Background and methods: The pathophysiology of diarrhea in patients with the carcinoid syndrome is not understood. Possible causes include tumor production of neurohumoral substances, such as serotonin and substance P, which stimulate small-bowel and colonic motility, and intestinal abnormalities, such as lymphangiectasia and bacterial overgrowth. We undertook this study to determine whether carcinoid diarrhea is associated with abnormal motor function in the small intestine and colon. We measured the gastric, small-bowel, and colonic transit of radiolabeled solid residue and estimated the volume of the ascending colon in 16 patients with the carcinoid syndrome and diarrhea and 16 normal subjects. We also measured colonic tone and phasic pressure activity by intracolonic multilumen manometry and with an electronic barostat in seven patients and six normal subjects.
Results: The patients with the carcinoid syndrome had elevated 24-hour urinary excretion of 5-hydroxyindoleacetic acid and elevated fasting plasma serotonin concentrations. Transit times in the small bowel and colon were two times (P < 0.001) and six times (P = 0.001) faster in the patients than in the normal subjects. The volume of the ascending colon was approximately 50 percent smaller in the patients than in the normal subjects (P < 0.001). The patients had normal fasting colonic tone; their mean postprandial colonic tone was markedly increased as compared with the values in the normal subjects (mean increase, 41 percent vs. 24 percent; P = 0.03).
Conclusions: Patients with the carcinoid syndrome who have diarrhea have major alterations in gut motor function that affect both the small intestine and colon.