiac — proceed to colonoscopy with random biopsies of the right and left colon, not just sigmoidoscopy.

Microscopic colitis is a histologic diagnosis — but you must first exclude mimics with targeted labs and stool studies before colonoscopy.

Initial laboratory panel:

Stool studies — required before scoping:

Imaging:

Diagnostic Workup — Colonoscopy with Biopsies (Confirmatory)

— Disease can be patchy and right-sided predominant; sigmoidoscopy alone misses up to 40% of cases

— Allows simultaneous evaluation for IBD, malignancy, polyps, ischemia

— Macroscopically normal mucosa in the majority

— Subtle findings sometimes: erythema, edema, altered vascular pattern, "cat-scratch" linear mucosal tears (more in CC), pseudomembranes (rare CC variant)

— Any ulceration, friability, cobblestoning → look for IBD instead

— At least 2 biopsies from each of: ascending, transverse, descending, and sigmoid colon (≥8 total)

— Place biopsies from right and left colon in separate jars to localize disease and increase yield

— Rectal-only biopsies are inadequate

— Lymphocytic colitis: ≥20 intraepithelial lymphocytes per 100 surface epithelial cells (normal <5), with surface epithelial damage and lamina propria mononuclear infiltrate

— Collagenous colitis: subepithelial collagen band >10 μm (normal <3 μm), often with similar lymphocytic infiltrate

— Incomplete MC (MCi): features suggestive but not meeting full criteria; managed similarly

— Consider bile acid diarrhea evaluation (75SeHCAT where available, or empiric cholestyramine trial) in non-responders — coexists in ~40%

— Re-check celiac serologies if not yet done

CCS pearl: In a CCS case of chronic diarrhea in a 65-year-old woman, the correct sequence is: stool studies → labs (TSH, celiac, CRP) → colonoscopy with random biopsies throughout the colon (not sigmoidoscopy). Ordering "flexible sigmoidoscopy" alone will lose points and miss right-sided microscopic colitis.

Definitive diagnosis of microscopic colitis requires colonoscopy with segmental random biopsies and histopathologic confirmation.

Why colonoscopy (not flex sig):

Endoscopic appearance:

Biopsy strategy (high-yield for boards):

Histologic criteria:

Adjunctive workup when MC confirmed:

Risk Stratification and First-Line Management Logic

— Mild: <3 watery stools/day, no nocturnal symptoms, no incontinence — try lifestyle/trigger removal first

— Moderate-severe (active disease): ≥3 watery stools/day or ≥1 nocturnal stool/day or significant urgency/incontinence/weight loss — initiate pharmacotherapy

— Step 1 — Remove triggers: discontinue offending drugs (PPI, NSAID, SSRI, statin if temporally related); counsel smoking cessation; avoid caffeine, lactose, alcohol; trial low-fat/low-residue diet

— Step 2 — Symptomatic only (mild): loperamide 2–16 mg/day titrated

— Step 3 — First-line induction: budesonide 9 mg PO daily × 6–8 weeks for moderate-severe or loperamide failures

— Step 4 — Maintenance: budesonide 3–6 mg/day for relapsers (many patients)

— Step 5 — Refractory: bile acid sequestrants, mesalamine (limited efficacy), immunomodulators (azathioprine, anti-TNF), surgical referral (rare)

— Always stop the most likely drug trigger first — symptom resolution in 4–8 weeks confirms drug-induced MC and may obviate chronic therapy

— Combination smoking + PPI + SSRI use predicts higher relapse rate; address all three

— ~80% remission with budesonide induction; 60–80% relapse within 6 months of stopping

— Reassure: MC does not increase colorectal cancer risk and does not progress to IBD

Step 3 management: In a Step 3 ambulatory vignette, the first move is not budesonide — it is discontinuing the PPI/NSAID/SSRI and counseling smoking cessation. Jumping to corticosteroids before trigger removal is a classic distractor.

Treatment is driven by symptom severity, drug triggers, and comorbidity, not by LC vs CC subtype (they respond similarly).

