Front line ultrasound imaging of the feline urinary tract

Introduction

The well-defined point-of-care method for rapid ultrasound scanning of small animals, known as Global FAST or GFAST (Focused Assessment with Sonography in Trauma/Triage) is now widely used in the veterinary community. The technique includes protocols for abdominal scanning (AFAST), thoracic scanning (TFAST), and lung assessment (Vet BLUE, or veterinary Bedside Lung Ultrasound Examination). The idea behind GFAST was to develop a method for standardized ultrasound examination specifically tailored for veterinary patients that would answer clinical questions that might differ from what complete abdominal ultrasound and comprehensive echocardiography were trying to achieve. It should be emphasized that the AFAST, TFAST and Vet BLUE examinations are not the same as "flashing" the abdomen, thorax, and lungs.

The GFAST technique uses defined acoustic windows (i.e., views), which include target-organ interrogation and specific, standardized probe maneuvers. This article focuses on assessment of the feline urinary bladder and considers potential findings using the AFAST Cysto-Colic View (CC), including identification of free fluid and obvious, easily detected bladder abnormalities. The recording of findings on goal-directed templates gives value to the objective examination.

Firstly, however, a word of caution, the veterinary point-of-care ultrasound (V-POCUS) movement lends itself to "satisfaction of search error" through selective imaging (picking and choosing). Without following a standardized global protocol, the clinician will miss pathology and fail to integrate other important ultrasound findings 1 2 3 4 5. The idea is that the GFAST ultrasound approach serves as an extension of the physical exam, as it is designed as a standardized, achievable format for the non-specialist radiologist veterinarian, and is intended to be the clinician’s first-line choice of imaging modality, i.e., it is a quick assessment test.

This paper offers an introduction to first-line use of GFAST to assess feline lower urinary tract disease. The AFAST technique is used for general assessment of the abdomen, including a free-fluid scoring system, and is a target-organ approach involving the urinary bladder. A subsequent paper will focus on using the approach to assess patients with kidney disease. TFAST and Vet BLUE methods should also be employed when staging feline patients and for overall volume status. The GFAST approach should be part of the work-up for all cats with urinary tract signs and those with urinary obstruction, but using it as a first-line imaging test may also detect incidental and unexpected findings within the urinary tract.

The AFAST examination

The external landmarks for the respective AFAST views that are part of the abdominal fluid scoring system are shown in Figure 1 and Figure 2. The standardized approach is necessary, beginning with the Diaphragmatico-Hepatic View (DH), followed by the least gravity-dependent Spleno-Renal View (SR) in right lateral recumbency (or the Hepato-Renal View (HR) in left lateral recumbency) followed by the Cysto-Colic View (CC) and then ending at the most gravity dependent Hepato-Renal Umbilical View (HRU) (or the Spleno-Renal Umbilical View (SRU) in left lateral recumbency). The standardized order ensures that the patient's thorax is first screened (i.e., with the DH view) for any obvious intrathoracic problems, such as pleural and pericardial effusion, that could increase patient risk when restrained. The final AFAST view ends at the most gravity-dependent region, the respective umbilical view, where abdominocentesis can be performed (after completing the AFAST) if effusion is detected.

The AFAST target-organ approach

AFAST allows for sonographic assessment of easily recognized urinary tract-related conditions. The sonographer merely has to decide whether the urinary bladder is unremarkable or abnormal, and when abnormal, direct further imaging and a more streamlined approach for definitive diagnosis. Achievable abnormal AFAST findings are detailed in (Table 1). The normal appearance of the bladder and abdominal urethra are demonstrated in Table 2.

Table 2. It is vital that the clinician is familiar with the normal appearance of the feline bladder and abdominal urethra before identifying potential abnormalities.

The AFAST is performed by fanning (interrogating in longitudinal planes) followed by rocking cranially and returning to the starting point at each of the respective views. Therefore the Cysto-Colic (CC) view interrogates the urinary bladder in longitudinal planes while searching for free fluid in the gravity-dependent region termed the Cysto-Colic Pouch. Note that the feline urethra differs from that of dogs, in that it has a substantial length which may be imaged intra-abdominally. The Spleno-Renal (SR) and Hepato-Renal (HR) views provide soft tissue information on the left and right kidneys and are used to search for free retroperitoneal and peritoneal fluid, and are also important for complete first-line evaluation of the feline urinary tract. This part of the assessment (which can be performed with the animal standing or in lateral or sternal recumbency) will be covered in the subsequent paper.

The AFAST CC view allows the urinary bladder to be surveyed for not only the presence, but also the degree, of sediment, which is especially helpful in urinary tract disease and urinary obstructed felines. Monitoring the amount of sediment can be helpful to assess the subsequent response to therapy (including dietary interventions), and – in cats with urinary obstruction – the degree of sediment helps determine the need for bladder lavage. Other possible findings include the presence of thrombi (blood clots), cystic calculi, bladder wall abnormalities and the location of a urinary catheter when placed. Table 3 illustrates some of the most useful normal and abnormal findings that can be detected using this technique.

Table 3. Ultrasound findings of the feline urinary bladder and urethra.

