Equine Health Update EHU Vol 19 Issue 3 - Page 27

EQUINE | Proceedings surface of the colon is visible and its shape is hard to ap- preciate. Thickening of the colon can be very helpful in the diag- nosis of a colon torsion. While this is often not a diag- nostic challenge, there are times when it can speed the decision for surgery. Thickness can be deceptive - the echogenic mucosal lining dominates the visual field and it takes some time to appreciate the echo lucent outer layers. Greater than 9mm thickness is considered rea- sonably sensitive and quite specific for colon torsion [1]. The colon can be like the dog in the night time (Conan Doyle 1892) – the curious finding being that the colon isn’t there. This can lead to the erroneous conclusion that there is a small intestinal abnormality as copious amounts of small intestine are apparent in the ventral abdomen. However careful assessment will reveal nor- mal diameter, normal mural thickness and normal motil- ity. In reality the colon is obstructed, gas distended, and hence has “floated” in the abdomen, leaving the small intestine below it in the ultrasound window. The small intestine is an interesting tissue, usually iden- tified ventrally; either at the cranial end of the abdo- men or caudally, either side of the prepuce in a gelding, right up towards the pelvis. The small intestine cannot be visualised in every case but the more distended it is the more there is to visualise. The diameter of the intes- tine can be measured easily – as a general rule > 5cm is distended. However the diameter of the intestine var- ies along its length. As well as measurement of the di- ameter, the shape should be appreciated. If it is an odd shape contouring to its surroundings it is probably not distended, even if 7cm in the long direction. However if it is 4cm diameter but quite round then it probably is distended. Motility can be appreciated simply by keeping the probe still and watching the motility. In distended small intestine a further sign of lack of motility is separation of the gut contents into fluid and solid. Remember the ultrasound image is upside down, so a round tube with a white half at the top and a black half at the bottom is what you are looking for. Intestinal distension with a separation of the contents is almost diagnostic of ob- struction, and there should be a very good reason if the horse is not going to go to surgery. The intestinal wall thickness can also be measured. This is particularly helpful in foals, to help differentiate en- teritis (where surgery is frequently fatal) from strangu- lating obstruction (where medicine is invariably fatal). To measure the gut wall thickness reliably a higher fre- quency probe is preferable. High quality images of foal intestine can be obtained with a “tendon” probe. In a foal thickness of >2mm is suspicious and >5mm is of- ten observed, with clear identification of two layers of the intestinal wall. Lawsonia intracellularis infection is a common condition in the UK, affecting the weanling. Afflicted animals often have implausibly low protein lev- els, often less than 20g/l total, and extremely thickened small intestine, often >1cm. Ultrasound can be very valuable for the identification of abdominal fluid. Sometimes a pocket of fluid can be imaged and a needle used to tap the exact spot. More usually ultrasound can be used to identify where the co- lon is, which is where attempts at abdominocentesis are hopeless. This is often at the most dependent part of the abdomen, where blind abdominocentesis is recom- mended. Generally further caudal is more promising, or much further cranially. If there is no large pocket of fluid, aim for the small intestine. There is often fluid between the pieces of small intestine, but there is none under a 100kg colon. The echogenicity of the fluid is also quite interesting. Haemoperitoneum is quite characteristic with swirling echogenic fluid. Finally ultrasonography allows quantification of the commonest cause of failure to get a sample of abdominal fluid – that you are using too short a needle. The abdominal fat can be measured • Volume 19 no 3 • September 2017 • 27