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MEGACOLON
44. MEGACOLON Joseph C. Glennon, V.M.D., Dip. A.C.V.S.
1. What is the anatomy of the colon? The colon is divided into three regionally continuous portions: ascending, transverse, and descending. The ascending portion begins at the ileocolic valve on the right side of the abdomen and runs cranially ending at the right colic flexure (hepatic flexure). The transverse colon starts at the right colic flexure, runs transversely from right to left, and ends at the left colonic flexure (splenic flexure). The descending colon begins at the left colonic flexure and runs caudally along the left side of the abdomen to the pelvic inlet, where it becomes the rectum. The descending colon is the longest segment. The cecum is a diverticulum of the proximal colon separated by a cecocolic orifice located just distal to the ileocecal sphincter. 2. What is the blood supply to the colon? The ileocolic artery, which arises from the cranial mesenteric artery, supplies the proximal portion. The branches of the ileocolic artery supply the cecum, the right colic artery, the ascending colon, and part of the transverse colon; and the middle colic artery supplies part of the transverse and half of the descending colon. The remainder of the descending colon is supplied by the left colic artery (some anastomoses with the middle colic artery), and the blood supply from these vessels to the colon is by numerous vasa recti. The caudal mesenteric artery also gives rise to the cranial rectal artery, which primarily supplies the rectum; however, it also supplies a short segment of the terminal colon via the vasa recti. Venous drainage is similar to the arterial supply. 3. What is the nerve supply to the colon? The intrinsic portion is from the submucosal (Meissner’s) plexus and myenteric (Auerbach’s) plexus. The intrinsic innervation is responsible for intestinal contractions that occur even in the absence of extrinsic innervation. The extrinsic portion is from parasympathetic and sympathetic innervation. The parasympathetic (cholinergic) innervation is from the vagus and pelvic nerves. The sympathetic (adrenergic) innervation is from the paravertebral sympathetic trunk via the abdominal sympathetic ganglia. Parasympathetic innervation stimulates smooth muscle contraction, and sympathetic innervation suppresses smooth muscle activity. 4. What is the function of the colon? The proximal colon functions to absorb water from the fecal material via an osmotic gradient created by the active absorption of sodium. During this process, bicarbonate ions are secreted in exchange for chloride ions. Potassium is lost from extracellular fluid by an active transport mechanism, secretion of mucus, and desquamation of mucosal epithelial cells. The distal colon functions to store the dehydrated fecal material. Numerous nutrients including vitamin K are synthesized in varying degrees by colonic bacteria. This is of little significance in the canine and feline. 5. What is megacolon? Enlarged colon. It is not a specific disease but a descriptive term for the clinical signs of chronic constipation associated with increased colonic diameter and progressive colonic inertia (hypomotility). It represents the end stage of a condition that usually results in intractable constipation.
Megacolon
173
6. What is the difference between constipation and obstipation? Much discomfort. Constipation is “crowding together of feces,” whereas obstipation is unrelievable constipation. 7. What are the causes of megacolon? • Idiopathic: Unknown etiology. Generalized dysfunction of colonic smooth muscle, or intrinsic/extrinsic innervation. Most commonly seen in middle age to older cats; rare in the dog. • Congenital: Sacral agenesis or dysgenesis (Manx cats); segmental agangliosis (two cats); absent myenteric ganglia (two cats) • Acquired: Spinal cord trauma; pelvic fracture nonunion; chronic sacroiliac luxation, perineal hernia; perianal fistula; proctitis; rectal strictures; anal sac disease; chronic colonic inflammation; neoplasia; foreign body; dietary indiscretion; behavioral; endocrine (hypothyroidism); atresia ani; electrolyte imbalances (hypercalcemia, hypokalemia) 8. What is Hirschsprung’s disease? A congenital disease in humans characterized by absence of ganglionic segments in the wall of the colon. The aganglionic segment remains tonically contracted and the colon proximal to this segment is dilated. This condition has not been documented in the dog or cat. 9. What are the common clinical signs of megacolon? Clinical signs include tenesmus, anorexia, depression, weakness, vomiting, weight loss, and distended or possibly painful abdomen. Scant amount of feces or paradoxical diarrhea may occur due to liquid fecal matter passing around fecal concretions. 10. What is passage of bloody feces? Hematochezia. 11. What are the common physical examination findings? Lean body condition, poor hair coat, depression, and dehydration. Abdominal palpation reveals a firm, large tubular structure (feces filled colon) in the abdomen. The abdomen may be painful. Digital rectal examination reveals hard feces within the rectum. 12. What other diagnostics may be useful? • Radiography: Distended colon filled with feces. The presence of pelvic, sacral, or coccygeal lesions should be noted. Positive contrast enema can help identify strictures or neoplasia. • Ultrasonography and colonoscopy; used to identify strictures or neoplasia. 13. What is the treatment of megacolon? It must be determined whether the megacolon is idiopathic, congenital, or acquired. If possible, the underlying etiology should be addressed first. • Medical: Dehydration, electrolyte and acid-base imbalances need to be corrected. For immediate relief, evacuation of feces can be accomplished by stool softeners, enemas, and or manually (digitally or with instrumentation). General anesthesia may be required. Long-term management consists of high-fiber diets, stool softeners, bulk laxatives (canned pumpkin), and periodic enemas. Prokinetic drugs (cisapride) can be used once the colon has been evacuated; however, it appears to have limited usefulness. • Surgery: Indicated in cases of chronic constipation that are refractory to medical therapy. Idiopathic megacolon in the cat is best managed by subtotal colectomy, which has minimal complications and good long-term results. Subtotal colectomy in the dog is also minimally complicated; however, chronic diarrhea, electrolyte imbalances, and bacterial overgrowth of the small intestine may occur, resulting in a less than favorable outcome.
