Residency · Residency · Gastroenterology

Celiac Disease and Malabsorption Syndromes

Celiac Disease

Pathophysiology

Celiac disease is an immune-mediated enteropathy triggered by exposure to gluten, specifically the gliadin fraction, in genetically predisposed individuals. The genetic susceptibility is conferred by HLA-DQ2, present in 95% of patients, or HLA-DQ8, present in the remaining 5%. However, these alleles are necessary but not sufficient for disease development, as 30 to 40% of the general population carries these alleles without developing celiac disease.

The pathophysiologic cascade begins when tissue transglutaminase (tTG) deamidates gliadin peptides, enhancing their binding affinity to HLA-DQ2 or DQ8 molecules on antigen-presenting cells. This triggers a dual immune response. The Th1 response activates CD8-positive intraepithelial lymphocytes and releases inflammatory cytokines, particularly interferon-gamma, which drive villous atrophy and mucosal destruction. The Th2 response involves CD4-positive T cells that stimulate B cell production of antibodies, including anti-tissue transglutaminase, anti-endomysial, and anti-deamidated gliadin antibodies, which serve as the foundation for serologic diagnosis. Interleukin-15 is a key cytokine that drives the expansion and activation of intraepithelial lymphocytes and represents a target for emerging therapeutic strategies.

Epidemiology

The prevalence of celiac disease is approximately 1% globally, ranging from 1 in 100 to 1 in 200, though an estimated 85% of affected individuals remain undiagnosed. There is a female predominance of 2:1. Several conditions are associated with a higher prevalence of celiac disease, including type 1 diabetes mellitus (3 to 16%), autoimmune thyroiditis (2 to 7%), IgA deficiency (2 to 3%), Down syndrome (5 to 12%), Turner syndrome (2 to 10%), and first-degree relatives of affected individuals (10 to 15%). Patients with celiac disease carry an increased risk of lymphoma (specifically enteropathy-associated T-cell lymphoma, or EATL), small bowel adenocarcinoma, osteoporosis, and infertility.

Clinical Presentation

Classic (Intestinal)

The classic intestinal presentation of celiac disease includes chronic diarrhea, steatorrhea, weight loss, bloating, and abdominal pain. Malabsorption manifests as iron deficiency anemia, folate deficiency, vitamin D and calcium malabsorption leading to osteopenia and osteoporosis, and, less commonly, vitamin B12 deficiency, since the terminal ileum is typically spared. In children, failure to thrive is a hallmark presentation.

Non-Classic (Extraintestinal)

The non-classic or extraintestinal presentation is increasingly recognized and has become the more common mode of presentation in adults. Iron deficiency anemia is the single most common presentation in the adult population. Dermatitis herpetiformis, an intensely pruritic papulovesicular rash distributed over the elbows, knees, and buttocks, is pathognomonic for celiac disease and demonstrates granular IgA deposits at the dermal papillae on direct immunofluorescence. Other extraintestinal manifestations include elevated transaminases (celiac hepatitis, typically less than 5 times the upper limit of normal, which resolves with a gluten-free diet), peripheral neuropathy, cerebellar ataxia (gluten ataxia), dental enamel defects, aphthous ulcers, osteoporosis or osteopenia, infertility and recurrent miscarriage, and depression and anxiety.

Seronegative Celiac Disease

Seronegative celiac disease accounts for 5 to 10% of celiac disease cases and is characterized by negative tTG-IgA with characteristic histologic findings. This entity is often associated with IgA deficiency, making it imperative to check a total IgA level with every initial celiac screening. When IgA deficiency is confirmed, IgG-based assays such as deamidated gliadin peptide IgG or tTG-IgG should be used.

Diagnostic Approach

Serologic Testing (while on gluten-containing diet)

Serologic testing must be performed while the patient is consuming a gluten-containing diet to avoid false-negative results. The tTG-IgA assay is the recommended first-line screening test, with a sensitivity of 93 to 98% and specificity of 95 to 98%. A total serum IgA level must be checked concurrently to exclude IgA deficiency. When IgA deficiency is present, the deamidated gliadin peptide IgG assay is used, offering a sensitivity of 88 to 95%. The endomysial antibody IgA has a sensitivity of 90 to 98% and a near-perfect specificity approaching 100%, making it a valuable confirmatory test, though it is observer-dependent and more labor-intensive. When tTG-IgA exceeds 10 times the upper limit of normal with a positive endomysial antibody, the ESPGHAN criteria allow diagnosis in children without biopsy; however, the ACG still requires biopsy confirmation in adults.

