The main causes of the chronic diarrhea are following;
1) Irritable bowel syndrome; is a sort of functional diarrhea in which no abnormality is found.
2) Inflammatory bowel disease (IBD); is an autoimmune-mediated type of diarrhea and it can involve other organs in the body. The biopsy of colon and small intestines need to be taken to establish the diagnosis with investigations like CRP, ANA, HLA B 27, etc. There are two types of IBD;
a) Crohn's disease
b) Ulcerative colitis.
You would need a colonoscopy for the evaluation.
3) Microscopic colitis; Microscopic colitis is characterized by chronic watery (secretory) diarrhea without bleeding. It usually occurs in middle-aged patients. Two different types of microscopic colitis have been generally recognized:
a) Lymphocytic colitis
b) Collagenous colitis without lymphocytic infiltration of the surface epithelium
Collagenous and lymphocytic colitis produce a similar clinical picture characterized by non-bloody chronic watery (secretory) diarrhea of up to two liters daily. So the total amount of stools per day has to ascertained.
4) Malabsorption syndromes; The classic manifestations of malabsorption are pale, greasy, voluminous, foul-smelling stools, and weight loss despite adequate food intake.
5) Chronic infections; Some persisting infections (C. difficile, Aeromonas, Plesiomonas, Campylobacter, Giardia, Amebae, etc.) can be associated with chronic diarrhea.
You should be investigated for IBD, chronic infections, or for secretory diarrhea (by estimating total amount of stools per 24 hours).
Gastrinoma and VIPoma also should be considered for prolonged secretory diarrhea and a high index of suspicion is required. Secretory diarrhea characteristically continues despite fasting, is associated with stool volumes more than 1 liter/day. Selective testing for plasma peptides such as gastrin, calcitonin, vasoactive intestinal polypeptide, and somatostatin, as well as urine excretion of 5-hydroxyindole acetic acid, metanephrine, or histamine and other tests of endocrine function, such as measurement of thyroid-stimulating hormone and serum thyroxine levels or an adrenocorticotropin-stimulation test for adrenal insufficiency, can be considered by your physician. An aspirate of small bowel contents for quantitative culture or breath tests with glucose or isotopically labeled xylose can be used to establish the presence of small bowel bacterial overgrowth
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