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Megacolon
14. Is presurgical bowel preparation necessary? In most cases, immediate preoperative enemas are not necessary. If an enema is required, it is preferable that it be performed 3 to 4 days prior to surgery so the bowel is not inflamed and there is a minimum of dry feces at the time of surgery. 15. Are presurgical antibiotics necessary? Parenteral prophylactic antibiotics effective against gram-negative aerobic and anaerobic organisms should be administered. 16. Should the ileocolic valve be preserved? The ileocolic valve minimizes access of colonic bacteria to the small intestine. Removal of the valve can result in bacterial overgrowth in the small intestine and deconjugation of bile acids. This does not appear to be a problem in the cat; therefore preservation is not necessary. Removal allows for a tension-free ileocolic anastomosis. In the dog, bacterial overgrowth in the ileum causes long-term problems; therefore preservation of the valve is recommended. 17. How much of the descending colon should be removed? As much as possible. In general, the colon is transected approximately 2-4 cm cranial to the pubis. 18. What vessels should be transected? If the ileocolic valve is removed, transect the ileal arteries and veins, ileocolic arteries and veins, caudal mesenteric artery and vein, and the cranial rectal artery and vein. If the ileocolic valve is preserved, transect the right colic, middle colic, and caudal mesenteric vessels. In the canine, do not transect the caudal mesenteric artery and vein or the cranial rectal artery and vein. Transect only the left colic artery and vein and the vasa recta from the cranial rectal artery. 19. How should the anastomosis be performed? If removing the ileocolic valve, an end-to-end ileocolostomy is performed. The luminal disparity can be corrected by transecting the smaller lumen bowel at an oblique angle or removing a wedge from the animesenteric border and transecting the larger lumen bowel at a perpendicular angle. If the ileocolic valve is preserved, an end-to-end colocolostomy is performed. Side-to-end or side-to-side anastomosis is not necessary. 20. What type of suture and pattern should be employed? In nondebilitated patients, a 3-0 or 4-0 monofilament absorbable material (polydioxanone, poliglecaprone25, or polyglyconate) is recommended. In debilitated or hypoalbuminemic patients, a 3-0 or 4-0 monofilament nonabsorbable material (nylon, polybutester, polypropylene) is preferred. In all cases, full-thickness simple interrupted pattern is utilized. 21. Can internal staplers be used? A ring type (end-to-end anastomosis) stapler can be used. Anastomosis can be accomplished by inserting the instrument through an incision in the antimesenteric border of the cecum or colon. Upon completion of the anastomosis, the antimesenteric enterotomy can be closed with a linear stapler or by suture. The instrument can also be introduced transanally; however, in the cat the diameter of the anus and pelvic canal may be too small to accommodate the instrument. 22. What is the postoperative care? Provided there is no gross abdominal contamination during surgery, antibiotics are not indicated beyond the perioperative period. Hydration status should be carefully monitored for several days after surgery because the patient will have loose stools. Fluid supplementation via intravenous or subcutaneous route may be necessary. Analgesics should be administered as necessary.
