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GUESSTIMATING Urolith Composition

BY DR ANNABEL ROBERTSON - APRIL 20, 2020 - URINARY & KIDNEY

Have you ever wondered how to guesstimate urolith composition?

Struvite stones in dogs and cats can be managed medically with c/d Multicare +/- appropriate antibiotics.

So then, how can we determine which urolith type is most likely without actually analysing it?

Fortunately, several factors can give you clues as to which stone type you may be managing.

1. Patient Signalment

Age-Uroliths in immature dogs (< 1 year old) are primarily struvite (MAP) associated with a urinary tract infection. In fact, struvites account for just over 60% of uroliths in this age category. Urates are also more prevalent in this age group, and are often associated with congenital portocaval shunts.1 Calcium oxalate uroliths are far more common in middle aged to older dogs, with dogs presenting with a mean age of 8.5 years.1

 

Cats with CaOX tend to be older than those with struvite. In fact, they had the highest risk of developing CaOX between 7-10 years of age (67 times as likely to develop this stone versus a cat 1-2 years of age).2 Whereas, cats 4-7 years of age had the highest risk of developing struvite (10 times as likely to develop this stone versus a cat 1-2 years of age).2

 

Breed- When it comes to stones types there are some distinct breed predispositions. Check out the infographic. You will notice smaller breed dogs typically have a higher incidence of forming calcium oxalate.

 

Gender- Struvite uroliths in dogs are almost always associated with a urinary tract infection (UTI) by urease producing bacteria. These infections are most common in female dogs due to their anatomy of a short urethra in close proximity to the anus. Whilst struvite uroliths are more common in female dogs, calcium oxalate, urate and cystine are found predominantly in male dogs. Check out this blog for tips on dissolving struvite stones.

 

Male cats have a 1.4 times higher risk of developing CaOX than female cats, whereas female cats were 1.4 times more likely to have a struvite urolith than male cats.2

Uroliths breed predispositions chart

2. Underlying medical problem

It’s always best practice to rule out underlying diseases that could be contributing to stone formation. For instance, does the dog have hyperadrenocorticism? This condition can result in hypercalcaemia and hypercalciuria and would increase your suspicion of a CaOX urolith.

Dogs with hyperadrenocorticism are 10x as likely to develop a CaOX urolith compared to healthy dog.5

Certain medications including furosemide and corticosteroids and supplements such as Vitamin D can also result in hypercalciuria. Spending the time to review the patients file and take a thorough history will reveal clues. Not all patients with a CaOX will have hypercalcaemia but it’s always worth checking total and ionised calcium, particularly if there is any reason to suspect a CaOX urolith.

If there is underlying liver disease or reason to consider a portosystemic shunt, then a urate urolith would be highest on the differential list.

Risk factors for struvite and calcium oxalate uroliths in dogs

Risk factors for struvite and calcium oxalate formation in dogs

3. Urinalysis

Crystals on sediment- This could provide a clue about the stone type you might be looking at. However, one can still be tripped up by compound uroliths. Likewise, the absence of crystals does not mean a urolith isn’t present.

 

Urine pH- Struvite rapidly precipitates in more alkaline pH. The urease splitting bacteria are responsible for urinary alkalinisation as well as increasing the availability of ammonium ions to build the stone.  CaOX are more likely to precipitate in a more acidic urine pH, but can also form over a wide pH range. pH must therefore be interpreted in light of other findings.

watchout 1

Bacteria- Staphylococcus spp (cocci) and Proteus spp (rod shaped) are common causes of UTI’s in dogs. Both these bacteria produce high levels of urease which alkalinises the urine. If you see bacteria on sediment examination or you have a positive urine culture, this should increase your suspicion of a struvite urolith.

 

Check out this blog for more tips on how to get the most out of urinalysis.

4. Radiographic Appearance

Most uroliths in dogs and cats are composed of either struvite or calcium oxalate. Helpfully these look quite different on radiographic appearance.  Struvite are usually relatively large radiopaque stones with a smooth or slightly rough appearance. CaOX are typically the most radiopaque urolith with a similar opacity to bone and present as multiple small to medium size stones. Remember to always take an orthogonal view. Urate stones can often be missed on radiographs as they appear radiolucent.  Further imaging such as a contrast urethrocystogram or bladder ultrasound will be required to detect these uroliths.  

watchout 2

5. Minnesota Urolith Centre App

You are no doubt familiar with the Minnesota Urolith Centre which has been performing analysis of uroliths from dogs and cats around the world since the early 1980s. This work has been partially sponsored by an educational grant from Hill’s. What you may not be familiar with is their app, MN Urolith, which is available to download free on mobile devices. Whilst you still need to log your urolith submission online via their website https://urolithcenter.org the app has many handy features and enables you to:

  • Track submissions and access results
  • View management recommendations for different uroliths
  • See the typical radiographic appearance of different uroliths
  • Have fun with the CALCulator
Minnesota Urolith Center App

The CALCulator function will show the likelihood of different urolith types based on the huge database of uroliths analysed at MUC. Have a go entering the signalment of the pet you are treating.

 

This information can help guide nutritional therapy whilst you are awaiting results of stone analysis.   

 

Hopefully you will have found these tips helpful. It is important to remember these are general guidelines and there will always be exceptions. If you are suspicious of a struvite urolith it is worth trialing dissolution as per the ACVIM guidelines. If a stone fails to dissolve then it should be removed and submitted for analysis. Removal can be achieved by urohydropropulsion or cystotomy depending on stone size.

Do you have a tricky case and need our help? Have you received urolith results but are unsure on the long term dietary recommendation? Contact us at the Hill’s Helpline 1800 800 733.

Link to register for Hill's Global Symposium on urinary disease

Dr Annabel Robertson BVSc (Hons) MANZCVSc MBA, Hills Technical Services Veterinarian

Annabel graduated from the University of Melbourne with honours in 2008. Since then she has worked as a small animal veterinarian in private practice in Australia and the UK. She also completed an internship in small animal medicine during this time and sat her membership in small animal internal medicine in 2012, and finished her MBA in 2015. Annabel joined the Hill’s team in 2015 as a technical services vet in Melbourne.

Dr Annabel Robertson

References:

  1. Lulich JP, Osborne CA, Bartges JW. Canine Lower Urinary Tract Disorders. In: Ettinger SJ, Feldman EC, editors. Textbook of Veterinary Internal Medicine, 4th edn. WB Saunders, Philadelphia, 2000:1747-1783.
  2. Lekcharoensuk C, Lulich JP, Osborne CA et al. Association between patient-related factors and risk of calcium oxalate and magnesium ammonium phosphate urolithiasis in cats, J Am Vet Med Assoc, 2000;217:520-525.
  3. Nolan BG, Labato MA, Calcium oxalate urolithiasis. In: Bonagura JD, Twedt DC, eds. Kirk’s Current Veterinary Therapy XV. Philadelphia: Elsevier Saunders, 2014, pp.897-901.
  4. Osborne CA, Bartges JW, Lulich JP et al. Canine Urolithiasis. In: Hand MS, Thatcher CD, Remillard RL, Roudebush P, editors. Small Animal Clinical Nutrition. 5th edn. Walsworth Publishing Company, Topeka, Kansas, 2010: 605-688.
  5. Hess RC, Kass PH, Ward CR. Association between hyperadrenocorticism and development of calcium-containing uroliths in dogs with urolithiasis. J Am Vet Med Assoc 1998;212:1889-1891.