Abstract
BACKGROUND: Chronic secretory diarrhoea is a diagnostic challenge with a broad differential and significant impact on patient's quality of life. While common causes include microscopic colitis, bile acid diarrhoea, and laxative use, rarer aetiologies such as vasoactive intestinal peptide (VIP)-secreting neuroendocrine tumours (VIPomas) must be considered when standard investigations fail. CASE PRESENTATION: We present a 35-year-old woman with a two-year history of progressively worsening, fasting-persistent, high-volume watery diarrhoea leading to severe electrolyte abnormalities and weight loss requiring resuscitation in intensive care. Extensive biochemical, endoscopic, and radiological investigations-including faecal analysis, colonoscopy, neuroendocrine markers, multiphase CT, endoscopic ultrasound, and Ga-68 Dotatate PET imaging-failed to identify an underlying cause. Serum VIP levels remained within the normal range. Despite the absence of a definitive diagnosis, empirical treatment with the somatostatin analogue octreotide led to rapid and sustained symptom resolution. The patient was subsequently maintained on long-acting lanreotide with complete remission. Notably, diarrhoea recurred upon cessation of therapy, again resolving with reinitiation. After 4 years, the patient self-ceased lanreotide without symptom recurrence, and follow-up imaging remained unremarkable. DISCUSSION: This case highlights a diagnostic dilemma: clinical and biochemical features were highly suggestive of a VIPoma, yet no tumour was identified despite repeated advanced imaging and biochemical workup. The patient's remarkable therapeutic response to somatostatin analogue therapy, in the absence of confirmed neuroendocrine neoplasia, suggests that somatostatin analogues may have a broader role in the management of idiopathic secretory diarrhoea than currently appreciated. CONCLUSION: We present a rare case of chronic secretory diarrhoea with suspected but unproven VIPoma, demonstrating sustained and reproducible response to somatostatin analogue therapy. This case supports the consideration of therapeutic trials of somatostatin analogues in refractory secretory diarrhoea of unknown origin.