QUARTERLY BEAT / DECEMBER 2023 ///
/// QUARTERLY BEAT / DECEMBER 2023
A CLINICALLY RELEVANT REVIEW Imaging the Blocked Cat: BLOCKED CAT, STAT!
uroliths are seen. 3 Small stones or mineralized urinary bladder debris are not always visible on radiographs, especially when the urinary bladder size is small. Other radiographic findings are variable and are dependent on the severity of various sequelae. Renal size, shape, and margination are usually normal. Small and irregularly shaped kidneys may indicate chronic kidney disease, renal cortical infarction, atrophy secondary to prior obstruction, or, more uncommonly, renal hypoplasia/ dysplasia. Renomegaly inconsistently occurs as a consequence of ascending pressure from the urethral obstruction itself, but can also indicate acute kidney injury, pyelonephritis, hydronephrosis from uroliths, feline infectious peritonitis, perinephric pseudocyst, or renal neoplasia such as lymphoma. Discussed further in the ultrasound section, retroperitoneal and peritoneal effusion/steatitis is common and manifests as soft tissue opaque wisps superimposed over the surrounding fat. Effusion can make it harder to visualize the kidneys, ureters, and urinary bladder due to border effacement. The urinary bladder is often large in cats with UO. However, a normal or small bladder size does not rule out the presence of obstruction. In the absence of peritoneal effusion, not visualizing the urinary bladder prior to urethral catheterization should raise concern for urinary bladder rupture, a finding that should be further interrogated with ultrasonography. RETROGRADE CYSTOGRAPHY AND URETHROGRAPHY Positive contrast radiographic studies have fallen out of favor in veterinary medicine due to more advanced imaging such as ultrasonography and computed tomography. However, these studies still hold immense value for evaluating the urinary tract for rupture and evaluating the urethra for stones. Urethral rupture presents a particular diagnostic challenge as tears are not easily identified with ultrasound and resulting urine leakage can occur into the surrounding soft tissues in addition to or instead of the peritoneal space. Clinically, urethral tears are suspected during difficult or unsuccessful catheterization and can result from mural inflammation and/or improper unblocking technique. A urethral tear should also be considered in any animal with persistent or worsening azotemia following unblocking.
retroperitoneal and peritoneal fat stores, especially when they are overweight. Excessive fat stores can displace the kidneys ventrally and medially. Additionally, the ureters are sometimes visible dorsally and laterally as thin and wispy soft tissue opaque tubes coursing between the kidneys and urinary bladder that should not be confused with retroperitoneal effusion. Increased fat stores also increase conspicuity of the deep circumflex iliac arteries and veins that lie dorsal to the L6 vertebral body. On lateral radiographs, the summation effect of these vessels viewed end on can be confused with faint calculi. Some cats have increased fat in the renal hilus which will appear as a focal region of lucency. Finally, cat kidneys are smaller as compared to those of the dog, with a renal length measuring approximately 2-3 times the length of the body of the L2 (L2L) vertebral body. The kidneys of younger and intact cats (up to 3.2x L2L) are usually larger as compared to their older and neutered counterparts (~1.9-2.6x L2L). Finally, over one-third of cats have a mineral opaque os penis visible on radiographs that should not be confused with a mineralized plug or urethrolith. 1 The os penis is more easily seen in older cats suggesting mineralization or size may increase with age, and its presence is not correlated to increased risk for feline urinary tract disease or UO. 1 ABDOMINAL RADIOGRAPHIC FINDINGS Abdominal radiographs are most useful for screening cats for radiopaque urinary calculi within the kidneys, ureters, urinary bladder, or urethra. Cranial and caudal landmarks of properly collimated abdominal radiographs are typically the diaphragm and femoral heads, respectively. However, including or obtaining a separate radiograph centered on the perineal soft tissues helps evaluate for stones or mineralized plugs in the urethra. Timing of radiography before or after urethral unblocking carries advantages and disadvantages either way. A recent study 2 demonstrated that preprocedural radiographs improve visualization of mineralized plugs or stones in the urethra that may be dislodged during unblocking. However, post-procedural radiographs are usually more comfortable for the