93 - Colon Flashcards
Where is the caecal orifice located in dogs and in cats?
Dogs = 1cm distral to ileocolic orifice
Cats = Adjacent to the ileocolic orifice
What are the distinct sections of the colon?
Ascending
Transverse
Descending
Transverse is between the right and left colic flexures
Where does the mesocolon origniate?
Origniates at the ileocecocolic junction = 2-3cm long at orign
Describe the arterial blood supply to the colon
Cranial mesenteric => Common trunk =>
- Ileocolic - Supplies ascending colon
- Right colic - Supplies ascending and transverse colon
- Middle colic - Supplies transverse and proximal half descending colon
Caudal mesenteric =>
- Left colic a (supples distal half of descending colon)
- Caudal rectal (provides majority of supply to the rectum)
Arteries form arcades from which vasa recta penetrate the muscular layer
Cranial rectal supplying the majority of the terminal colon and rectum (middle/caudal supply relatively insignificant amounts)
Describe the venous drainage from the colon
Left colic vein (Caudal) => Caudal mesenteric vein => Portal vein
Caecal vein and right colic vein => Ileocolic vein
Joined by middle colic vein 1cm before enters portal vein
Describe the lymphatic drainage of the colon
Rich network of lacteals on serosal surfaces
Subserous and submucosal lymphatic plexuses
Drain into Right, middle and left colic LNs
Innervation of the colon?
Autonomic
Cranial and caudal mesenteric plexuses
Travel with mesenteric blood supply
What are the layers of the colon?
What differences are there between the colon and SI?
Mucosa, submucosa, muscularlis (2 layers), serosa
- Mucosa consists of Columnar and cuboidal epithelium arranged in parallel crypts
- Goblet cells are numberous (produce mucus)
- Ebterichromaffin cells (distinct to colon)
- No villi or aggregated LNs
- Lymphoglandular complexes ~ 3mm diameter (only in caecum in cats)
What are the three phase of healing in the colon?
Which is the most critical?
Lag phase = Day 0-3/4
Proliferative phase = Day 4-14
Maturation phase = Day 17 onwards
Lag phase is most critical
- 100% of strength at the surgical site is provided by the closure method
- Majority of dehisence occurs in the first 72-96hrs
Briefly describe the process during colonic healing
Lag phase
- Creates a barrier (fibrin clot)
- Mucus plays role in plugging wound
- Neutrophils predominate (macs and monos later)
Proliferative phase
- Fibroblasts proliferate and are major cell by day 4
- Immature collagen produced.
- TGF beta, FGF and PDGF major cytokines
- Angiodenesis occurs providing O2 to the wound
- Delivery of nutrients for healing/collagen synthesis
Maturation phase
- AKA remodelling
- Day 17 onwards
- Reorganisation and remodelling of collagen
- Type III reduces - Thick bundles of collagen form
- Macs and fibroblasts reduce
At what stage is colonic wound bursting strength near normal post op?
10-17 days
How does mucosal healing occur?
- Completely re-epithelialises within 3 days when apposition achieved
- Heals through hyperplasia and migration
What is the normal collagen content of the submucosa?
Type I = 68%
Type III = 20%
Type V = 12%
1, 3, 5
How does submucosal healing occur?
- Smooth muscle and submucosa contibute to collagen production in addition to fibroblasts
- Collagenolysis through degredation by MMPs due to weakness
- Weakness lasts for minimum 3-4 days (Lag phase)
- MMP activity decreases after day 3
- Then wound strength rapidly begins to increase.
- Wound strength increases to 30% after 48 hours
- Return to 75% strength at 4 months
Basically, weakeness propagates MMP activity for 3-4 days until collagen begins to be deposited in the wound. After this the increase in wound strength is exponential, but not 100% for several months.
Name 5 local and 8 systemic factors that are detremental to GI healing
How does hypoxia impact wound healing?
