Vipomas are a type of pancreatic endocrine tumor that arises from islet cells. Of these tumors, 50 to 75% are malignant, and some may be quite large (7 cm) at diagnosis. In about 6%, vipoma occurs as part of multiple endocrine neoplasia.
(See also Overview of Pancreatic Endocrine Tumors.)
The major symptoms of vipoma are prolonged massive watery diarrhea (fasting stool volume > 750 to 1000 mL/day and nonfasting volumes of > 3000 mL/day) and symptoms of hypokalemia, metabolic acidosis, and dehydration. In half of patients, diarrhea is constant; in the rest, diarrhea severity varies over time. About 33% of patients have diarrhea < 1 year before diagnosis, but 25% have diarrhea ≥ 5 years before diagnosis.
Lethargy, muscular weakness, nausea, vomiting, and crampy abdominal pain occur frequently.
Flushing similar to that of carcinoid syndrome occurs in 20% of patients during attacks of diarrhea.
Diagnosis of vipoma requires demonstration of secretory diarrhea (stool osmolality is close to plasma osmolality, and twice the sum of sodium and potassium concentration in the stool accounts for all measured stool osmolality). Other causes of secretory diarrhea and, in particular, laxative abuse must be excluded (see testing of diarrhea). In such patients, serum VIP levels should be measured (ideally during a bout of diarrhea). Markedly elevated levels establish the diagnosis, but mild elevations may occur with short bowel syndrome and inflammatory diseases. Patients with elevated VIP levels should have tumor localization studies, such as endoscopic ultrasonography, PET, and octreotide scintigraphy or arteriography to localize metastases.
Electrolytes and complete blood count should be measured. Hyperglycemia and impaired glucose tolerance occur in ≤ 50% of patients. Hypercalcemia occurs in 50% of patients.
Initially, fluids and electrolytes must be replaced. Bicarbonate must be given to replace fecal loss and avoid acidosis. Because fecal losses of water and electrolytes increase as rehydration is achieved, continual IV replacement may become difficult.
Octreotide usually controls diarrhea, but large doses may be needed. Responders may benefit from a long-acting octreotide formulation given 20 to 30 mg IM once a month. Patients using octreotide may also need to take supplemental pancreatic enzymes because octreotide suppresses pancreatic enzyme secretion.
Tumor resection is curative in 50% of patients with a localized tumor. In patients with metastatic tumor, resection of all visible tumor may provide temporary relief of symptoms. The combination of streptozocin and doxorubicin may reduce diarrhea and tumor mass if objective response occurs (in 50 to 60%). Newer chemotherapies under investigation for vipoma include temozolomide-based regimens, everolimus, or sunitinib. Chemotherapy is not curative.
More than half of vipomas are malignant.
Copious watery diarrhea (often 1 to 3 L/day) is common, often resulting in electrolyte abnormalities and/or dehydration.
Patients with confirmed watery diarrhea should have their serum vasoactive intestinal peptide (VIP) levels measured (ideally during a bout of diarrhea).
Localize tumors with endoscopic ultrasonography, positron emission tomography (PET), or octreotide scintigraphy or arteriography.
Remove tumors surgically when possible and suppress diarrhea with octreotide.