Upper Endoscopy with Duodenal Biopsies

Upper endoscopy with duodenal biopsies remains the gold standard for confirming the diagnosis. The biopsy protocol requires a minimum of 4 biopsies from the distal duodenum (D2) and 2 biopsies from the duodenal bulb (D1), as 10 to 13% of cases have pathology isolated to the bulb. Endoscopic findings suggestive of celiac disease include scalloping of duodenal folds, loss of folds, a mosaic mucosal pattern, and visible submucosal vasculature, though a normal-appearing endoscopy does not exclude celiac disease.

The modified Marsh classification grades histologic severity. Type 0 represents normal pre-infiltrative mucosa. Type 1 (infiltrative) shows increased intraepithelial lymphocytes exceeding 25 per 100 enterocytes with normal villous architecture. Type 2 (hyperplastic) adds crypt hyperplasia to the increased lymphocytes. Type 3 encompasses villous atrophy: 3a with partial villous atrophy, 3b with subtotal villous atrophy, and 3c with total villous atrophy and flat mucosa. Marsh type 3 findings in combination with positive serology are diagnostic, while Marsh types 1 and 2 are supportive but not specific for celiac disease.

<image>A histopathological comparison panel showing the Marsh classification stages of celiac disease. Four panels arranged left to right showing progressive changes: Panel 1 "Normal/Marsh 0": tall, finger-like villi with normal villous-to-crypt ratio (3:1 to 5:1), scattered intraepithelial lymphocytes (<25/100 enterocytes), normal crypt depth. Panel 2 "Marsh 1-2": preserved villous architecture but increased IELs (>25/100 enterocytes highlighted with dots), crypt hyperplasia with elongated crypts. Panel 3 "Marsh 3a-3b": partial to subtotal villous atrophy with blunted, widened villi, markedly increased IELs, and hypertrophic crypts. Panel 4 "Marsh 3c": total villous atrophy with flat mucosa, complete loss of villous architecture, maximally hypertrophic crypts, dense IEL infiltration. Use H&E stain coloring. Include labeled arrows for: villus height, crypt depth, IELs (small dark cells within epithelium), and villous-to-crypt ratio for each stage. Include an inset showing normal vs. increased IEL density at high magnification.</image>

HLA Testing

HLA-DQ2 and DQ8 testing offers an exceptionally high negative predictive value exceeding 99%, making it most useful for ruling out celiac disease in equivocal cases where serology and histology are discordant. It is not useful for ruling in celiac disease, given that 30 to 40% of the general population carries these alleles.

Treatment

Strict Gluten-Free Diet (GFD)

The cornerstone of treatment is a lifelong strict gluten-free diet, which eliminates wheat, barley, rye, and contaminated oats. Pure or uncontaminated oats are tolerated by more than 95% of celiac patients at intakes of 50 to 70 g per day. The FDA defines gluten-free labeling as containing less than 20 parts per million of gluten. Dietitian consultation is essential, as hidden sources of gluten are ubiquitous in sauces, processed foods, medications, and communion wafers. Symptom improvement typically occurs within days to weeks, serologic normalization within 6 to 12 months, and histologic recovery within 12 to 24 months, though 30 to 40% of patients have persistent villous atrophy at 2 years despite dietary adherence.

Monitoring

Monitoring involves serial tTG-IgA measurements at 6 and 12 months and then annually, with falling titers correlating with dietary adherence, though not perfectly with histologic recovery. The ACG recommends follow-up biopsy to confirm mucosal healing, particularly in adults, with timing typically at 12 to 24 months after initiating a gluten-free diet. A DEXA scan should be obtained at diagnosis and repeated in 1 to 2 years. Micronutrient assessment should include iron, folate, vitamin B12, vitamin D, calcium, zinc, and copper.

Refractory Celiac Disease (RCD)

Refractory celiac disease is defined as persistent symptoms and villous atrophy despite strict adherence to a gluten-free diet for more than 12 months, verified by a dietitian. Before establishing this diagnosis, inadvertent gluten exposure must be excluded, as it is the most common cause of non-response. Other conditions that should be considered include microscopic colitis, small intestinal bacterial overgrowth, lactose intolerance, and overlap with irritable bowel syndrome.