Rectal Tumors
175
Food should be offered within the first 24 hours after surgery to help maintain nutritional status. Tenesmus, soft to liquid stool, and melena should be anticipated for several days after surgery. 23. What are the surgical complications associated with subtotal colectomy? Dehiscence, leakage, peritonitis, stricture formation, ischemic bowel necrosis, and abscess formation are possible but uncommon. 24. Will the patient have diarrhea or fecal incontinence? A soft formed stool should be expected along with an increased frequency of defecation (34 times/day). In cats, the stool generally will become more firm over 6 weeks. Occasionally, it may take 4-6 months for the stool to become firm. Rarely will diarrhea persist. Persistent diarrhea may result from removal of the ileocolic valve allowing for bacterial overgrowth, bile salt deconjugation, and steatorrhea. True fecal incontinence should not occur. 25. Will there be recurrent constipation? Not commonly. 26. Will there be any nutritional, biochemical, or electrolyte imbalances? No. The enteric function in cats with subtotal colectomy was found to be similar to that of normal cats. Restoration of appetite and weight should be anticipated. 27. What is the prognosis? Subtotal colectomy in cats with idiopathic megacolon is associated with a very favorable prognosis. Subtotal colectomy in the dog may result in persistent diarrhea. BIBLIOGRAPHY 1. Bertoy RW: Megacolon in the cat. Vet Clin North Am Small Anim Pract 32:901-915, 2002. 2. Hasler AH, Washabau RJ: Cisapride stimulates contraction of idiopathic megacolonic smooth muscles in cats. J Vet Intern Med 11:313-318, 1997. 3. Holt DE, Brockman D: Large intestine. In Slatter D, editor: Textbook of small animal surgery, ed 3, Philadelphia, 2002, W.B. Saunders, pp 665-682. 4. Kudisch M, Pavletic MM: Subtotal colectomy with surgical stapling instruments via trans-cecal approach for treatment of acquired megacolon in cats. Vet Surg 22: 457-463, 1993. 5. Washabau RJ, Holt D: Pathogenesis, diagnosis, and therapy of feline idiopathic megacolon. Vet Clin North Am Small Anim Pract 29:589-603, 1999.
45. RECTAL TUMORS Barbara R. Gores, D.V.M., Dip. A.C.V.S.
1. How common are rectal tumors? Primary tumors of the colon and rectum are common; the rectum is more frequently affected than the colon. Tumors of the colon and rectum represent 36-60% of all canine intestinal neoplasia and 15% of all feline intestinal neoplasia. In the dog, twice as many tumors are malignant as opposed to benign. In the cat, tumors are almost always malignant.
1. What is the anatomy of the colon? The colon is divided into three regionally continuous portions: ascending, transverse, and descending. The ascending portion begins at the ileocolic valve on the right side of the abdomen and runs cranially ending at the right colic flexure (hepatic flexure). The transverse colon starts at the right colic flexure, runs transversely from right to left, and ends at the left colonic flexure (splenic flexure). The descending colon begins at the left colonic flexure and runs caudally along the left side of the abdomen to the pelvic inlet, where it becomes the rectum. The descending colon is the longest segment. The cecum is a diverticulum of the proximal colon separated by a cecocolic orifice located just distal to the ileocecal sphincter. 2. What is the blood supply to the colon? The ileocolic artery, which arises from the cranial mesenteric artery, supplies the proximal portion. The branches of the ileocolic artery supply the cecum, the right colic artery, the ascending colon, and part of the transverse colon; and the middle colic artery supplies part of the transverse and half of the descending colon. The remainder of the descending colon is supplied by the left colic artery (some anastomoses with the middle colic artery), and the blood supply from these vessels to the colon is by numerous vasa recti. The caudal mesenteric artery also gives rise to the cranial rectal artery, which primarily supplies the rectum; however, it also supplies a short segment of the terminal colon via the vasa recti. Venous drainage is similar to the arterial supply. 3. What is the nerve supply to the colon? The intrinsic portion is from the submucosal (Meissner’s) plexus and myenteric (Auerbach’s) plexus. The intrinsic innervation is responsible for intestinal contractions that occur even in the absence of extrinsic innervation. The extrinsic portion is from parasympathetic and sympathetic innervation. The parasympathetic (cholinergic) innervation is from the vagus and pelvic nerves. The sympathetic (adrenergic) innervation is from the paravertebral sympathetic trunk via the abdominal sympathetic ganglia. Parasympathetic innervation stimulates smooth muscle contraction, and sympathetic innervation suppresses smooth muscle activity. 4. What is the function of the colon? The proximal colon functions to absorb water from the fecal material via an osmotic gradient created by the active absorption of sodium. During this process, bicarbonate ions are secreted in exchange for chloride ions. Potassium is lost from extracellular fluid by an active transport mechanism, secretion of mucus, and desquamation of mucosal epithelial cells. The distal colon functions to store the dehydrated fecal material. Numerous nutrients including vitamin K are synthesized in varying degrees by colonic bacteria. This is of little significance in the canine and feline. 5. What is megacolon? Enlarged colon. It is not a specific disease but a descriptive term for the clinical signs of chronic constipation associated with increased colonic diameter and progressive colonic inertia (hypomotility). It represents the end stage of a condition that usually results in intractable constipation.
