ABSTRACT: A limited ultrasound examination to demonstrate pleural, pericardial fluid or abdominal fluid can allow the clinician to perform a diagnostic or therapeutic pleurocentesis in unstable patients. The unstable patient must, however, be stabilised (using intravenous fluids and oxygen supplementation, for example! before a lengthy ultrasound examination is undertaken. Radiography and ultrasonography are complementary diagnostic techniques. Over-reliance on ultrasonographic examination alone will lead to many conditions being overlooked. Many diseases have a similar appearance on ultrasonography and. therefore, whilst ultrasonography is useful for locating pathology, a definitive diagnosis can only be reached by obtaining tissue samples for cytology, histopathology or culture via ultrasound guided needle aspiration or 'trucut' biopsy or surgical exploration.

Ultrasonography has been employed in the diagnosis of abdominal and thoracic disease in small animals for almost 40 years. Once the preserve of referral institutions, it is now widely available in general practice. This article briefly reviews the principles of diagnostic ultrasound in small animals and focuses on the aspects and goals in the clinical setting in which are of particular significance to the VN.

Ultrasound examination

A detailed description of important ultrasound findings is beyond the scope of this article; hence it will focus on several aspects which are relevant to the welfare and management of the patient: triage, purpose, limitations and pitfalls.

Triage

In the unstable patient presenting for respiratory distress, distended abdomen or collapse, a limited ultrasound examination can provide rapid relevant information to the clinician. This examination can be performed with the animal standing, in sternal recumbency, or if a cat, on the examiners’ lap or at the cage side.

The goals of the examination are to establish whether there is pleural or pericardial fluid (Figures 1a &1b) or abdominal fluid (as a result of bleeding from a mass or intestinal rupture) or an obvious mass is present. This should allow the clinician to perform a – diagnostic or therapeutic pleurocentesis or abdominocentesis as required.

Figures 1a and 1b: Ultrasound has been used to confirm the presence of pleural effusion (arrow) in this dog; but the radiographic study is the most informative imaging technique

Ultrasound has been used to confirm the presence of pleural effusion (arrow) in this dog (Figure 1a); but the radiographic study (Figure 1b) is the most informative imaging technique, however, and shows a blunt-ending bronchus which is highly suspicious for lung lobe torsion.

Figure 1b.

Lung lobe torsion was confirmed at surgery and the lobe removed. Although ultrasound can be used to evaluate for lung lobe torsion in a patient with difficulty breathing, the technique can be very challenging and speculative.

Occasionally, this brief examination allows an assessment to be made as to whether pleural fluid is likely to be present as a consequence of cardiac disease, or not. Assessment of the pleural fluid volume is helpful as this will guide the clinician in draining the chest. Although ultrasound-guided drainage can be helpful, it can stress and prolong the drainage procedure and in large volumes a partial safe drainage and further stabilisation may be preferred to complete drainage in a high risk patient.

Bear in mind that the presence of pericardial effusion does not necessarily imply that it is significant, or requires drainage. It may simply be the consequence of the underlying disease (FIP, lymphoma or cardiac disease). Similarly, identifying the bladder on ultrasound does not imply that it is intact or rules out a urethral tear.

Purpose

The primary presenting complaint should never be forgotten. Additional – sometimes dramatic – findings may be recognised during the ultrasound examination and can distract the clinician and VN. These could lead to inappropriate or unnecessary invasive steps being taken.

Figures 2a & 2b are from a one-year-old male domestic short-hair cat that presented for severe vomiting. The cat had raised kidney parameters. The kidney (white arrowheads) has a striking appearance with a very bright ring (black arrowheads) at the junction between the cortex and the medulla. The significance of this is uncertain – it may just represent fat, a normal finding in some male cats.

Figures 2a & 2b: Images from a one-year-old male domestic short hair cat that presented for severe vomiting

Of greater significance is the shadowing structure which resembles ‘Mickey Mouse’ ears (arrowheads) within the small intestine which was a small intestinal foreign body. The normal intestine behind the obstruction is shown by the asterisk.

The raised kidney parameters represented severe dehydration (pre-renal azotaemia), a clinical finding which should be apparent to both clinician and nurse and both should not be distracted by the ultrasound appearance of the kidney.

Limitations and pitfalls

Ultrasound and radiography are complementary, and the widespread availability of ultrasound has unfortunately led to radiographic examination of the abdomen being discarded. This is a risky strategy. It is not possible by simply using ultrasound (except by sophisticated methods) to establish whether a dog or cat is in congestive heart failure (pulmonary oedema). Thoracic radiography remains the method of choice.

Similarly the presence of free air in the chest or abdomen (pneumothorax or pneumoabdomen) is easily overlooked, and unless radiographs are available, the clinician may miss the opportunity to intervene in a life-threatening situation.

