Definition

A gastrointestinal disorder characterized by persistently increased large bowel diameter associated with chronic constipation/obstipation and low-to-absent colonic motility.

Pathophysiology

• Acquired megacolon results from chronic colonic fecal impaction that leads to excessive absorption of fecal water and solidified fecal concretions.
• Prolonged distension of the colon results in irreversible changes in colonic motility that leads to colonic inertia.
• Congenital absence of colonic ganglionic cells (Hirschsprung's disease) is not clearly documented in small animals.
• The pathogenesis of idiopathic megacolon in cats likely involves a disturbance of normal colonic smooth muscle function.

Systems Affected

Gastrointestinal

Genetics

N/A

Incidence/Prevalence

Unknown

Geographic Distribution

N/A

Signalment

Signs

Causes

• Idiopathic-cats.
• Mechanical obstruction-pelvic fracture malunion, foreign body or improper diet (especially bones), stricture, prostatic disease, perineal hernia, neoplasia, anal or rectal atresia.
• Causes of dyschezia-anorectal disease (anal sacculitis, anal sac abscess, perianal fistula, proctitis), trauma (fractured pelvis, fractured limb, dislocated hip, perianal bite wound or laceration, perineal abscess).
• Metabolic disorders-hypokalemia, hypocalcemia, severe dehydration.
• Drugs-vincristine, barium, antacids, sucralfate, anticholinergics.
• Neurologic/neuromuscular disease-congenital abnormalities of the caudal spine (especially Manx cats), paraplegia, spinal cord disease, intervertebral disc disease, dysautonomia, sacral nerve disease, sacral nerve trauma (e.g., tail fracture/pull injury), trauma to colonic innervation.

Risk Factors

• Conditions leading to inability to posture (limb and pelvic fractures, neuromuscular disease, etc.) or rectoanal pain.
• Prior pelvic fractures.
• Possible association with low physical activity and obesity.
• Perineal hernias.

Differential Diagnosis

• Other causes of palpable colonic masses (e.g., lymphoma, carcinoma, intussusception)-distinguish on the basis of texture, rectal examination, imaging, and mucosal biopsy.
• Dysuria/stranguria-exclude by palpation of the bladder and colon, and by urinalysis.
• Tenesmus due to inflammation of the colonic mucosa (colitis)-exclude by palpation, rectal examination, and endoscopic imaging with mucosal biopsy.

CBC/Biochemistry/Urinalysis

• May show evidence of dehydration (elevated packed cell volume, total protein) and stress leukogram.
• Electrolyte abnormalities may develop depending on duration of obstipation; may be prerenal azotemia with dehydration.
• Urinalysis-no consistent changes; important to confirm normal renal function in dehydrated animals and to rule out lower urinary tract disease as a differential diagnosis.

Other Laboratory Tests

N/A

Imaging

• Abdominal/pelvic radiographs to identify any underlying causes.
• Can easily see the enlarged, fecal-filled colon on survey abdominal radiographs.
• Abdominal ultrasound may identify mural or obstructive masses.

Diagnostic Procedures

May need colonoscopy to rule out mural or intraluminal obstructive lesions.

Pathologic Findings

• The most severe dilation typically occurs in the transverse and descending colon, although the entire length of the colon can be involved.
• The colon is usually histologically unremarkable with megacolon.

Appropriate Health Care

• Inpatient medical management; surgery may be indicated if recurrent/severe problem.
• Medical therapy-restore normal hydration, followed by anesthesia and manual evacuation of the colon using warm water enemas, water-soluble jelly, and gentle extraction of feces with a gloved finger or sponge forceps; do not traumatize the colonic mucosa.
• Continue long-term therapy at home.

Nursing Care

• Most patients require parenteral fluid support to correct dehydration.
• Intravenous administration of balanced electrolyte solutions is the preferred route.

Activity

• Encourage activity and exercise.
• Restriction indicated in the postoperative period if surgery is performed.

