5: THE ABDOMEN

5 THE ABDOMEN



Clinical importance of the abdomen


The equine clinician spends a large amount of time attending to medical and surgical conditions of the equine abdomen. This is principally due to the occurrence of colic (abdominal pain) which can arise from any of the abdominal organs. Colic is believed to be the single most important clinical cause of death in the horse. It is also the most common acute emergency. Approximately 90% of cases may respond to medical treatment, particularly pain relief, supportive therapy and prevention of dehydration.


Diagnostic techniques used in the examination of the abdomen include palpation, auscultation, percussion and rectal examination. Radiography is useful in foals and endoscopy is useful for investigation of stomach disorders, particularly gastric ulceration. Ultrasonography is particularly useful in cases of intussusception and for viewing colonic displacements. The ultimate diagnostic techniques are laparoscopy and laparotomy (surgical opening of the abdomen). For laparotomy, the horse is usually placed in dorsal recumbency and surgery will involve a midline incision. In this way most of the small intestine can be exteriorised but not the rostral 1 metre and the last 15–20 cm. The apex and part of the body of the caecum can be lifted out. In addition, the left ventral colon, left dorsal colon and parts of the right dorsal colon and right ventral colon can be lifted out. As in a rectal examination, identification of the various parts of the large bowel is fundamental in cases of displacement or distortion. The major help in this is the presence of the taeniae in the wall of the large intestine; the caecum and the right and left ventral colon all have 4, the pelvic flexure 1, the left dorsal colon 1, and the right dorsal colon has 3 taeniae.


One of the key factors in problems of the digestive system is the inability of the horse to vomit. Once inside the digestive tract, the ingesta (no matter how indigestible or toxic) is there to stay. It is, however, the peculiar anatomy and disposition of the large intestine which especially predisposes to colic. Special features in this respect are the four components of the large colon (two dorsal and two ventral parts) of different calibres and weights; the flexures (particularly the pelvic flexure); the blind-ending sac of the caecum; the long small intestine, which is freely movable; and the considerable narrowing of the lumen from the large colon to the small colon. The left ventral and left dorsal colon are freely movable. The left colon can become displaced to the lateral surface of the caecum, next to the body wall. The pelvic flexure may be displaced forward to the sternum, lateral to the caecum. The left colon can also be displaced to a position between the dorsal body wall and the suspensory ligament of the spleen.


Diet, management, and the presence of particularly virulent parasitic infestations of the gut around the branches of the cranial mesenteric arteries are significant factors in the occurence of colic. Intestinal parasites are less of a common problem these days, thanks to the effective endoparasiticides and better management. Colic can be classified as mild, moderate or severe abdominal pain. It is usually associated with an obstruction to the passage of contents, caused by a failure of gut motility. This causes a rise in intra-abdominal tension, and pain results from the tensions in the mesenteries. Ischaemia of the affected bowel is followed by gut stasis, irritation of the mucosal surfaces and eventually necrosis and inflammation. Several sorts of colic are described. The details are beyond the scope of this introduction but they include spasmodic, impactive, flatulent, obstructive, non-strangulation infarction, enteritis and idiopathic cases.


Displacements of organs, particularly large bowel displacements and torsions, can occur in several sorts of colic. Spasmodic colic may be responsible for 40% of cases and may be associated with increased vagal tone. The condition of ‘grass sickness’ (equine dysautonomia) in horses is essentially an impaired motility of the gut, resulting in spasmodic colic. Impactive colic is often the result of abnormal feeding, including coarse feed, and failure of a proper fresh water supply. Poor dentition leading to poor mastication, lack of regular worm treatments and failure to remove foreign bodies from the diet, can contribute. Inactivity (i.e. following box rest for orthopaedic conditions) can also contribute to the problem.