Severity stratification (Hjortswang criteria, pragmatic version):

Stepwise management algorithm:

Practical decision points:

Counseling expectations:

Pharmacotherapy — First-Line Drug Regimens

— Budesonide 9 mg PO once daily × 6–8 weeks

— Ileal-release formulation preferred (Entocort, Uceris); high first-pass hepatic metabolism → low systemic glucocorticoid burden

— Expect symptomatic response within 2 weeks; clinical remission by 8 weeks in ~80%

— Budesonide 3–6 mg/day as the lowest effective dose, often continued 6–12 months or longer

— Reassess periodically with attempts to taper

— Long-term low-dose budesonide is well tolerated but requires bone density monitoring at baseline and every 1–2 years, plus calcium + vitamin D supplementation

— Loperamide 2–16 mg/day; cheap, safe, useful as bridge or in mild cases — but not disease-modifying

— Diphenoxylate-atropine alternative

— Cholestyramine 4 g 1–4 times daily, or colestipol, or colesevelam (better tolerated)

— Separate from other meds by 4 hours (binds many drugs including levothyroxine, warfarin)

Board pearl: Prednisone is inferior to budesonide in microscopic colitis — higher systemic side effects and higher relapse rates. If the question offers prednisone vs budesonide for induction, always pick budesonide.

Budesonide is the unequivocal first-line, guideline-recommended pharmacotherapy (AGA strong recommendation, high-quality evidence) for both LC and CC.

Induction regimen:

Maintenance regimen (for the ~60–80% who relapse after induction):

Antidiarrheals (adjunct or mild disease):

Bile acid sequestrants (especially when bile acid diarrhea coexists):

Mesalamine: previously used; inferior to budesonide in head-to-head trials; not preferred first-line

Bismuth subsalicylate: modest evidence; alternative if budesonide contraindicated

Refractory disease: azathioprine/6-MP, methotrexate, anti-TNF (infliximab, adalimumab), vedolizumab — refer to GI

Expanded Pharmacology — Refractory Disease and Drug Pitfalls

— Budesonide-refractory: failure to achieve clinical response after 8 weeks of 9 mg/day

— Budesonide-dependent: relapse on tapering below ~6 mg/day or within weeks of discontinuation

— Re-biopsy to confirm persistent histologic disease and exclude IBD evolution

— Evaluate for coexisting bile acid diarrhea (empiric cholestyramine/colesevelam trial × 2–4 weeks)

— Reassess celiac disease (repeat serologies and duodenal biopsies on gluten)

— Review for lactose/fructose intolerance, SIBO

— Cholestyramine 4 g BID–QID or colesevelam 1.875 g BID

— Azathioprine 2–2.5 mg/kg/day or 6-MP 1–1.5 mg/kg/day — check TPMT activity before starting

— Methotrexate — limited but some evidence

— Anti-TNF (infliximab, adalimumab) — for severe refractory disease

— Vedolizumab — gut-selective integrin inhibitor; emerging evidence in MC

— Diverting ileostomy or colectomy — last resort, rarely needed

— Budesonide + CYP3A4 inhibitors (clarithromycin, ketoconazole, ritonavir, grapefruit juice) → increased systemic steroid exposure; avoid or use with caution

— Long-term budesonide → screen for glaucoma, cataracts, hyperglycemia, adrenal suppression (rare but reported), osteoporosis

— Counsel against abrupt discontinuation after prolonged use; taper

— NSAIDs — perpetuate disease; use acetaminophen for pain

— PPIs — deprescribe whenever possible; use H2 blocker (famotidine) if acid suppression truly indicated (avoid ranitidine — withdrawn and implicated)

— SSRIs — switch class (e.g., to bupropion or SNRI) if temporally linked

Key distinction: Budesonide treats the inflammation; loperamide treats the symptom. In a refractory patient already on loperamide, adding budesonide is correct — not increasing loperamide. Symptom suppression without mucosal healing is not adequate long-term management.

