Malignancy and diarrhea

Infectious causes dominate the majority of causes for acute diarrhea, and its often what we first consider based on investigations, and possible treatments. However, with diarrhea lasting more than 2 weeks, persistent and chronic diarrhea are considered, and after the infectious group we have to think about other classes of disease.

Malignancy is an uncommon but ominous cause of diarrhea. This association can result from several ways, and categorizing the type of diarrhea can be helpful. Typical groups include the following:

1. Watery diarrhea - secetory vs. osmotic

2. Fatty diarrhea

3. Inflammatory diarrhea

Watery diarrhea can be identified on history, where patients tend to have large volume, frequent stools, without solid material. Patients will often describe diarrhea despite not eating, and having to wake in the night to have a bowel movement in secretory/osmotic causes. Although its often not necessary, the stool osmolality gap can be calculated to differentiate between osmotic and secretory where a gap of greater than 125 suggests a osmotic cause. Cancers causing secretory diarrhea include the islet cell tumours which secrete hormones including: gastrinoma, glucagonoma, vasoactive intestinal peptides tumours, and pancreatic polypeptide tumours. Increased seratonin is also felt to be the cause of secretory diarrhea that occurs in carcinoid syndrome. Patients with medullary thyroid cancer complain of diarrhea as a ommon symptom in metastatic disease present in 40% of patients. The mechanism is controversial, but calcitonin production is thought to play a role. The diarrhea in this disease was clasically considered a secretory diarrhea, though some studies have found an increased electrolyte gap in the stool suggesting an osmotic cause. Lymphoma (when present in the gut) and villous adenomas of the bowel are also felt to produce a secretory pattern of diarrhea.

Fatty diarrhea is diagnosed using a 72h quantitative fecal fat test. Additional sudan staining can confirm the presence of fat in the stool. Patients will often say that they have greasy, oily stool that doesnt go down the toilet after flushing because it floats. Malabsorption from multiple causes can present like this, the most common likely being celiac disease. However, again malignancy can cause a similar presentation. Pancreatic cancer that results in exocrine dysfuntion can impair fat digestion in the bowel and lead to fatty diarrhea. Biliary obstruction from pancreatic cancer will decrease the release of bile salts in to the bowels and again impair fat absorption as a result, contributing to diarrhea. Somatostatin secreting tumours are documented as causing fatty diarrhea, these are islet cell tumours that produce octreotide, inhibit pancreatic function and bicarbonate release, which results in fat malabsorption. 

Many patients with cancer are on chemotherapy and are therefore immunosuppressed. Diarrhea may be a direct result of chemotherapeutic agents, or newly acquired infection. Our first focus of care in diarrhea should be to rule out infection and avoid exacerbating medications given these things can be life threatening and are treatable. 

Gastroenterology guidelines for chronic diarrhea