The urethra is the tube that runs from the bladder to the tip of the penis in a male, and into the vagina in a female, through which the urine passes when an animal urinates.
A urethral obstruction, or “blocked bladder”, occurs when a bladder stone (calculus) or stones become lodged in the urethra, thereby preventing urine from passing through the urethra. The kidneys continue to produce urine, which accumulates in the bladder, and, try as it may, the animal is unable to empty the bladder, resulting in an extremely uncomfortable and potentially life-threatening condition.
A calculus is a crystalline structure that can vary in size from a single crystal, visible only under a microscope, to a mass of crystals the size of a tennis ball. The most common causes of calculi are dietry imbalances and chronic low-grade bladder infections that go unnoticed by the owner, and probably by the animal. Some animal breeds have a hereditary predisposition to forming calculi.
Urethral obstruction occurs almost exclusively in male animals, due to the fact that the male urethra is a long, narrow structure compared to the female urethra which is short and wide.
At about two thirds of the length of the urethra in male dogs and cats there is a narrowing, and this is where the calculi tend to lodge. In cats the obstruction is generally caused by a number of calculi the size of very fine sea sand, while in dogs it is generally caused by a single calculus the size of a small pea. Apart from the calculus or calculi causing the obstruction there will almost invariably be more calculi further up the urethra and in the bladder.
The clearing of the urethral obstruction is a medical emergency, firstly to relieve the animal’s discomfort, and secondly to prevent or minimize secondary complications.
The animal is anaesthetized and an attempt is made to flush the calculi from the urethra, ideally out of the end of the urethra, or where this is not possible, back into the bladder, from where they can later be medically or surgically removed. This is often successful in cats, and less often in dogs.
In dogs we almost invariably have to resort to making an incision, half way between the scrotum and the end of the penis, through the body of the penis and into the urethra to remove the calculus. This wound is not stitched and for a while the dog will pass urine through the wound and through the end of the penis. This procedure is called a urethrotomy.
The calculi are then analysed and depending on the composition of the calculus, an attempt is made to make any remaining calculi dissolve and prevent further calculi from forming, either by changing the animal’s diet or by medical management.
There are a number of possible secondary complications to urethral obstruction. If the obstruction is not cleared in time, the bladder will rupture, requiring an operation to repair the bladder, provided that the animal is in a strong enough condition to make such surgery possible.
Due to the “back pressure” of the urine the kidneys will be damaged, resulting in a certain degree of temporary or permanent kidney failure. After removal of the calculus, all animals will be in intensive care until the kidneys are functional.
Due to excessive stretching of the bladder muscle, the bladder may be paralysed, resulting in an animal that is unable to pass urine voluntarily even though the obstruction has been removed. These animals have a permanently full bladder and drip urine continuously. Fortunately this paralysis is often temporary.
Some calculi are too large to be managed medically or by a change of diet. In these cases an operation is required to remove the calculi from the bladder.
Some animals suffer from repeated obstructions, either due to a large number of calculi, or to narrowing of the urethra due to damage caused by the calculi and attempts to remove them. In such cases an operation is required, whereby the penis is amputated on the bladder side of the narrowing of the urethra. In a dog the urethra is stitched to the skin between the anus and scrotum. In a cat the scrotum is removed and the urethra is stitched to the skin where the scrotum was removed. This procedure is called a perineal urethrostomy, and results in a relatively large opening through which the animal can pass the calculi that would have lodged at the narrowing of the urethra. The animal retains full control of voluntary urination, since this depends mainly on a muscle at the outlet of the bladder.