Defining refractoriness:

Approach to refractory MC (after confirming diagnosis and removing all triggers):

Escalation therapies (GI-directed):

Drug interactions and safety:

Drugs to avoid or minimize in MC patients:

Special Populations — Elderly and Renal/Hepatic Impairment

— Elderly have blunted thirst response and reduced renal concentrating ability

— Chronic diarrhea → hypokalemia, hyponatremia, prerenal AKI, non-anion-gap metabolic acidosis

— Check BMP at diagnosis and after 2–4 weeks of therapy

— Aggressive symptomatic control prevents falls, syncope, and hospitalization

— Deprescribe PPIs (often inappropriately continued), NSAIDs, SSRIs when feasible

— Review statins — if temporally associated and CV risk allows, consider alternative or pause

— Beware digoxin, lithium toxicity with volume depletion

— Levothyroxine absorption decreased by diarrhea and by bile acid sequestrants — monitor TSH

— Loperamide: hepatic metabolism, generally safe in CKD; caution at high doses (cardiac conduction effects)

— Budesonide: no renal dose adjustment

— Cholestyramine/colesevelam: safe in CKD; colesevelam preferred for tolerability

— Budesonide undergoes extensive first-pass hepatic metabolism — in moderate-severe hepatic impairment, systemic exposure increases substantially; use with caution, consider dose reduction, and monitor for steroid toxicity

— Avoid budesonide + strong CYP3A4 inhibitors in cirrhotics

— Baseline DEXA in elderly women starting long-term budesonide

— Calcium 1200 mg/day + vitamin D 800–1000 IU/day; consider bisphosphonate if osteoporotic

Step 3 management: In an 80-year-old woman with MC, cirrhosis, and ongoing diarrhea on PPI/NSAID, the correct first step is stop the PPI and NSAID, replete electrolytes, start low-dose budesonide with hepatology input, and add calcium/vitamin D — not start high-dose budesonide immediately.

Microscopic colitis is predominantly a disease of older adults — median age at diagnosis ~65; geriatric considerations are central to management.

Volume and electrolyte vulnerability:

Polypharmacy review (essential in every elderly MC patient):

Renal impairment:

Hepatic impairment:

Bone health:

Special Populations — Pregnancy, Younger Adults, and Immunocompromised

— Budesonide is Category B/relatively safe; widely used in IBD during pregnancy with reassuring safety data — preferred corticosteroid given low systemic exposure

— Loperamide: limited data; generally avoided in first trimester, can use cautiously later if needed

— Cholestyramine: not absorbed systemically — safe, but may impair fat-soluble vitamin and folate absorption → supplement

— Mesalamine: safe in pregnancy

— Avoid azathioprine initiation in pregnancy if possible (continue if already controlling disease); avoid methotrexate (teratogenic — discontinue ≥3 months pre-conception)

— Budesonide compatible with breastfeeding (minimal milk transfer)

— Loperamide compatible

— Consider secondary causes more aggressively: celiac disease, autoimmune enteropathy, drug-induced (checkpoint inhibitor colitis, mycophenolate)

— Checkpoint inhibitor colitis can produce MC-like histology — distinct management (hold ICI, high-dose steroids, possibly infliximab/vedolizumab) — coordinate with oncology

— Extremely rare; if suspected, refer to pediatric GI; consider autoimmune enteropathy and IPEX-like syndromes in infants

— Always exclude CMV, cryptosporidium, Giardia, microsporidia, C. difficile before attributing diarrhea to MC

— Consider mycophenolate-induced colitis in transplant recipients — histology overlaps with MC; management is drug dose reduction or switch

— Be cautious with immunomodulator escalation; infection screening (TB, hep B) before anti-TNF

Board pearl: A patient on pembrolizumab or nivolumab with new chronic watery diarrhea and biopsies showing increased intraepithelial lymphocytes is immune checkpoint inhibitor colitis, not idiopathic MC — hold the ICI, start steroids, and consult oncology/GI. Do not just deprescribe a PPI.

Microscopic colitis is rare in pregnancy (peak age >60), but cases occur in women of reproductive age, particularly with autoimmune overlap.

