QUARTERLY BEAT / DECEMBER 2023 ///
/// QUARTERLY BEAT / DECEMBER 2023
IS THERE PERITONEAL OR RETROPERITONEAL EFFUSION? • Peritoneal effusion is very common in cats with UO, with an incidence ranging between 33% and 93% across four studies 5,7,8,9 . Retroperitoneal effusion is reported in up to 36% of patients 5 . Improving ultrasound technology combined with increasing clinician skill level with POCUS have likely in part contributed to increasing rates of detection. Numerous fluid types are possible including modified transudate, exudate, urine, and hemorrhage. The presence of urine does not always indicate urinary rupture as transmural leakage is postulated to occur in animals as it does in humans. Peritoneal effusion can also occur and/or mildly progress immediately following cystocentesis. Since ultrasound is incapable of differentiating fluid types based on their appearance alone, abdominocentesis may be performed for fluid analysis and cytology. However, this may not be necessary all the time since many cases of peritoneal effusion are self- limiting with supportive care. IS THERE EVIDENCE OF URINARY BLADDER RUPTURE? • The accurate diagnosis of urinary bladder rupture with ultrasound is challenging. An accurate diagnosis requires visualization of a defect in or thinning of the bladder wall in the presence of confirmed uroperitoneum. A false negative exam is possible as urinary wall thickening (whether pathologic or secondary to insufficient urine volume) can hide defects in the wall. On the contrary, a false positive may occur if the edge shadowing artifact is misinterpreted as a rent in the apex of the urinary bladder wall. It is also important to remember that uroperitoneum can occur with rupture of the proximal urethra and uroretroperitoneum can occur with rupture of the ureters. While radiograph or CT contrast studies are still the gold standard for diagnosing rupture anywhere throughout the urinary tract, ultrasonographic contrast cystography using agitated saline is quite helpful at confirming urinary bladder or proximal urethral rupture 10 . Agitated saline is created by rapidly injecting saline (32 mL) admixed with room air (3 mL) back and forth between two 35mL syringes via a three-way stopcock. The author finds smaller syringes easier to work with.
ULTRASONOGRAPHIC APPEARANCE THE NORMAL FELINE URINARY TRACT
of information gleaned from these rapid exams has expanded. The purpose of this session is to review the role of POCUS in feline patients presenting with UO. Although experienced clinicians can glean a lot of information from focused ultrasound exams, they should never be relied on for a definitive diagnosis or to rule out a condition. The primary purpose of POCUS is to identify potential “game changers” and confirm them through other more definitive diagnostics. Clinicians can expect to find a lot of ultrasonographic abnormalities in blocked cats, even when the underlying cause is idiopathic cystitis. Abnormalities are the result of ascending pressure throughout the genitourinary tract and the inflammatory nature of most underlying etiologies. A retrospective study 5 described the following common findings among 87 cats with naturally occurring UO (and without cystocentesis): echogenic urinary sediment, bladder wall thickening, pericystic effusion, hyperechoic percystic fat, and increased urinary echoes. Specific to the kidneys, mild renal pelvic dilation, renomegaly, perirenal effusion, hyperechoic perirenal fat, and ureteral dilation were commonly observed. Importantly, the authors found no association among any findings and the risk for short-term recurrent UO. Some of these findings overlap with more insidious etiologies such as pyelonephritis, hydronephrosis, and urinary bladder rupture. Furthermore, therapies such as sedation and aggressive fluid therapy can induce abnormal findings such as renal pelvic dilation and peritoneal effusion. This is partly why POCUS cannot be relied on for a definitive diagnosis. Fortunately, most of these findings are transient and resolve with supportive care. Previously reported point-of-care ultrasound exam techniques 2,6 may be used to evaluate blocked cats. Transducer pressure should be minimized since excessive amounts can displace peritoneal effusion outside of the field-of-view, increasing the potential for a false negative, especially when the patient is standing 3 . Excessive transducer pressure can also distort or displace soft tissue structures located in the near field. If not already part of a clinician’s routine POCUS exam, both kidneys should be evaluated. When evaluating POCUS locations containing the urinary bladder and kidneys, more time can be spent fanning or sweeping through these target organs in both longitudinal and transverse planes. Evaluation of the ureters and the majority of the viewable urethra are beyond the intended purpose of a standard POCUS exam. If the urinary bladder is small or not identified and a urethral catheter is in place, 10-20 mL of sterile saline can be instilled to improve visualization of the wall and intraluminal contents. When evaluating the urinary tract, the following clinical questions can guide the clinician’s POCUS of the typical blocked cat. WHAT CLINICAL QUESTIONS CAN BE ASKED AND ANSWERED WITH ABDOMINAL POCUS
