Pyelonephritis (PN)
PN is a renal disorder affecting tubules,
intrestitium, and renal pelvis and is one of the most common diseases of the
kidney. The term
urinary tract infection (UTI) implies involvement of either the bladder
(cystitis) or the kidney and their collecting system (pyelonephritis), or both.
UTIs are extremely common disorders.
It occurs in two forms:
1. Acute PN is acute pyogenic infection.
2. Chronic PN is a more complex disorder: bacterial infection plays a
dominant role, but other factors (vesicoureteral reflux, obstruction) are
involved in its pathogenesis.
Etiopathogenesis
·
The dominant etiologic agents are the gram-negative
bacilli that are normal inhabitants of the intestinal tract: E.coli (Proteus,
Klebsiella and Enterobacter), Str. fecalis etc.
·
In most patients with UTI, the infecting organisms are
derived from the patient’s own fecal flora. This is
thus a form of endogenous infection.
·
There are two routs by which bacteria can reach the
kidneys:
a) Through the bloodstream
(hematogenous).
b) From the lower urinary tract
(ascending infection).
·
Although obstruction is an important predisposing
factor in the pathogenesis of ascending infection, it is incompetence of the
vesicoureteral orifice that allows bacteria to ascend the ureter into the
pelvis.
Acute Pyelonephritis
Morphology
·
The hallmarks of acute PN are patchy interstitial
suppurative inflammation and tubular necrosis.
·
Macroscopically, the kidneys show variable numbers of small, yellowish white
cortical abscesses, which are usually spherical, under 2 mm in diameter, and
are sometimes surrounded by a zone of hyperemia; 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
hyperemic or covered with a fibrinopurulent exudate.
·
Histologically: the neutrophilic infiltration is limited to the
interstitial tissue. Some tubules destroyed: abscesses formed; other tubules
filled by puss cells. Glomeruli usually unaffected.
·
Clinical features. Classically, acute pyelonephritis has an acute onset with chills,
fever, loin pain, lumbar tenderness, dysuria and frequency of micturition.
Urine will show bacteria, pus cells and pus cell casts in the urinary sediment.
·
Three complications of acute PN are encountered in
special circumstances.
- Papillary necrosis is seen mainly
in diabetics and in those with urinary tract obstruction. Papillary
necrosis is usually bilateral, but may be unilateral.
- Pyonephrosis is seen when
there is total or almost complete obstruction, particularly when it is
high in the urinary tract (pelvis filled with puss).
- Perinephric abscess implies
extension of suppurative inflammation through the renal capsule into the
perinephric tissue.
·
At the acute phase of PN, healing occurs. The
neutrophilic infiltration is replaced by macrophages, plasma cells, and (later)
lymphocytes. The inflammatory foci are eventually replaced by scars. The
pyelonephritic scar is almost always associated with inflammation, fibrosis,
and deformation of the underlying calyx and pelvis.
·
Uncomplicated acute PN usually follows a benign
course, and the symptoms disappear within a few days after the institution of
appropriate antibiotic therapy. In the presence of unrelieved urinary
obstruction, diabetes mellitus acute PN may be more serious, leading to
repeated septicemic episodes.
Chronic
Pyelonephritis (CPN)
Chronic PN is a chronic tubulointerstitial renal disorder in which
chronic tubulointerstitial inflammation and renal scarring are associated with
pathologic involvement of the calyces and pelvis.
Etiopathogenesis
Two types of chronic pyelonephritis are described:
·
Reflux nephropathy. Reflux of urine
from the bladder into one or both the ureters during micturition is the major
cause of chronic pyelonephritis. Vesicoureteric reflux is particularly common
in children, especially in girls, due to congenital absence or shortening of
the intravesical portion of the ureter so
that ureter is not compressed during the act of micturition. Reflux results in
increase in pressure in the renal pelvis so that the urine is forced into renal
tubules, which are eventually followed by damage to the kidney and scar
formation.