Pregnancy considerations:

Lactation:

Younger adults with MC:

Pediatrics:

Immunocompromised hosts:

Complications and Adverse Outcomes

— Dehydration and electrolyte derangements: hypokalemia, hyponatremia, hypomagnesemia, non-anion-gap metabolic acidosis from bicarb loss

— Prerenal acute kidney injury, especially in elderly on ACEi/ARB/diuretics

— Weight loss and sarcopenia in elderly (modest but functionally significant)

— Fecal incontinence — major quality-of-life burden, social isolation

— Falls and syncope from orthostasis

— Sleep disruption from nocturnal diarrhea → depression, fatigue, work disability

— Colonic perforation in collagenous colitis — rare; reported during colonoscopy due to thick subepithelial collagen band and reduced mucosal compliance ("cat-scratch" tears can rarely extend) — insufflate gently

— Toxic megacolon — very rare

— Budesonide long-term: hyperglycemia, glaucoma, cataracts, osteopenia, adrenal suppression (uncommon), increased infection risk

— Loperamide overuse: ileus, QT prolongation/torsades (high-dose abuse), constipation

— Cholestyramine: constipation, bloating, fat-soluble vitamin deficiency, drug binding (separate dosing!)

— Azathioprine: myelosuppression (check TPMT), pancreatitis, hepatotoxicity, lymphoma risk

— Anti-TNF: infection, reactivation TB/HBV, demyelination

— Anxiety, depression, social withdrawal — screen with PHQ-2/GAD-2 at follow-up

Key distinction: Unlike ulcerative colitis, MC does not require surveillance colonoscopy for dysplasia — there is no increased CRC risk. Routine CRC screening intervals apply (per USPSTF). Don't over-scope these patients.

Microscopic colitis is a benign disease in terms of mortality — no increased risk of colorectal cancer, no progression to IBD, no increased all-cause mortality. But morbidity is substantial and frequently underestimated.

Disease-related complications:

Rare but reported:

Treatment-related adverse outcomes:

Psychosocial sequelae:

When to Escalate Care — Inpatient Triage, Consults, and Referral

— Severe dehydration with orthostatic hypotension or AKI not correctable in clinic

— Symptomatic electrolyte derangements: K <3.0, Na <130, severe metabolic acidosis

— Inability to tolerate oral intake

— Fall, syncope, altered mental status from volume depletion

— Suspected complication (toxic megacolon, perforation — rare)

— Diagnostic uncertainty with concerning features (hematochezia, fever, severe pain) warranting expedited inpatient workup

— All patients require GI for diagnostic colonoscopy with biopsies

— Refractory disease despite 8 weeks of budesonide and trigger removal

— Budesonide-dependent disease requiring immunomodulator/biologic

— Atypical histology or overlap features (suspected IBD)

— Need for re-biopsy

— Rheumatology: significant autoimmune overlap requiring shared management

— Endocrinology: difficult-to-control thyroid disease coexisting

— Dietitian: persistent symptoms despite therapy, weight loss, suspected food triggers, bile acid diarrhea diet counseling

— Mental health: depression/anxiety from chronic illness burden

— Surgery: extraordinarily rare, only for refractory disease with diverting ileostomy considered

— Hospital discharge: ensure GI follow-up within 2–4 weeks, medication reconciliation removing offending agents, electrolyte recheck in 1 week

— Reconcile primary-care med list — deprescribed PPI/NSAID/SSRI must not silently restart at next refill cycle

CCS pearl: In a CCS scenario with an elderly MC patient admitted for dehydration, the discharge order set must include: (1) stop the PPI, (2) start budesonide 9 mg daily, (3) BMP in 1 week, (4) GI follow-up in 2 weeks, (5) smoking cessation counseling. Missing any of these costs points.

MC is overwhelmingly an outpatient diagnosis and outpatient disease, but specific triggers mandate higher-level care.

