C h a p t e r 2 5
Disorders of Renal Function
635
to treat urinary tract infections. Most stones that are
less than 5 mm in diameter pass spontaneously. All
urine should be strained during an attack in the hope of
retrieving the stone for chemical analysis and determi-
nation of type. This information, along with a careful
history and laboratory tests, provide the basis for long-
term preventive measures.
A major goal of treatment in persons who have passed
kidney stones or have had them removed is to prevent
their recurrence. Prevention requires investigation
into the cause of stone formation using urine tests,
blood chemistries, and stone analysis. Underlying
disease conditions, such as hyperparathyroidism, are
treated. Adequate fluid intake reduces the concentra-
tion of stone-forming crystals in the urine and needs
to be encouraged. Calcium supplementation with
calcium salts such as calcium carbonate and calcium
phosphate also may be used to bind oxalate in the
intestine and decrease its absorption. Thiazide diuret-
ics lower urinary calcium by increasing tubular reab-
sorption so that less remains in the urine. Drugs that
bind calcium in the gut (e.g., cellulose phosphate)
may be used to inhibit calcium absorption and uri-
nary excretion.
Depending on the type of stone that is formed,
dietary changes, medications, or both may be used to
alter the concentration of stone-forming elements in the
urine (Table 25-2). For example, persons who form cal-
cium oxalate stones may need to decrease their intake
of foods that are high in oxalate (e.g., spinach, Swiss
chard, cocoa, chocolate, pecans, peanuts). Because of
associated electrolyte disturbances and altered urine
chemistry, obese persons are predisposed to hyperuri-
cemia, gout, hypercitraturia, and uric acid stones.
31
Weight loss may improve or undermine management of
these stones; however, it can be detrimental if associ-
ated with a high animal protein diet, laxative abuse,
rapid loss of lean tissue, or poor hydration. High acid
diets, such as the Atkins diet, increase the risk of uric
acid stones.
Measures to change the pH of the urine also can
influence kidney stone formation. In persons who lose
the ability to acidify or lower the pH of their urine, there
is an increase in the divalent and trivalent forms of urine
phosphate that combine with calcium to form calcium
phosphate stones. The formation of uric acid stones is
increased in acid urine; stone formation can be reduced
by raising the pH of urine to 6.0 to 6.5 with potassium
alkali (e.g., potassium citrate) salts.
In some cases, stone removal may be necessary.
Several methods are available for removing kidney
stones: ureteroscopic removal, percutaneous removal,
and extracorporeal lithotripsy.
28
All of these proce-
dures eliminate the need for an open surgical procedure,
which is another form of treatment. Open stone surgery
may be required to remove large calculi or those that are
resistant to other forms of removal. A nonsurgical treat-
ment, called
extracorporeal shock-wave lithotripsy,
uses
acoustic shock waves to fragment calculi into sandlike
particles that are passed in the urine over the next few
days. Because of the large amount of stone particles that
are generated during the procedure, a ureteral stent (i.e.,
a tubelike device used to hold the ureter open) may be
inserted to ensure adequate urine drainage.
SUMMARY CONCEPTS
■■
Obstruction of urine flow can occur at any
level of the urinary tract. Among the causes
of urinary tract obstruction are developmental
defects, pregnancy, infection and inflammation,
kidney stones, neurologic defects, and prostatic
hypertrophy.
■■
Obstructive disorders produce stasis of urine,
increase the risk for infection and calculi
formation, and produce progressive dilation
of the renal collecting ducts and renal tubular
structures, which cause renal atrophy.
■■
Hydronephrosis refers to urine-filled dilation
of the renal pelvis and calyces associated
with progressive atrophy of the kidney due
to obstruction of urine outflow. Unilateral
hydronephrosis may remain silent for long
periods because the unaffected kidney can
maintain adequate function. With partial
bilateral obstruction, the earliest manifestation
is an inability to concentrate urine, reflected
by polyuria and nocturia. Complete bilateral
obstruction results in oliguria, anuria, and renal
failure.
■■
Kidney stones are a major cause of upper urinary
tract obstruction.The development of kidney
stones is influenced by the concentration of stone
components in the urine, the ability of the stone
components to complex and form stones, and
the presence of substances that inhibit stone
formation.
■■
There are four types of kidney stones: calcium
(i.e., oxalate and phosphate) stones, which are
associated with increased serum calcium levels;
magnesium ammonium phosphate (i.e., struvite)
stones, which are associated with urinary tract
infections; uric acid stones, which are related to
elevated uric acid levels; and cystine stones, which
are seen in cystinuria.Treatment measures depend
on stone type and include adequate fluid intake
to prevent urine saturation, dietary modification
to decrease intake of stone-forming constituents,
treatment of urinary tract infections, measures
to change urine pH, and the use of diuretics that
decrease the calcium concentration of urine.