RCD type I is characterized by a normal intraepithelial lymphocyte phenotype with polyclonal, surface CD3-positive, CD8-positive cells. It responds to immunosuppressive therapy with budesonide or azathioprine and carries a 5-year survival of 80 to 96%. RCD type II is characterized by an aberrant intraepithelial lymphocyte phenotype with monoclonal, intracellular CD3-positive but surface CD3-negative, CD8-negative cells. This is a pre-lymphomatous condition with a 5-year survival of only 40 to 58% and a risk of progression to EATL of 33 to 52%. Flow cytometry of duodenal biopsies demonstrating aberrant intraepithelial lymphocytes exceeding 20% suggests RCD type II. Treatment options for RCD type II include cladribine and autologous stem cell transplant (experimental), with close monitoring for development of EATL.

Malabsorption — Systematic Approach

Pathophysiologic Categories

Malabsorption is categorized into three pathophysiologic groups. Intraluminal or digestive causes include pancreatic exocrine insufficiency and bile salt deficiency, where the problem lies in inadequate breakdown of nutrients within the intestinal lumen. Mucosal or absorptive causes include celiac disease, tropical sprue, Whipple disease, small bowel Crohn disease, radiation enteritis, and graft-versus-host disease, where the absorptive surface itself is damaged. Post-mucosal or transport causes include intestinal lymphangiectasia and congestive heart failure, where nutrient transport after absorption is impaired.

Clinical Clues to Etiology

The pattern of malabsorption provides important clues to the underlying etiology. Fat malabsorption produces steatorrhea with pale, bulky, foul-smelling, floating stools, weight loss, and deficiency of fat-soluble vitamins A, D, E, and K. Carbohydrate malabsorption produces osmotic diarrhea, bloating, and flatulence, with a low stool pH below 5.5. Protein malabsorption manifests as edema, hypoalbuminemia, and muscle wasting. Specific nutrient deficiency patterns can further localize the lesion: iron malabsorption suggests proximal small bowel disease, while vitamin B12 and bile salt malabsorption point to terminal ileal pathology.

Diagnostic Testing

The diagnostic workup for malabsorption includes several key tests. Fecal fat can be assessed qualitatively using Sudan staining or quantitatively through a 72-hour fecal fat collection, with greater than 7 g per day on a 100 g fat diet being the gold standard for confirming steatorrhea. Fecal elastase-1 below 200 mcg/g suggests pancreatic exocrine insufficiency, with values below 100 mcg/g indicating severe insufficiency, though falsely low values may occur with watery diarrhea. The D-xylose absorption test distinguishes intraluminal from mucosal causes: normal absorption suggests an intraluminal cause, while abnormal absorption suggests mucosal disease. Hydrogen breath tests for lactose, fructose, or glucose (the latter for small intestinal bacterial overgrowth) help identify specific malabsorptive conditions. SeHCAT scanning or measurement of serum 7-alpha-hydroxy-4-cholesten-3-one (C4) assesses bile acid malabsorption. Small bowel imaging with CT or MR enterography evaluates for Crohn disease, tumors, and anatomic abnormalities.

<image>A diagnostic flowchart for the evaluation of chronic malabsorption. Start with "Chronic diarrhea with weight loss/nutrient deficiency." First step: "Confirm malabsorption" with boxes for fecal fat (qualitative or quantitative), low fat-soluble vitamins (A, D, E, K), low albumin/prealbumin. Second tier: "Localize the defect" branching into three pathways: (1) "Intraluminal/digestive" — test fecal elastase, consider secretin-stimulated MRCP, treat with PERT; (2) "Mucosal/absorptive" — check tTG-IgA, perform EGD with small bowel biopsies, consider capsule endoscopy/CT enterography; (3) "Post-mucosal/lymphatic" — check albumin, consider CT for lymphadenopathy/lymphangiectasia, fat-pad biopsy for amyloid. Include specific diagnoses under each pathway: celiac, tropical sprue, Whipple, SIBO, Crohn under mucosal; pancreatic insufficiency, bile acid malabsorption under intraluminal; intestinal lymphangiectasia, cardiac disease under post-mucosal. Use color-coded boxes: orange for intraluminal, blue for mucosal, purple for post-mucosal.</image>

Other Important Causes of Malabsorption

Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth is defined as an excessive bacterial population in the small intestine causing malabsorption symptoms. Risk factors include anatomic predispositions (blind loop syndrome, strictures, diverticulosis, post-surgical anatomy), motility disorders (scleroderma, diabetes, opioid use), hypochlorhydria (from PPI use or atrophic gastritis), and immune deficiency. Diagnosis is achieved through a glucose or lactulose hydrogen breath test, with glucose being preferred (sensitivity 62%, specificity 83%), or through small bowel aspirate culture showing more than 10 to the third power colony-forming units per milliliter, which is the gold standard but is invasive and rarely performed. First-line treatment is rifaximin 550 mg three times daily for 14 days for non-constipating SIBO, with alternatives including ciprofloxacin, metronidazole, amoxicillin-clavulanate, doxycycline, and neomycin (particularly for methane-predominant overgrowth). Recurrence is common at 40 to 50%, and addressing the underlying predisposing condition is essential, with cyclical antibiotics sometimes required.