The procedure is technically simple considering most patients already have an indwelling urinary catheter in place. Most water-soluble iodinated contrast agents can be used. The author prefers non-ionic monomers due to their lower osmolality and reasonable viscosity. Examples include iohexol and ioversol with a concentration ranging between 240-350 mg/ mL. Diluting the contrast 1:1 or 1:2 with sterile saline is less irritating and improves visualization of the urethral wall. If both cystography and urethrography are to be performed, cystography is typically performed first. The patient should be heavily sedated or placed under general anesthesia. If not already done so, a urinary catheter should be placed in a sterile fashion such that the tip terminates just inside the urinary bladder neck. Preprocedural radiographs are obtained to confirm proper catheter placement and to establish a baseline. The urinary bladder should be drained and then refilled with dilute iodinated contrast until it is moderately distended based on palpation and radiographic appearance. The typical cystogram dose for most cats ranges between 2-5ml/kg. The author typically starts with 2ml/kg and then obtains radiographs. Subsequent doses of 1-2ml/kg are administered with repeat radiographs performed after each dose. Retrograde filling of the ureters can occur with overfilling of the urinary bladder. Leakage of iodinated contrast into the peritoneal space confirms rupture, as evidenced by discrete streaks of contrast and/or a diffuse increase in opacity throughout the entire peritoneal space. Once the cystogram is complete, the urethrogram may be performed by repositioning the urinary catheter into the distalmost aspect of the penile urethra. The urinary catheter should be left distended as the pressure helps dilate and fill the urethra with contrast. In fact, if the urinary catheter diameter is considerably smaller than the urethral luminal diameter, inadvertent filling of the urethra with contrast can occur during the cystogram. Once the catheter is properly placed, 5 mL of the same diluted contrast is injected into the urethra. Radiographs should ideally be obtained during injection after the final 1-2 mL to ensure adequate filling and distention of the urethra. This may require gentle clamping of the tip of the prepuce to ensure the catheter does not move retrograde during injection. Obviously, hands should be kept out of the primary X-ray beam and protected from scatter radiation with the use of lead gloves and/or shields. The normal feline penile urethra is smaller in diameter than the other sections and should not be confused with a stricture. Similar to that above, a urethral tear is confirmed if there is extravasation of contrast into the surrounding soft tissues or peritoneal space. Calculi are seen as persistent radiolucent filling defects in the contrast column that do not clear with repeat boluses of contrast. While air bubbles appear as similar filling defects, they are easily cleared with repeat injections of contrast. As mentioned above, iodinated contrast is irritating to the mucosal layer of the urinary tract. Therefore, all contrast should be removed upon completion of the study. If no tear is identified, the author additionally flushes the urethra (following a urethrogram) or lavages the urinary bladder (following a cystogram) with a small amount of sterile saline to dilute any residual contrast.
The ultrasonographic appearance of the feline urinary tract differs from dogs in many ways. While more superficially located, excessive transducer pressure can make finding feline kidneys harder due to their mobile nature as compared to the dog. Cat kidneys are more oval in shape with a consistent size across breeds, ranging between 3.0-4.5 cm in length. Renal size may be slightly larger in cats that are intact, male, and/or Maine Coon. Additionally, the right kidney may be slightly longer than the left in some cats. Fat deposition in the renal cortex and sinus and excretion into the urine changes the expected ultrasonographic appearance of the urinary tract as compared to the dog. Fat is hyperechoic and hence increases the echogenicity of any parenchyma in which it is contained. Increased renal hilar fat results in hyperechoic tissue in and adjacent to the renal sinus, which attenuates the beam and sometimes causes faint distal acoustic shadowing, mimicking mineralization. The echogenicity of the renal cortex is highly variable and often hyperechoic to the liver and/ or spleen, especially in older neutered male cats. As a result, increased renal cortical echogenicity cannot always be presumed to be pathologic as is seen with pyelonephritis, tubulointerstitial or glomerular nephritis, feline infectious peritonitis, or renal lymphoma. The outer portion of the renal medullae in some cats contains a well-demarcated hyperechoic band, termed a “medullary rim.” While a medullary rim is reported with numerous pathologies, it should not be over-interpreted in an otherwise normal cat. The renal pelvis is inconsistently seen within the renal sinus, and normally measures less than 1-2 mm in thickness. However, mild renal pelvic dilation up to 3.2 mm in one or both kidneys may be normal in some feline patients, especially when receiving fluid diuresis. The normal ureters are uncommonly seen in cats as they measure < 0.4 mm in diameter. Similar to the dog, the feline urinary bladder wall measures 1-2 mm in thickness. An elongated urinary bladder neck is common in the cat. Finally, normal cats without lower urinary tract disease can have a mild to moderate amount of hyperechoic urinary bladder debris from lipiduria 4 . Curiously, the degree of lipiduria is independent of body condition score. Unfortunately, fat is hard to distinguish from cellular/proteinaceous debris, sediment, and hemorrhage. Although lipid debris does not typically settle out with gravity or cause distal acoustic shadowing, the author relies heavily on the presence or absence of lower urinary signs as well as urinalysis results to interpret urinary bladder debris in the cat. ABDOMINAL POCUS In general practice and emergency medicine, point-of-care ultrasound (POCUS) exams have gained popularity as a valuable adjunct triage tool. Unlike full diagnostic ultrasound exams, POCUS techniques are designed to answer specific clinical questions in a yes or no format to help guide the initial diagnostic and therapeutic plan. For example, abdominal POCUS was originally designed to identify abnormal free fluid most likely to be hemorrhage in human trauma patients (i.e., Focused Assessment with Sonography for Trauma, or FAST). Soon after, clinicians realized POCUS also aided in the bedside evaluation of non-traumatized patients. As POCUS has gained popularity in both human and veterinary medicine, the amount
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