Indications for inpatient admission:

Indications for GI consultation (outpatient or inpatient):

Indications for other consults:

Transition-of-care touchpoints:

Key Differentials — Other Causes of Chronic Watery Diarrhea (GI Category)

— Younger patients (typically <50), symptoms tied to meals/stress, no nocturnal diarrhea, no weight loss

— Rome IV criteria, normal biopsies — but biopsy still indicated if any red flag

— Treat with dietary modification, antispasmodics, eluxadoline, rifaximin

— Iron deficiency, weight loss, steatorrhea, dermatitis herpetiformis

— Tissue transglutaminase IgA positive; duodenal biopsy confirms villous atrophy

— Coexists with MC in ~5–10%; consider when MC fails standard therapy

— Post-cholecystectomy, ileal disease/resection, idiopathic

— Responds dramatically to cholestyramine/colesevelam — empiric trial is diagnostic

— Coexists with MC in up to 40% — consider in budesonide non-responders

— Bloody diarrhea, elevated CRP, markedly elevated fecal calprotectin (>250), endoscopic ulceration/cobblestoning, characteristic histology

— Bloating, postprandial symptoms; hydrogen breath testing

— Steatorrhea, weight loss, low fecal elastase; chronic pancreatitis or post-Whipple

— Older adults, sigmoid disease, sometimes responds to mesalamine

— Always exclude with full colonoscopy in any chronic diarrhea workup >50

Key distinction: Same symptom (watery diarrhea), different mucosa: IBS = normal scope + normal biopsies, MC = normal scope + abnormal biopsies, IBD = abnormal scope + abnormal biopsies. Biopsy is what separates IBS from MC — and skipping it is the most common diagnostic miss in board questions.

Within the differential of chronic non-bloody watery diarrhea in adults, MC competes with several gastrointestinal mimics. Biopsy is the discriminator.

Irritable bowel syndrome — diarrhea predominant (IBS-D):

Celiac disease:

Bile acid diarrhea (BAD):

Inflammatory bowel disease (UC, Crohn):

Lactose/fructose intolerance, SIBO:

Pancreatic exocrine insufficiency:

Diverticular-associated colitis, segmental colitis associated with diverticulosis (SCAD):

Colorectal cancer / villous adenoma:

Key Differentials — Non-GI and Systemic Causes

— Weight loss, palpitations, tremor, heat intolerance, anxiety

— TSH suppressed, free T4 elevated

— Always check TSH in chronic diarrhea workup — both as a mimic and as an autoimmune overlap with MC

— Long-standing diabetes, nocturnal symptoms, often coexists with gastroparesis

— Diagnosis of exclusion; consider after ruling out MC, celiac, SIBO, pancreatic insufficiency, metformin/GLP-1 effects

— Fatigue, hyperpigmentation, hypotension, hyponatremia/hyperkalemia, eosinophilia

— Cosyntropin stimulation test

— Flushing, wheezing, right-sided valvular disease, secretory diarrhea

— 24-hour urine 5-HIAA, chromogranin A; cross-sectional imaging

— Massive secretory diarrhea (VIPoma — "pancreatic cholera"), hypokalemia, achlorhydria

— Serum VIP, gastrin levels; rare but classic Step 3 distractors

— Metformin, magnesium-containing antacids, laxatives (overt or surreptitious), colchicine, antibiotics, chemotherapy, GLP-1 agonists, orlistat

— Stool osmotic gap helps separate osmotic (>50–100, often laxative or carbohydrate-related) from secretory

— Giardia (camping, daycare, immunoglobulin deficiency), Cryptosporidium (immunocompromised), HIV enteropathy, C. difficile (post-antibiotic, healthcare exposure), tropical sprue

Board pearl: Surreptitious laxative abuse is the classic Step 3 distractor for chronic watery diarrhea in a young woman with eating-disorder features — stool osmotic gap, urine laxative screen, melanosis coli on colonoscopy. Histology does NOT show MC features.

Beyond primary GI disease, several systemic and endocrine conditions produce chronic watery diarrhea and can mimic MC, especially in older patients with autoimmune backgrounds.

