Most bladder stones can be seen on plain x-rays.

This week, we had several discussions around the clinic about bladder stones in cats and dogs. I thought this might be a good topic, as these often take owners totally by surprise.

Urinary stones can form from many different inorganic compounds and can make trouble anywhere along the urinary tract. Common types include struvite (magnesium-ammonium-phosphate, sometimes still referred to as “triple phosphate”), calcium oxalate (“CaOx,” which come in monohydrate or dihydrate varieties), urate, and cystine. Struvite and CaOx constitute by far the majority of stones seen in cats and dogs. Each type develops based on fairly specific conditions, such as urine pH, the presence of bacteria (struvite in dogs), or a primary disease process (urate with liver disease). In addition, breed-specific mutations may permit large amounts of the precursor compounds that lead to stone formation; this is true of cystine (Newfies, Dachshunds, and English Bulldogs) and urate (Dalmatians).

How they form. Urinary crystals, which are generally innocuous, and the more troublesome stones form from their constituent compounds under certain physiologic conditions. First, there must be an abundance of the primary compounds in the urine. Second, a specific urine pH can encourage or discourage stone formation; this can come in handy when planning treatment or prevention. Finally, factors that allow the urine to sit for long periods in the bladder or kidneys (like decreased bladder tone, or infrequent access to suitable places to urinate) appear to permit stones to form.

There is some debate about the specific steps in stone formation. Three competing theories contend that:

  1. Stones form from “supersaturation” of crystalloids that are common in the urine.
  2. The presence of an abnormal substance triggers stone formation from otherwise harmless crystals.
  3. The presence of a promoter or absence of an inhibitor allows stones to form under conditions that would not otherwise lead to their formation.

Reality may be a combination of all three.

Clinical signs. When stones form in the bladder, clinical signs can include blood in the urine, recurrent urinary tract infections, and non-specific signs of abdominal discomfort. If stones form in the ureters or urethra, they may block these structures, causing pain, either the inability to pass urine (an emergency event) or frequent urination in small amounts, and straining/vocalizing when trying to urinate. Obstruction can lead to severe acute renal failure.

Diagnosis. Many types of stones can be seen on standard x-rays, where they appear as mineral-dense structures in the urinary tract; cystine and urate stones, however, do not show up on plain x-rays and are diagnosed in other ways. Ultrasound is a great way to diagnose stones in the bladder, ureters, and kidneys, as all types of stones reflect the sound waves equally well. It is less good for seeing stones in the urethra, as the pelvic bones cast large shadows in the area. A camera inserted in the bladder (cystoscopy) is also an effective way to diagnose stones; and small ones may be retrieved with the scope for analysis for treatment.

As part of the diagnosis, it is ideal to identify the type of stones that are present. Often we can rule out urate or cystine if the stones were visible on x-ray. But it is crucial to differentiate between struvite and CaOx, as the treatments are very different. We can often get a clue about the type of stones from the type of crystals in the urine. It stands to reason that, if there are struvite crystals, the stones are likely struvite. It’s tricky to rely entirely on this supposition, however, because multiple types of stones can form together.

It should be noted that, while urinalysis can identify urinary crystals, their presence – even in high numbers – does not indicate the presence of stones. However, since crystals and stones form under the same conditions, crystals in the urine can signal the failure of any dietary preventative measures that may have been instituted.

Treatment. There are several treatment options based on the type of stones:

Surgery: Most larger stones (except struvite or urate) must be removed surgically. If a stone is in the kidney, it may be more desirable to leave it, as surgery on a kidney can cause the loss of up to 20% of its function units (nephrons). The risk of leaving these kidney stones is that, if dislodged, they can obstruct the ureter on their way to the bladder. If the stone is found in the ureter or urethra, surgery may be an emergency, as obstruction of these structures can be life-threatening.

Voiding Urohydropulsion (VU): Smaller stones in the bladder may be flushed out with a procedure called VU, which entails placing a large quantity of fluid in the bladder and forcing it (and the stones) out through the urethra. One complication is that a stone may get lodged in the urethra; so it’s a good idea to plan for surgery as back up. Any stone removes surgically or via VU should be submitted to analysis to prepare for additional treatment.

Dissolution Protocol: For struvite or urate stones in the bladder, it is technically possible to dissolve them. This requires a dietary change that alters the pH of the urine (lower for struvite, higher for urate) and increases water intake. Antibiotics are warranted for dogs with struvite stones, as urease-producing bacteria are implicated in stone formation in most canine cases.

Prevention: In at-risk pets, we try to alter the diet in a way that removes precursor compounds and changes the urine pH and concentration to levels that do not encourage stone formation. In some cases, these preventative steps will reduce or eliminate future stone formation. In many cases, however, additional stones form anyway.

Urinary stones can be very frustrating for owners, as diagnosis requires more that a lab test, prevention and treatment can be unrewarding, and the concern over life-threatening obstruction can be tiring. As we learn more about the pathway to stone formation, we will get closer to effective means of prevention.