Describe acute and chronic pyelonephritis. Add a note on chronic kidney disease and its systemic consequences.

Urinary Tract Infection and Acute Pyelonephritis

• Predisposing factors — female sex (a short urethra), urinary obstruction, catheterisation and instrumentation, vesicoureteric reflux, diabetes, pregnancy, calculi, immunosuppression
• Morphology of acute pyelonephritis — patchy suppurative inflammation with abscesses, and characteristic neutrophil casts within tubules; the glomeruli are typically spared
• Complications — papillary necrosis (especially in diabetes, obstruction and analgesic abuse), perinephric abscess, pyonephrosis, and septicaemia

Chronic Pyelonephritis and Reflux Nephropathy

• Chronic tubulointerstitial inflammation with scarring that involves the pelvis and calyces — this involvement of the collecting system is what distinguishes it from other causes of interstitial scarring
• Two forms — reflux-associated (the commoner; begins in childhood with vesicoureteric reflux and infection) and obstructive
• Naked eye — irregular, asymmetrical, coarse, U-shaped cortical scars overlying dilated, blunted, deformed calyces, characteristically at the upper and lower poles where compound papillae permit intrarenal reflux
• Microscopy — interstitial fibrosis and chronic inflammation; tubular atrophy with "thyroidisation" (dilated tubules containing pink colloid-like casts); periglomerular fibrosis; and later secondary focal segmental glomerulosclerosis

Chronic Kidney Disease

• Leading causes — diabetic nephropathy (the commonest worldwide and in India), hypertensive nephrosclerosis, chronic glomerulonephritis, chronic pyelonephritis and reflux nephropathy, polycystic kidney disease, obstructive uropathy
• CKD of unknown aetiology is a recognised problem in agricultural communities in parts of India and Sri Lanka, and is under active investigation

Consequences of Chronic Kidney Disease

Renal Papillary Necrosis

• Ischaemic coagulative necrosis of the renal papillae, which are supplied by the vasa recta and lie in a relatively hypoxic medulla
• Four classical causes — mnemonic "postcards" in full, but the four to state are: pyelonephritis, obstruction, sickle cell disease, and diabetes; plus analgesic abuse (phenacetin, NSAIDs)
• Naked eye — grey-white to yellow necrotic papillae with a sharp demarcation from viable tissue; sloughed papillae may be passed in the urine
• Presents with fever, loin pain, haematuria and sometimes ureteric colic from a sloughed papilla, which can obstruct and be mistaken for a stone
• "Ring shadow" or "signet ring" sign on urography where contrast surrounds the detached papilla

Applied Aspects

• Distinguish acute from chronic — small kidneys on ultrasound, anaemia, renal bone disease and a long history indicate chronic disease. The exceptions with normal or large kidneys despite chronic disease are diabetes, amyloidosis, polycystic disease, HIV and obstruction
• Slow the progression — strict blood pressure control, ACE inhibitors or ARBs (which reduce intraglomerular pressure and proteinuria), glycaemic control, treatment of the cause, and avoidance of nephrotoxins
• Screening in diabetes and hypertension by annual urine albumin:creatinine ratio and eGFR — cheap, and detects disease while it is still modifiable
• Vesicoureteric reflux in children must be detected early, since scarring is largely established before the age of 5; prompt treatment of urinary infection in a child, with imaging, prevents a lifetime of renal impairment
• Never ignore urinary infection in pregnancy, including asymptomatic bacteriuria, which progresses to pyelonephritis in up to 30% and causes preterm labour
• Renal replacement therapy is expensive and unevenly available in India, which makes prevention and early detection far more valuable than treatment
• Anaemia of chronic kidney disease is treated with erythropoietin and iron, targeting a haemoglobin around 10 to 11 g/dL — full correction increases thrombotic risk
• Phosphate binders, vitamin D analogues and dietary phosphate restriction manage renal bone disease; hyperphosphataemia also drives vascular calcification
• Vaccinate against hepatitis B early, since response falls as renal function declines and dialysis carries a transmission risk
• Refer early to nephrology — late referral is associated with worse outcomes, emergency dialysis and lost opportunities for transplantation
• Emphysematous pyelonephritis — gas-forming infection almost exclusively in diabetics; a surgical emergency with a high mortality
• Xanthogranulomatous pyelonephritis — a chronic destructive infection with lipid-laden foamy macrophages forming a mass that mimics carcinoma; associated with Proteus and staghorn calculi
• Treat asymptomatic bacteriuria only in pregnancy and before urological instrumentation; treating it otherwise breeds resistance without benefit
• Renal tuberculosis must be considered in India with sterile pyuria, and requires early morning urine samples for acid-fast bacilli and culture
• Analgesic nephropathy from long-term combined analgesic use causes chronic interstitial nephritis with papillary necrosis, and predisposes to urothelial carcinoma
• Dietary protein and salt restriction reduce symptoms and slow progression, but must be balanced against the risk of malnutrition, which itself predicts poor outcome on dialysis