Pyelonephritis

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Acute pyelonephritis

Acute pyelonephritis is caused by bacterial infection with organisms entering the kidney by two routes:
Ascending infection from the lower urinary tract (most common). Predisposing factors that lead to
ascending urinary tract infection are pregnancy, diabetes mellitus, stasis of urine, structural defects of the urinary tract, and reflux of urine from bladder into ureters (vesicoureteric reflux).
Bloodstream spread in bacteraemic or septicaemic states (unusual). Although less common,
this seems to be the most likely cause in elderly patients who develop pyrexia of unknown origin,
often with rigors, and acute renal failure.

Macroscopically the kidneys show variable numbers of small, yellowish white cortical abcesses, which
are usually spherical, under 2 mm in diameter, and are sometimes surrounded by a zone of hyperaemia;
the cortical abscesses are often most prominent on the sub-capsular surface, after the capsule has been
stripped away. In the medulla the abscesses tend to be in the form of yellowish
white linear streaks that converge on the papilla.

The pelvicalyceal mucosa is hyperaemic or covered with a fibrinopurulent exudate. Histologically the
kidney shows focal infiltration with neutrophils.

Chronic pyelonephritis

Chronic pyelonephritis is characterized by chronic interstitial inflammation associated
with large scars of the kidney
Chronic pyelonephritis is a common cause of end-stage chronic renal failure, accounting for
about 15% of all cases. The disease is characterized by interstitial chronic inflammation and scarring,
which destroys nephrons. The areas of scarring are associated with distortion of the pelvicalyceal
system of the kidney. Renal-induced hypertension may develop and hypertensive-induced vascular
damage can increase renal damage.
There are two forms of chronic pyelonephritis:

reflux-associated and obstructive. In the most common form, reflux-associated chronic
pyelonephritis, reflux of urine from the bladder up the ureters predisposes to recurrent bouts
of inflammation, leading to scarring. This occurs in childhood, and disease becomes manifest
in early adult life.
In obstructive chronic pyelonephritis, recurrent episodes of infection occur in a kidney
in which there is obstruction to the pelvicalyceal drainage. The obstruction, which can be at any
level in the lower urinary tract, may be due either to anatomical abnormality or to renal tract stone.
Kidneys have irregular areas of scarring, seen as depressed areas, 1-2 cm in size. The scars
are sited over a club-shaped distorted renal calyx and are associated with fibrous scarring of the
renal papilla. The most common site for these areas of scarring is the renal calyces
at the poles of the kidney.
Histologically the kidney has irregular areas of interstitial fibrosis with chronic inflammatory
cell infiltration. Tubules are atrophic or may be dilated and contain proteinaceous material.
Glomeruli show periglomerular fibrosis and many demonstrate complete hyalinization.


Tuberculous pyelonephritis

Tuberculous pyelonephritis may lead to destruction of the whole kidney
Tuberculous pyelonephritis is characterized by white caseous material filling the
pelvicalyceal system, which may be unilaterally or bilaterally affected.
Infection is initially renal but, over a period of months or years, enlarges and
ruptures into the pelvicalyceal system, releasing tubercle bacilli into the lower urinary tract.
This can lead to the development of tuberculous ureteritis, cystitis and, in the male,
prostatitis and epididymo-orchitis.

With time, extension of caseous granulomatous inflammation leads to destruction of cortex
and medulla, so that at the end-stage of the disease the kidneys are reduced to cystic masses
of partially calcified caseous material; if both kidneys are affected,
chronic renal failure results.
This pattern of renal involvement in TB is distinct from renal involvement in rapidly
progressive miliary TB, in which the kidney is just one of many organs that receive large
numbers of tubercle bacilli, spread from a fulminating lung infection. In miliary TB
there are very large numbers of small tuberculous granulomas scattered throughout both kidneys.
The patient usually dies before the individual granulomas can enlarge and show much caseation.
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