Urolithiasis
What is urolithiasis?
Urolithiasis is the long-winded Latin term to describe the development of bladder and kidney stones. The term ‘uro’ describes the urinary system, while ‘lith’ means stone.
The urinary system acts as the filtration system for the body. Waste products as well as excess fluid are excreted through the urinary system in the form of urine. The urinary system extends from the kidneys out through the little tubes known as ureters, which empty into the bladder, which in turn empties outside the body via the urethra.
Stones usually develop in the kidneys or bladder, but can get caught anywhere in between. These stones are technically termed urinary calculi or uroliths, but are more commonly known as kidney or bladder stones and are described based on their location within the urinary system. The problems that arise from these stones are determined by their location and the symptoms they cause. Different types of urinary stones exist depending on the mineral composition of each stone.
How do uroliths develop?
Urolithiasis occurs when diluted minerals in the patient’s urine precipitate (solidify) to form crystals when there is a suitable environment for their formation. Uroliths can consist of one main type of mineral or a mixture of various minerals. Changes in urine pH (whether increased acidity or alkalinity) can promote the formation of stones that favour the particular environment. Struvite stones, for example, form in alkaline urine.
Other factors that have been attributed to the formation of urinary calculi include the weakening of the patient’s natural abilities to minimise the formation of stones; the presence of excess salts in the patient’s urine as a result of any of the diverse pathologic processes; delays in urine transit from the body; and the presence of a nidus around which crystallisation can concentrate, such as in urinary tract infections.
What types of uroliths are there?
Struvite stones are the most commonly observed calculi in dogs, accounting for approximately 50% of all urolithiasis cases of their lower urinary tract. In cats, struvite urethral plugs account for an estimated 80% of all urethral plugs in the species. Struvites favour alkaline urine conditions and constitute magnesium ammonium phosphate. Urinary tract infections are a major contributing factor for the development of this stone. If a pet eats more protein than their recommended requirements, they will be predisposed to developing this stone. Evidence also suggests that incidences of struvites are higher in pets who eat throughout the day – this is thought to promote the continuous presence of high levels of minerals in the urinary bladder. The distal urethral diameter in tom cats predisposes them to the higher incidences of struvite urethral plugs.
Calcium oxalate stones represent approximately 35% and 55% of stones in the lower urinary tract of dogs and cats respectively. Miniature schnauzers, Lhasa apso, shih tzu and Yorkshire terriers are the most commonly affected breeds of dogs, while in cats, the Burmese, Himalayans and Persians are over represented. These uroliths develop in an acidic environment in the urinary bladder. Calcium oxalate stones form when there is excessive calcium (hypercalciuria), oxalates (hyperoxaliuria), or low citrate (hypocitraturia) in the urine. Often, as a result of this type of urolith, patients will have distended bladders with complete blockage of the urethra.
Urate stones and cystine stones are two other types of less frequently encountered urinary stones in small animal medicine. They readily form in acidic urine. Dalmatians, which have defects in uric acid transport systems in the liver, are commonly affected by urate stones. Cystine stones are thought to be a result of inherited kidney defects which lead to excessive elimination of cysteine in urine. Male dachshunds are commonly affected by this type of stone. Both stones are generally not visible on direct x-rays, so ultrasonography is the better method of their diagnosis.
Other types of uroliths not described in this article also do exist in practice however.
What can I expect to see if my pet has a problem with bladder or kidney stones?
Signs of urolithiasis vary greatly in practice according to the area the stones are found. Typically, signs can range from inappropriate voiding, difficulties in urinating, frequent urination in small volumes, and/or blood in the urine. When the urethra is completely blocked, a patient can be seen straining several times without successful urine voiding. A blocked urethra can present as a medical emergency. On abdominal palpation, a distended urinary bladder will be identified with complete blockage. In other instances, abdominal palpation will only reveal pain in the abdominal area as well as hard stone masses. In scenarios where there are complications as a result of the presence of stones in the kidney, systemic signs will be evident to the veterinarian.
How will the vet diagnose urinary stones?
The vet will use imaging to confirm their suspicion of the presence of urinary calculi. Radiographs, ultrasound scans and sometimes contrast radiography are the most commonly used imaging tests in practice. It is important to remember that not all stones will be visible on a simple x-ray; hence the pet owner must be prepared for the possibilities of a false negative result. Once a stone is identified, its characterisation ensues. Knowing which type of stone the patient has informs how they will be treated. Blood tests are useful in detecting other predisposing factors such as elevated blood calcium levels with calcium oxalate stones. It’s also important to ensure that the patient does not have an underlying urinary tract infection.
My pet has uroliths, what now?
The treatment and management of uroliths in pets will depend on several factors, namely the severity, location and type of stone involved. Treatment can be classified as either medical or surgical, oftentimes a combination of both. Not all urinary stones require surgical intervention. With modern science, correcting the urine pH as well as correct dietary management have been shown to dissolve certain uroliths successfully. Due to the high risk of recurrence of the condition, a good maintenance diet is often prescribed for life. Access to other types of foods must be prohibited when dietary management is undertaken.
Surgical management is employed for those stones that do not respond well to diet management alone, and for emergency situations caused by the stones. At times surgical intervention will involve structural changes to the urinary system in order to reduce the risk of repeat blockages – particularly of the urethra in males. Laser therapy (lithotripsy), a highly advanced procedure, can also be used to dissolve uroliths. Lithotripsy is minimally invasive for the patient. Proper post-surgery aftercare improves the chances of surgical success. Regardless of how small the procedure might look for the patient, adequate rest and adherence to the discharge instructions post operation is of critical importance.
Prognosis
Prognosis is good when accurate treatment is given in time and before the development of other complications as a result of urinary stones. However, most stones have a high probability to recur within three years of treatment. Maintenance on the best food is thus required in a bid to minimise the chances of recurrence.
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