South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
ventral aspect of the abdomen. Remember to examine potential hernia sites such as the epiploic
foramen (cranial abdomen) and the inguinal region/scrotum in stallions/colts.
Numerous studies have investigated the effect of sedation on motility of the intestine. Use of
alpha-2 agonists is common in both investigation of colic and for ease of examination during
ultrasonography. Sedation generally reduces motility in the large colon and small intestine; fasting
also reduces motility of small intestine.
Large colon
The large intestine lies adjacent to the body wall and contains predominantly gas and ingesta; the
acoustic shadowing produced by gas contents prevents imaging of deeper structures. The large
colon has the same wall thickness and layering as the small intestine, but fewer contractions (26/minute). The sternal flexure is imaged in the cranioventral abdomen; the diaphragmatic flexure
cannot be seen. The pelvic flexure can be identified both transrectally and in the left caudal
abdomen by transcutaneous ultrasound. The ventral and dorsal colons are generally identified
according to location within the abdomen, as presence and width of sacculations is not consistent.
On the right hand side, the division between the right dorsal and right ventral colon can be seen
medial to the liver between ICS 9 and 13. The division between the left ventral colon and right
ventral colon is on the abdominal floor close to the midline. The left dorsal colon is medial to the
spleen.
Large intestine often becomes distended regardless of the disease process, creating significant
acoustic shadowing due to the gas contents and making differentiation of the disease processes
difficult. Primary tympany and secondary tympany due to colon torsion can be differentiated by
identification of increased wall thickness due to vascular occlusion associated with strangulating
lesions. Non-strangulating displacement results in colon distension with gas and ingesta but no
thickening of the wall.
Colon torsions have reduced intestinal motility, increased wall thickness and allow identification
of ventral colon (sacculated) in the dorsal abdomen. Increased wall thickness may be subtle
initially and sequential examinations are an important part of monitoring when decision making
for medical vs surgery management. When choosing a location to measure large colon quickly, a
recent study identified the ventral abdominal site as the most useful to accurately predict presence
of a large colon torsion (Pease et al 2003). Colon wall thickness > 9 mm had a sensitivity of 67%
and specificity of 100% for colon torsion (ie no false positives). It is also a useful place to
determine the best site for paracentesis.
Colon vasculature courses in the mesentery along the medial aspect of the colon, therefore is not
visible in the normal horse. If the vessels are visible against the body wall, this indicates a
displacement/torsion. They are seen as 2 or more hypoechoic structures running horizontally
along the right body wall.
Ultrasonography of nephrosplenic entrapment will identify gas-filled colon between the spleen
and left kidney, causing the dorsal border of the spleen to be displaced ventrally and with a
horizontal border. Be aware of false positives associated with this technique. Thickened colon
wall associated with colitis can sometimes be mistaken for distended small intestine due to the
circular appearance; look closely for fluid (diarrhoea) moving between the gut walls.
Caecum
The caecum fills the right dorsal and caudal abdomen. The base has a smooth, curvilinear outline
with gas echoes and reverberation artefacts in the paralumbar fossa and 17-16 ICS. The gas
prevents visualisation of the contents of the caecum. In cases of colic or diarrhoea, the caecum is
likely to be fluid-filled and may have increased wall thickness (normal wall thickness 4 mm).
Mucosal folds may be seen if there is significant wall oedema.
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