·
Obstructive
pyelonephritis. Obstruction to the
outflow of urine at different levels predisposes the kidney to infection.
Recurrent episodes of such obstruction and infection result in renal damage and
scarring.
Morphology
·
Gross examination. The kidneys are usually small and contracted (weighing less
than 100 gm) showing unequal reduction; if bilateral, the involvement is
asymmetric. The surface of the kidney is irregularly scarred; the capsule can
be stripped off with difficulty due to adherence to scars. There is generally
dilatation of pelvis and blunted calyces. This contrasts with chronic
glomerulonephritis, in which the kidneys are diffusely and symmetrically
scarred.
·
The microscopic
changes involve predominantly tubules and interstitium.
·
The tubules show atrophy in some areas and hypertrophy in others, or
dilatation. Dilated tubules may be filled with colloid crystals, producing thyroidisation of
tubules (thyroid-like).
·
Interstitium. There is chronic interstitial inflammatory reaction,
chiefly composed of lymphocytes, plasma cells and macrophages with pronounced interstitial fibrosis. Xanthogranulomatous pyelonephritis is an
uncommon variant characterised by collection of foamy macrophages admixed with other inflammatory cells and giant cells.
·
Pelvicalyceal system. The renal pelvis and calyces are dilated. And show marked
chronic inflammation and fibrosis.
·
Blood vessels. Blood vessels entrapped in the scarred areas show
obliterative endarteritis.
·
Glomeruli. There is often
periglomerular fibrosis. In advanced
cases, there may be hyalinisation of glomeruli.
·
Clinical features. Chronic pyelonephritis often has an insidious onset.
The patients present with clinical picture of chronic renal failure or with
symptoms of hypertension.
·
Chronic obstructive
PN may be insidious in onset or may present the clinical
manifestations of acute recurrent PN with back pain, fever, frequent pyuria,
and bacteriuria.
Infections of the lower
urinary tract
·
Infections in the lower urinary tract are predisposed by
obstruction and stasis.
·
Lower urinary tract infection is usually due to
Gram-negative coliform bacilli, e.g. E. coli and Proteus, which are normally in
the large bowel; because they have a short urethra, women are particularly
prone to developing ascending infections.
·
In men, lower urinary tract infection is usually
associated with structural abnormalities of the lower urinary tract and stasis
due to obstruction.
·
Diabetes mellitus also predisposes to infection.
Morphology
·
The pelvicalyceal system is dark reddish brown as a
result of acute inflammation of the usually smooth creamy mucosal lining due to
bacterial infection.
·
The kidney is also congested and some small scattered
abscesses are present in the cortex and medulla (acute pyelonephritis).
·
Obstruction of the drainage of urine from the kidney
causes hydronephrosis.
·
Obstruction, one of the most important consequences of
disease of the lower urinary tract, may occur at any place in the tract: renal
pelvis (calculi, tumors), pelviureteric junction (stricture, calculi, extrinsic
compression), ureter (calculi, extrinsic compression -pregnancy, tumor,
fibrosis), bladder (tumor, calculi); urethra (prostatic hyperplasia or
carcinoma, urethral valves, urethral stricture).
·
If obstruction occurs in the urethra, the bladder
develops dilatation and secondary hypertrophy of muscle in its wall. This
predisposes to development of out pouching of the bladder mucosa
(diverticulae).
·
If obstruction occurs in a ureter, there is dilatation
of the ureter (megaureter), with progressive dilatation of the renal
pelvicalyceal system, termed hydronephrosis. Fluid entering the collecting
ducts cannot empty into the renal pelvis and there is intrarenal resorption of
fluid. At this stage, if the obstruction is relieved, renal, function returns
to normal. However, if obstruction persists, there is atrophy of renal tubules,
glomerular hyalinization, and fibrosis. As an end-stage, the renal parenchyma
becomes severely atrophic and renal function is permanently impaired.
·
Urinary tract obstruction also predisposes to
infection and stone formation.