93. Colon Flashcards
branches of caudal mesenteric artery
- left colic (feeds descending colon)2. cranial rectal
where does the transverse colon lie in relation to the root of the mesentery
cranial to cranial mesenteric artery and the root of the mesentery
at what point does the descending colon become the rectum
anatomically indistinguishablenear pelvic inlet2 cm CRANIAL to pelvic brim but 1 cm CAUDAL from caudal mesenteric artery
branches of the cranial mesenteric artery
- jejunal, ileal 2. common colic artery (ileocolic–cecal and antimesenteric, right and middle colic)
what is the vasa recti
arteries connect to the colon via vasa recti (short irregular branches that bifurcate upon entering the intestine
2 arterial networks of the colon
- subserous2. mural (direct and plexiform anastomoses that lie predominately in the submucosa)
cells present in the epithelial mucosa layer of the colon
columnar epitheliumcuboidal epitheliumnumerous goblet cellsenterochromaffin cells (5%)NO VILLINO aggregated lymph folliclesinstead have relatively large lymphoglandular complexes
autonomic plexuses of the abdominal region
- cranial mesenteric plexus–main artery common colic –supplies large intestine–cranial mesenteric ganglion2. caudal mesenteric plexus–left colic artery: lumbar splanchnic nerves supplies paired hypogastric nerves–cranial hemorrhoidal artery: aortic plexus supplied distal colon
key functions of the colon
- storing fecal material2. reservoir for complex essential microbial ecosystem3. maintain fluid/electrolyte balance through resorting water/Na/Cl/fatty acids (secretes K/HCO3/mucus)
area of colon with most absorptive capacity
proximal (segmental/mixing motility)(distally the colon is primarily involved with fecal storage but still modulates fecal water content)in general, absorbs 1.5L/day of fluid
T/Fcolonocytes are able to switch from absorption to secretion when stimulated by hormones/secretagogues
TRUE–aldosterone (favors absorption)–other hormones may favor secretiondisruption of this balance of ion transport leads to disease (constipation or diarrhea); under control of complex endocrine, autocrine, paracrine and neuronal stimuli
T/Fmost Na absorption in colon is electroneutral path
TRUEthrough Na/H exchangers (three types)ENHANCED Na absorption is through an electrogenic route (maintains electrochemical gradient—Na/Cl co transport)
net movement of water occurs through which process
osmotically driven active absorption and secretionoccurs via paracellular and transepithelial routes (aquaporin-4)
what ion pump is responsible for maintaining homeostasis essential to secretion of mucus
Na/2Cl/K co transporter type 1 protein in the basolateral membrane
where do short chain fatty acids come from in the colon?
products of colonic bacterial fermentation of dietary fiber (butyrate, acetate, propionate)absorbed with Na/Clhave a marked trophic effect on colonic epithelium and stimulate Na absorption through acidification and activation of apical Na/H transportalso stimulate HCO3 release/secretion and thus absorption of Clalso prevent colonic irritation
motility in colon
segmental–mixing: proximal and midcolonpropulsive aboral peristalsis –distal colonunder intrinsic plexus control:–myenteric (Auerbach): btwn long and circular muscle layers–submucous (Meissner): in colonic submucosaPSNS: preganglion vagus, pelvic n STIMULATESYM: superior and inferior mesenteric plexus, hypogastric n INHIBIT
innate local defense of colon
–impermeable barrier–rapidly renewing–constantly moving–mucus–cryptdins (alpha defensins)–lysozyme–phospholipase–chemokinesPREVENTS COLONIZATION/INVASION of bacT
adaptive immunity of the colon
part of a more specific, long term response for memory and rapid elimination of any pathogen1. antigen presenting cells: M (microfold) cells: no villi or enzymes, just proinflm cytokines and invaginate to present pathogen to T and B cellsD (dendritic) cells: interact with M, B, T cells; can also penetrate lumen to sample antigens2. intraepithelial lymphocyte:express CD8 alpha alpha (like mucosal T cells)
3 phases of GI wound healing
- lag (inflammation/debridement)–some separate these stages: 0-4 days post op includes fibrin clot, PMN, then macros, minimal wound strength and collagenase activity is high2. proliferation: 4-14d; fibroblasts prolif and make collagen III > I; under cytosine influence (FGF, TGF beta, PDGF); angiogenesis3. maturation (and remodel): day 17 onward; less type III collagen. All types I, III, IV is present in GI wound healing
most critical stage of healing in which dehiscence or breakdown of repair is most likely to occur
LAG phase (inflammation and debridement phase)3-4 days post op72-96 hours
collagen content of GI submucosa
I 70%III 20%IV 10%made by fibroblast and smooth muscle cells
when does the colon have 75% of its strength return
4 months30% strength after 48hr
factors negatively affecting wound healing of the colon
LOCAL–wound shear stress/tension (intraluminal transport and peristalsis)–down regulated vascular perfusion in shock–poor wound apposition–infection (anaerobic/aerobic bacterial flora present)SYSTEMIC–hypovolemic–nutrient deficiency (Zn, Iron needed for protein synthesis)–chemotx–immunocompromised state
at what PaO2 will collagen NOT occur
below 40 mm Hg collagen formation will not occur below 10 mm Hg angiogenesis and epithelial hyperplasia will fail
methods for improving colonic wound healing
–correct hypovolemia–minimal tension–preserve local blood supply–use vacularized tissue wraps (omentoplasty +/- vascular rectus abdominis muscle flap)–colonic reinforcement with biomaterial (SIS)–experimental VEGF administration to promote angiogenesis