PaO2 critical
If PaO2 < 40mmHg = Failure of collagen deposition
If PaO2 < 10 mmHg = Failure of angiogenesis and epithelialisation
Wound tension and hypoperfusion can lead to hypoxia
Considerations for anaemic patients?
PCV as low as 15% does not appear to affect healing
BUT
Blood transfusion impairs colonic healing
(Impaired function and migrations of macs into the wound)
How do Zinc and Iron affect healing?
Zinc and Iron important to proliferation and remodelling phases
Deficiency => Reduced fibroblast activity and poor collagen production
How does infection impact healing?
Prolongs the lag phase
Endotoxins e.g. Lipopolysaccharide (from E.coli)
- Induces collagenase synthesis
- Enhances break down
When it comes to corticosteroid treatment, what factor is most important to healing?
CHRONIC TREATMENT is MORE likley to impair healing
(High dose treatment does not seem to have significant effect)
Name 5 Methods for improving colonic wound healing
- Omentalisation
- Rectus abdominis muscle flaps
- Reinforcement with porcine SIS (provides collagen matrix)
- Amniotic membrane incisional patch (collagen matrix, increases angiogenesis and fibroblast proliferation)
- VEGF (improves angiogenesis)
Name 6 important functions of the colon
- Electrolyte transport
- Water Transport
- Secretion of mucus
- Absorption of Short Chain Fatty Acids
- Faecal storage
- Immunity
How are electrolytes resporbed in the colon?
- Na/K ATPase pump and K+ ion pump in brush border create electrochemical gradient
- 1.5 L/day fluid absorbed to maintain excretion of NaCl into the faeces
- Solutes absorbed by electroneutral or electrogenic transport
Which structures in the colon are responsible for water transport?
- Aquaporins 3 +4 (Water channel proteins) in the basolateral membrane
- Cystic fibrosis transmembrane regulator (Cl- selective channel)
Osmotically driven
What is the function of mucus in the colon?
What secretes it?
What regulates secretion?
Mucus protects epithelium from abrasion and bacterial invasion
Secreted by Goblet and Columnar cells (Different compostion)
Cystic fibrosis transmembrane receptor regulates mucus secretion
Regulated by autocrine, paracrine and neuronal stimuli
Alterations => Increased secretion = D+ or excessive absorption = constipation
Where do short chain fatty acids come from?
What is their role?
SCFAs are a product of colonic bacterial fermentation
- Metabolised by colonocytes (nutritional)
- Stimulate Na absorption by acidification and activation of Na/H+ apical membrane
- Stimulate scretion of HCO3- and increase Cl absorption to regulate colonic pH
- Prevent colonic inflammaiton by reducing ionisation of bile acids and long chain fattty acids
How is faecal storage controlled?
Myenteric (Auerbach) plexus in muscles
Submucous (Meissner) plexus in submucosa
Parasympathetic => Pelvic nerve => Defecation
Sympathetic => Superior mesenteric and hypogastric plexus => Store
What specifically delays transit time in cats?
Retrograde peristalsis in the proximal colon
What limits the response of commensal bacteria to prevent colitis?
Tregs
(Provide post thymic education to foreign antigen)
Regulatory myeloid cells also inhibit T-cell proliferation in response to intestinal bacteria
Which local mechanisms provide a barrier to infection in the colon
Impermiable colonocytes
Rapid renewal of epithelium
Protection by mucous
Constant movement
Alpha defensins, lysosomes, phospholopases and chemokines provide protection, reduced bacterial numbers and prevent colonisation
Phagocytic cells can be mobilised when needed
What are M-cells and D-cells?
M-cells = Microfold cells
- Cells within an invagination in the basolateral membrane
- Move proteins, pathogens and particals transepthelially to subendothelial lymphoid cells to produce relevent cyto/chemokines
D-cells = Dendritic calls
- Interact with M-cells or directly penetrate epithelium to sample antigens
- Migrate to prime T-helps
- Can induce IgG and IgA causing chronic inflammation
Adaptive immunity
What feature if characteristic of colonic intraepithelial lymphocytes?
