Watery diarrhea, hypokalemia, and achlorhydria is seen in which of the following tumors
a. VIPomas are endocrine tumors usually arising from pancreatic islets that secrete VIP and cause a syndrome of profound watery diarrhea, hypokalemia, and achlorhydria.
b. The diarrhea persists despite fasting (which qualifies it as a secretory diarrhea) and despite nasogastric aspiration (which differentiates it from the diarrhea of ZES).
c. The diagnostic triad in Verner-Morrison syndrome is a secretory diarrhea, high levels of circulating VIP, and a pancreatic tumor.
d. Diarrhea volumes are often massive, 3 to 5 L/day, and the diagnosis of VIPoma is unlikely if stool volume is less than 700 mL/day.
e. Conditions to be considered in the differential diagnosis are laxative abuse, bacterial and parasitic diarrhea, carcinoid syndrome (which has an elevated level of 5-hydroxyindole acetic acid in urine), and ZES (which has an elevated serum gastrin level).
f. Of these, VIPomas alone show elevated levels of VIP; normal levels are lower than 200 pg/mL, and VIPoma patients have levels ranging from 225 to 2000 pg/mL.
g. The best localization is achieved either by SRS or by endoscopic ultrasound. SRS detected 91% of primary tumors and 75% of metastases.
i. About 50% of patients have metastatic spread by the time of diagnosis. If abdominal studies fail to locate the tumor, thoracic CT is performed because as many as 10% of the tumors are intrathoracic.