Front line ultrasound imaging of the feline kidney

Introduction

Global FAST (an acronym for Focused Assessment with Sonography for Trauma) is a well-defined point of care ultrasound protocol that was first created as a triage and post-interventional screening test by human trauma surgeons in the 1990s, and then progressed into a non-trauma and monitoring imaging tool. The technique has now been developed for the veterinary field, and comprises abdominal FAST (AFAST), thoracic FAST (TFAST), and Vet BLUE (brief lung ultrasound examination) methods for assessing the small animal patient, although the latter two techniques will not be discussed in any detail in this paper.

The G-FAST approach includes target-organ interrogation (abdomen and thoracic organs including heart and lung) and involves 15 specific, standardized probe maneuvers (Figure 1); when performed by a competent person the entire procedure can be accomplished in around six minutes. This article focuses on findings at the AFAST Spleno-Renal (SR) and Hepato-Renal (HR) views (as described below) which allow easy detection of soft tissue renal and adjacent ureteral abnormalities, as well as free fluid within the peritoneal cavity and retroperitoneal space. The recording of findings on goal-directed templates gives value to the examinations and enables clear objectives to be achieved.

Importantly, a word of caution should be sounded. The veterinary point-of-care ultrasound (V-POCUS) movement lends itself to "satisfaction of search error" through selective imaging (picking and choosing) and will often fulfill a preconceived clinical bias, thus missing other important imaging information. This is potentially dangerous; a clinician would never selectively perform a physical examination, so without following a standardized global protocol whilst scanning, the clinician may not only miss pathology but will also fail to integrate other important G-FAST findings into the overall assessment of the patient 1 2 3 4 5 6. The mindset for those using ultrasound is that the G-FAST approach serves as an extension of the physical exam, as it is a standardized, achievable format that can be easily utilized by the non-specialist radiologist veterinarian and should be the first-line imaging modality; in other words, it is a new quick assessment test.

AFAST can be used for general abdominal evaluation, and includes a standardized fluid scoring system for assessment of free fluid; the target-organ approach involves visualization of the kidneys and adjacent ureters and retroperitoneal space. TFAST and Vet BLUE can be combined with this, and may be used for staging renal patients and assessing their overall volume status, as well as urine production and output.

How does the AFAST examination work?

The AFAST order for scanning is standardized, as shown in Figure 2. It begins 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 view (SRU) in left lateral recumbency). This standardized manner ensures that the patient's thorax is first screened at the DH view for any obvious intrathoracic problems, such as pleural and pericardial effusion, that could increase patient risk with restraint. The final AFAST view, which incorporates use of the abdominal fluid scoring system, ends at the most gravity-dependent region, the respective umbilical view, where abdominocentesis can be performed if effusion is detected; this should be done only after completing the AFAST exam.

The AFAST is performed by fanning (interrogating in longitudinal/sagittal planes) followed by rocking cranially and returning to the starting point at each of the respective AFAST views; this is because anatomy is more generally recognizable in the longitudinal planes, and the presence of ascites is independent of probe orientation 7. Therefore the SR (and HR) views interrogate the kidneys for obvious soft tissue abnormalities in longitudinal (sagittal) orientation while searching for free fluid in the adjacent retroperitoneal space and peritoneal cavity. The AFAST technique also allows imaging of the urinary bladder and the adjacent urethra. Generally, most experienced sonographers will image both kidneys via a single SR or HR view (Figure 3) depending on which side the animal is lying. If the kidneys are not readily visualized the HR 5^th Bonus View (or SR 5^th Bonus View) is used. These views are not part of the abdominal fluid scoring system, but provide soft tissue information on the respective kidney, its retroperitoneal space and adjacent liver and soft tissue. Transverse orientation is considered an add-on skill once image acquisition is mastered in longitudinal (sagittal) planes.

Although ultrasound imaging is typically undertaken with the patient in lateral recumbency, the G-FAST technique can also be performed with the animal in a standing or sternal posture, as image acquisition is independent of patient positioning. This approach has less impact and is safer for patients that are respiratory fragile, are potentially unstable in their hemodynamics, or if the animal is stressed. Standing (or sternal recumbency) is actually preferred for most cats, and is better for evaluating pleural and pericardial effusion and pneumothorax; however, gravity-dependency, which affects where free fluid pools, and where sediment and intraluminal pathology settles, will differ depending on the chosen position, and must be acknowledged by the sonographer.

Since standing or sternal recumbency is often much less stressful for the cat; if no free fluid is detected, lateral recumbency is unnecessary. However this position is required for fluid scoring, so if a patient is fluid positive, it should be moved into either left or right lateral recumbency and a three-minute waiting period imposed (to allow for fluid redistribution) before rescanning to allow for fluid scoring 8.

What is the AFAST target-organ approach?

