Simple obstruction: an obstruction of the intestinal lumen without compromise to the blood supply. However, fluid is continually secreted into the small intestine and is not absorbed. As the intraluminal pressure increases intestinal mucosal capillaries are compressed. The progression of clinical signs is relatively slow compared to strangulating obstructions, particularly in the case of large colon/ small colon impactions. The prognosis is good for large colon simple obstruction, and fair for simple obstruction of the small intestine, cecum, and small colon. This assumes the horse has been referred within a reasonable period of time. The prognosis is lower for foals because of the difficulty of managing critically foals, and their propensity to develop adhesions.
Simple Obstruction of the Large Colon
• Pelvic flexure impactions: These are the most common of the large colon obstructions. Early feed impactions are best treated medically, with laxatives, analgesics, and intravenous fluids in refractory cases to maintain hydration. In cases that don't respond (increasing pain, deteriorating systemic condition, lack of progress on softening the impaction), treatment is by laparotomy, pelvic flexure enterotomy, and lavage.
• Sand impactions: Diagnosis can be made by rectal palpation of the mass and mixing feces with water and allowing the sample to stand. Sand typically accumulates in the right dorsal colon and transverse colon. Surgical treatment is by enterotomy and evacuation, as above. Incidence of this disorder can be decreased by decreasing pasture animal density (to prevent the grass from being so short that the animals are eating dirt with it), turning animals onto pasture after they've been fed, and regular administration of bulk laxatives.
• Enteroliths: These most commonly occur on the west coast. Diagnosis may be made by radiography, but these horses are generally taken to surgery because of signs of large colon obstruction (gas distension, bands, and impaction). Enteroliths are typically located in the right dorsal colon, transverse colon, or small colon. Initial evacuation of intestinal contents via a pelvic flexure enterotomy is followed in most cases by an incision in the right dorsal colon (because of the size of the enterolith). If an enterolith has a flat side, look for more.
• Cecal impactions: Impaction of the cecum can be very difficult to recognize. Horses usually exhibit low grade pain and may be off feed. Ingesta can still pass over the top of the impaction to the large colon, so fecal material is still passed. Medical treatment may be rewarding if these are caught early, but surgical exploration is usually indicated. Surgical treatment involves enterotomy and removal of the mass of ingesta from the cecum. The cecum should be by-passed via an ileocolostomy if the impaction is primary (cecal impaction occurred in the absence of another disease process). Cecal impactions that develop secondary to another disease process (typically a painful musculoskeletal condition) must be treated surgically as soon as they are recognized, but don't necessarily require a by-pass.
• Nephrosplenic entrapment: the nephrosplenic ligament runs from the lateral border of the left kidney to the medial surface of the spleen. The left colon migrates over this ligament, resulting in partial obstruction. Several treatments are available. There are several treatment options, including rolling the horse, or surgical correction. For the rolling procedure, the horse is dropped on its right side, so the displaced colon is uppermost. The horse is rolled into dorsal recumbency, then to its left side. The clinician can attempt to palpate the horse while recumbent to make sure the entrapment is resolved. If not, the rolling procedure can be repeated, or the horse is taken to surgery. Administration of phenylephrine may facilitate the process by reducing the size of the spleen.
• Other large colon displacements: The large colon is only fixed in position at the attachment of the right dorsal colon to the dorsal body wall, and the right ventral colon to the cecum, giving the rest of the large colon a lot of freedom of movement. A number of different configurations of displacement may occur, and are all treated by surgical manipulation. It is not important to understand each anatomic configuration of displacement.
Simple Obstruction of the Small Colon
• Obstruction of the proximal small colon results in signs similar to large colon obstruction. Distal small colon obstruction results in similar signs, with the addition of tenesmus. Foals often have meconium impactions shortly after birth that are effectively treated with enemas. Rarely, laparotomy with infusion and massage of the meconium is necessary to relieve the obstruction. Obstruction of the small colon can also occur in adults, with small enteroliths. Fecal impactions can also occur, particularly in miniature horses. Small colon enterotomies should be made through the anti-mesenteric band.