What are the potential causes of functional dyspepsia in a 36‑year‑old man with persistent epigastric dyspepsia despite adequate proton pump inhibitor therapy?

What Causes Functional Dyspepsia?

Functional dyspepsia is a disorder of gut-brain interaction caused by multiple interconnected pathophysiological mechanisms, not by the absence of pathology—it represents real dysfunction in how the stomach, duodenum, and nervous system communicate and function together. 1

Core Pathophysiological Mechanisms

The British Society of Gastroenterology 2022 guidelines clearly establish that FD arises from several distinct but overlapping mechanisms 1:

Motor Dysfunction

• Delayed gastric emptying occurs in a subset of patients, particularly those with early satiation, bloating, postprandial fullness, nausea and vomiting—though the association is weaker than previously thought 1
• Impaired fundic accommodation (the stomach's inability to relax properly after eating) associates with reduced drinking capacity, early satiation, postprandial fullness and weight loss 1
• Rapid gastric emptying has also been identified in some FD patients and may represent an underrecognized therapeutic target 1
• Recent evidence suggests FD and gastroparesis may exist on the same spectrum of gastric neuromuscular dysfunction, sharing pathological features like loss of interstitial cells of Cajal 1

Visceral Hypersensitivity

• Mechanical hypersensitivity to gastric distension increases after meals and correlates with postprandial pain, fullness, bloating and belching 1
• Chemical hypersensitivity to acid (both exogenous and endogenous) worsens dyspeptic symptoms, particularly nausea, even when patients secrete normal amounts of gastric acid 1
• This explains why some patients have persistent symptoms despite adequate PPI therapy—the issue isn't just acid production but duodenal acid clearance and sensitivity 1

Low-Grade Inflammation and Immune Dysfunction

• Duodenal eosinophilia is now an accepted pathophysiological finding in FD 1, 2
• Meta-analyses show increased eosinophils and mast cells in both stomach and duodenum compared to healthy controls 1
• This low-grade mucosal inflammation, especially in the duodenum, activates TRPV1 receptors, releasing neurotransmitters (substance P, calcitonin gene-related peptide) that increase visceral sensitivity 1

Psychological Factors

• Anxiety and depression frequently associate with FD, though causality remains unconfirmed 1
• Stress upregulates the hypothalamic-pituitary-adrenal axis, increasing corticotrophin-releasing hormone, which activates local inflammatory processes affecting epithelial permeability, immune function and the microbiome 1
• Anxiety specifically associates with duodenal eosinophilia 1
• Recent evidence shows anxiety and depression status reduce therapeutic efficacy of PPIs on both GERD and FD symptoms 3

Microbiome Alterations

• Changes in gastric and small bowel microbiome contribute to symptom generation 1
• Mucus-associated Porphyromonas is lower in FD patients and correlates with both symptoms and duodenal eosinophils 4
• Long-term PPI therapy may induce duodenal dysbiosis, with increased Streptococcus 4

Post-Infectious Triggers

• Acute gastroenteritis increases odds of developing FD almost threefold at 6+ months post-infection 1
• This represents "postinfection FD" similar to post-infectious IBS 1

Genetic Predisposition

• Weak heritability (~5%) with considerable clinical and genetic overlap with IBS, GERD, mood disorders, and even rheumatological conditions 1
• Genes involved in synaptic transmission likely play a role 1

Why Symptoms Persist Despite PPI Therapy

In your 36-year-old patient with persistent symptoms on adequate PPI therapy, consider:

1. PPI reduces acid reflux episodes but increases "non-acid" reflux—the patient continues having reflux, just less acidic 5
2. Chemical hypersensitivity to duodenal contents (not just acid) may be the primary driver 1
3. Impaired duodenal motility and acid clearance means even normal acid secretion causes symptoms 1
4. Underlying anxiety or depression reduces PPI therapeutic efficacy 3
5. Duodenal eosinophilia and low-grade inflammation are not addressed by acid suppression 1, 2
6. Motor dysfunction (delayed emptying, impaired accommodation) requires different therapeutic approaches 1

Critical Clinical Pitfall

The most common cause of "refractory" symptoms is misdiagnosis—the problem isn't treatment failure but diagnostic error 5. In patients unresponsive to PPI, consider functional heartburn, achalasia, eosinophilic esophagitis, or other non-GERD diagnoses 5, 6. However, true FD with PPI refractoriness reflects the multifactorial pathophysiology described above, requiring neuromodulators (tricyclic antidepressants), prokinetics, or psychological therapies rather than escalating acid suppression 1, 7.