Hyperthyroidism:

Diabetic diarrhea / autonomic neuropathy:

Addison disease (primary adrenal insufficiency):

Carcinoid syndrome:

VIPoma, gastrinoma, somatostatinoma:

Medication-induced diarrhea (non-MC mechanism):

Chronic infections:

Mastocytosis, amyloidosis, lymphoma — rare infiltrative mimics

Secondary Prevention, Discharge Planning, and Long-Term Plan

— Budesonide 9 mg daily × 6–8 weeks for induction, with tapered maintenance plan written before stopping

— Loperamide PRN for breakthrough symptoms (specify max dose)

— Calcium 1200 mg/day + vitamin D 800–1000 IU/day if on prolonged budesonide

— Discontinue offending agents: PPI, NSAIDs, SSRIs, statins (if temporally linked and CV risk permits), ranitidine

— Replace PPI with famotidine if acid suppression truly needed; reassess GERD indication

— Replace NSAID with acetaminophen ± topical NSAID for musculoskeletal pain

— Smoking cessation — pharmacotherapy (varenicline or NRT) + counseling; smokers have worse remission rates

— Avoid caffeine, alcohol, lactose, artificial sweeteners during active disease

— Adequate oral hydration with electrolyte solutions during flares

— Symptom-based; no surveillance colonoscopy required (MC does not increase CRC risk — continue standard age-based USPSTF screening)

— Recheck TSH and celiac serology if not improving despite therapy

— DEXA scan at baseline and every 1–2 years on chronic budesonide

— Eye exam yearly (cataract/glaucoma screening) on prolonged steroids

— Annual influenza, pneumococcal (PCV20 or PCV15+PPSV23), shingles (Shingrix), COVID, hepatitis B

— Screen for latent TB and HBV before biologics

Step 3 management: The single most cost-effective long-term intervention in MC is deprescribing. Every clinic visit should include a med-reconciliation check to ensure the PPI or NSAID didn't quietly creep back onto the list at a refill.

Microscopic colitis is a relapsing-remitting disease — 60–80% of patients relapse within 6 months of budesonide discontinuation. Long-term planning is core to Step 3 management.

Discharge or post-diagnosis medication list:

Lifestyle prescriptions:

Long-term monitoring strategy:

Vaccinations (if escalating to immunomodulators/biologics):

Follow-Up, Monitoring, and Patient Counseling

— 2–4 weeks post-diagnosis: assess symptom response to budesonide, recheck electrolytes/renal function, reinforce trigger avoidance

— 8 weeks (end of induction): assess clinical remission (<3 stools/day, no nocturnal symptoms, no incontinence); plan taper or maintenance

— 3 months: assess maintenance therapy tolerance and relapse

— 6–12 months: attempt budesonide taper if remission sustained; long-term clinic visits every 6–12 months

— Earlier follow-up if flare, new symptoms, or medication changes

— Stool frequency/consistency diary (or smartphone tracker) — best outcome measure

— BMP at 2–4 weeks and as needed for diarrhea flares

— Glucose/HbA1c if on prolonged budesonide, especially in diabetics

— DEXA every 1–2 years on chronic steroids

— TSH annually if levothyroxine-treated (absorption affected by diarrhea and sequestrants)

— Set expectations: response in 2 weeks, remission in 8 weeks, high relapse rate when stopping therapy

— MC does not cause cancer, does not progress to IBD, does not shorten life — explicit reassurance reduces anxiety

— Symptom recurrence warrants prompt budesonide restart, not just loperamide self-titration

— Pelvic floor PT for refractory fecal urgency/incontinence

— Mental health support for depression/anxiety related to chronic illness

— Dietitian for fiber/diet optimization, especially if bile acid diarrhea or celiac coexists

— Symptom diary, hydration plan, sick-day rules (when to call, when to ER)

— Written taper schedule provided at induction completion

Board pearl: Patients often stop budesonide on their own when they feel well, then relapse 4–8 weeks later. The visit at week 8 must include a written maintenance/taper plan, not just "good — keep going."

