The most common duodenal carcinoids are gastrin-producing G-cell tumors (~two thirds) followed by somatostatin-producing D-cell tumors (~one fifth), which rarely produce systemic manifestations of somatostatin excess.[5,31,33]
Gastrin production from G-cell carcinoids (also called gastrinomas if serum gastrin levels are elevated) results in ZES in approximately one-third of the cases of duodenal G-cell tumors. Although duodenal G-cell carcinoids may occur sporadically, 90% of patients with MEN1 develop them. The clinical manifestations of serum gastrin elevation include:
- Abdominal pain.
- Hemorrhage from multiple and recurrent peptic ulcers.
- Gastroesophageal reflux caused by excess acid production.
- Diarrhea from hypergastrinemia.
The most common symptom is abdominal pain; the combination of abdominal pain and diarrhea is present in approximately 50% of patients. In contrast to sporadic gastrinomas, which are usually solitary lesions, gastrinomas in patients with MEN1-ZES are usually multiple and smaller than 5 mm in size.
Somatostatin-producing D-cell tumors occur exclusively in and around the ampulla of Vater, and as many as 50% of patients with D-cell carcinoids have NF1. Most of the patients with NF1 are black women, and their tumors are exclusively located in the periampullary region.[15,32] As a result of their location, these tumors may cause local obstructive symptoms and signs such as jaundice, pancreatitis, or hemorrhage. Although D-cell carcinoids produce somatostatin, systemic manifestations of excess somatostatin such as steatorrhea, diarrhea, diabetes mellitus, hypochlorhydria and achlorhydria, anemia, and cholelithiasis are rare.
Jejunal and ileal carcinoids
Most jejunal and ileal carcinoids are argentaffin-positive, substance P–containing, and serotonin-producing EC-cell tumors that generate carcinoid syndrome when hepatic or retroperitoneal nodal metastases are present. L-cell, glucagon-like polypeptide-producing, and pancreatic polypeptide- and polypeptide YY-producing tumors occur less frequently. Ileal carcinoids develop preferentially in the terminal ileum. Jejunal and ileal carcinoids occur equally in men and women at a mean age of 65.4 years. Similar to all carcinoids, jejunal and ileal carcinoids vary in their biologic behavior and ability to metastasize. Typically, EC-cell carcinoids of the small intestine metastasize to lymph nodes and the liver. Patients with these lesions may be asymptomatic. The primary tumor may cause small intestinal obstruction, ischemia, or bleeding, and some patients may complain of a long history of intermittent crampy abdominal pain, weight loss, fatigue, abdominal distention, diarrhea, or nausea and vomiting.[5,23,35]
At the time of diagnosis, ileal NETs (i.e., carcinoids plus poorly differentiated neuroendocrine carcinomas) are commonly larger than 2 cm and have metastasized to regional lymph nodes; in as many as 40% of cases, the tumors are multifocal. Immunocytochemically, the cells contain serotonin, substance P, kallikrein, and catecholamine. Approximately 20% of patients with ileal NETs have regional lymph node and liver metastases. Most GI carcinoids secrete their bioactive peptides and amines into the portal circulation, and the effects of these biochemical mediators are diminished or negated by hepatic detoxification; accordingly, carcinoid syndrome (e.g., flush, diarrhea, and endocardial fibrosis) occurs only in patients with liver metastases because hepatic detoxification of serotonin is bypassed.