Figures 3a – 3d demonstrate a perforated duodenal ulcer in cat. The ultrasound findings are very subtle; there is thickening (arrows) of small area of the duodenum (A), which is itself difficult to distinguish from the surrounding peritonitis.

Figures 3a- 3d: A perforated duodenal ulcer in a cat

The free gas (B) in the abdomen was identified as a ‘reverberation’ artifact, dirty bright lines tailing off from the transducer and obscuring the structures beneath it. The free gas which has leaked from the intestine into the abdomen is more obvious on the radiographs (C), and a special view (D), obtained as a horizontal beam view with the cat lying on its side, the plate against the cat’s back and the beam centred on the sternum. This shows the free gas rising to the highest part of the abdomen.

< p>Figure 3e shows a gastric ulcer from a different patient and demonstrates how small ulcers may be. The important lesson is that an over-reliance on ultrasound alone will lead to these life- threatening conditions being missed. Ultrasound and radiography are complementary diagnostic techniques

Figure 3e: Gastric ulcers can be very small

Ultrasound certainly assists in locating intestinal foreign bodies and gall stones; but body size and intestinal gas may obscure specific regions of interest which can be targeted if a radiograph is available.

In the case history illustrated in Figures 4a – 4c, the dog presented with jaundice and a painful abdomen. A conscious ultrasound (A) identified a dilated common hepatic duct – the duct which carries bile from the gall bladder to the duodenum and which, if obstructed, can cause severe jaundice. However, a cause for the obstruction could not be identified.

Figures 4a – 4c: This dog presented with jaundice and a painful abdomen

Pain relief and sedation were instituted and an abdominal radiograph (C) taken. A large gallstone (calculus) was seen superimposed on the stomach in the region where the common hepatic duct joins the duodenum (arrowhead) and additional small stones were present in the gall bladder.

With this information – and the abdomen relaxed – the calculus could be identified on ultrasound (B). This case demonstrates the importance of obtaining abdominal radiographs in addition to ultrasound and the importance of pain relief and sedation as dictated by the clinical status of the patient.

Ultrasound is also very useful for assessing the structural appearance of changes; but is not specific enough to identify the underlying cause or differentiate between neoplasia, inflammation, infection and cystic changes. Three examples of intestinal disease in the cat are presented to demonstrate this in Figures 5a-5d.

Figures 5a· 5d: Small intestinal disease in the cat

All demonstrate bowel wall thickening and all appear similar, yet each have a different underlying cause. Ultrasound should not replace endoscopy or surgical biopsy.

The schematic in Figure 5a demonstrates the normal layered appearance of the small intestine in the cat. It resembles a ‘road’ with central reservation and ‘hard shoulder’. Intestinal disease may result in die layers appearing thicker as in Figures 5b, 5c and 5d which all appear very similar but have different underlying causes.

Ultrasound may not be specific for the type of underlying disease. In these examples: (B) was caused by a lymphoma, (C)   was the result of fibrosis/scarring and (D)   was the consequence of inflammatory bowel disease. Biopsy is required to differentiate between these diseases and to determine the diagnosis. It is not possible to rely on ultrasound alone.

There is a great temptation to consider all fluid-containing structures or cavities as representing an abscess. Abscesses are relatively uncommon in the cat and the dog. Cystic change – including cystic change associated with neoplasia – can be quite dramatic (Figure 6).

Figure 6: A peri-renal pseudocyst (a large cyst-like structure around the kidney) in a cat

Spectacular lesions may, in fact, have little clinical significance. There may also be a striking difference between the appearance of a lesion on ultrasound and at surgery. Any assessment and expectation regarding how easily or how necessary it is to remove a structure surgically should be approached with caution.

In Figure 6, the kidney (arrowheads) is small and bright and surrounded by fluid (asterisk) which appears dark (anechoic). Although this is a striking image there is little benefit in aspirating the fluid in most cases. These cats often have chronic renal disease which is managed medically and the cyst is usually not managed surgically.

Conclusion

Appropriately calibrated diagnostic ultrasound is an invaluable technique in diagnosing disease in small animal patients (Table 1). The importance of patient welfare and management and expectations of the procedure should be controlled by establishing sound systems of practice. 

Author

Andrew Holloway BVSc CertSAM DVDI DipECVDI MRCVS

Andrew qualified from the University of Pretoria in 1992. He spent eight years in general practice before joining the University of Cambridge as a visiting resident in radiology; and in 2006, joined the Animal Health Trust as clinical radiologist. He holds the RCVS and European Diplomas in Veterinary Diagnostic Imaging.

To cite this article use either

DOI: 10 1111/j.2045-0648.2012.00163.X or Veterinary Nursing Journal Vol 27 pp 128-131

Veterinary Nursing Journal • VOL 27 • April 2012 •