The transverse colon is often the site of impaction with enteroliths and faeces and, in the foal, with meconium. Enteroliths are caused by the precipitation of magnesium and ammonium phosphate crystals around a nucleus such as a stone, nail or other foreign body. As they increase in size they are particularly prone to getting stuck in the small intestine and transverse colon. Impaction is very common in the caecum and in the pelvic flexure of the colon. Caecal impaction, if severe, may be followed by caecal perforation. Pelvic flexure impaction is often caused by a combination of coarse roughage, sand and gas. The stomach can become impacted with corn and bedding, and the duodenum can be impacted with coarse ingesta.


Flatulent colic is a result of excessive production of gas associated with fermentation of the feed (usually of poor quality). Excessive gas production can be detected by listening for a ‘ping’ when simultaneous percussion and auscultation is carried out. It can be relieved by caecal trocarisation in which a trocar and a cannula are inserted into the caecum though the right paralumbar fossa.


Obstructive more colic and infarction is usually a result of nematode parasites. The 4th and 5th stages of Strongylus vulgaris damage the blood supply by migrating through the walls of the cranial mesenteric artery. The resultant thrombi cause intra-vascular occlusion of the vessels and ischaemia. The ileum may become thickened (hypertrophic) in response to persistent parasitic infestation.


A particular form of obstruction occurs with sand (picked up with the food) and also with the ingestion of foreign materials (e.g. plastic bags). Idiopathic colic (where there is at present no known cause) may, perhaps, include colitis X. Colitis can also result from prolonged drug therapy (e.g. right dorsal colitis with phenylbutazone therapy). In addition, strangulation of the bowel can occur and also torsion. Both will cause impaction and obstruction. Small intestinal volvulus occurs when a segment of the small intestine rotates around the long axis of its mesentery. A 180 degree rotation of the jejunum may also occur, and frequently the ileum is involved because of its fixed attachment at the ileo-caeco-colic junction. Large bowel volvulus involves a rotation of the dorsal and ventral parts of the colon on their horizontal axes and this also frequently involves the caecum.


Enteritis is not common in the horse but there is, on occasion, an acute severe enteritis in horses associated with clostridial infections. It causes an acute colic and is often diagnosed only at surgery or post-mortem examination. There are up to 10 parasites that may live in the large bowel and cause irritation which may lead to diarrhoea. Severe diarrhoea, with irritation and constant straining, may lead to intussusception. Sudden dietary changes and parasitic infestation are usually a cause of this condition. There is also a real possibility of salmonellosis in any horse that is stressed or has been subjected to surgery. Several other conditions may also cause acute enteritis such as Potomac horse fever in certain parts of the world. Chronic inflammatory bowel disease can also occur. It usually involves internal parasites but also may involve strangles, Rhodococcus equi or even M. paratuberculosis. A naso-gastric (stomach) tube can be used to deliver large volumes of oral rehydration therapy.


Dysfunctions of the liver can be investigated by percutaneous liver biopsy in the right 12–13th intercostal space, with sedation and local anaesthesia. It should be carried out between lines drawn from the tuber coxae to the point of the shoulder and to the point of the olecranon. The liver normally lies deep to intercostal spaces 10–14 but may atrophy considerably with age. Liver disease is quite common in the horse, probably as a result of eating plants of the genus Senecio, particularly ragwort. It is often associated with damage to the bile ducts. It causes weight loss, and investigations should include analysis of liver enzymes and liver function tests. In severe cases, hepatic encephalopathy can occur; nervous signs result from circulating waste products. Complete liver failure is quite rare, as there is a huge functional reserve in the equine liver, and over 70% has to be damaged before this will occur.


The horse is particularly prone to peritonitis, probably because of the fragility of the gut. It can be primary or secondary, diffuse or localized. It is usually acute and diffuse and follows gastro-intestinal disease. Haemoperitoneum follows migration of strongyle larvae through the cranial mesenteric vessels, or foaling or rupture of the spleen or liver following traumatic accidents.


Uroperitoneum may result from rupture of the bladder. In a foal, during the first week of life, this must be differentiated from other types of colic which are caused by distension. If an accumulation of peritoneal fluid is suspected, abdominocentesis can be performed. Needles are used to drain fluid from a site about 5 cm caudal to the xiphoid cartilage in the right paramedian position (this avoids penetrating the spleen). Abdominal ultrasound can be used. (The normal approach for paracentesis of the abdomen is midline through the linea alba at the most dependent area.)


