The case of the mixed urinary stone - Veterinary Practice
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The case of the mixed urinary stone

LEE DANKS in this first in a new series of monthly columns from Royal Canin exploring a wide range of clinical conundrums, looks at how to manage a case of mixed urinary stones

HAVING provided technical advice alongside our nutritional advisers for nigh on four months now, I’m confronted with the daily reminder that urinary patients in the real world rarely present as textbook cases.

Not a surprise to those at the coalface, I’m sure. In this article I’ll be discussing a mixed urolithiasis case and the conundrum faced when deciding which diet to feed for now and in the long term.

“Bud”, an eight-year-old male Miniature Schnauzer presents to you with reports of “dark” urine for the last two weeks. He’s “generally unwell and rarely finishing his food” as described by the owner and has developed a recent habit of stopping for a wee every 20 steps while out on a walk.

Concurrently you note that he has severe periodontal disease, has a body condition score of 7/9 (he loves sharing meals with his senior citizen owner!) and those ears don’t smell too fantastic either.

On collecting a minimum database, including radiographs and full urinalysis, your work-up and management plan leads you down the route of a cystotomy, as repeat x-rays show persistent radio-opaque bodies in the bladder.

The three stones you removed have been whizzed off to the lab and the analysis is just in: uroliths of varied composition, approximately 60% struvite with a calcium oxalate nidus.

So what are your next steps in managing Bud? How do we prevent recurrence, particularly where a dissolution diet has been used in the (albeit two-week) period between presentation and surgery?

The starting point is to remember that the urine is essentially an aqueous vehicle by which the body voids metabolic wastes.

The urinary environment is a complex one and under the influence of numerous physical and chemical factors.

In certain conditions, with the presence of certain waste products, a precipitate will form. If the “right” conditions continue, the crystal precipitate can aggregate to form stones over time.

Regardless of the influence of electrolyte imbalances, bacterial infections, organ dysfunction outside of the urinary tract or even environmental, behavioural and lifestyle factors of the individual animal, one factor common to the formation of all types of stones is the urinary concentration.

A bladder which doesn’t have a reasonable amount of fluid moving through it is much more likely to contain an oversaturated urine sample. It stands to reason then, that regardless of the type of stone (and crystal) we’re aiming to prevent, promoting diuresis by means of increased water consumption will help.

Simply encouraging our pets to increase their water intakes by environmental modifications or (more reliably) via dietary methods is the answer.

Regardless of what else we might recommend for this Schnauzer case, feeding a higher sodium level (where there’s a dry diet preference) or moving toward a wet diet will make a difference to reduce urinary solute concentration.

Increasing water turnover is central to the Relative Supersaturation (RSS) concept developed in human urology in the 1960s.

Some veterinary diet manufacturers recognise this concept which extends beyond the “induced polyuria” mentioned above.

The presence and interaction of the many electrolytes which can be present in the urine are important too. Bud the Schnauzer, with a combination of two precipitates in his bladder, has had a decent amount of calcium, oxalate, magnesium, ammonium and phosphate ions present at a certain point in time to precipitate his stones.

The formulation of a diet can either directly or indirectly influence the presence of these ions. For example, I can feed Bud with a lowmagnesium diet which will reduce his predisposition to struvite stones, as will my efforts to moderate the protein he’s fed.

By reducing protein we lower the production of urea, which becomes available as a substrate for urease enzymes which are produced by the bacteria which compound Bud’s urinary issues.

Urea is broken down into ammonia in the presence of (common) bacterial infections. Many subtle tweaks to a diet formula can change its ability to create a urinary environment less (or more) likely to form a precipitate of varied minerals.

The next consideration is the age-old reference to urinary pH. Just as with other ions, the presence of hydrogen influences the interactions which happen between solutes in the urine.

As the chemical bonds which bring magnesium together with the ammonium and phosphate aren’t particularly strong in the struvite crystal, acidification (generally <6.5)/ the presence of hydrogen ions can separate one molecule from the next, effectively dissolving the stone or crystal over time.

It is by combining a trifecta of pH modification, minimising urine mineral content alongside increasing urine water turnover that we can be said to fully address every aspect possible for a diet when it comes to supporting urinary health.

Moreover, by combining this approach to consider both struvite and oxalate precipitates, one dietary formula is able to prevent the recurrence of both stone types.

Technically, what we’ve diagnosed with our Schnauzer’s urolith analysis is a compound stone, which reveals a nidus of different composition to its shell.

Struvite-on-oxalate is the most common type of compound urolith but struvite about a urate, calcium phosphate or silica nidus are also reported to us on the Royal Canin helpline.

If we’re to stick to correct terminology, it’s the stone which is less than 70% of a single mineral and without an obvious nidus which is classified as mixed. When composition is over 70% of any one type, the stone is named according to that single mineral.

In the case of a compound urinary stone, management should be focused primarily on combating the composition of the nidus, which was the seeding issue.

Bud may have been fed on an S/O (struvite/oxalate) diet as soon as he was recognised as a urinary case, getting the best of both worlds: combining a struvite dissolution diet with one that prevents recurrence of both struvite and oxalate. Urinary S/O is the recommendation I would stick to.

Given Bud’s concurrent weight issues I might place even greater emphasis on using the moisture in a wet formula for energy dilution or even reach for a moderate calorie or small dog formula given dental health considerations.

All in all, real-life cases keep us rightly on our toes and adaptive at all times. Next month, we’ll continue the urinary theme further and delve into the more atypical or “metabolic” uroliths.

  • For further reading visit vetportal. (or vetportal. for Ireland).

References and further reading:

Caney, S., Cortadellas, O., Dhumeaux, M. and Nickel, R. (2014) Practical Management of Urinary Tract Disease. Veterinary Focus special edition, Royal Canin, Aimargues France.

Hesse, A. and Neiger, R. (2009) A Colour Handbook of Urinary Stones in Small Animal Medicine, Manson Publishing, London, UK.

McNeill (ed) (2014) Veterinary Focus: Lower Urinary Tract Disease, Buena Media Plus, Boulogne, France.

Moore, A. (2007) Quantitative analysis of urinary calculi in dogs and cats. Veterinary Focus 17 (1): 22-27.

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