Diet

• Many patients require a low-residue producing diet; bulk-forming fiber diets can worsen or lead to recurrence of colonic fecal distension.
• A high-fiber diet is occasionally helpful.
• A more palatable, maintenance-type diet can be supplemented with fiber-enriched foods (pumpkin) or products containing fermentable fiber such as Metamucil.

Client Education

• In idiopathic disease or with severe colonic injury, medical therapy is often life-long and can be frustrating to clients.
• Increased activity of the cat with daily or alternate-day subcutaneous fluid therapy can help minimize recurrences in many cats.
• Recurrence of megacolon is common.
• Surgery (subtotal colectomy) is indicated if medical therapy fails.

Surgical Considerations

• An underlying obstructive cause requires surgical correction.
• Avoid enema administration/colonic evacuation prior to subtotal colectomy.
• Subtotal colectomy with ileorectal or colorectal anastomosis-treatment of choice for idiopathic megacolon refractory to medical management.
• Colectomy may also be required with obstructive megacolon caused by irreversible changes in colonic motility.

Drug(s) Of Choice

• Can improve colonic motility in less severe cases with cisapride, a prokinetic gastrointestinal drug (dogs, 0.3–0.5 mg/kg PO q8–12h; cats, 2.5–10 mg/cat q8–12h). Metoclopramide does not affect colonic motility and should not be used in cats with megacolon.
• Stool softeners (e.g., lactulose, 1 mL/4.5 kg PO q8–12h to effect) are recommended in conjunction with cisapride and diet.
• Broad-spectrum antibiotics are recommended prior to surgery to reduce the potential for bacterial sepsis .

Contraindications

• Sodium phosphate retention enemas (e.g., Fleet; C.B. Fleet Co., Inc.)-because of their association with severe hypocalcemia.
• Mineral oil and white petrolatum-because of danger of fatal lipoid aspiration pneumonia due to lack of taste.

Precautions

Common hairball laxatives (e.g., Laxatone, Cat-a-Lax) are typically ineffective.

Possible Interactions

N/A

Alternative Drug(s)

Docusate sodium can be used as a stool softener in place of lactulose.

Patient Monitoring

• Following colonic resection and anastomosis-for 3–5 days check for signs of dehiscence and peritonitis.
• Clinical deterioration warrants abdominocentesis and/or peritoneal lavage to detect anastomotic leakage.
• Continue fluid support until the patient is willing to eat and drink.

Prevention/Avoidance

• Repair pelvic fractures that narrow the pelvic canal.
• Avoid exposure to foreign bodies and feeding bones.

Possible Complications

• Recurrence or persistence-most common.
• Potential surgical complications include peritonitis, persistent diarrhea, fecal incontinence, stricture formation, and recurrence of obstipation.
• Traumatic perforation of the colon is a serious complication of overzealous fecal evacuation.

Expected Course and Prognosis

• Historically, medical management has been unrewarding for the long term.
• Cisapride appears to improve the prognosis with medical management in some patients, but may not suffice in severe or long-standing cases.
• Postoperative diarrhea-expected; typically resolves within 6 weeks (80% of cats with idiopathic megacolon undergoing subtotal colectomy) but can persist for several months; stools become more formed as the ileum adapts by increasing reservoir capacity and water absorption.
• Subtotal colectomy is well tolerated by cats; constipation recurrence rates are typically low.

Associated Conditions

Perineal hernia

Age-Related Factors

Concurrent medical conditions (e.g., chronic renal insufficiency, hyperthyroidism) may occur with idiopathic megacolon, because many cats are old.

Zoonotic Potential

N/A

Pregnancy/Fertility/Breeding

• The effect of cisapride on the fetus is unknown.
• Patients would be at increased risk for dystocia if they carried a pregnancy to term.

See Also

• Constipation and Obstipation
• Dyschezia and Hematochezia
• Perineal Hernia

Author Albert E. Jergens

Consulting Editor Stanley L. Marks

Client Education Handout Available Online