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U N I T 7
Kidney and Urinary Tract Function
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.)