Bladder obstruction, ascites and retroperitoneal effusion

Cats with urinary obstruction commonly have ascites associated with the obstruction 6 11 12 and retroperitoneal effusion. In the most detailed study to date (to the author's knowledge) ~ 60% of obstructed cats were positive for pericystic fluid (analogous to the AFAST CC View) and ~35% were positive for retroperitoneal effusion 6. It is important to be aware that the clinical course for the great majority of such cats is unaltered, in that with standard care the ascites and retroperitoneal effusion will resolve in time as the patient recovers 6. Sampling and testing the effusion may support a diagnosis of uroabdomen, but medical therapy, rather than surgical intervention, is appropriate in such cases. There is speculation as to why the effusion develops in such cases, but the author proposes that it is caused by tissue inflammation and backpressure of urine against the urinary bladder wall and renal capsule 13. Using the abdominal fluid scoring (AFS) system with the AFAST-applied technique provides not only an objective semi-quantification of the volume (usually scored between 0-4, although the system can be modified for smaller volumes), but also specifies positive and negative regions 1 14 15 16. The scoring system provides distinct advantages over use of subjective terms such as trivial, mild, moderate and severe (which have been employed to describe the fluid), and allows monitoring of affected cats as necessary, including during daily patient rounds and recheck evaluations. From the author’s experience, the free fluid usually resolves 24-36 hours after the obstruction is removed and the patient successfully resuscitated.

Sampling accessible free fluid

When free fluid is detected on ultrasound scan and is safely accessible it must be sampled to accurately characterize it; fluid analysis and cytology should be performed to better direct care and diagnostics. When urinary tract rupture is suspected, it is helpful to compare serum creatinine or potassium levels to that of the effusion. Importantly, ultrasound cannot accurately characterize free fluid, and with larger volume effusions abdominocentesis is generally performed immediately after competing AFAST at its most gravity-dependent umbilical view, where free intra-abdominal fluid is pocketed.

Urinary bladder volume estimation

The use of longitudinal (sagittal) and transverse measurements with the AFAST CC view can provide estimations for urinary bladder volume and, with serial measurements over time, assessment of urine output 17. The bladder is measured in longitudinal orientation at its largest oval for the measurements of length (L) and height (H) in centimeters. The probe is then rotated 90 degrees for the measurement of width (W). The formula of L x H x W x 0.625 gives an estimation of the bladder volume in milliliters (Figure 3a) (Figure 3b). This provides an indirect non-invasive option to gain this important clinical information, especially in cats at risk for or in renal failure.

Bladder masses

A not-uncommon scenario in small animal practice is when the veterinarian detects a suspect bladder mass when performing cytocentesis on a cat. Such cases should be staged with GFAST, partly in order to provide a much better dialogue with the client. Two simple scenarios can illustrate the point:

(i) The clinician discovers a bladder mass during cystocentesis, aborts the procedure and returns to tell the client that there is bad news, i.e., that there is likely a neoplastic process, and that an expensive work-up is recommended. If the cat is stable, the client may opt to go home to "think about it", and does not return for the work-up. This leads to a setback in the veterinarian – client relationship and can leave the client guilt-ridden at home, wondering what the best option should be for their cat.

(ii) The clinician discovers a bladder mass but returns to the exam room having already performed a GFAST assessment. The dialogue may be more upbeat than the first scenario if the scan suggests that the mass appears to be localized, with no obvious renal pelvic dilation or masses detected, no liver masses, no lung masses (from a Vet BLUE scan) and no pleural or pericardial effusion. If the cat is cooperative, TFAST echo views may also allow detection of unremarkable heart chambers. The clinician can then recommend that appropriate further tests are the next step. Conversely, if serious findings are detected on the scan, such as lung nodules 18, then the clinician should move to discuss palliative care, helping both client and pet as best as possible. Using the GFAST approach, the veterinarian-client bond becomes even stronger.

GFAST for assessing patient volume status

The feline species as a whole seems to be more susceptible to fluid volume overload, including cats with urinary obstruction 19; this may result in pulmonary edema, hepatic venous congestion, pleural or pericardial effusion, or any combination of the above 20. Obtaining a baseline GFAST on such patients on presentation is invaluable. The integration of findings during TFAST and Vet BLUE are helpful in determining if left- versus right-side volume overload is occurring. Moreover, and importantly, echo views are not needed in many patients, as the so-called "fallback views" can be sufficient. Left-sided congestive heart overload/failure results in cardiogenic lung edema and is either readily detected and scored using Vet BLUE or excluded 20 21 22. Right-sided congestive heart overload/failure results in hepatic venous congestion, which again can be readily detected by characterizing the size of the caudal vena cava and its associated hepatic veins. Moreover, pleural and pericardial effusion can occur concurrently with either condition and this can be assessed during TFAST 15 23 24 25 26. Integration of echo findings during TFAST, the characterization of the caudal vena cava, and lungs during Vet BLUE increase the probability of an accurate assessment 3.

Recording the results

Goal-directed templates are imperative to clearly convey objectives and for recording patient data that may be measured and compared initially and with future studies. Published examples may be accessed through the website FASTVet.com 1 15 27 28.

Conclusion

Abdominal ultrasound scans using a standardized approach should be the clinician’s first-line imaging choice when presented with a cat that has potential bladder disease or abdominal trauma cases. Using the AFAST method, with defined acoustic windows with specific, consistent probe maneuvers allows targeted organ interrogation and should permit rapid assessment of cases and appropriate ongoing treatment as necessary.