Megacolon
173
6. What is the difference between constipation and obstipation? Much discomfort. Constipation is “crowding together of feces,” whereas obstipation is unrelievable constipation. 7. What are the causes of megacolon? • Idiopathic: Unknown etiology. Generalized dysfunction of colonic smooth muscle, or intrinsic/extrinsic innervation. Most commonly seen in middle age to older cats; rare in the dog. • Congenital: Sacral agenesis or dysgenesis (Manx cats); segmental agangliosis (two cats); absent myenteric ganglia (two cats) • Acquired: Spinal cord trauma; pelvic fracture nonunion; chronic sacroiliac luxation, perineal hernia; perianal fistula; proctitis; rectal strictures; anal sac disease; chronic colonic inflammation; neoplasia; foreign body; dietary indiscretion; behavioral; endocrine (hypothyroidism); atresia ani; electrolyte imbalances (hypercalcemia, hypokalemia) 8. What is Hirschsprung’s disease? A congenital disease in humans characterized by absence of ganglionic segments in the wall of the colon. The aganglionic segment remains tonically contracted and the colon proximal to this segment is dilated. This condition has not been documented in the dog or cat. 9. What are the common clinical signs of megacolon? Clinical signs include tenesmus, anorexia, depression, weakness, vomiting, weight loss, and distended or possibly painful abdomen. Scant amount of feces or paradoxical diarrhea may occur due to liquid fecal matter passing around fecal concretions. 10. What is passage of bloody feces? Hematochezia. 11. What are the common physical examination findings? Lean body condition, poor hair coat, depression, and dehydration. Abdominal palpation reveals a firm, large tubular structure (feces filled colon) in the abdomen. The abdomen may be painful. Digital rectal examination reveals hard feces within the rectum. 12. What other diagnostics may be useful? • Radiography: Distended colon filled with feces. The presence of pelvic, sacral, or coccygeal lesions should be noted. Positive contrast enema can help identify strictures or neoplasia. • Ultrasonography and colonoscopy; used to identify strictures or neoplasia. 13. What is the treatment of megacolon? It must be determined whether the megacolon is idiopathic, congenital, or acquired. If possible, the underlying etiology should be addressed first. • Medical: Dehydration, electrolyte and acid-base imbalances need to be corrected. For immediate relief, evacuation of feces can be accomplished by stool softeners, enemas, and or manually (digitally or with instrumentation). General anesthesia may be required. Long-term management consists of high-fiber diets, stool softeners, bulk laxatives (canned pumpkin), and periodic enemas. Prokinetic drugs (cisapride) can be used once the colon has been evacuated; however, it appears to have limited usefulness. • Surgery: Indicated in cases of chronic constipation that are refractory to medical therapy. Idiopathic megacolon in the cat is best managed by subtotal colectomy, which has minimal complications and good long-term results. Subtotal colectomy in the dog is also minimally complicated; however, chronic diarrhea, electrolyte imbalances, and bacterial overgrowth of the small intestine may occur, resulting in a less than favorable outcome.
