Did you just say that my cat has stones in his body?

Dr. Kari Mundschenk

Just For Cats Veterinary Hospital

Over the past 20 years, there has been an increase in urinary stones from cats being sent to stone analysis laboratories.  The causes are varied and often unknown yet overall, there are some common risk factors.  This talk is focused on just one aspect of renal disease, urolithiasis or stones in the urinary tract.  Hopefully, one will come away with an understanding of risk factors, possible prevention and methods we use to monitor a cat with stones. 

In the past, diet was determined to be the main treatment to get rid of stones.  Everyone thought all you had to do was feed a low ash, acidified diet and the problem will just go away.  Unfortunately, this has been proven to not be the case; in fact, there is now an increasing incidence of Calcium Oxalate stones which do not dissolve with dietary treatment.

Urolithiasis is not a single disease.  It is a combination of genetics, physiology, diet and the environment.  In order for a urolith to form, one must have an increased amount of promoters and a decreased amount of inhibitors in an oversaturated environment.  This leads to crystal nucleation which when combined with retention and subsequent aggregation, growth and addition, leads to a visible urolith.

Examples of the promoters of urolith formation include urinary tract infections, hypovolemia (ie blood loss, dehydration), disorders which lead to an increase in blood calcium, genetics, liver disease, vitamin/mineral excess, altered urine pH (the former are often influenced by diet), sequelae to drug administration and inflammation.

Signs of urolithiasis often depend on their location.  For example stones in the urethra may go unnoticed if they are passing or they may cause a complete blockage whereas stones in the kidneys or ureters, may only be found when examining other aspects of the abdomen.  All can potentially be life threatening if a complete blockage occurs.

In recent years in cats, the most common stone types present are Calcium Oxalate followed by struvite (aka MAP magnesium ammonium phosphate), calcium phosphate, purine and mixed/compound stones.  As with snowflakes, stones can vary within the particular type and the appearance on radiograph or at surgery does not necessarily assist with diagnosis.  All stones removed at surgery, are sent for analysis to assist with treatment recommendations.

All cats presented with possible feline urinary tract disease must have a full urinalysis with culture and radiology.  Blood work is also indicated if there is a concern about urinary blockage.  Oftentimes, crystals may be present in normal cat urine yet crystals are not synonymous with the presence of stones.  In fact, as stones form, there will actually be fewer crystals.  There is significance to the presence of crystals.  They indicate there is a risk factor for a urolith or plug (a combination of crystals + inflammatory debris), they assist with predicting mineral composition (radiographs are better), and they do assist as an index of therapeutic response.

As with diagnosis, treatment is tailored to the type of urolith present, its location and taking into account preventing further stone formation.  As was seen in the '80's, trying to prevent stone formation by decreasing ash and the use of acidifying diets, is now causing problems with oxalate stone formation.  Currently, the primary preventative method is based on obtaining a neutral to mildly acidic urine and keeping the relative super saturation (RSS) high enough to inhibit an environment for stones to form.

The need for urolith removal and the type of therapy depends on the effect of the urolith on the patient (asymptomatic, recurrent infection, dysuria, urinary obstruction), the characteristics of the urolith (mineral composition, size, contour, number and location), and the familiarity of removal techniques and availability of specialized equipment.  Surgery is not always indicated and depending on the situation a novel therapy and management may be the best option.  Many times hospitalization with fluid therapy and medications to affect the urinary tract is required in order to decrease the risk of damage to the kidneys and correct electrolyte imbalances.

In all cases, these kitties need to be monitored on an ongoing basis and treated accordingly.  For some stones, it may just be a urinalysis and others may require radiographs or urinary culture.  The frequency of followup visits is dependent upon the cause and location of the primary stone problem.  In all aspects, urinary concentration and getting moisture into the system are major factors in controlling these stones.