Canine Constipation Study Guide

Overview and Clinical Importance

Constipation is defined as the infrequent or difficult evacuation of feces, which are typically dry and hard. This is a common clinical problem in small animal practice that ranges from mild, self-limiting episodes to severe, life-threatening conditions requiring emergency intervention. Understanding the pathophysiology, diagnostic approach, and treatment options is essential for NAVLE success and clinical practice.

Obstipation represents intractable constipation characterized by an inability to evacuate the mass of dry, hard feces, with impaction potentially extending from the rectum to the ileocolic valve. When obstipation results in permanent dilation and hypomotility of the colon, the condition is termed megacolon. While megacolon is more common in cats, it does occur in dogs and carries significant clinical implications.

Pathophysiology

Normal Colonic Function

The colon serves two primary functions: water and electrolyte absorption and fecal storage. Peristaltic waves are responsible for the aboral movement of fecal material in the colon. Giant, migrating waves occur intermittently throughout the day and move material farther and more rapidly, constituting the gastrocolic reflex that is common after meal ingestion.

Mechanism of Constipation

When fecal material remains in the colon for an extended period, water continues to be absorbed, making the feces progressively drier, harder, and more difficult to pass. A reduction or loss of peristaltic waves may contribute to constipation, as can increased segmentation wave activity. The resulting fecal concretions become increasingly impacted and can eventually be described as rock-hard masses.

Progression to Megacolon

Prolonged retention of feces and distention of the colon results in damage to smooth muscle and nerves, ultimately leading to colonic inertia. The colon becomes three to four times larger than normal and loses its ability to contract effectively. This represents irreversible damage and is classified as dilated megacolon. In contrast, hypertrophic megacolon results from outlet obstruction such as pelvic fracture malunion, causing the colon to hypertrophy in response to the obstruction.

Etiology and Predisposing Factors

Constipation can be classified based on the location of the underlying cause:

Clinical Presentation

Classic Clinical Signs

Tenesmus: Straining to defecate with little or no result, or producing only small amounts of liquid fecal matter mixed with blood or mucus

Dyschezia: Painful or difficult defecation

Hard, dry feces: Often smaller than normal or ribbon-like if obstruction is present

Infrequent defecation: No bowel movement for greater than 48-72 hours

Behavioral signs: Circling, scooting, frequent squatting, crying out during defecation attempts

Signs of Severe Constipation or Obstipation

• Lethargy and depression
• Anorexia and weight loss
• Vomiting (may occur with severe straining)
• Dehydration
• Abdominal distension and discomfort
• Paradoxical diarrhea (liquid feces passing around impacted mass)

Diagnostic Approach

History

Obtain detailed information about the last normal bowel movement, diet (especially bone consumption), previous trauma history, medications, and any signs of distress, vomiting, lethargy, or bloating.

Physical Examination Findings

• Abdominal palpation: Firm, distended colon palpable; may be painful
• Digital rectal examination: Hard fecal mass, assess for strictures, tumors, foreign bodies, prostate size, pelvic canal narrowing
• Perineal examination: Look for perineal hernia (swelling lateral to anus)
• Neurologic examination: Evaluate tail tone, anal tone, perineal reflex, hindlimb function

Diagnostic Imaging

Abdominal Radiography

Radiographs are the primary diagnostic tool and confirm the extent of fecal impaction. Key findings include a colon distended with fecal material that appears as soft tissue opacity with gas mixed in. Megacolon is diagnosed when the colon diameter exceeds twice the length of the L5 vertebral body on lateral abdominal radiographs. Radiographs also help identify pelvic fractures, masses, prostatomegaly, and radiopaque foreign bodies.

Additional Diagnostics

• Bloodwork (CBC, chemistry, urinalysis): Evaluate hydration status, electrolytes (K+, Ca++), renal function, screen for metabolic causes
• Abdominal ultrasound: Assess prostate, identify masses, evaluate bladder position (perineal hernia)
• Thyroid panel: If hypothyroidism suspected
• Colonoscopy: For intraluminal lesions, strictures, or biopsy collection
• Barium enema: Rarely needed; can evaluate for strictures or masses

Treatment

Treatment Goals

• Remove impacted feces
• Correct dehydration and electrolyte abnormalities
• Identify and treat underlying cause
• Prevent recurrence

Mild Constipation - Outpatient Management

For patients with mild to moderate fecal impaction without systemic signs, outpatient management may be appropriate:

• Increase water intake and ensure adequate hydration
• Dietary modification (high-fiber diet or low-residue diet depending on cause)
• Oral laxatives
• Increase exercise

Moderate to Severe Constipation - Medical Management

Patients with significant fecal impaction require more aggressive intervention:

Rehydration

Intravenous or subcutaneous fluid therapy should be initiated before attempting deobstipation. Dehydrated patients should not receive laxatives until adequately rehydrated, as this can worsen fluid deficits.

Enemas

Enemas soften impacted feces and stimulate defecation. They should be warmed and administered slowly via lubricated rubber catheter under sedation. CRITICAL: Phosphate enemas (Fleet enemas) are CONTRAINDICATED in dogs and cats due to the risk of severe, potentially fatal hyperphosphatemia, hypocalcemia, hypernatremia, and hyperosmolality.

Manual Deobstipation

For severely impacted patients, manual extraction under general anesthesia may be required. The patient should be intubated due to risk of vomiting during colonic manipulation. Fecal masses are gently fragmented and milked toward the rectum for digital removal. Multiple procedures may be necessary.

Pharmacotherapy

Surgical Treatment

Surgery is indicated when medical management fails or when there is an identified mechanical obstruction requiring correction.

Subtotal Colectomy

Subtotal colectomy involves removal of 90-95% of the colon and is the definitive treatment for refractory megacolon. The procedure is most commonly performed in cats but can be done in dogs with variable results. Preservation of the ileocolic junction is recommended when possible to minimize postoperative diarrhea and small intestinal bacterial overgrowth.

In a UK study of 8 dogs with acquired hypertrophic megacolon that underwent subtotal colectomy with preservation of the ileocolic junction, 7 of 8 dogs showed clinical improvement at discharge and returned to normal defecation in 5-10 weeks. These dogs survived 11-48 months post-surgery. One dog died from septic peritonitis, highlighting the significant risks of this procedure.

Other Surgical Procedures

• Perineal herniorrhaphy: For constipation secondary to perineal hernia; concurrent castration is essential
• Pelvic osteotomy: For pelvic fracture malunion if megacolon is less than 6 months duration
• Castration: For prostatomegaly-induced constipation in intact males

Prognosis

"CONSTIPATE" - Causes of Constipation: C - Colon diseases (stricture, neoplasia) O - Obstruction (foreign body, pelvic fracture) N - Neurologic disease (spinal cord, dysautonomia) S - Sedentary lifestyle T - Trauma (old pelvic fractures) I - Insufficient water/dehydration P - Prostate enlargement A - Anorectal disease (perineal hernia) T - Toxins/drugs (opioids, antihistamines) E - Electrolyte abnormalities (hypokalemia, hypercalcemia)

Exam Focus: The NAVLE loves to test the Fleet enema contraindication! Never forget: Phosphate enemas (Fleet) are TOXIC to dogs and cats and can cause fatal hyperphosphatemia and secondary hypocalcemia. Safe alternatives include warm water/saline, DSS, mineral oil (rectal only), and lactulose enemas.