External hernias are protrusions of an organ or part of an organ through a normal opening in the wall of the cavity. Umbilical, inguinal and perineal are the usual types. In young animals the congenital hernias can be left if small or surgically repaired if large. Most inguinal hernias fall into this group. Umbilical hernias occur in about 2% of thoroughbred horses, are more common in females than in males, and have a strong hereditary component. Diaphragmatic hernias are internal hernias. They can occur as congenital defects but are more likely as a result of trauma, as when a horse is trapped over a gate or fence or jump, resulting in an increase in intra-abdominal pressure. If abdominal contents pass through one of the normal openings in the diaphragm (caval, oesophageal or aortic hiatuses) colic may result.


It is also possible to have an internal herniation of abdominal viscera through the epiploic foramen. Through this foramen, the greater peritoneal cavity communicates with the lesser cavity or omental bursa. It lies in the right dorsal quadrant of the abdomen, situated on the ventral surface of the liver, dorsal to the portal fissure. Dorsal to the foramen lie the caudate process of the liver, hepatic portal vein, gastro-pancreatic fold and the caudal vena cava. Ventrally, there are relations to the right lobe of the pancreas, the gastro-pancreatic fold and the portal vein. Rostrally, lies the hepatoduodenal ligament. Caudally, lie the pancreas and mesoduodenum. There is no real age association for this internal hernia, but there are suggestions that the condition may be more common in the horse that crib bites or wind sucks (8 times more likely). In old horses, this foramen may be 10 cm long, mainly due to the atrophy of the right lobe of the liver. This atrophy may predispose to internal herniation. In these animals there is the possibility of a segment of small intestine passing through the foramen. The intestine may enter the omental bursa from right to left, or may slip through the foramen from left to right. The amount of intestine that slips through varies from several centimetres to all of the small intestine.


All the boundaries of the abdomen may have ruptures which do not traverse the natural openings but traverse defects in the walls or membranes. Complex internal ruptures may occur, as when the gastro-splenic ligament is torn and the distal jejunum goes from a caudal to a cranial direction. Mesocolic rupture may occur during parturition, involving the mesentery of the small colon. It is a complication of rectal prolapse often accompanied by prolapse of bladder, uterus, vagina and intestine or any combination thereof.


Abdominal abscesses usually involve the mesenteric lymph nodes and are associated with the usual pus-forming bacteria of the horse – streptococci, E.coli, salmonella or Rhodococcus equi. Abdominal neoplasia is rare. Alimentary lymphoma is seen infrequently, but lipomas are a common cause of colic, especially in grey horses. Intestinal or other alimentary melanomas are also common in grey horses.


Clinical considerations for the spine in the abdominal region are dealt with in the section on the spine in Chapter 8 (p. 269).



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Fig. 5.1 Surface features of the abdomen: left lateral view. The hair has been shaved over the palpable bony prominences. The umbilicus lies in the median plane, just cranial to the prepuce, and its position in the female is shown in Fig. 5.55.



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Fig. 5.2 Bones related to the abdomen: left lateral view. The palpable bony prominences shown in Fig. 5.1 are coloured red, except for the tuber sacrale.



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Fig. 5.3 The cutaneous muscle of the trunk: left lateral view. The muscle lies in the superficial fascia; superficially it is closely adherent to the dermis and forms the ‘panniculus carnosus’. Deep to it, the superficial fascia contains extensive fat deposits (‘panniculus adiposus’). The cutaneous nerves are shown in Figs. 5.4 and 5.10. See also Fig. 5.9.



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Fig. 5.4 The external oblique abdominal muscle and cutaneous nerves of the thoracic and abdominal wall: left lateral view. The cutaneous muscles have been removed. Further details of the nerves are shown in Fig. 5.10.

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Jul 8, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on 5: THE ABDOMEN

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