Acute and chronic pyelonephritis
Acute pyelonephritis is one of the commonest subtype of tubulointestitial renal diseases. There is acute inflammation in the renal tubules and the interstitium due to an infectious cause. The infection settles in the renal parenchyma leading to acute inflammation with neutrophils and edema in the interstitium. Then the neutrophils invade the renal tubules causing tubulitis, renal tubular destruction and formation of microabscesses in the renal parenchyma. Next the infection spreads through the entire renal parenchyma, usually sparing the glomeruli.
Macroscopic appearance of acute pyelonephritis:
Surface of the kidney is dull and opaque. Small multiple subcapsular abscesses may be seen. Cut surface of the kidney shows multiple small abscesses or streaks of pus in the renal cortex. Features of complications may or may not be present.
Microscopic appearance of acute pyelonephritis:
Edema and neutrophil infiltration of the interstitium can be seen under the light microscope. Also there is tubulitis, microabscesses formation and pus cell casts in the renal tubules.
Complications of acute pyelonephritis:
- Renal papillary necrosis.
- Pyonephrosis - accumulation of purulent exudate in the kidney. This is usually due to associated complete or almost complete obstruction in the pelvis/ureters preventing drainage of the exudate.
- Perinephric abscesses – extension of suppurative exudate through the renal capsule into the perinephric tissue.
In most patients, acute pyelonephritis follows an uncomplicated course with treatment. However, patients with obstruction, diabetes mellitus and immunodeficiency can have complications.
- Fever (usually high fever with chills and rigors).
- Pain in the costovertibral angle on the affected side.
Presence of pus cell casts in urine analysis will show renal involvement. In lower urinary tract involvement there is only pus cells, no casts.
Chronic pyelonephritis is characterized by chronic inflammatory infiltration of the interstitium, interstitial scarring and tubular atrophy. In advanced cases there is glomerular sclerosis secondary to severe tubular involvement. Chronic pyelonephritis frequently progresses into end stage renal failure.
Chronic pyelonephritis could be due to two main causes:
- Reflux nephropathy - associated with chronic vesicoureteric reflux and intrarenal reflux. Recurrent acute pyelonephritis lead to chronic pyelonephritis eventually.
- Chronic obstructive pyelonephritis - obstruction in the renal tract (e.g, calculi, posterior urethral valves) predisposes to infection. Here too recurrent acute pyelonephritis lead to chronic pyelonephritis.
Affected kidney is smaller in size (contracted) and the surface shows coarse scarring. If both kidneys are affected appearance is asymmetrical. Cut sections of the kidney show calyces are deformed and blunted. Coarse scarring involving the corticomedullary regions, and more prominent in upper and lower poles (if reflux associated). In obstruction associated cases there could be features of hydronephrosis too. (Compare the macroscopic features of chronic pyelonephritis with chronic glomerulonephritis)
Microscopy of chronic pyelonephritis:
Interstitium will show chronic inflammatory infilatration and fibrosis.There will be renal tubular atrophy with accumulation of secretions giving rise to thyroid like appearance – thyroidization.
The glomeruli are relatively spared. But there can be periglomerular sclerosis and in advanced cases glomerular sclerosis. The blood vessels usually show hypertension associated vascular changes (hyaline arteriolosclerosis).
Usually the patients are asymptomatic and patient may eventually present with features of end stage renal disease such as frothy urine, oliguria or anuria, generalized edema, fatiguability and uremic symptoms. Asymptomatic patients may also be picked by when investigating for hypertension, presence of proteinuria in routine urine examinations. But the patient could show episodes of features of acute pyelonephritis.
A type of chronic pyelonephritis characterized by presence of a chronic inflammatory infiltration with numerous foamy macrophages. Macroscopically they can form a tumor like mass mimicking renal cell carcinoma. Xanthogranulomatous pyelonephritis is associated with diabetes mellitus and obstruction related infections caused by proteus.
Toxic tubulointerstitial nephritis /nephropathy
Usually the appearance is nonspecific inflammation and fibrosis in the interstitium with tubular changes. Severe cases may even look like chronic pyelonephritis. Some cases may have interstitial fibrosis without significant inflammation. Toxins could be drugs or heavy metals.