Porth's Essentials of Pathophysiology, 4e

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Kidney and Urinary Tract Function

U N I T 7

Agenesis, Hypoplasia, and Dysplasia Defects in development of renal structures are frequently observed during the 1st year of life, when they collectively represent a significant cause of morbidity and mortality. Renal agenesis, hypogenesis, and dysplasia account for a significant portion of these defects. 3 Renal Agenesis. The term agenesis refers to the com- plete failure of an organ to develop. Total agenesis of both kidneys is incompatible with extrauterine life. Infants are stillborn or die shortly after birth. Newborns with renal agenesis often have characteristic facial features, called Potter syndrome, that are caused by fetal compression due to a marked reduction in amniotic fluid levels. 1,2,4 The eyes are widely separated and have epicanthic folds, the ears are set low, the nose is broad and flat, and the chin is receding. The most life-threatening component of Potter syndrome is pulmonary hypoplasia , which is caused by inadequate stimulation from the amniotic fluid and by compression of the chest wall. Bilateral renal agenesis, which has an incidence of 1 in 3000 births with a male predominance, is usually suspected when maternal ultra- sonography demonstrates oligohydramnios, nonvisual- ization of the bladder, and absent kidneys. 2 Unilateral agenesis is more common than bilateral agenesis and is compatible with life if no other abnor- mality is present. The opposite kidney usually is enlarged as a result of compensatory hypertrophy. The disorder frequently goes undetected, unless discovered by chance observation on prenatal ultrasound. Renal Hypoplasia. In renal hypoplasia , the kidneys are small in size and have less than the normal number of calyces and nephrons. Hypoplasia may affect one or both kidneys. 2–4 When unilateral, the condition is usu- ally discovered during examination for another urinary tract problem or hypertension. When both kidneys are affected, there is progressive development of renal fail- ure. A history of polyuria and polydipsia is common. Renal Dysplasia. A developmental disorder, renal dys- plasia, is characterized by maldifferentiated primitive structures, primarily of the renal tubules. The condition can affect all or only part of the kidney. If the entire kidney is affected with multiple cysts, the condition is referred to as a multicystic dysplastic kidney disorder (MCDK) . 2–4 Bilateral MCDKs cause oligohydramnios and Potter syndrome and are incompatible with life. Multicystic dysplastic kidney disorder is generally unilateral. 2 In most children with MCDK, a palpable mass is discovered shortly after birth, although small multicystic kidneys may not be apparent until years later. Management is controversial, with complete cyst regression occurring in nearly half of children by age 7 years. 2 The risk of hypertension and Wilms tumors of the affected kidney is approximately 1 in 333. 2 Because of the risk of these neoplasms, annual follow-up with sonography and blood pressure measurement is usually recommended. The most important aspect of follow-up is to make certain that the unaffected kidney is function- ing properly.

Alterations in Kidney Position and Form During embryonic life, the kidneys can develop in an abnormal location, usually just above the pelvic rim or within the pelvis. Because of the abnormal position, kinking of the ureters and obstruction of urine flow may occur. One of the most common alterations in kidney form is an abnormality called a horseshoe kidney . This abnor- mality occurs in approximately 1 in every 500 to 1000 persons. 4,5 In this disorder, the upper or lower poles of the two kidneys are fused, producing a horseshoe- shape structure that is continuous along the midline of the body anterior to the great vessels. Most horse- shoe kidneys are fused at the lower poles (Fig. 25-1). The condition usually does not cause problems because its collecting system develops normally and the ureters enter the bladder. 1 If urinary flow is impeded, signs and symptoms and/or infection may appear. Cystic Diseases of the Kidney Renal cysts are epithelium-lined cavities filled with fluid or semisolid material. The cysts may be single or multiple, can vary in size from microscopic to several centimeters in diameter, and can be symptomatic or asymptomatic. Although they may arise as a developmental abnormality or be acquired later in life, most forms are hereditary. 4,5 The inherited cystic kidney diseases, which are single- gene disorders and are inherited as mendelian traits, include autosomal dominant polycystic kidney disease,

FIGURE 25-1. Horseshoe kidney.The kidneys are fused at the lower pole. (From Jennette JC.The kidney. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2012:757.)