Kidney and Ureteral Stones in Cats – Diagnosis, Clinical Consequences and Prevention of Recurrence
- Ecommercy
- 2026-09-21
- 14:56
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Upper urinary tract urolithiasis is a significant clinical concern in cats, not only because of the potential for renal parenchymal damage, but particularly due to the risk of nephrolith migration into the ureter, which may result in partial or complete obstruction. Cats with nephrolithiasis may remain asymptomatic for prolonged periods; however, migration of a calculus can rapidly lead to acute kidney injury, electrolyte disturbances and a clinical emergency requiring prompt intervention.
During a webinar dedicated to feline nephrolithiasis, vet Magdalena Firlej-Oliwa discussed the pathophysiology of the most common types of uroliths, the limitations of relying solely on urinalysis for diagnosis, the clinical consequences of ureteral obstruction, and the importance of adequate hydration in reducing the risk of recurrence.
Watch the full webinar
Nephroliths, Ureteroliths and Uroliths – The Clinical Significance of Stone Location
When discussing mineral deposits within the urinary tract, it is important to define their location precisely.
A nephrolith is a stone located within the kidney, whereas a stone present in the ureter is referred to as a ureterolith. The term urolith has a broader meaning and refers to stones located anywhere within the urinary tract.
From a clinical perspective, the location of the stone is of critical importance.
A nephrolith that remains within the kidney may be an incidental finding on diagnostic imaging and cause no clinical signs for a prolonged period. The situation changes when the stone migrates towards or into the ureter.
The feline ureter has a very narrow diameter, meaning that even a relatively small stone can impair urine flow. The resulting obstruction may be partial or complete, leading to progressive dilation of the renal pelvis and ureter and, ultimately, damage to the renal parenchyma.
Unilateral Urolithiasis Does Not Necessarily Cause Azotaemia
An important diagnostic consideration is that unilateral ureteral obstruction does not necessarily result in a marked increase in renal parameters.
If the contralateral kidney maintains adequate function, it may compensate for reduced filtration on the affected side for a certain period of time.
As a result, the patient may:
- continue to urinate normally,
- have normal or only mildly elevated creatinine levels,
- show no typical signs of uraemia,
- yet still have significant disease affecting one kidney or ureter.
In clinical practice, this means that normal renal parameters do not rule out unilateral ureterolithiasis or partial ureteral obstruction.
Ultrasonography may sometimes reveal evidence of a previous disease process, such as renal atrophy, ureteral dilation or degenerative changes, even when there is no history of a previous acute episode.
Hydronephrosis as an Indicator of Impaired Urine Flow
Dilation of the renal pelvis and calyces should always prompt further investigation to identify the underlying cause of impaired urine outflow.
A ureterolith is the most obvious potential cause; however, ureteral obstruction is not necessarily associated with urolithiasis alone.
Other possible causes include:
- blood clots,
- inflammatory material,
- external compression,
- masses,
- anatomical abnormalities of the ureter.
Chronic complete obstruction can result in severe atrophy of the renal parenchyma. In advanced cases, ultrasonography may reveal a markedly dilated fluid-filled collecting system surrounded by only a thin layer of residual renal tissue.
Calcium Oxalate Stones – The Most Common Type of Urolithiasis in Cats
The webinar placed particular emphasis on calcium oxalate (CaOx) stones.
Data presented during the webinar, based on analyses of urinary stones submitted to reference laboratories, indicate that calcium oxalate accounts for a substantial proportion of uroliths identified in cats. However, an important limitation should be considered when interpreting these data: the statistics include uroliths collected from the entire urinary tract, rather than nephroliths alone.
From a clinical perspective, CaOx urolithiasis is particularly challenging because once a calcium oxalate stone has formed, it cannot be dissolved through dietary management or modification of urinary pH.
Dietary management therefore plays a key role primarily in reducing the risk of recurrence.
The Importance of Hydration in Calcium Oxalate Urolithiasis
During the webinar, calcium oxalate (CaOx) urolithiasis was discussed primarily as a condition strongly associated with concentrated urine and inadequate water intake.
The lower the urine volume, the higher the concentration of minerals that may reach supersaturation and precipitate as crystals.
For this reason, increasing water intake is one of the key elements of preventive management.
From a nutritional perspective, this primarily means favouring diets with a high moisture content. A typical wet diet contains approximately 75% water or more, allowing cats to obtain a substantial proportion of their daily fluid intake directly from their food.
It is not always necessary to add large amounts of extra water to every meal. What matters most is the cat’s total daily water intake and maintaining adequately diluted urine.
Urinary pH Should Not Be the Sole Target of Therapy
One of the more common issues in clinical practice is placing too much emphasis on urinary pH.
Lowering urinary pH may be appropriate in certain cases of struvite urolithiasis; however, prolonged and uncontrolled urinary acidification is not without potential consequences.
