Sometimes it's the diseases you already have that can cause trouble for your kidneys. The most common culprits include diabetes and hypertension. A few patients develop kidney failure secondary to potassium deficiency.
People with either kind of diabetes (insulin-dependent and non-insulin-dependent) may get kidney failure after a decade or more of suffering from this disease. People with diabetes now comprise the largest group of patients starting dialysis in the United States and account for a large portion of the deaths from kidney failure. Diabetic kidney disease is relatively easy to detect in the early stages, because traces of protein appear in the urine. However, only one-third of subjects with traces of protein in their urine (microalbuminuria) will go on to develop full-blown kidney disease, with substantial amounts of protein in the urine. People with diabetes can and should test their urine for protein at least once a month. (Details of this test are given in Chapter 3.)
There is now evidence that close control of blood glucose levels in people with insulin-dependent diabetes somewhat reduces the incidence of kidney failure, though only at the cost of more frequent attacks of hypoglycemia. (see Chapter 16.)
For people with diabetes who are overweight (which many are), whether they are insulin-dependent or not, 20 percent weight loss, especially if combined with smoking cessation and increased exercise, can be extraordinarily beneficial. Not only does blood pressure decrease, but also the levels of blood fats (like cholesterol) fall; furthermore, in those who have kidney disease, kidney function improves and urinary protein loss diminishes. Thus all of the complications of diabetes are reduced.
High blood pressure is one of the most common disorders in the United States. The majority of people over 50 suffer from it. Thanks to a persistent campaign by the American Heart Association and others, the importance of controlling blood pressure is more and more widely known, and most patients now get at least some treatment for hypertension. Undertreated or untreated, hypertension can lead to heart failure, strokes, and kidney failure. It was widely assumed in the past that hypertension causes kidney failure.
However, a recent analysis of 10 large trials shows that controlling blood pressure in nonmalignant hypertension (the commonest kind) doesn't make any difference in the development of kidney failure. Among 26,521 people with high blood pressure followed for an average of five years, only 317 developed kidney failure. Patients who received antihy-pertensive drugs, who consequently had lower blood pressure than the others, did not have a significant reduction in their chances of getting kidney failure.
It is also widely held that African Americans are more susceptible than whites to kidney disease from hypertension. Dr. Norman Kaplan, an authority on high blood pressure, has recently summarized this issue, and concludes that it is true that most nondiabetic hypertensive African Americans with mild to moderate kidney failure have primarily hypertensive kidney disease. Yet only 10 percent of African Americans with kidney disease can be said to have hypertension as their primary disorder.
Thus the question as to just how often high blood pressure causes kidney failure remains unsettled, in contrast to the widespread view that it is a major cause. Whatever the case may be, it is still highly important for your overall health to treat hypertension effectively with the help of your doctor.
Pregnancy unquestionably makes hypertension worse. Consequently pregnant women should be closely monitored for the development or worsening of hypertension. Late in pregnancy, hypertension may be a sign of preeclampsia or eclampsia, serious complications. If you are pregnant, your blood pressure should be checked frequently.
Treatment of hypertension includes both lifestyle changes and drugs. Protein in the urine is an early sign of kidney damage from high blood pressure. Control of blood pressure is now known to be one of the most important features in the treatment of chronic kidney disease and may in fact stop progression altogether. But first kidney disease and hypertension have to be recognized.
Even in the absence of kidney disease, blood pressure should be maintained below 130/80, according to recent recommendations.
Potassium deficiency is another cause of kidney failure, though it is uncommon. Kidney function decreases in severe potassium deficiency and may never recover. Chronic diarrhea or overuse of laxatives can induce chronic potassium deficiency and renal failure. The ability of the kidneys to produce highly concentrated urine, and to decrease the output of urine in response to dehydration is characteristically impaired in patients with potassium deficiency, so these patients tend to excrete large volumes of urine (despite their reduced kidney function). Replenishing potassium stores usually restores kidney function, but not always.
Laura Melton came to Johns Hopkins at the age of 56. She told us that in order to lose weight, she had been taking laxatives every day for 30 years and still takes 2 to 3 pills daily. She had first exhibited urinary protein in an exam 11 years earlier. Three years before, she had 500 mg per day of urinary protein (normal urinary protein amounts to less than 150 mg per day) and a slightly elevated serum creatinine level (1.5 mg per dl; normal levels in women are below 1.3 mg per dl). Urinary protein had increased to 2.56 g per day. A kidney biopsy (removal of plug of kidney tissue with a needle) 10 months earlier showed glomerulosclerosis, meaning scarring of the glomeruli. Laura had had high blood pressure for three years, which had been treated irregularly. High serum cholesterol and triglyceride levels had been noted for at least 14 years, but had been treated only irregularly. Because of intermittent potassium deficiency, she was taking a potassium chloride supplement daily, but also taking a diuretic, hydrochlorothiazide, to lose weight. This diuretic causes increased potassium excretion (see Chapters 8 and 12), as well as increased salt excretion. Her physical exam was normal except for hypertension (160/100). Laboratory data showed mild renal insufficiency and serum potassium at the lower limit of normal. By the next visit, and for three years thereafter, while she showed up only occasionally, serum potassium was always subnormal, despite continuance of a potassium supplement. She admitted that she could not stop abusing laxatives, even at serious risk to her long-term health. At her last visit, after five years of follow-up, her renal failure had gotten only slightly worse. Subsequently she was lost to follow-up.
Laura's story illustrates how long-standing potassium deficiency (in this case self-induced) can cause kidney failure.
One of the most common causes of renal disease in my patients, as well as in the United States as a whole, is glomerular disease (glomerulonephritis or glomerulosclerosis). Glomerular diseases include a large list of disorders with varying causes, presentation, and predicted outcomes. These disorders can be treated with drugs, a discussion outside the scope of this book.
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