Primary gastric lymphomas mostoftenarederivedfromthemucosa-associatedlymphoidtissuewhosedevelopmentisinducedbychronicgastritis.
Low-orintermediate-gradeneuroendocrine (carcinoid) tumors arisefromthediffusecomponentsoftheendocrinesystemandaremostcommoninthegastrointestinaltract,particularlythesmallintestine.Tumorsofthesmallintestinetendtobemostaggressive,whilethoseoftheappendixarealmostalwaysbenign.
Gastrointestinal stromal tumor (GIST), themostcommonmesenchymaltumoroftheabdomen,occursmostofteninthestomach.GISTsarisefrombenignpacemakercells,alsoknownastheinterstitial cells of Cajal. Amajorityoftumorshaveactivatingmutationsineitherthec-KITorthePDGFRAtyrosinekinasesandrespondtokinaseinhibitors.
The small intestine and colon are the sites of a wide variety of diseases, many of which affect nutrient and water transport. Perturbation of these processes may cause malabsorption and diarrhea. The intestines are also the principal site where the immune system interfaces with a diverse array of antigens present in food and gut microbes. Recent advances have defined the composition and diversity of the gut microbiome, which is largely unculturable, using molecular approaches. One common trend is the association of reduced microbial diversity in disease. The gut microbiome is also exquisitely sensitive to diet, which may partly explain the association between food allergy and other diet-induced diseases and altered composition of the microbiome. Thus, it is not surprising that the small intestine and colon frequently are involved by infectious and inflammatory processes. The colon is the most common site of gastrointestinal neoplasia in Western populations.
Obstruction of the gastrointestinal tract may occur at any level, but the small intestine is most often involved because of its relatively narrow lumen. Collectively, hernias, intestinal adhesions, intussusception, and volvulus account for 80% of mechanical obstructions (
Fig. 15.18 ), while tumors and infarction account for most of the remainder. The clinical manifestations of intestinal obstruction include abdominal pain and distention, vomiting, and constipation. Surgical intervention is usually required in cases involving http://ebooksmedicine.net
Fig. 15.18 Intestinal obstruction. The four major mechanical causes of intestinal obstruction are (1) herniation of a segment in the umbilical or inguinal regions, (2) adhesion between loops of intestine, (3) volvulus, and (4) intussusception.
mechanical obstruction or severe infarction. Volvulus and adhesions between bowel loops are well illustrated in
Fig.
15.18 . Brief comments are made next about the other causes of obstruction.