patient due to decreased pain and increased compliance following unblocking and sedation/analgesic drug administration. Post-procedural radiographs also help evaluate the terminal location of the indwelling urinary catheter. The majority of cystoliths and urethroliths are composed of either struvite or calcium oxalate, both of which are radiopaque. Struvite is the predominate composition of urethral plugs whereas calcium oxalate is the predominate composition of nephroliths and urethroliths. While differences have been reported, the shape and margination (smooth vs rough) of stones cannot be reliably used to determine a stone’s composition. Less common uroliths such as urate and cystine have historically been considered radiolucent and hence not seen on plain film radiographs. However, recent evidence suggests urate and cystine calculi are detected with digital radiography and hence should not be excluded as a differential if mineral opaque
MARC SEITZ , DVM, DACVR, DABVP, BCSCP (Canine and Feline Practice)
Assistant Clinical Professor of Diagnostic Imaging Mississippi State University, Mississippi State, MS
SMALL ANIMAL WEBINAR
Ever imagined what it would be like to visualize a feline urethral obstruction during a diagnostic imaging procedure? Dr. Marc Seitz, DACVR, DABVP (Canine and Feline Practice) reviews what you need to know when it comes to imaging the feline urethral obstruction – from abdominal radiographs to POCUS. If you missed Dr. Marc Seitz on November 8, 2023 entitled Imaging the Blocked Cat: A Clinically Relevant Review, read the highlights below!
WATCH FULL WEBINAR
I NTRODUCTION Feline urethral obstruction (UO) is a common complication of feline lower urinary tract disease (FLUTD) in male cats. Common causes include mucous or crystalline urethral plugs, mural inflammation or spasms, and urethroliths. In many cases, a definitive structural cause of the UO is not identified and termed “idiopathic.” Etiologies contributing to obstruction include feline idiopathic cystitis, crystalluria, calculi, urinary infections (more common in cats older than 10 years of age), and neoplasia. Much of what is considered standard of care remains unchanged. Goals of therapy include patient stabilization, restoration of urethral patency, a diagnostic work-up to identify the underlying cause and etiology, supportive care, and long-term management of the underlying disease. These proceedings will review the role of diagnostic imaging in feline patients with UOs, including abdominal radiography, iodinated contrast studies, and abdominal point-of-care ultrasound (POCUS). EMERGENCY STABILIZATION Feline UO has the potential to cause severe morbidity and even mortality due to the resultant hyperkalemia, metabolic acidosis, cardiovascular compromise, arrhythmias, uremia, and acute kidney injury. Once obstruction occurs, these metabolic derangements occur within 24 hours and, if left untreated, death ensues within 3-6 days. Despite this, evidence and clinical experience demonstrate
that only a small percentage (~12%) of patients present severely metabolically affected. Still, most patients do benefit from some form of stabilization. As a result, patient stabilization should take priority over any diagnostic imaging procedures as feline UO is easily diagnosed during the physical examination. Fluid therapy in the form of a balanced isotonic crystalloids is the cornerstone of stabilization. Patients also benefit from pain medication and/or anxiolytic drugs. In cases of moderate to severe hyperkalemia, therapies that augment fluid therapy in the lowering of serum potassium include insulin, dextrose, and/or albuterol. Additionally, calcium gluconate can improve the resting membrane potential of cardiomyocytes until hyperkalemia improves. Once stabilized, diagnostic imaging procedures such as radiography, contrast radiography, and abdominal POCUS may be more safely performed. Still, it is reasonable and often more ideal to restore urethral patency prior to imaging.
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RADIOGRAPHIC APPEARANCE OF THE NORMAL FELINE URINARY TRACT
Abdominal radiographs are an essential part of the blocked cat work- up. When evaluating radiographs, a systematic approach should be used to ensure all peritoneal, retroperitoneal, and extra-abdominal structures are evaluated. However, the genitourinary tract and surrounding soft tissues should be closely scrutinized. While there are numerous radiographic species differences as compared to the dog, the most important involves fat. Cats typically have increased
Webinar Highlights
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