AFAST allows for sonographic assessment of easily recognized renal and other urinary tract-related conditions, since most are fluid-related, and a key strength of the ultrasound modality is in imaging fluid. The sonographer merely has to decide whether the kidneys are unremarkable or abnormal, and when abnormal, direct further imaging and a more streamlined diagnostic plan for definitive diagnosis. In other words, the non-expert sonographer should be able to identify cases that would otherwise be missed without expert ultrasound or computed tomography imaging. The AFAST (and indeed the entire G-FAST) approach should be of the mindset “do the kidneys – and other structures in the abdomen and thorax – deviate from what is expected in normalcy?” rather than “what is the diagnosis?” With point-of-care ultrasound it is important to understand the limitation that renal appearance does not always indicate normalcy, and that a radiologist evaluation is the gold standard for ultrasound. Clinical questions to be asked during the procedure are shown in Box 1, and achievable abnormal AFAST findings are discussed below and summarized in Table 2. As a measure of how useful and effective this technique can be for renal problems, the reader is referred to the findings of a retrospective study that reviewed ultrasonographic findings in cats with acute kidney injury which identified various abnormalities and quantified them, as shown in Table 2 9.

What does the normal kidney look like?

A normal sagittal view of the kidney (Figure 4a) (Figure 4b) should identify three areas:

1) A bright central echo complex (the renal sinus and peripelvic fat)

2) A hypoechoic medullary region surrounding the pelvis

3) A peripheral cortical zone of intermediate echogenicity

Importantly, a normal sonographic appearance does not necessarily indicate normal renal function; conversely, an abnormal sonographic appearance does not necessarily indicate abnormal function. The most commonly accepted normal range of dimensions for feline kidneys are:

• Length (L) 3.0-4.5 cm
• Width (W) 2.2-2.8 cm
• Height (H) 1.9-2.5 cm
  
  

What abnormal kidney findings may be detected?

Mineralization and renal calculi

The ease with which mineralization and calculi are detected with ultrasound is variable (Figure 5), and radiography is generally considered a better imaging modality for these cases. If large enough, nephroliths have clean shadowing. Note that the peripelvic fat can mimic mineralization, which may or may not shadow. The twinkle artifact may be helpful in such cases using color flow Doppler 10.

Pyelectasia

Dilation of the renal pelvis is usually easily detected with AFAST (Figure 6). The width of the feline renal pelvis can be measured and assessed as follows;

• Normal < 2 mm
• Mild dilation < 4 mm
• Moderate dilation 5-10 mm
• Severe dilation > 10 mm
  
  

Hydronephrosis

Hydronephrosis, or distension of the kidney, is severe pyelectasia with blunting of the renal papilla (Figure 7) and is usually easily identified on imaging.

Cysts

This term encompasses cortical cysts (Figure 8), polycystic kidneys (usually seen in Persian cats (Figure 9)) and perinephric pseudocysts (Figure 10), the latter being more common in older cats and cats with chronic renal failure. There may be a possible sex predilection, with males more commonly affected. AFAST should readily detect all types of renal cyst.

Nephromegaly

Kidney length should be assessed in all cases. The maximum length in the sagittal plane should be less than 4.5 cm; if kidney enlargement is detected (Figure 11) the rule-outs include lymphoma, acute kidney injury (AKI), and hepatic shunts.

Renal and retroperitoneal masses

If a retroperitoneal shadow is noted (Figure 12) it is essential to differentiate between a mass and a hematoma; this can be done using color flow Doppler to assess for the presence or absence of pulsatile flow in the mass, with the absence of flow indicating a hematoma. It may be appropriate to consider a coagulation profile if a hematoma is detected. If a renal or retroperitoneal mass is detected the animal should be staged using the G-FAST Approach, as discussed below.

Perirenal fluid

This is seen as fluid rounded within the renal capsule (Figure 13) and is not part of the abdominal fluid score. Again if fluid is detected the patient should be staged with the G-FAST Approach, with the rule-outs including renal failure, especially AKI.

Abnormal architecture

If any abnormal renal architecture is found during imaging (Figure 14) it is essential to look for further obvious soft tissue abnormalities in the other abdominal organs during the AFAST process; the heart, pleural and pericardial effusion should also be reviewed via TFAST and the lung surfaces evaluated using the Vet BLUE technique.

Infarction

An infarct is usually easy to identify using AFAST (Figure 15). When staging an affected animal the clinician should include TFAST echo views and Vet BLUE to look for any evidence of a “wedge” sign in the lungs, indicative of pulmonary thromboembolism (PTE).

Peritoneal fluid

Free fluid (Figure 16) is generally triangulated because the fluid is outside the renal capsule. If found the maximum dimension can be recorded. Peritoneal fluid can be semi-quantified using the AFAST-applied Abdominal Fluid Score (AFS) as described below. Note that cats with urinary obstruction commonly have ascites associated with the obstruction 11 12 13. In the most detailed study to date to the author's knowledge, ~ 60% of obstructed felines were positive for pericystic fluid (analogous to the AFAST CC View) and ~35% were positive for retroperitoneal effusion 13. It is important to be aware that the clinical course for the great majority of these cats is unaltered from standard care, and that the ascites and retroperitoneal effusion will usually resolve with time as the cat recovers, typically 24-36 hours after the obstruction is relieved and the patient successfully resuscitated 13. Sampling and testing the effusion may support a diagnosis of uroabdomen, however, the uroabdomen is most often treated medically. The author proposes that the cause for the effusion lies in inflammation and backpressure of urine against the urinary bladder wall and renal capsule over alternative speculation 14.