Follow-up cadence (typical outpatient flow):

Monitoring parameters:

Counseling pearls:

Rehab and quality of life:

Patient self-management tools:

Ethical, Legal, and Patient Safety Considerations

— Stopping a PPI in a patient with prior GI bleed or Barrett esophagus requires shared decision-making — weigh diarrhea morbidity against acid-related disease recurrence

— Document risk-benefit conversation when deprescribing chronic SSRIs (worsening depression, suicidality), NSAIDs (uncontrolled arthritis), or statins (CV risk)

— Offer alternative therapy whenever possible (famotidine for PPI; acetaminophen for NSAID; alternative-class antidepressant for SSRI in coordination with prescriber)

— Hospital-initiated PPIs frequently continue indefinitely after discharge — perform medication reconciliation at every transition

— When a patient is discharged with new budesonide, communicate the taper plan and indication explicitly to the PCP to prevent indefinite continuation or premature discontinuation

— Bile acid sequestrants bind concomitant medications (levothyroxine, warfarin, digoxin) — counsel on 4-hour separation and inform the pharmacy

— Apply Beers criteria: avoid high-dose loperamide in patients with QT-prolonging drugs; minimize anticholinergics

— Adrenal suppression risk after prolonged budesonide — counsel on stress-dose considerations before surgery or major illness

— Ethical obligation to address tobacco — affects both MC outcomes and overall mortality; offer pharmacotherapy at every visit

— Fall risk from orthostasis and nocturia — assess home safety

— Driving safety on high-dose loperamide is generally preserved (does not cross BBB significantly) — but counsel on opioid co-use

— Histologic diagnosis must be in the chart — "presumed MC" without biopsy is not adequate for chronic steroid therapy

Step 3 management: A discharge order to "continue budesonide indefinitely" without taper plan, follow-up, or bone health monitoring is a patient safety event waiting to happen — always specify duration, follow-up, and monitoring at the time of prescribing.

Despite being a "benign" condition, MC management carries several Step 3-flavored ethical and safety touchpoints.

Deprescribing and informed consent:

Transition-of-care risks:

Polypharmacy and the elderly:

Smoking cessation:

Patient safety pearls:

Documentation and billing:

High-Yield Associations and Rapid-Fire Clinical Facts

— LC: ≥20 IELs per 100 epithelial cells

— CC: subepithelial collagen band >10 μm

— Chronic watery non-bloody diarrhea >4 weeks

— Macroscopically normal colon

— Full colonoscopy with random biopsies of right and left colon (NOT sigmoidoscopy alone)

— PPIs (lansoprazole), NSAIDs, SSRIs (sertraline), statins, ranitidine, acarbose, ticlopidine, clozapine, carbamazepine, mycophenolate, checkpoint inhibitors

— Celiac disease (5–10% comorbid)

— Autoimmune thyroid disease

— Rheumatoid arthritis, Sjögren, T1DM, psoriasis

— Smoking (current and former) — worsens disease and response

— Bile acid diarrhea coexists in ~40%

— Budesonide 9 mg/day × 6–8 weeks = first-line; ~80% remission

— Loperamide for mild disease or adjunct

— Cholestyramine/colesevelam for bile acid diarrhea overlap or as adjunct

— Prednisone is INFERIOR to budesonide — never the right answer

— Refractory: azathioprine, anti-TNF, vedolizumab

— Does NOT increase colorectal cancer risk → no surveillance scope needed

— Does NOT progress to IBD

— Does NOT cause hematochezia or fever → these mean something else

— Does NOT shorten life expectancy

— Secretory pattern (osmotic gap <50), persists with fasting

— Calprotectin mildly elevated (rarely >250)

Key distinction: "Normal colonoscopy in chronic diarrhea" is NOT a normal result until biopsies are reviewed — this is the single highest-yield teaching point for both Step 2 and Step 3 in MC.

Demographics: Middle-aged to older women (peak 60–70s); F:M ~3:1 for CC, ~2:1 for LC.

Histologic cutoffs:

Diagnostic must-haves:

Top drug triggers — memorize:

Key associations:

Treatment one-liners:

What MC does NOT do:

Stool clues:

Endoscopy clue: rare "cat-scratch" linear tears in collagenous colitis from rigid mucosa.

