Uroliths are one potential cause of lower urinary tract signs in cats. Management varies by type but can be guided by clinical suspicion based on appropriate history, signalment, and diagnostics.
Feline lower urinary tract disease is a frustrating condition for cats, owners, and veterinarians. While feline idiopathic cystitis is the most common cause of lower urinary tract disease,1 urolithiasis is also common. During a session at the 2022 American Association of Feline Practitioners (AAFP) conference, Anne-Marie Germain, DVM, of Village Cat Clinic in Cambridge, Ontario discussed the management and prevention of feline urolithiasis.
When it comes to creating a differential diagnosis list for causes of inappropriate elimination, “Never underestimate the power of a good history,” said Germain. “One really important thing I think we miss is confirming presence of lower urinary tract signs,” she continued. Lower urinary tract (LUT) signs include pollakiuria, dysuria, stranguria, overgrooming of the abdomen, and vocalization when voiding urine. While not all cats with uroliths will have LUT signs, the presence or absence of these signs will change the differential list. Hematuria is another clinical indicator that uroliths may be present, though it is not pathognomonic.
Additionally, Germain reminds veterinarians that they have to speak the same language as clients. When an owner says their cat is urinating more, it is important to clarify whether they are referring to the volume of urination or the frequency. Germain suggests comparing the sizes of the balls of urine in the litter box to known objects, such as fruits. Uroliths may be present with both polyuria and pollakiuria, but other differentials will vary between the two presentations.
The signalment of cats should also help to shape the differential diagnosis list. FIC is uncommon in cats less than 1 year of age and older than 10 years of age.2 Struvite uroliths are more common in cats less than 7 years of age and rare over the age of 10.3 Calcium oxalate uroliths are more common in cats over the age of 7.3 These are the two most common types of uroliths in cats, but stones composed of or containing urate, apatite, dried solidified blood (DSB), and silica are also possible.3 There is a breed predisposition for urate stones in Egyptian Mau, Siamese, and Ocicats.3
Once a suspicion of urolithiasis is formed based on signalment, history, clinical signs, and physical examination, diagnostics should be undertaken to determine if stones are in fact present. “There’s a huge discrepancy in what we do in theory and what we actually do,” said Germain. Ultrasound, which is often used to aid in obtaining urine samples by cystocentesis can help determine if stones are present and may be the only way to diagnose non-radiopaque stones, such as urates.
However, radiographs are essential for the diagnosis of uroliths as they can aid in predicting stone composition and allow for measurement of stones to track dissolution.4 Because struvite stones are the only type of stone that can be dissolved by therapeutic dissolution diets, predicting the likelihood of a struvite stone can help to prepare owners for the chance of successful dissolution. Calcium oxalate stones are the most radiodense, tend to be round to spiculated and often have multiple stones (more than 3-5).4,5 Additionally, the vast majority of renoliths are calcium-based, so in patients with renoliths, calcium-containing uroliths in the bladder are more likely.5 Struvite stones are also radiodense, but tend to be ovoid and occur in smaller numbers, usually less than 3 stones.4,5 The Urolith Center at the University of Minnesota has an app that can help to predict stone type based on radiographic appearance and patient data.
Urinalysis results can also provide important information to predict stone type but should be interpreted in light of collection method, storage, and handling. The pH can provide important information as to the type of stone, as struvite stones tend to form close to neutral pH.5 Crystals may or may not be present. Germain reminds practitioners to “Never rule out a stone if there aren’t crystals in the urine.”
Medical management of uroliths should always be attempted first.4,5 Dissolution diets are effective for struvite uroliths, and clinical signs should improve quickly after starting the diet. “They can dissolve as fast as one week,” shared Germain. In general, struvite stones should be reduced by 50% in the first two weeks, and most will be gone within 4 weeks. Germain prefers to radiograph patients every 2 weeks to monitor dissolution but noted that this is not financially feasible for some clients. In these cases, waiting 4-6 weeks to radiograph is appropriate if the patient’s clinical signs are improving.
While using a dissolution diet, pain medication to address associated discomfort from stones should be utilized. Additionally, while most struvite stones in cats are sterile, stones do predispose to infection, which should be treated if present. Finally, environmental modification to reduce patient stress should also be introduced.
If medical dissolution fails in the face of appropriate dietary compliance, environmental management to reduce stressors, and confirmation that a urinary tract infection is not present, stone removal should be pursued. Both the American College of Veterinary Internal Medicine and AAFP consensus statements on urolith management recommend minimally invasive procedures as the preferred option.4,5 These include voiding urohydropulsion, cystoscopic basket retrieval, intracorporeal lithotripsy, and percutaneous cystolithotomy.4 Cystotomy should be the last intervention but is necessary in some patients, especially in male cats, whose anatomy limits the use of many non-invasive options.
One essential piece of information when it comes to urolith prevention is knowing what type of stone the patient had. All stones that are retrieved physically from a patient should be sent to a laboratory for quantitative analysis. “There’s no urolith too small,” said Germain, “That sand and grit is precious.”
Once the type of stone is identified, an appropriate therapeutic maintenance diet can be selected. There are numerous diets on the market and some diets that manage multiple concurrent conditions. Germain encourages veterinarians to “Reach out to the different companies because [diets] aren’t all the same.” Additionally, knowing the type of stone may prompt investigation of concurrent medical conditions, such as hypercalcemia or portosystemic shunts. Management of these conditions may contribute to prevention of future uroliths.
A primary goal in urolith prevention is dilution of the urine, which increases frequency of urination and creates a flushing action and dilutes the concentration of stone precursors. There are many ways to try to dilute urine. The primary way is to try to increase voluntary water intake, which can be difficult in some feline patients. Using running water fountains, adding water to the food, feeding a canned diet, increasing the frequency of meals, and decreasing calories can all be strategies to dilute urine. Additionally, increasing the number of litterboxes available and utilizing pain medication for arthritic cats may also help to increase the frequency of urination. Diluting the urine can be difficult in some cats and multiple strategies may need to be used concurrently to maximize success.
Boatright, a 2013 graduate of the University of Pennsylvania, is a practicing veterinarian and freelance speaker and author in western Pennsylvania. She is passionate about mentorship, education, and addressing common sources of stress for veterinary teams and recent graduates. Outside of clinical practice, Dr. Boatright is actively involved in organized veterinary medicine at the local, state, and national levels.