53 Small intestinal obstruction

Pathophysiology

Types of obstruction

The small intestine may be physically or functionally obstructed.

Functional obstruction is due to lack of motility but there is not physical obstruction. Many animals experience postoperative ileus  (POI), especially horses and humans). During POI, inflammation disrupts normal nerve function.  Animals can also get ileus from enteritis (inflammation of the intestines).  Hypocalcemia is another common cause of functional obstruction – no smooth muscle action means no peristalsis.

Physical obstruction can occur due to intraluminal or extraluminal obstruction.  Intraluminal obstructions can be due to food material, tumors, enteroliths, foreign bodies and the like. Extraluminal obstructions can be due to gut twists and displacements, tumors, adhesions, hernias and changes in other peritoneal structures that pinch the gut closed.

All animals can develop intussusceptions – one part of the bowel telescopes (folds into) another part. This is particularly common in younger animals with diarrhea and in older animals with tumors. Brown Swiss cattle of all ages are predisposed (no idea why).

Intestinal torsions are also common – the gut twists on itself. These are most common in younger animals.

Dog and cats are prone to foreign body obstructions. Cats like to eat string and thread. These linear foreign bodies can then “plicate” or “fold up” the SI as they get stuck in the SI.

In cattle, a relatively more recent form of physical obstruction is blood clot disease – hemorrhagic gastroenteritis. We believe this is a clostridial or fungal problem causing bleeding into the gut lumen. It almost always causes death.

Pathophysiology

An obstruction in the small intestine stimulates local contractions to push the obstruction distally. Secretions are increased to “liquefy” the contents. Scant soft feces may be seen in partial obstructions as fluid but not ingesta can pass.

With complete obstructions, the secretions eventually back up into the stomach and are vomited in small animal patients. In horses, the stomach may rupture unless a nasogastric tube is placed to remove the fluid.

In ruminants, the secretions back up into the abomasum. Abomasal fluid eventually refluxes back into the rumen. A low pH in the rumen will also shut down motility of the forestomachs.

Diagnostics

The most common finding is vomiting or refluxing. Vomiting cats and dogs that do not respond to maropitant are likely to have a physical obstruction.

Abdominal pain is related to intestinal distension or traction on the mesentery. In smaller animals, abdominal palpation may be useful but animals may be too painful to allow good palpation.

Animals with SI obstructions are not usually bloated.

Animals are often sick and are dehydrated with electrolyte and acid base abnormalities. With loss of proximal SI fluids  in the vomitus (which is high in HCl, sodium bicarbonate and electrolytes), serum levels become low in chloride and other electrolytes.  More proximal lesions lead to more dehydration. More distal lesions mean more opportunity for fluid and electrolyte resorption and are generally associated with less dehydration and electrolyte derangements.

Radiographs are useful in smaller animals, particularly contrast radiographs. Ultrasound can occasionally identify lesions such as intussusceptions.

Functional obstruction is typically a systemic problem and affected animals may show other systemic signs such as fevers and depression.

Therapies

Treatment of functional obstruction includes supportive care and time, iv lidocaine or other motility agents, and surgery to decompress the intestines. Moving the fluid out of the SI is often enough to let them restart normal activity.

Impactions (dried out feces) can be treated by improving hydration and gut fluid. Agents such as epsom salts can be fed to an animal to help passage of dried out ingesta. These agents pull water into the gut lumen using osmotic forces. Mineral oil and other lubricants can help the ingesta slide along the gut.  Foreign body physical obstructions generally require surgical removal.