Adult-onset focal nodular non-exophytic nesidioblastosis mimicking insulinoma detected by endoscopic ultrasonography: A case report.

Endogenous hyperinsulinemic hypoglycemia in adults is most commonly caused by insulinoma, whereas adult-onset nesidioblastosis represents a rare etiology that can closely mimic insulinoma both clinically and biochemically. In the present study, a rare case of focal, non-exophytic, nodular adult-onset nesidioblastosis that was initially diagnosed as insulinoma was reported. A 25-year-old non-diabetic woman presented with a one-year history of recurrent fasting and postprandial hypoglycemia episodes. The patient fulfilled Whipple's triad and a supervised 72-h fast verified endogenous hyperinsulinism. Conventional imaging modalities, including computed tomography and magnetic resonance imaging, failed to reveal a pancreatic lesion. However, endoscopic ultrasonography (EUS) detected a 7.3x7.1 mm non-exophytic, hypoechoic nodular lesion in the pancreatic tail. Based on the above findings, a preoperative diagnosis of insulinoma was considered and the patient underwent spleen-preserving distal pancreatectomy. Histopathological evaluation revealed focal nodular nesidioblastosis, characterized by ductulo-insular complexes, architectural disorganization, absence of a fibrous capsule and hyperplastic islet cells with a low Ki-67 proliferative index. The patient has remained free of hypoglycemic episodes for >18 months postoperatively. This case underscores the diagnostic challenge of distinguishing focal adult-onset nesidioblastosis from insulinoma before surgery, particularly when a small, non-exophytic pancreatic lesion could be identified by EUS.