Urolithiasis

Question 1

What is the signification of crystalluria in urinalysis

Answer 1

The presence of crystalluria implies current or recent supersaturation of urine with the calculogenic crystalloid materials that have been detected

Question 2

What are the causes of supersaturation

Answer 2

Normal finding in healthy cats with very concentrated urine

Ingestion of a certain diet

Changes in the pH

Temperature of the urine during storage
- the storage of urine before analysis will have a big effect on crystal precipitation

Question 3

What is the link betwwen crystalluria and urolithiasis

Answer 3

The finding of crystalluria is a risk factor for the development of urolithiasis but the two are clearly not synonymous
- many individuals display microscopic crystalluria
- in the majority of cases, crystalluria is not associated with disease and crystals present within the urine are usually voided

Many normal, healthy cats will have crystals in their urine
- cats have concentrated, often supersaturated urine

Question 4

Give exemple of associations between crystalluria and disease

Answer 4

Crystalluria may be associated with:
- development of urolithiasis
- development of matrix-crystalline urethral plugs
- metabolic abnormalities (e.g., urate crystals in cats with prtosystemic shunts)

Question 5

What is the most common form of crystalluria recognized in cats

Answer 5

Struvite crystalluria is the most common form of crystalluria encountered in cats

Question 6

How would you explain the rise in prevalence of calcium oxalate uroliths that has been reported over the past decade

Answer 6

It is not necessarily a genuine increase in incidence of calcium oxalate urolith, but rather a relative rise seen simply due to the decline in the proportion of struvite uroliths encountered

In other words, the widespread adoption of relatively magnesium-restricted acidifying diets has not led to a large population of cats with oxalate crystalluria but has simply contributed to a decline in the risk for struvite crystalluria and struvite urolithiasis

Question 7

Which types of uroliths are dominant in cats

Answer 7

Struvite (= magnesium ammonium phosphate) and calcium oxalate are the dominant forms seen in cats

Question 8

Which factors affect crystal formation and growth

Answer 8

Frequency and adequacy of bladder emptying

Presence of certain mucoproteins, cellular debris or foreign material (e.g., bacteria) that can promote crystallization and formation of a urolith nidus

Presence and balance of various promoters and inhibitors of crystallization in urine
- For exemple, magnesium is a component of struvite and thus favors struvite crystal formation, but conversely magnesium helps to inhibit oxalate crystal formation

The urine pH

Question 9

Explain what is the mainstay of treatment for urolith formation

Answer 9

Urolith formation is a complex situation involving the genetics of the cat, the diet, saturation of the urine and complex promotors and inhibitors

The mainstay of treatment will be the reduction of urine supersaturation with the crystalloid material

Question 10

Which risk factors of urolith formation can you cite

Answer 10

Breed
- Persians, Burmese and Himalayans seem at risk for calcium oxalate stone formation

Age
- middle-aged cats

Cats with underlying conditions
- pyelonephritis
- hypercalcemia

Obesity and a sedentary lifestyle

Neutering (but this likely relates to obesity and inactivity)

Question 11

Explain how crystal precipitation can occur in supersaturated urine

Answer 11

Below the saturation product (Ksp) for any given crystalloid, urine will be unsaturated
- in this state crystals can neither form nor grow
- dissolution of some existing stones will occur (e.g., struvite not calcium oxalate)

At the saturation product (Ksp), the urine becomes fully saturated with the solute and enters a zone of “metastable supersaturation”
- the concentration of the solute is such that if there is a pre-existing crystal in the urine, growth of the crystal will occur
- spontaneous crystallization (spontaneous nucleation) cannot occur

Only if the concentration of a solute exceeds the formation product (Kfp) will spontaneous homogenous crystallization and nucleation occur

However, heterogenous crystallization and nucleation can occur in the metastable zone
- a urolith promoter (e.g., impurities in urine, cellular debris, matrix glycoproteins) can act as the nidus for precipitation of another crystal and form the nucleus of a urolith
- heterogenous crystalization

Formation product and the propensity for a solute to precipitate are influenced by the temperature and pH of the urine, and the presence and concentration of both inhibitors and promoters of crystallization

Once nucleation has occured, crystal growth can proceed under supersaturated conditions (either metastable or unstable) via crystal growth and aggregation

Question 12

Give exemple of calcium oxalate inhibitors

Answer 12

Citrate

Magnesium

Nephrocalcin

Osteopontin

Question 13

What are the three types of clinical presentation for lower urinary tract urolithiasis

Answer 13

Lower urinary tract uroloithiasis can be:
- clinically silent

- may result in signs of cystitis

- may result in urethral obstruction

Question 14

What are the clinical presentation of nephrolithiasis

Answer 14

Nephrolithiasis may be:
- clinically silent

- may be associated with renal disease

- may be associated with chronic pyelonephritis

- may be associated with development of ureterolithiasis which may cause ureteral obstruction

Question 15

Describe the clinical presentation for bilateral ureteral obstruction

Answer 15

Bilateral ureteral obstruction is a cause of acute, severe azotemia
- cats present with AKI and may be oliguric