174
Megacolon
14. Is presurgical bowel preparation necessary? In most cases, immediate preoperative enemas are not necessary. If an enema is required, it is preferable that it be performed 3 to 4 days prior to surgery so the bowel is not inflamed and there is a minimum of dry feces at the time of surgery. 15. Are presurgical antibiotics necessary? Parenteral prophylactic antibiotics effective against gram-negative aerobic and anaerobic organisms should be administered. 16. Should the ileocolic valve be preserved? The ileocolic valve minimizes access of colonic bacteria to the small intestine. Removal of the valve can result in bacterial overgrowth in the small intestine and deconjugation of bile acids. This does not appear to be a problem in the cat; therefore preservation is not necessary. Removal allows for a tension-free ileocolic anastomosis. In the dog, bacterial overgrowth in the ileum causes long-term problems; therefore preservation of the valve is recommended. 17. How much of the descending colon should be removed? As much as possible. In general, the colon is transected approximately 2-4 cm cranial to the pubis. 18. What vessels should be transected? If the ileocolic valve is removed, transect the ileal arteries and veins, ileocolic arteries and veins, caudal mesenteric artery and vein, and the cranial rectal artery and vein. If the ileocolic valve is preserved, transect the right colic, middle colic, and caudal mesenteric vessels. In the canine, do not transect the caudal mesenteric artery and vein or the cranial rectal artery and vein. Transect only the left colic artery and vein and the vasa recta from the cranial rectal artery. 19. How should the anastomosis be performed? If removing the ileocolic valve, an end-to-end ileocolostomy is performed. The luminal disparity can be corrected by transecting the smaller lumen bowel at an oblique angle or removing a wedge from the animesenteric border and transecting the larger lumen bowel at a perpendicular angle. If the ileocolic valve is preserved, an end-to-end colocolostomy is performed. Side-to-end or side-to-side anastomosis is not necessary. 20. What type of suture and pattern should be employed? In nondebilitated patients, a 3-0 or 4-0 monofilament absorbable material (polydioxanone, poliglecaprone25, or polyglyconate) is recommended. In debilitated or hypoalbuminemic patients, a 3-0 or 4-0 monofilament nonabsorbable material (nylon, polybutester, polypropylene) is preferred. In all cases, full-thickness simple interrupted pattern is utilized. 21. Can internal staplers be used? A ring type (end-to-end anastomosis) stapler can be used. Anastomosis can be accomplished by inserting the instrument through an incision in the antimesenteric border of the cecum or colon. Upon completion of the anastomosis, the antimesenteric enterotomy can be closed with a linear stapler or by suture. The instrument can also be introduced transanally; however, in the cat the diameter of the anus and pelvic canal may be too small to accommodate the instrument. 22. What is the postoperative care? Provided there is no gross abdominal contamination during surgery, antibiotics are not indicated beyond the perioperative period. Hydration status should be carefully monitored for several days after surgery because the patient will have loose stools. Fluid supplementation via intravenous or subcutaneous route may be necessary. Analgesics should be administered as necessary.
Rectal Tumors
175
Food should be offered within the first 24 hours after surgery to help maintain nutritional status. Tenesmus, soft to liquid stool, and melena should be anticipated for several days after surgery. 23. What are the surgical complications associated with subtotal colectomy? Dehiscence, leakage, peritonitis, stricture formation, ischemic bowel necrosis, and abscess formation are possible but uncommon. 24. Will the patient have diarrhea or fecal incontinence? A soft formed stool should be expected along with an increased frequency of defecation (34 times/day). In cats, the stool generally will become more firm over 6 weeks. Occasionally, it may take 4-6 months for the stool to become firm. Rarely will diarrhea persist. Persistent diarrhea may result from removal of the ileocolic valve allowing for bacterial overgrowth, bile salt deconjugation, and steatorrhea. True fecal incontinence should not occur. 25. Will there be recurrent constipation? Not commonly. 26. Will there be any nutritional, biochemical, or electrolyte imbalances? No. The enteric function in cats with subtotal colectomy was found to be similar to that of normal cats. Restoration of appetite and weight should be anticipated. 27. What is the prognosis? Subtotal colectomy in cats with idiopathic megacolon is associated with a very favorable prognosis. Subtotal colectomy in the dog may result in persistent diarrhea. BIBLIOGRAPHY 1. Bertoy RW: Megacolon in the cat. Vet Clin North Am Small Anim Pract 32:901-915, 2002. 2. Hasler AH, Washabau RJ: Cisapride stimulates contraction of idiopathic megacolonic smooth muscles in cats. J Vet Intern Med 11:313-318, 1997. 3. Holt DE, Brockman D: Large intestine. In Slatter D, editor: Textbook of small animal surgery, ed 3, Philadelphia, 2002, W.B. Saunders, pp 665-682. 4. Kudisch M, Pavletic MM: Subtotal colectomy with surgical stapling instruments via trans-cecal approach for treatment of acquired megacolon in cats. Vet Surg 22: 457-463, 1993. 5. Washabau RJ, Holt D: Pathogenesis, diagnosis, and therapy of feline idiopathic megacolon. Vet Clin North Am Small Anim Pract 29:589-603, 1999.
45. RECTAL TUMORS Barbara R. Gores, D.V.M., Dip. A.C.V.S.
1. How common are rectal tumors? Primary tumors of the colon and rectum are common; the rectum is more frequently affected than the colon. Tumors of the colon and rectum represent 36-60% of all canine intestinal neoplasia and 15% of all feline intestinal neoplasia. In the dog, twice as many tumors are malignant as opposed to benign. In the cat, tumors are almost always malignant.