Updates in the Medical Management of Colic: Moving Beyond Mineral Oil

1. Introduction

Colic remains one of the leading reasons for which horse owners consult their veterinarians. While exact numbers are difficult to obtain and vary geographically, reported risks have ranged from 3 to 10 episodes of colic per 100 horse-years.¹ Fortunately, less than 5% of horses with colic have a surgical lesion. Therefore, medical treatment is indicated for the vast majority of horses with colic. While there is unfortunately no new “silver bullet” treatment for colic, this paper will review the main medical treatments currently in use: fluid therapy, laxatives, and analgesics.

2. Fluid Therapy

Fluid therapy is used in horses for a variety of reasons, but the basic goals are to replace losses, support the cardiovascular system, and maintain hydration in the face of ongoing losses. In horses with colic due to intestinal impactions, fluids are commonly given in an effort to promote hydration of the colonic contents. Fluids may be given either intravenously or enterally. Intravenous fluids are more expensive, require an indwelling intravenous catheter with the associated risk of thrombophlebitis, and may be impractical on the farm. However, horses with diarrhea, signs of hypovolemic shock, or nasogastric reflux greater than 2 to 3 L will generally necessitate IV fluids to maintain hydration. In hospital settings, high volume intravenous fluids (two to three times maintenance rate or 100–150 mL/kg/d) are often given under the premise that there will be net flux of fluid into the gut lumen resulting in softening of impacted material. However, there is conflicting evidence as to the effect of IV fluid volumes on fecal hydration. In one study of normal horses, 5 L/h of IV fluids for 12 hours (approximately 100 mL/kg/d or twice maintenance) did not show any effect on hydration of right dorsal colon contents or feces.² In another, study comparing fluid rates and means of administration to dehydrated horses, fecal and systemic hydration were restored at twice maintenance IV fluids; however, there was no additional benefit seen when increasing the rate to three times maintenance. In fact, horses given three times maintenance IV fluids had increased urine production and sodium loss, which could contribute to electrolyte abnormalities as well as rebound dehydration when fluids are discontinued.³ There is in fact too much of a good thing!

Rapid transit of fluids through the small intestine makes enteral fluids a logical option in treatment of large colon impactions. Fluids delivered via nasogastric tube will exit the stomach within 15 minutes and reach the cecum and large intestine within 1 to 2 hours in most normal horses. It is also thought that intermittent bolus delivery may result in more fluid being delivered to the colon, as it will overwhelm the small intestine’s capacity for absorption.² Nasogastric administration may also stimulate the gastrocolic reflex, thus, aiding with overall gastrointestinal motility. There is evidence that leaving an indwelling nasogastric tube in place for 72 hours does delay gastric emptying, so repeat intubation may be better despite its inconvenience and behavioral effects.⁴ Enteral fluids are overwhelmingly less expensive and easier to prepare than IV fluids, as they do not need to be sterile. Enteral fluid therapy is also slightly more forgiving than intravenous fluids in terms of rate and electrolyte composition. However, large volumes of plain water or hypotonic solutions may cause marked electrolyte abnormalities including hyponatremia, hypokalemia, and hypocalcemia.^2,5 Isotonic fluids, however, are generally well tolerated with no significant effects on plasma biochemistry values except for a mild hemodilution when using a potassium rich solution with 6 g NaCl and 3 g KCl per liter of water.⁵ Most average size horses can tolerate 6 to 10 L/h of intragastric fluids; however, there does seem to be some individual variation. Horses with a significant amount of ingesta may show signs of discomfort when large volumes (5 L) of fluid are administered. Fortunately, this generally resolves with time, walking, or decompression via the nasogastric tube. Abdominal distension can also be seen with high volume enteral fluids but is generally well tolerated by most horses.⁶ Many horses treated with enteral fluids for an impaction will develop self-limiting diarrhea due to excretion of fluids as the impaction resolves, and cecal rupture has also been reported.⁷

Administration of enteral fluids has been evaluated in several studies in normal horses. In one study of adult horses, plain water given at once (50 mL/kg/day), twice (100 mL/kg/day), and three times (150 mL/kg/day) maintenance administered via nasogastric tube over four treatment periods every 6 hours has been shown to be safe and effective at restoring intestinal hydration with a volume-dependent effect on fecal volume.3 In another study, a balanced electrolyte solution given continuously at a rate of 10 L/h via nasogastric tube was shown to be more effective in hydrating ingesta that an identical rate of fluids given intravenously along with one intragastric dose of magnesium sulfate (1 g/kg in 1 L water). There was also a trend of more fecal production in the horses treated with enteral fluids, and there were less systemic effects.⁶ This balanced electrolyte solution contained 5.27 g NaCl, 0.37 g KCl, and 3.78 g NaHCO3 per 1 L of water resulting in a solution with 135 mmol Na/L, 5 mmol K/L, 95 mmol Cl/L, and 45 mmol HOC3/L. In normal horses, hydration was measured in the feces and right dorsal colon of horses with indwelling fistulas in a crossover design of six treatments. The balanced electrolyte solution and sodium sulfate resulted in the best hydration of right dorsal colon (RDC) contents, while sodium sulfate, magnesium sulfate, and balanced electrolyte solution resulted in the most hydrated feces. Sodium sulfate caused hypocalcemia and hypernatremia while plain water caused hyponatremia leaving the balanced electrolyte solution as the safest and most effective option.²

Enteral fluids have also been shown to be effective in management of clinical cases of nonstrangulating large colon lesions. In a retrospective analysis of 147 horses with large colon impactions where all were treated with IV fluids (8.2–14.3 mL/kg/h) and only 49 (33.3%) received any enteral treatment, the mean time to resolution of impaction was 48 hours with a range from 1 to 6 days. Additionally, 24 horses (16.3%) required surgical intervention.⁸ This contrasts with another study of 108 horses with large colon impactions or displacements where horses were given either enteral fluids alone (8 –10 L of a potassium-rich isotonic electrolyte solution every 2 hours) or enteral fluids simultaneously with intravenous fluids (2 mL/kg/h lactated Ringer’s solution). Both groups had a mean time to resolution of approximately 24 hours for impactions and 14 hours for displacements with no significant electrolyte abnormalities. The overall success rate was 99% for impactions and 83% for displacements.⁵ Another retrospective study of 53 horses found that impactions treated with enteral fluids resolved faster, had shorter hospitalizations (4 vs 7 days), and lower mean hospital bills (£483 vs £2006) than horses treated with intravenous fluids.⁹ The bulk of the evidence supports treatment of colic with enteral fluids whenever possible.

Enteral fluids can also be used to correct mild electrolyte abnormalities. A potassium-rich balanced electrolyte solution safely corrected mild hypokalemia in horses with large colon impactions and displacements.⁵ Administration of 1 g/kg body weight of NaHCO3 has been shown to increase cecal pH, which may be useful in the treatment of horses with grain overload.¹⁰

3. Cathartics/Laxatives

Despite its routine use, there is little literature to support the use of mineral oil in horses. Mineral oil will lubricate ingesta but does not treat dehydration. It is most useful as a marker of transit time, as it should be seen in the feces of a normal horse 12 to 24 hours after administration. Mineral oil has also been advocated for use as a cathartic in cases of intoxication. Recent work, however, suggests that its use in cases of cantharidin toxicity may be contraindicated. Rats treated with mineral oil had increased absorption of cantharidin with increased morbidity and mortality relative to negative controls or those treated with other adsorbents.¹¹ [...]