INTRODUCTION

Diarrhea is a very common clinical sign observed in critically ill canine and feline patients. Diarrhea is defined as an increase in fecal mass caused by an increase in fecal water or solid content. This is usually associated with an increase in frequency, fluidity, or volume of feces. In a 20-kg dog, approximately 2.5 L of fluid enters the duodenum each day, and about 98% of the fluid entering the intestine is absorbed.¹ Diarrhea in the critical care setting is often overlooked and overshadowed by the primary disease process. However, diarrhea can lead to severe aberrations in nutrient, acid-base, fluid, and electrolyte balance. Without proper attention it can lead to the deterioration of the patient’s condition. Diarrhea may be associated with patient discomfort, local dermatitis, catheter or catheter site infections, and potentially bacterial translocation if the integrity of the intestinal mucosa is altered. Consideration of the most likely cause is important because it allows the clinician to decide which diagnostic modalities are indicated for proper investigation of the diarrhea. Three broad etiologic categories may be used when considering diarrhea: iatrogenic causes, primary gastrointestinal (GI) causes, and other diseases secondarily causing diarrhea.

PATHOPHYSIOLOGIC MECHANISMS OF DIARRHEA

There are several categorization schemes for diarrhea, with great overlap among the classifications. One of the most commonly used classification schemes arranges the pathophysiologic mechanisms underlying diarrhea as follows: osmotic diarrhea, secretory diarrhea, diarrhea resulting from altered permeability, and diarrhea resulting from deranged motility.

Osmotic diarrhea is caused by the presence of excess luminal osmoles, leading to fluid retention and fluid draw into the intestinal lumen. Most causes of a diarrhea have an osmotic component.

Secretory diarrhea is caused by a net increase in intestinal fluid secretion. This results from either an absolute increase in intestinal secretion or a relative increase caused by a decrease in intestinal absorption.

Normal intestinal physiology and systemic health are dependent on the semipermeable nature of the intestinal mucosa. Nutrients, electrolytes, and fluid are absorbed and secreted, and the mucosa and immune system of the intestine inhibit translocation of bacteria and bacterial toxins. However, microscopic and macroscopic damage to either the epithelial cells or epithelial cell junctions can lead to altered intestinal permeability. Not only are vital substances lost into the intestinal lumen, but the altered permeability leaves the intestine vulnerable to translocation of potentially fatal bacteria and their products.

Alterations in intestinal motility are probably the least understood of the causes of diarrhea. Motility alterations leading to diarrhea include either increased peristaltic contractions or decreased segmental contractions.¹ Even within this classification scheme, significant overlap occurs among the groups.

In animals with primary GI causes of diarrhea, historical questions may provide evidence that allows anatomic localization of the disease to either the small or large bowel. This differentiation will allow a more accurate formation of differential diagnoses and subsequent diagnostic testing. Historical and clinical differences usually noted between small and large bowel diarrhea are illustrated in Table 132-1.

Table 132-1 Differentiation of Diarrhea Based on Anatomic Location

IATROGENIC CAUSES OF DIARRHEA

Iatrogenic causes of diarrhea are likely more common than is realized and should be ruled out to facilitate clinical improvement. Diarrhea is a common side effect of several classes of drugs used in critically ill patients (Box 132-1). Antimicrobial agents may cause diarrhea as a direct result of drug formulation or properties, or as a result of alterations in intestinal microbacterial flora. Most of the chemotherapeutic agents have direct toxic effects against the rapidly dividing cells of the intestinal crypts, leading to villous blunting and altered absorption. Other classes of drugs such as antiarrhythmic agents, lactulose, and proton pump inhibitors may also be associated with diarrhea.

Acute or abrupt changes in the diet are not uncommon in hospitalized or critically ill patients. Anorexic animals are often coaxed to eat with canned diets and other potentially novel foods. Enteral tube feeding is commonly employed in critically ill patients. The osmotic and caloric properties of these diets may exceed the digestive and absorptive capacities of the intestine and lead to osmotic diarrhea. Furthermore, prolonged quiescence of the intestine from either anorexia or parenteral nutrition can lead to villous atrophy and decreased absorptive function when enteral feeding is initiated.