Chronic kidney disease is a common diagnosis in veterinary medicine. It’s a disease for which we have no cure, only the ability to slow it down and manage the other symptoms. Ultimately, it’s fatal. The really nasty part is that we have a hard time finding it before a vast amount of irreversible damage is already done. I’ll give a little background into what’s happening with kidney disease, then talk about what we can do to treat it.
The kidney is a remarkable organ. It filters waste products out of our blood so that they can be excreted in the urine. They manage the balance of water and electrolytes (salts) in our bodies. They also make a hormone that tells the bone marrow to produce red blood cells. (Red blood cells carry oxygen throughout our bodies.) The kidneys you’re born with are the kidneys you have for the rest of your life. They don’t regrow, and “healing” is a limited ability for this important organ.
On a microscopic level, the kidney is quite complex. The illustration below shows the arrangement of blood flow and filtering in the functional unit of the kidney: the nephron (neff-ron). Tiny little arteries carry blood into a tangled ball of blood vessels called the glomerulus. Around that ball is a cup-like structure called Bowman’s Capsule. The vessels in the glomerulus are designed so that waste products and water are leaked out. Some other things the body prefers to keep are also filtered out. These wastes and other things are then collected by the Bowman’s Capsule. Bowman’s Capsule is connected to a big U-shaped tube (cut off in this illustration) that gives the kidney a chance to recollect the good things that were leaked out of the glomerulus. Those things are reabsorbed or transported back into the blood. Some new things are excreted, while others are traded out and sent into this U-shaped tube. Eventually, the U-shaped tube hooks up with a bigger tube that travels to the middle of the kidney. In the middle of the kidney, all of this waste material and whatever water the body is getting rid of is collected, then piped down to the bladder. Of course, this waste fluid is urine.
In chronic kidney disease, a variety of different things can go wrong, but in the end, it’s the nephrons that are destroyed. As nephrons are destroyed, the kidney begins to lose its ability to do the work it needs to do. This can be a very slow process, taking years in some cases. As with most things in the body, there’s some redundancy in the kidney to allow for a certain amount of damage without the body suffering for it. The kidneys can do their job well enough even with some loss of nephrons. Eventually, there’s a threshold after which the kidneys can’t keep up and things start to go very wrong. We can’t easily detect a problem until things start to go wrong, which is why this is such a nasty diagnosis to make.
Perfect kidneys are at 100% function. As we talk about percent damage, keep in mind that we’re referring to the grand total sum of both kidneys. It’s possible that one kidney can be damaged more than the other, or undamaged, but we aren’t distinguishing for the purposes of this post. The end result of damage to the kidneys is that the nephrons are destroyed, leaving scar tissue where functioning kidney used to be.
When 2/3 of the kidneys’ functional capacity is gone, leaving only 33% working, the kidneys lose their ability to manage water balance in the body. Water is allowed to leave the bloodstream at a rate that will dehydrate the animal. This is the earliest sign of chronic kidney disease. It can be subtle. The urine will seem more dilute. Pets will begin to drink more water and pass more urine. In truth, the urine production goes up because of the inappropriate loss of water, so pets drink more to remain well-hydrated. We can detect this low urine concentration when we run a urinalysis. Sometimes, we can detect protein loss in the urine, too, which is a sign of trouble. A pet may begin losing weight.
When 3/4 of the kidneys’ functional capacity is gone, leaving only 255 working, the kidneys can no longer keep up with the removal of waste products from the blood. Byproducts of metabolism begin to build up in the bloodstream. We can detect these rising values on bloodwork, namely the serum chemistry. As these waste products rise, we begin to see effects in the animal. Appetite drops because of nausea and irritation of the stomach lining, followed by vomiting. Dehydration and weight loss occur, as well as significant loss of muscle mass. At very high levels, these wastes can directly impair brain function, too. Other effects of advancing kidney disease can include high blood pressure and anemia. Sadly, many of the pets we diagnose arrive with lots of these clinical signs.
There are lots of treatments available for renal disease, but remember that none of them are a cure. Chronic kidney disease is managed, not fixed. We lower the protein intake of a pet’s diet to decrease the waste-filtering workload of the kidney. We also keep the phosphorus low in the foods we feed to help prevent some other complex but highly damaging changes in calcium and phosphorus levels in the blood and bone. There’s an oral medicine to help with that problem as well. We can give injections of the hormone that makes red blood cells be produced. We can give fluids under the skin to help flush the wastes out through what’s left of the kidney. Some animals respond very well to these treatments. Cats tend to do better for longer periods of time than dogs. Each patient is different in terms of response to treatment and the rate the disease progresses. Some patients survive only a few months. Others can go years before we lose the fight. We manage the patient, not the numbers on the lab work, with the goal being good quality of life. The sooner we start treatment, the better most animals do!
Running lab work is the ONLY way we have to watch for the emergence of chronic kidney disease. The clinical signs that owners see are present early on, too, but they start out subtly. Most of us don’t notice a gradual change like this until it’s further along. Annual urinalysis helps us detect the changes that signal early kidney disease. Bloodwork may also show subtle signs earlier in the process, so that’s also a valuable tool.
The take home message here is this: RUN ANNUAL LABWORK ON YOUR PETS! It’s the best way we have to watch for early signs so we can begin treatment early.
Please ask questions. This is a complex disease that I could talk about for pages and pages, so I simplified the basic discussion for today. I’m happy to expand on any aspect if there’s interest!