Renomegaly may occur with hydronephrosis

Abdominal pain and pain on renal palpation may be noted
- but it can be very subtle in cats

Question 16

Explain the big kidney, little kidney syndrome

Answer 16

A finding of big kidney, little kidney may occur where one kidney is hydronephrotic but the other has been previously obstructed and is end-stage

Clinical signs are often non-specific with weight loss, inappetence, vomiting and PU/PD if there is bilateral renal impairment

Question 17

What are the advantages and disadvantages of abdominal radiography for the diagnosis of ureterolithiasis

Answer 17

The vast majority of uroliths (i.e., calcium oxalate and struvite) are radio-opaque
- BUT, dried blood calculi will be radiotransparent
- The ureteroliths are often in the retroperitoneal region
- the colon with fecal material could overly the area where uroliths would be
- the sensitivity of radiography for the diagnosis of ureterolithiasis is 81%

Question 18

What are the ultrasonographic signs of ureterolithiasis

Answer 18

Abdominal US has a sensitivity of 77%

In most cases, renal pelvic dilation or hydronephrosis is identified
- Ureteral proximal dilation may also be identified

Question 19

Describe the common presentation of struvite on abdominal radiography

Answer 19

Struvite tend to be large stones and tend to be fewer in number

Question 20

Describe the common presentation of calcium oxalate on radiography

Answer 20

Calcium oxalate tend to be smaller and tend to be more of them

Question 21

What is the only technique able to give the exact nature of a urolith

Answer 21

Quantitaive analysis by x-ray diffraction

Question 22

Explain how you can manage urolithiasis

Answer 22

Initial management of urolithiasis may be either surgical (struvite or oxalate) or medical (dietary dissolution of struvite) depending on the type of stone present

Question 23

What is the principal point for dissolution of struvite stones and prevention of their recurrence

Answer 23

It relies on under-saturating the urine for the mineral constituents and increasing their solubility by controlling the urine pH

Question 24

What is the principal point in preventing recurrence of oxalate stones

Answer 24

It relies on reducing supersaturation of urine with calcium and oxalate, which can result in spontaneous crystal formation and growth

Question 25

What is the goal in the management of all lower urinary tract conditions including urolithiasis

Answer 25

Diluting the urine
- water intake is the key

Question 26

Which factors are important in calcium oxalate prevention and how to achieve them

Answer 26

Urine volume and frequency of urination are important factors in long-term management

The use of wet diet and encouraging the cat to drink through the provision of flavored waters or water fountains may all be of benefit
- For cats that have a strong preference for dry food and cannot be transitioned to a wet diet, a dry diet that contains a moderately increased salt content is beneficial

Question 27

Which factors are important in struvite prevention

Answer 27

The urine pH markedly affects the solubility of struvite crystals

A diet designed to both reduced the concentration of the crystalloid precursors of struvite and produce mildly acidic urine is ideal

Maximizing water intake will also assist in management and prevention

Question 28

Which points are important to prevent struvite and oxalate uroliths formation in cats (general guidelines)

Answer 28

Maintain optimum body condition and avoid excess weight and obesity

Encourage exercise

Encourage drinking

Encourage frequent urination through the use of multiple clean litter trays using the cat’s preferred litter substrate

Question 29

How can you prevent recurrence of uroliths

Answer 29

Management of hypercalcemia is vital if present

Dietary treatment should be instigated in order to produce less saturated urine of ideal pH
- urine SG should be checked regularly
- ideally, a wet diet is fed

Analgesia should be provided

Question 30

What are the known and probable diet-related risk factors for struvite uroliths. What are the consequences for diet manipulation

Answer 30

Diet-related risk factors
- alkaline urine
- excess protein (ammonium precursor)
- excess phosphate
- excess magnesium
- excess carbohydrate
- increased fibre
- decreased urine volume

Diet manipulations
- urine pH 6.2-6.4 (similar to natural diet)
- reduced magnesium
- reduced phosphate
- reduced protein
- wet diet and encourage drinking

Question 31

What are the known diet-related risk factors for oxalate uroliths. What are the consequences for diet manipulation

Answer 31

Known diet-related risk factors
- Acid urine
- excess calcium or citamin D
- low magnesium
- low phosphate
- excess oxalate
- reduced citrate
- excess vitamin C (oxalate precursor)
- B6 deficiency (promotes oxalate production)
- decreased urine volume

Diet manipulation
- urine pH 6.6-6.8
- Not calcium, vitamin D, oxalate supplemented
- added citrate
- replete in magnesium and vitamin B6
- wet diet and encourage drinking

Question 32

What can you say about dried solidified blood calculi

Answer 32

They seem specific of cats (not found in dogs)

They appear to increase in frequency

These calculi can be found anywhere in the urinary tract

They are not radio-dense in most cases
- when an upper urinary tract obstruction is present but no radiodense calculi are noted, DSB calculi should be considered as a cause