How is fluid assessed on imaging?

The abdominal fluid scoring (AFS) system has been developed as an objective semi-quantification method to evaluate the volume of free fluid detected within the abdominal cavity during AFAST, and can be used for hemorrhage, uroabdomen or ascites. Lack of space precludes an in-depth discussion of the system in this paper, but it essentially scores free fluid from 0 to 4, and also specifies positive and negative regions within the abdomen 1 8 15 16. If the abdomen is negative for free fluid at all four AFAST views a zero score is applied; a maximum score of 4 indicates positive free fluid at all four views. This method allows differentiation between small volumes of fluid (scoring 1 or 2) and large volumes of fluid (scoring 3 or 4) and the clinician can then act accordingly. The author recommends a modification of the system in cats; if the maximum fluid pocket dimensions are under 5 mm, or there are linear fluid stripes of < 5mm, a score of "1/2" is applied; if over this a score of 1 is required 16 23.

This system provides distinct advantages over “trivial”, “mild”, “moderate” and “severe” volume characterization. Serial use of the AFS system is a means to regularly monitor the progress of cats with free peritoneal fluid, including during daily patient rounds and recheck evaluations.

When free fluid is safely accessible it must be sampled for accurate characterization, followed by fluid analysis and cytology to better direct care and diagnostics. When urinary tract rupture is suspected, comparative serum creatinine or potassium to the effusion should be performed. Importantly, ultrasound cannot accurately characterize free fluid; and in 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. Safe sampling may be acquired in the retroperitoneal space as well as smaller volumes within the peritoneal cavity by sonographers with more advanced ultrasound-guided skills.

What about staging for renal masses and nephromegaly?

It is strongly advisable that any cat with a suspect renal mass should be staged with the G-FAST Approach to provide a better dialogue with the client. This avoids leaving an owner to make the decision as to whether or not they should proceed with further investigations. Remember that not all renal masses are neoplastic, and infectious, metabolic and other conditions must also be considered. A G-FAST sonographic examination may suggest that the renal mass or nephrogmegaly appears to be localized, and that there are no obvious other abdominal masses. If a Vet BLUE examination excludes lung masses and also confirms that there is no pleural or pericardial effusion, this is a positive step. If the cat allows TFAST echo views, (to confirm unremarkable cardiac chamber sizes), so much the better, and the clinician can discuss appropriate further diagnostics. Conversely, if serious findings are detected, such as lung nodules 17, then a different diagnostic plan is necessary. As with renal masses, do bear in mind that not all lung nodules are neoplastic – for example, they can be due to fungal disease, which still may be treatable. Using the G-FAST approach as a first line diagnostic tool will allow the veterinarian to help both client and pet as best as possible.

What about G-FAST for assessing patient volume status?

The cat as a species seems to be more susceptible to fluid volume overload (especially when receiving intravenous fluid therapy for urinary obstruction and/or renal failure 18) that may result in any combination of pulmonary edema, hepatic venous congestion, pleural effusion and pericardial effusion 19. Obtaining a baseline G-FAST on a patient is invaluable upon presentation, as integrating findings from TFAST and Vet BLUE are helpful in determining if left- versus right-side volume strain/overload/failure is occurring. Moreover, and importantly, echo views are not needed in many patients when using the so-called "G-FAST non-echo fallback views". Left-sided congestive heart failure results in cardiogenic lung edema and is readily ruled out or detected (96% sensitivity) and quantified using Vet BLUE 19 20 21. Right-sided congestive heart failure results in caudal vena cava distension and hepatic venous congestion, which is readily detected at the AFAST-TFAST Diaphragmatico-Hepatic View (DH). Pleural and pericardial effusion can occur concurrently with either left or right sided failure, and can be detected during TFAST 15 19 22 23 24 25. Therefore integration of echo findings from TFAST and characterization of the caudal vena cava and assessment of the lungs during Vet BLUE increase the probability for accurate assessment of the patient 3.

Recording 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. Goal-directed template examples are published 1 25 26 27 and may be accessed through the website FASTVet.com.

In conclusion, if the question is “when should renal disease patients be scanned?” the answer is that the G-FAST approach should be part of the work-up for ALL cats with renal or urinary tract signs, including those with urinary obstruction. Using it as a first line-imaging test will also potentially detect incidental and unexpected findings, not only within the entire urinary tract but also the rest of the abdomen and the thorax, including the heart and lungs. Essentially the technique allows the clinician to rapidly assess a patient on initial presentation and make objective decisions as to the next steps in diagnostics and treatment, and can ultimately make the difference between life and death.