T/Fappositional pattern of colonic closure leads to less scar than inverting or everting suture patterns
trueappositional has less scar and less reduction in luminal diameter(though simple continuous may decrease surgical time and achieve better histological alignment of layers with minimal tissue handling–has also been reported)
closure options for colon sx
- hand sewn—simple interrupted (preferred) or simple continuous SINGLE layer appositional2. GIA with TA–eversion3. EEA (transcecal, through enterotomy, transrectal) –inversion4. skin staples (small intestinal described, not clinically described for large intestine)5. biofragmentable anatosmotic ring (polyglycolic acid and barium)—performed in cats with subtotal colectomy (limited sizes available—may have serosal tearing)6. sutureless closure (laser, cyanoacrylates, fibrin glue)–NOT clinically experimented
colonogram contrast agent and recommendations
- soapy enema 2-4 hours prior2. barium diluted 50:50 warm water give 10 ml/kg trans rectally (use balloon catheter to prevent leak)can also use double contrast (provide air after evacuating barium)
prep for endoscopy/colonoscopy
- fast 24-48 hr2. oral cleansing solution (may need orogastric tube) 20–30 ml/kg +/- repeat dose 2 hrs later3. two warm water enemas 20 ml/kg morning of scope and one hour before procedure4. left lateral recumbency so transverse colon drains out
choice of perioperative antibiotics based on contaminants present
coliforms and anaerobes–higher generation cephalosporin(Cefoxitin)–combo first gen ceph with anaerobic Ab (cefazolin, metronidazole)
Define parker kerr suture pattern
—continuous cushing over a Doyen clamp, tightened when the clamp is removed–followed by inverted continuous lembert pattern (oversewn)
indications for colonic R&A
–neoplasia–megacolon–perforation–trauma–chronic intussusception (irreducible/necrotic)–colonic torsion
difference btwn subtotal vs total colectom
subtotal–preserve ileocolic junction (cut 1-2 cm aboral to ileocolic jxn)total–resect including ileocolic jxn (cut 1-2 oral to ileocecal fold)importance of ileocolic junction is debated: though to prevent colonic reflux into SI; removal may lead to colonic bacT in SI and lead to SIBO–>diarrhea
T/Fsingle surgical field approach should be used in cats when using a EEA stapler
TRUEno long term complications in 15 cats with single field approach vs 2/10 complications with dual field approach and transrectal application of EEA
two types of colostomy
- end on colostomy (flank exteriorized)2. loop colostomyall render patient fecally incontinent
what term has been given to acquired megacolon that developed from mechanical obstruction (pelvic stenosis, neoplasm, FB, etc)
hypertrophic megacolonpotentially reversible if cause of obstruction is removed early in disease process (within 6 months of initial trauma)(most common cause pelvic fracture malunion)
causes of megacolon in dogs and cats
congenital===RAREacquired–extraluminal compression (prostatomegaly, pelvic deformity/chronic factures, pelvis masses, strictures–post neuter/OHE)–intraluminal obstruction (FB, neoplasia, stricture)–metabolic (hypoK, hypoT)–NM dysfunction (sacral spinal cord deformitis–Manx, ileus, dysautonomia, idiopathic)–failure to fix perineal hernia
feline idiopathic megacolon
62%middle aged male catsgeneralized dysfunction of long and circular muscle layer no evidence of physical or functional obstruction
megacolon diagnosis on lateral radiograph
normal colon is length of body of L2megacolon = colonic diameter >1.5x length of the body of L7
MOA of lactulose therapy for treatment of megacolon
osmotic laxativedisaccharideNOT hydrolysable to mammal Stays in colon for bacT fermentation into organic acids/osmostic/pulls water into stool
MOA of cisapride therapy for treatment of megacolon
benzamine prokineticcauses release of Ach from enteric neurons to stimulate contraction in descending colonworks better in cats with mild/moderate dzeffects may only be transientwithdrawn from market due to cardiotoxicity in people
how long does it take normal enteric plexuses to reestablish
8 weeks for normal enteric plexuses and motility to reestablish
diarrhea following total or subtotal colectomy
loose stools are expected bc of loss of absorptive capacity and decreased transit timepost op motility reestablished ~8 weeksremaining intestine adapts by increasing villus height and enterocyte number and density (may take months)
complications following colon surgery
–diarrhea with massive resection (may adapt over months and small proportion may have loose stools for years)–stricture at surgical site rare but reported within 3 weeks–recurrence of constipation (45% based on 22 cats)–recurrence of neoplasia–dehiscence/leakage/peritonitis
colonic/cecocolic volvulus
–young to middle aged medium to large breed dogs–prognosis depends on severity and prompt survival intervention
causes of reported colonic entrapment
–post OHE/neuter: resect fibrous band–good px–herniation through duodenocolic ligament–good px without intestinal strangulation
colonic neoplasia
majority malignantdogs: adenocarcinoma, GIST, leiomyocarcomacats: LSA, adenocarcinoma5-8 cm resection recommended (recurrence likely with incomplete margins)palliative use of colonic stents reported
MST in dogs with intestinal GIST vs leiomyosarcoma
MST GIST 37 monthsMST leiomyocarcom 8 months
classification of colonic duplication
- rectal, colon2. rectal, colon with other congenital abN (urogenital duplication, vertebral abN)1. spherical noncommunicating2. tubular noncommunicating3. tubular communicating