What role do they play in immunity?
Express CD8 alpha alpha
Role =
- Epithelial homeostaisis
- Cancer surveillance
- Defense against pathogens
What is the current standard of care for colonic suturing?
Single layer
Simple intterupted
Appositional
Swaged on round or taper cut
Monofilament
Absorbable
+/- antibiotic coating
BUT Simple continous had better histologic alignment of layers and reduced surgical time
What suture pattern should be used to oversew the caecum if hand suturing?
Parker-Kerr
Starting knot not tied
Single layer of cushing
Both ends pulled to tighten
Second layer of Lembert
Suture tied to original end
Describe the technique for using EEA stapler
- Incisions 2cm distal to caecuma and 2cm proximal to pelvic brim
- Furniss clamp used with straight needle to place purse string either end
- 3cm antemesenteric incision over the caecum
- Ovoid sizer passed normograde to proximal margin to measure bowel diameter
- Stapler intriduced transcaecally and into distal colonic/rectal segment
- Purse strings tied, ends brought together and instrument fired
- Cutting blade removed excess inverted tissue
- Stapler rotated gently to detach and then removed
- Instrument inspected to ensure forms a complete ring
- Leak test performed to assess for haemorrhage
- Caecal wound closed with TA stapler or appositional suture
- Omentalised
How is a biofragmentable ring applied?
What size is used?
Benefit to this approach?
Similar to EEA
First ring placed in oral resection site using holding device and purse string in place
Exposed half place in aboral segment and purse string tied
Ring snapped shut with digital pressure
Sloughing of entrapped intestinal tissue releases the ring which is passed in faeces
Size = 25 mm
Benefit = No caecal incision required
Describe the colopexy technique
Reduce prolapse
Ventral midline approach
Cranial traction on descending colon
Digital rectal exame to confirm appropriate reduction
Ensure cranial colon retracted sufficiently
Apposition with 2/0-3/0 PDS or Maxon
Simple apposition with partial thickness bites into submucosa and left abdo wall OR
Partial thickness incisions in colonic and abdominal wall (3-4 cm long)
Suture dorsal side first, then ventral
Epidural to reduce straining, stool softeners, recurrence = failure at site
Describe the Combined Abdominal Transanal Pull-Through colorectal amputation procuedure
Abdominal approach
- Ventral midline approach
- Ensure proximal colon can reach distal rectum
- Colectomy performed 5cm orad to proximal mass
- Both ends oversewn with Parker-Kerr
- Ends sutured together with 4 x 2/0 sutures leaving 1cm gap
- Abdo lavaged and closed
Transanal approach
- Perineal positioning
- Rectal wall everted through anus and rectum
- 1-1.5cm of distal rectum preserved
- Rectum incised and stay sutures placed to prevent retraction into abdomen
- Rectum mobilised with blunt dissectio along external surface
- Rectococcygeus transected
- Dissection continued until suture stumps of colon visible
- Pulled through to the anus
- Connecting sutures transected
- Oversew from proximal parker-kerr removed
- Colon anastomosed to distal rectum
- Simple interrupted apposition 2/0 or 3/0 PDS
Resection limited by tention on colonic vascular pedicles
What are the main causes of megacolon in dogs and cats?
Extraluminal compression
- Pelvic deformity/fractures
- Prostatomegaly
- Pelvic masses
- Strictures
- Post neutering adhesions
- Atresia ani
Intraluminal obstruction
- Foreign bodies
- Neoplasia
- Intraluminal strictures
Metabolic
- Hypokalaemia, Hypomagnaesemia, Hypercalcaemia
- Hypothyroidism in dogs
- Hyperthyroidism in cats
Neuromuscular abnormalities
- Sacral spinal cord deformities (Manx cats)
- Ileus
- Dysautomonia
- Idiopathic agangliosis. (cats)
- Pelvic nerve injury