Conditions that help reduce the risk of struvite formation may, at the same time, increase the risk of calcium oxalate crystallisation in predisposed patients.
Moreover, adjusting urinary pH alone does not provide complete protection against CaOx formation. Calcium oxalate crystals may also occur at pH values that would not intuitively be considered typical of this type of urolithiasis.
For this reason, clinical interpretation should extend well beyond a single urinary pH measurement.
Struvite Urolithiasis in Cats and Bacterial Urinary Tract Infection
Struvites, or magnesium ammonium phosphate stones, represent another important group of uroliths.
It is important to distinguish between the pathogenesis of struvite urolithiasis in dogs and cats.
In dogs, struvite stones are very often associated with urinary tract infections. In cats, however, the presence of struvite is not necessarily associated with bacterial infection.
This does not mean that the two conditions cannot occur concurrently.
Uroliths may increase the susceptibility of the urinary tract to bacterial colonisation, while in certain cases infection may contribute to stone formation. The relationship between the two should therefore be assessed on an individual basis, rather than automatically assuming that bacteriuria is the primary cause of every case of struvite urolithiasis.
Crystalluria Cannot Reliably Determine the Composition of a Nephrolith
One of the key diagnostic challenges is that there is no direct correlation between the crystals identified in urinary sediment and the composition of a stone located within the kidney.
For example, it is possible to:
- identify a nephrolith on ultrasound or radiography,
- find no evidence of crystalluria in the urinary sediment.
The absence of crystals in urine therefore does not rule out urolithiasis.
Likewise, the presence of a particular type of crystal does not necessarily mean that an existing stone has the same chemical composition.
Radiography has similar limitations. Both struvite and calcium oxalate stones may be clearly radiopaque, meaning that their radiographic appearance alone cannot reliably distinguish between the two.
If a stone is not available for direct analysis, clinical management should therefore be based on the overall clinical picture and epidemiological likelihood, rather than on urinary sediment findings alone.
A Nephrolith May Predispose Cats to Pyelonephritis
Urolithiasis does not necessarily have to cause mechanical obstruction to affect renal function.
The presence of a stone may promote bacterial colonisation of the renal collecting system and contribute to the development of pyelonephritis.
In clinical practice, several pathological processes may occur simultaneously:
urolithiasis → partial obstruction → urinary stasis → infection → inflammation and further renal damage
Therefore, in a patient with diagnosed urolithiasis, subsequent abnormalities should not automatically be attributed solely to the direct effects of the stone itself. Potential complications should always be considered and assessed.
Ureterolithiasis and Acute Kidney Injury
Migration of a stone into the ureter represents a critical point in the course of the disease.
Severe ureteral obstruction may lead to acute kidney injury (AKI).
A patient’s clinical condition can deteriorate rapidly. A cat that appeared clinically well one day may develop marked systemic signs the next.
The webinar emphasised that in such cases, prompt diagnosis and intervention are essential – the appropriate timeframe is measured in hours rather than days of continued observation.
Initial management should focus on stabilising the patient and addressing concurrent abnormalities, including:
- dehydration,
- pain,
- electrolyte disturbances,
- hypertension,
- bacterial infection, if present.
At the same time, options for restoring urine flow should be assessed.
Medical Management or Intervention?
In selected cases, medical management may be attempted to facilitate the passage of a ureterolith into the urinary bladder.
However, this approach requires close monitoring.
The key consideration is not simply how long medication is administered, but whether:
- the stone is changing position,
- urine flow remains unobstructed,
- renal pelvic dilation is not progressing,
- the patient’s laboratory parameters and clinical condition remain stable.
A lack of improvement or progression of the obstruction may require a prompt change in treatment strategy.
Interventional options discussed during the webinar included:
- ureteral stenting,
- placement of a subcutaneous ureteral bypass (SUB),
- selected lithotripsy techniques,
- nephrectomy in specific cases.
The choice of procedure depends on the location and nature of the obstruction, the patient’s clinical condition, the function of the contralateral kidney, and the feasibility of the individual procedure.
Assessment of the contralateral kidney is particularly important. Nephrectomy should only be considered when the remaining kidney is capable of providing adequate renal function.
What Can Happen After an Episode of AKI?
Acute kidney injury is potentially reversible; however, in cases associated with urolithiasis, a complete return to baseline renal function is not always possible.
Following successful restoration of urine flow, several outcomes are possible:
- recovery of normal or near-normal renal function,
- persistent renal damage with subsequent development of chronic kidney disease (CKD),
- progressive loss of renal function despite treatment.
In clinical practice, progression from an episode of acute kidney injury to chronic kidney disease is an important potential consequence of upper urinary tract urolithiasis.
The Role of Nutrition in Prevention
In cases of calcium oxalate (CaOx) urolithiasis, diet cannot be used to dissolve an existing stone.
Its primary role is to reduce the risk of further crystal and stone formation.