Board Question Stem Patterns

— 68-year-old woman on omeprazole, sertraline, and ibuprofen presents with 3 months of watery nocturnal diarrhea. Colonoscopy normal; biopsies show lymphocytic infiltrate.

— Answer: Discontinue offending medications first (NOT immediately start budesonide). If asked for pharmacotherapy: budesonide.

— 60-year-old woman with chronic watery diarrhea; sigmoidoscopy is normal. Next step?

— Answer: Full colonoscopy with random biopsies of right and left colon — sigmoidoscopy alone is insufficient because MC is patchy and right-sided.

— Confirmed MC, moderate-severe symptoms. Best induction therapy?

— Answer: Budesonide 9 mg/day. Prednisone is inferior — more side effects, more relapses.

— Patient with diagnosed MC fails budesonide. Next step?

— Answer: Reassess for celiac disease (tTG-IgA, duodenal biopsy on gluten) and bile acid diarrhea (empiric cholestyramine trial); consider re-biopsy.

— Patient with recurrent relapses despite budesonide; current smoker.

— Answer: Address smoking cessation (varenicline + counseling) alongside maintenance therapy.

— Patient with 5-year history of MC asks about cancer risk and scope frequency.

— Answer: MC does NOT increase CRC risk; follow age-based USPSTF screening, not IBD-style surveillance.

— 80-year-old with MC, 10 stools/day, orthostatic, K 2.9, Cr up from 0.9 to 1.8.

— Answer: Admit, IV fluids, replete potassium, start budesonide, deprescribe contributing drugs.

— Patient on pembrolizumab develops watery diarrhea; biopsies show increased IELs.

— Answer: Immune checkpoint inhibitor colitis — hold ICI, consult oncology/GI, high-dose steroids ± infliximab.

Board pearl: The "best next step" question in MC is almost always either "discontinue the offending drug" or "obtain colonoscopy with random biopsies" before it becomes "start budesonide." Read the stem for what hasn't been done yet.

Recognize these vignette archetypes — they recur on Step 3:

The "deprescribe first" stem:

The "biopsy or no biopsy" stem:

The "prednisone vs budesonide" distractor:

The "celiac overlap" stem:

The "smoking" stem:

The "no surveillance" stem:

The "elderly with AKI" stem:

The "ICI colitis" curveball:

One-Line Recap

Microscopic colitis is a histologic diagnosis of chronic non-bloody watery diarrhea in an older woman with a macroscopically normal colon, managed first by removing offending drugs (PPI, NSAID, SSRI) and counseling smoking cessation, then induced into remission with budesonide 9 mg daily for 6–8 weeks, with long-term maintenance for the majority who relapse — and it does NOT increase colorectal cancer risk.

Step 3 management: The two questions that separate a competent Step 3 answer from a wrong one are — "Did you stop the offending drug before reaching for steroids?" and "Did you biopsy a normal-looking colon?" Get those right and microscopic colitis becomes a predictable, manageable disease.

Diagnose: chronic watery non-bloody diarrhea >4 weeks → stool studies + celiac serology + TSH + colonoscopy with random biopsies of right and left colon; LC = ≥20 IELs/100 epithelial cells, CC = collagen band >10 μm.

Trigger removal first: PPI, NSAID, SSRI, statin, ranitidine; smoking cessation; lifestyle modification — this alone resolves many drug-induced cases.

Pharmacotherapy: budesonide 9 mg/day × 6–8 weeks (NOT prednisone) → ~80% remission; maintenance budesonide 3–6 mg/day for the 60–80% who relapse; loperamide for mild disease or breakthrough; cholestyramine/colesevelam for bile acid diarrhea overlap; refractory → azathioprine, anti-TNF, vedolizumab.

Long-term: no CRC surveillance needed (continue standard USPSTF screening); reassess celiac and bile acid diarrhea in non-responders; bone health monitoring on chronic budesonide; address autoimmune comorbidities (thyroid, celiac, RA).