Tropical Sprue

Tropical sprue is endemic to tropical regions including the Caribbean and South and Southeast Asia. The etiology is presumed to be infectious, supported by the disease's response to antibiotic therapy. Histologically, it produces villous atrophy similar to celiac disease but characteristically affects the entire small bowel, in contrast to celiac disease, which typically affects the proximal small bowel more severely. Treatment consists of tetracycline or trimethoprim-sulfamethoxazole for 3 to 6 months, supplemented with folate and vitamin B12 replacement.

Whipple Disease

Whipple disease is caused by Tropheryma whipplei, a gram-positive actinobacterium. The classic triad includes diarrhea and weight loss, arthralgias (which often precede gastrointestinal symptoms by years), and central nervous system involvement. Histologic examination of duodenal biopsies reveals PAS-positive, diastase-resistant macrophages within the lamina propria, and the diagnosis is confirmed by T. whipplei PCR on duodenal biopsy tissue. Treatment involves an induction phase of ceftriaxone 2 g intravenously daily for 2 weeks, followed by a maintenance phase of trimethoprim-sulfamethoxazole double-strength twice daily for 12 months.

Bile Acid Malabsorption (BAM)

Bile acid malabsorption is classified into three types. Type 1 results from ileal disease or resection, as seen in Crohn disease or radiation injury. Type 2, or primary/idiopathic bile acid malabsorption, involves defective FGF19/FXR feedback signaling and may account for 25 to 50% of patients diagnosed with diarrhea-predominant irritable bowel syndrome. Type 3 is secondary to other conditions including post-cholecystectomy state, celiac disease, SIBO, and chronic pancreatitis. Diagnosis is established by SeHCAT retention test with values below 15% (the gold standard, though not available in the United States), serum C4 levels exceeding 52 ng/mL, or an empiric trial of cholestyramine. Treatment consists of bile acid sequestrants: cholestyramine 4 g one to four times daily, colesevelam 625 mg two to three tablets twice daily, or colestipol.

Lactose Intolerance

Primary lactose intolerance results from a developmentally programmed decline in lactase activity affecting 65 to 70% of the world population, though it is rare in Northern European populations. Secondary lactose intolerance results from mucosal damage in conditions such as celiac disease, Crohn disease, or infectious enteritis. Diagnosis is achieved through a lactose hydrogen breath test or genetic testing for the LCT polymorphism at position -13910 C/T. Management consists of dietary lactose restriction, lactase enzyme supplementation, and use of lactose-free dairy products.

Key Clinical Pearls

• Always check total serum IgA when screening for celiac disease — IgA deficiency (2-3% of celiac patients) causes false-negative tTG-IgA
• Duodenal bulb biopsies (D1) are essential — 10-13% of celiac disease may be isolated to the bulb
• Negative HLA-DQ2/DQ8 virtually excludes celiac disease (NPV >99%)
• Persistent symptoms on GFD: most common cause is inadvertent gluten exposure; rule out before diagnosing refractory celiac disease
• RCD-II (aberrant IEL phenotype) is a pre-lymphomatous condition with 33-52% risk of EATL — requires aggressive surveillance
• SIBO should be considered in any patient with malabsorption and predisposing anatomy or motility disorder
• Bile acid malabsorption (especially type 2/idiopathic) is underdiagnosed and may account for 25-50% of IBS-D
• Fecal elastase <100 mcg/g is highly specific for pancreatic exocrine insufficiency

References

1. Rubio-Tapia A, et al. ACG Clinical Guidelines: Diagnosis and Management of Celiac Disease. Am J Gastroenterol. 2023;118(1):59-76.
2. Al-Toma A, et al. European Society for the Study of Coeliac Disease (ESsCD) guideline for coeliac disease and other gluten-related disorders. United European Gastroenterol J. 2019;7(5):583-613.
3. Pimentel M, et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020;115(2):165-178.
4. Camilleri M. Bile acid diarrhea: prevalence, pathogenesis, and therapy. Gut Liver. 2015;9(3):332-339.
5. Green PH, Cellier C. Celiac disease. N Engl J Med. 2007;357(17):1731-1743.

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