Ensuring adequate water intake is one of the key components of preventive management.
In practice, feeding a high-moisture wet diet can be particularly beneficial. However, dietary management should be considered as a whole rather than focusing on a single variable, such as urinary pH.
In cats with renal disease or a history of urolithiasis, dietary management should also be tailored according to:
- renal function,
- electrolyte profile,
- the presence or absence of infection,
- current urine specific gravity,
- concurrent diseases,
- the type of urolithiasis suspected or confirmed.
A monoprotein diet is not, in itself, a primary criterion when selecting a diet for a renal patient. The overall nutritional profile, adequate hydration and the patient’s clinical status are considerably more important.
Effective Prevention Requires Ongoing Monitoring, Not Just a One-Time Change in Diet
Patients with a history of urolithiasis remain at increased risk of recurrence and related complications.
Follow-up should be tailored to the individual patient, with the primary aim of detecting changes before clinically significant obstruction or acute kidney injury develops.
Depending on the individual case, monitoring may include:
- complete urinalysis with sediment examination,
- urine specific gravity,
- urine culture when indicated,
- renal parameters and electrolytes,
- follow-up ultrasonography,
- radiography in appropriately selected cases,
- assessment of blood pressure and the patient’s overall clinical condition.
Laboratory findings should always be interpreted in conjunction with the clinical picture and diagnostic imaging findings.
FAQ – Feline Nephrolithiasis in Clinical Practice
Can the absence of crystals in urine rule out nephrolithiasis?
No. In cats with stones located within the kidneys, urinary sediment may show no evidence of crystalluria. The absence of urinary crystals therefore does not rule out nephrolithiasis.
Can the composition of a stone be determined from radiographs?
Not with certainty. Both struvite and calcium oxalate stones can be clearly radiopaque. Radiography can help confirm the presence and location of certain uroliths, but it cannot replace direct analysis of stone composition.
Does the presence of calcium oxalate crystals mean that a nephrolith is composed of calcium oxalate?
Not necessarily. Crystalluria does not reliably reflect the composition of an existing stone. Direct analysis of a retrieved or spontaneously passed urolith provides the most reliable determination of its composition.
Can calcium oxalate stones be dissolved through dietary management?
No. Neither dietary management nor modification of urinary pH can dissolve an existing calcium oxalate (CaOx) stone. In these cases, nutrition plays a primarily preventive role by helping to reduce the risk of recurrence.
Does the presence of struvite stones in cats indicate a urinary tract infection?
No. Unlike the typical situation in dogs, struvite urolithiasis in cats is not necessarily associated with a urinary tract infection. However, infection and urolithiasis may occur concurrently.
Does a normal creatinine level rule out ureteral obstruction?
No. In cases of unilateral disease, the normally functioning contralateral kidney may compensate for the loss of function on the affected side, meaning that renal parameters may remain within the normal range for some time.
Can a cat with unilateral ureteral obstruction still urinate?
Yes. The contralateral kidney may continue to produce urine. Therefore, the presence of urine in the bladder or continued urination does not rule out unilateral ureteral obstruction.
Does every ureterolith require intervention?
No. In selected cases, medical management may be attempted, but only with close monitoring. Failure of the stone to progress, increasing urinary obstruction or deterioration in the patient’s clinical condition may indicate the need for intervention.
What is the most important aspect of dietary prevention of CaOx urolithiasis?
One of the most important measures is increasing water intake and reducing excessive urine concentration. In practice, feeding a high-moisture wet diet plays an important role.
Should a cat’s urine be acidified preventively after an episode of struvite urolithiasis?
No. Long-term urinary acidification should not be used without appropriate monitoring. Excessive lowering of urinary pH may increase the risk of calcium oxalate precipitation in predisposed patients.
Is a monoprotein diet necessary for cats with urolithiasis?
Not because of urolithiasis itself. A monoprotein diet may be appropriate for other indications, such as food hypersensitivity, but in the prevention of renal and urinary tract disorders, adequate hydration, the mineral composition of the diet and the individual patient’s clinical status are more important considerations.
Summary
Feline nephrolithiasis requires a much broader clinical assessment than simply determining whether crystals are present in the urine.
The greatest clinical concern is the risk of a nephrolith migrating into the ureter and causing an obstruction, which may lead to acute kidney injury. Normal creatinine levels, continued urination or the absence of crystalluria are not sufficient to rule out unilateral urinary tract disease.
Accurate diagnosis requires an integrated assessment of clinical findings, laboratory results and diagnostic imaging.
In cases of calcium oxalate urolithiasis, there is no dietary method available to dissolve an existing stone. Preventing recurrence is therefore particularly important, with adequate hydration and regular patient monitoring forming key components of long-term management.
A comprehensive discussion of the mechanisms of stone formation, ureteral obstruction, their impact on renal function and appropriate management strategies is available in the webinar recording with vet Magdalena Firlej-Oliwa.