One of the most painful conditions encountered in practice is urinary obstruction. The dog is suddenly found in great discomfort making frequent attempts to urinate but passing only a few drops of blood.
Urinary obstruction is a serious condition. If the condition is not treated quickly the dog soon becomes very depressed and is reluctant to stand. In these cases, the obstruction to the urinary flow has occurred in the canal from the bladder through the penis, the urethra.
A less acute condition occurs where a partial obstruction originates in the urinary bladder. In this case, the dog can pass urine but does so frequently and only in small quantities. This condition can occur in both male and female dogs.
The cause of these obstructions are urinary calculi. These are aggregations of crystalline material that form in the urinary bladder. In some cases, the calculi are large and stay within the bladder. Over a long period, they produce a chronic irritation of the bladder wall and predispose the tissues to infection.
The wall of the bladder can become many times thicker than normal and can lose its elasticity. The dog is then unable to retain urine for long periods and passes small quantities which are often blood-stained.
In male dogs, it is more common for large numbers of small calculi to form. These easily pass out of the bladder into the penile portion of the urethra. The dog has a boney structure within its penis, called the os penis, through which the urethra travels.
As the urethra is unable to expand within this bone, the calculi lodge at the beginning of the boney segment of the canal. The pressure of urine builds up in the distended bladder and no amount of straining on the part of the dog can force the calculus any further.
Treatment of urethral obstructions must be undertaken without delay, to relieve the dog’s discomfort and minimize the serious effects on the kidneys that follow the increased pressure of urine within the bladder.
The dog is anesthetized and attempts are made to pass a catheter past the obstruction. Sometimes small calculi can be gently pushed back into the bladder but more frequently an operation is necessary to remove the obstruction.
The urethra is opened at the site of the calculi and they are pushed out of the incision by passing a catheter through the penis. When the obstruction is freed the catheter is pushed past the incision and into the bladder and the wound is repaired.
Calculi in the bladder do not require such urgent treatment. The bladder is opened through an abdominal incision and all calculi are removed. The bladder is then flushed with a saline solution to remove the tiny sand-like deposits which can be the precursors of larger calculi.
The composition of urinary calculi varies widely. The most common type is a combination of magnesium and ammonium phosphate. Some are composed of amino acid, cystine, which is excreted in excessive amounts in the urine of some dogs because of an inherited metabolic defect.
Prevention of recurrent calculi formation may be possible once the exact composition of the stone is known.
Laboratory tests are made to analyze the stones taken from the patient. In some cases changing the acidity of the urine may prevent crystallization from occurring within the bladder. Where the stone is composed of protein material or urates an appropriate treatment is available.
In all cases, the dog should be encouraged to drink more water by adding salt to the food. The dog should be given the opportunity to pass urine frequently during the day so as to prevent stasis and sedimentation within the urinary bladder.