Large Colon Impaction - Horse

Anatomy

Impactions of the large colon occur where the luminal diameter narrows, especially the pelvic flexure and the right dorsal colon (RDC) (83 in Bliks). Food impactions occur most often at the pelvic flexure, the site of the myoelectrical pacemaker. Sand impactions occur at various sites in the large intestine.

Risk factors

Sudden restriction in exercise associated with musculoskeletal injury (84) Twice daily feeding of concentrate - large fluxes of fluid into and out of colon, associated with readily fermentable carbohydrate in the colon and increases in serum aldosterone. Fluid fluxes may cause dehydration of ingesta during aldosterone-stimulated net fluid flux out of the colon (32). Amitraz - acaricide associated with clinical cases of colon impaction (85,86) - may alter pelvic flexure pacemaker activity resulting in uncoordinated motility patterns between the left ventral and left dorsal colon and excessive retention of ingesta. Absorption of water from ingesta increases with time, dehydrating the contents of the colon and resulting in impaction Parasite migration in the region of the pacemaker (87) Limited exercise Poor dentition Coarse roughage Dehydration

Clinical signs

Slwo onset mild colic Reduced defaecation Faeces hard, dry and mucus-covered because of delaed transit Heart rate mildly elevated during painful episodes but often normal Colic signs typically well controlled with analgesics but become increasingly more severe and refractory if impaction not resolved

Diagnosis

See colic diagnosis in horses Firm mass in large colon but may underestimate extent of impaction because much of colon out of reach (83). Adjacent colon may be distended if impaction has resulted in complete obstruction. Impaction at other sites such as the transverse colon may not be palpable per rectum.

Treatment

Medical

See medical treatment of colic in horsesInitially intermittent abdominal pain controlled with analgesics: Flunixin meglumine IV every 6-12 hours Butorphanol IV every 6-8 hours Xylazine IV as needed Oral laxatives to soften the impaction: Liquid paraffin or mineral oil 2-4lites by nasogastric tube every 12 to 24 hours Anionic surfactant dioctyl sodium succinate (DSS) 6-12g/500kg diluted in 2-4litres of water by nasogastric tube every 12-24 hours Saline cathartics such as magnesium sulphate 0.1 mg/kg in 2-4litres by nasgastric tube may also be useful Prevent access to feed Aggressive oral and IV fluid therapy (2-4 times maintenance) if impactions persist(83)

Surgical

If impaction remains unresolved, pain becomes uncontrollable, or extensive gas distension of the colon occurs, surgery is indicated. Abodominocentesis can be used to monitor the onset of intestinal compromise.(83) At surgery the contents of the colon are evacuated via a pelvic flexure enterotomy.

Prognosis

Good for impactions that resolve medically (95% long term survival in one study) and fair in horses that require surgical intervention (58% long-term survival in the same study) (84)

Prevention

Frequent small feedings(32)