Metastatic Insulinoma Presenting With Post-Prandial Hypoglycemia

Background: Patients with an insulinoma, a type of pancreatic neuroendocrine tumor, typically present with fasting hypoglycemia (1). Occurrence of exclusively postprandial hypoglycemia as a result of a predominantly proinsulin-secreting metastatic neuroendocrine tumor is rare (2). Clinical Case: A 69-year-old man presented with episodes of postprandial blurry vision, sweating and confusion for the last two years that were becoming more frequent over the last several weeks. Self-monitoring of blood glucose at home revealed postprandial hypoglycemia (45-70mg/dl) and symptoms were consistent with Whipple’s triad. Continuous glucose monitoring over 14-days via Dexcom G6 showed no nocturnal or fasting hypoglycemia and revealed only postprandial hypoglycemia within one-two hours after meals. Laboratory measurements were performed at 8am in fasting state which revealed a blood glucose of 97mg/dl, insulin level 7.8 µIU/ml (2-21 µIU/ml), c-peptide 1.67 ng/ml (1.1-4.4 ng/ml) and elevated proinsulin level of 39 pmol/l (<8.0 pmol/l). An outpatient fast was conducted in the clinic, and when serum blood glucose dropped to 47mg/dL (21 hours after the initiation of the fast), insulin (6 µIU/ml) and c-peptide (2.0ng/ml) levels were detectable with an elevated proinsulin (20.8pmol/L) level. CT abdomen and pelvis showed a 1.6cm hyperenhancing lesion in the distal body of the pancreas. He underwent endoscopic ultrasonography with fine-needle aspiration, confirming the diagnosis of a pancreatic neuroendocrine tumor. Distal pancreatectomy and splenectomy were performed to resect the tumor which led to resolution of his symptoms. Pathology revealed a low grade well-differentiated neuroendocrine tumor with lymphovascular invasion and regional lymph node metastases. Conclusion: Pancreatic neuroendocrine tumor should be considered in post-prandial hypoglycemia, even in the absence of fasting hypoglycemia. Measuring proinsulin is essential in the diagnostic workup of insulinoma causing hypoglycemia References: 1. Placzkowski KA, Vella A, Thompson GB, Grant CS, Reading CC, Charboneau JW, et al. Secular trends in the presentation and management of functioning insulinoma at the Mayo Clinic, 1987-2007. J Clin Endocrinol Metab. 2009;94(4):1069-73.2. Murtha TD, Lupsa BC, Majumdar S, Jain D, Salem RR. A Systematic Review of Proinsulin-Secreting Pancreatic Neuroendocrine Tumors. J Gastrointest Surg. 2017;21(8):1335-41.