544 UNIT VIII RENAL AND UROLOGICAL DISORDERS
stones than people living in other parts of the coun- try because the hot, dry climate predisposes them to develop dehydration. Nephrolithiasis is more common in European Americans than in African Americans, and the disease is predominately found in males. Kidney stones most commonly develop in adults aged 20 to 49 years, with a peak incidence at age 35 to 45 years old. The mean age is 44.8 years in men and 40.9 years in women. A family history doubles the risk of kidney stones. Recurrence of nephrolithiasis is common. After suffering a kid- ney stone, individuals have a 52% chance of suffering another stone within 10 years. Etiology The exact cause of nephrolithiasis is unknown, but about 90% of patients who present with clinical man- ifestations have at least one metabolic risk factor: hypercalcemia, hyperoxaluria, hyperuricemia, hyper- parathyroidism, or gout. In addition, low fluid intake is a significant risk factor because dehydration enhances kidney stone formation. There is a genetic predisposi- tion, with more than 30 genetic variations associated with renal calculi development. Differences in intes- tinal calcium absorption, renal calcium transport, and renal phosphate transport have all been attributed to genetic variation. In patients without specific metabolic or genetic risk factors, nephrolithiasis is attributed to dietary habits, such as excessive calcium supplements and low fluid intake. Hypercalciuria and low fluid con- tent of the urine are the most common predisposing factors that lead to nephrolithiasis (see Box 22-1).
Protein is filtered out of the blood at the glomerulus.
Protein
Loss of protein from the bloodstream (hypoalbuminemia)
Low colloid oncotic pressure
Proteinuria
Edema
Nephrolithiasis Nephrolithiasis is the formation of stones, also called calculi, in the kidney. Calculi can form in the kidney and travel into the ureter, when it is then referred to as urolithiasis. Although pain is a presenting sign with all types of renal calculi, characteristics vary based upon the stone’s location. Epidemiology In the United States, the lifetime risk of developing nephrolithiasis is approximately 11% for men and 7% for women. Approximately 2 million patients seek health care for kidney stones each year. Dehydration increases susceptibility to kidney stone formation. For this reason, people living in the south and south- west United States have higher incidences of kidney CLINICAL CONCEPT Hypoalbuminemia, edema, and proteinuria are the three distinguishing features of nephrotic syndrome. Hyperlipidemia and HTN are also associated with nephrotic syndrome. FIGURE 22-8. Nephrotic syndrome. In nephrotic syndrome, the glomerulus is damaged, allowing proteins to be filtered out of the bloodstream. The major protein from the bloodstream that is lost is albumin; thus, hypoalbuminemia results. The loss of protein (albumin) from the bloodstream causes decreased colloid oncotic pressure, which leads to edema. Therefore, the signs of nephrotic syndrome are hypoalbuminemia, proteinuria, and edema.
BOX 22-1. Predisposing Factors of Nephrolithiasis
There are many predisposing factors for nephrolithiasis. Nephrolithiasis is usually caused by a number of different conditions that act together to cause precipitation of calculi in the kidney. • Age greater than 40 years • Male gender • Certain medications (e.g., sulfonamides, indinavir, acetazolamides) • Dietary factors (e.g., purines, calcium, oxalate) • Gastric bypass surgery • Geographic location (hot, arid climates) • Hypercalciuria • Hyperparathyroidism • Hyperuricemia • High-sodium diet • Inflammatory bowel disease • Inherited conditions (e.g., polycystic kidney disease, renal tubular acidosis) • Low hydration/low urine volume • Obesity • Proteus urinary tract infection
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