Nephrolithiasis

Overview

Urinary calculi anywhere in tract — COMMONEST cause of obstructive uropathy. Common in US/South Africa/INDIA/SE Asia. ~2% lifetime prevalence, M:F 2:1, peak 2nd-3rd decade. Classic: renal colic (pain as stone passes ureter) + haematuria.

Obstructive uropathy causes (context)

Intraluminal (calculi, tumours, sloughed papilla, clots, foreign body), Intramural (PUJ/vesicoureteric obstruction, urethral stricture/valves, inflammation, neuromuscular dysfunction), Extramural (pregnant uterus, retroperitoneal fibrosis, tumours, prostatic disease, trauma). 3 sequelae: hydronephrosis, hydroureter, bladder hypertrophy.

4 main stone types

1. Calcium stones (~75%, commonest)

Pure oxalate (50%), pure phosphate (5%), mixed (45%).

Etiology: ~50% idiopathic hypercalciuria (no hypercalcaemia); ~10% hypercalcaemia+hypercalciuria (hyperparathyroidism, absorptive/renal hypercalciuria); ~15% hyperuricosuria + normal Ca metabolism; ~25% truly idiopathic.

Pathogenesis: ion supersaturation vs urinary inhibitor imbalance; crystals precipitate on tubular lining/debris NIDUS, grow by deposition. Contributing: alkaline urine, low volume, ↑oxalate/uric acid excretion.

Morphology: small (<1cm), ovoid, hard, granular rough surface, DARK BROWN (old blood pigment, sharp-edge trauma).

2. Mixed (struvite) stones (~15%)

Mg-ammonium-Ca phosphate (“triple phosphate”).

Etiology: INFECTION-INDUCED — urease-producing organisms, mainly PROTEUS (also Klebsiella, Pseudomonas, Enterobacter). E. coli does NOT make urease → doesn’t cause this. Urease splits urea → ammonia → alkaline urine → struvite precipitation.

Morphology: yellow-white/grey, soft, friable, irregular. “STAGHORN STONE” = classic struvite example, takes shape of renal pelvis.

3. Uric acid stones (~6%)

RADIOLUCENT (key distinction from radio-opaque calcium stones — invisible on plain X-ray).

Etiology: hyperuricaemia/hyperuricosuria (primary gout, secondary gout from myeloproliferative disease/leukaemia esp. on chemo, uricosuric drugs — salicylates/probenecid). Acidic urine (pH<6) + low volume contribute.

Pathogenesis: uric acid solubility 200mg/dl at pH7 vs only 15mg/dl at pH5 — acidification directly favours precipitation. HYPERURICOSURIA = dominant factor; hyperuricaemia present in only ~half (urinary excretion matters more than blood level).

Morphology: smooth, yellowish-brown, hard, often multiple, laminated cut section.

4. Cystine stones (<2%)

Etiology: CYSTINURIA — genetic defect in cystine (+other amino acid) transport, renal tubular+intestinal mucosal cells.

Pathogenesis: cystine = LEAST soluble natural amino acid; excess excretion → crystals → stones.

Morphology: small, rounded, smooth, often multiple, yellowish, waxy.

Other rare types (<2%)

Inherited amino acid metabolism disorders — e.g. hereditary xanthinuria → xanthine stones.

Clinical relevance

4 stone types map to 4 distinct upstream processes (Ca metabolism/idiopathic hypercalciuria, urease infection, purine metabolism, specific amino acid transport defect) — stone composition on analysis/imaging is diagnostic of underlying systemic process, not just describes the stone. Struvite = infection-induced (specifically Proteus, NOT E.coli) — clinically actionable: won’t resolve/prevent via metabolic measures alone, eradicating causative organism is central. Uric acid stones’ radiolucency (vs calcium’s radio-opacity) = practical imaging point — normal plain X-ray + classic colic ≠ stone-free, uric acid stones invisible on that modality. pH-dependence of uric acid solubility (13-fold drop pH7→pH5) = direct mechanistic basis for urinary alkalinisation as treatment/prevention — physical chemistry translates directly into rational intervention.

Overview

Nephrolithiasis (urolithiasis) is formation of calculi anywhere in the urinary tract — the most common cause of obstructive uropathy. Particularly common in parts of the US, South Africa, India, and South-East Asia. ~2% lifetime prevalence, male:female 2:1, peak incidence 2nd–3rd decades. Classic presentation: colicky pain (renal colic) as a stone passes down the ureter, with haematuria.

Obstructive Uropathy — Causes (Context)

• Intraluminal: calculi, tumours (kidney/bladder cancer), sloughed renal papilla, blood clots, foreign body.
• Intramural: PUJ obstruction, vesicoureteric obstruction, urethral stricture/valves, inflammation (phimosis, cystitis), neuromuscular dysfunction.
• Extramural: pregnant uterus, retroperitoneal fibrosis, tumours (cervix, rectum, colon, caecum), prostatic disease, trauma.

Three anatomic sequelae of obstruction: hydronephrosis, hydroureter, bladder hypertrophy (see Hydronephrosis).

Four Main Types of Urinary Calculi

1. Calcium Stones (~75%, commonest)

Pure calcium oxalate (50%), pure calcium phosphate (5%), or mixed oxalate-phosphate (45%).

Etiology: ~50% idiopathic hypercalciuria (no hypercalcaemia); ~10% hypercalcaemia + hypercalciuria (hyperparathyroidism, absorptive or renal hypercalciuria); ~15% hyperuricosuria with normal calcium metabolism; ~25% genuinely idiopathic (no identifiable urinary abnormality).

Pathogenesis: imbalance between ion supersaturation and urinary inhibitor concentration; crystals precipitate on tubular lining or around debris acting as a nidus, then grow by progressive crystal deposition. Contributing factors: alkaline urine, low urinary volume, increased oxalate/uric acid excretion.

Morphology: small (<1 cm), ovoid, hard, granular rough surface, dark brown (old blood pigment from repeated sharp-edge trauma to the urothelium).

2. Mixed (Struvite) Stones (~15%)

Magnesium-ammonium-calcium phosphate (“triple phosphate”).

Etiology: infection-induced — urea-splitting, urease-producing organisms, chiefly Proteus (occasionally Klebsiella, Pseudomonas, Enterobacter); notably E. coli does NOT produce urease and does not cause this stone type. Bacterial urease splits urea, releasing ammonia and alkalinising urine, favouring struvite precipitation.

Morphology: yellow-white/grey, soft, friable, irregular. The classic “staghorn stone” — a large, solitary stone taking the shape of the renal pelvis it forms within — is the archetypal struvite stone.

3. Uric Acid Stones (~6%)

Radiolucent — a key practical distinction from radio-opaque calcium stones (invisible on plain X-ray, requiring other imaging).

Etiology: hyperuricaemia/hyperuricosuria (primary gout, or secondary gout from myeloproliferative disease/leukaemia especially during chemotherapy, or uricosuric drugs like salicylates/probenecid); acidic urine (pH <6) and low urinary volume contribute.

Pathogenesis: uric acid solubility falls sharply with acidity — 200 mg/dl at pH 7 versus only 15 mg/dl at pH 5 — so acidifying urine directly favours precipitation. Hyperuricosuria is the dominant factor; hyperuricaemia is present in only about half of cases (worth noting: urinary uric acid excretion matters more than blood level for stone formation).

Morphology: smooth, yellowish-brown, hard, often multiple; laminated on cut section.

4. Cystine Stones (<2%)

Etiology: cystinuria — a genetically-determined defect in cystine (and other amino acid) transport across renal tubular and small intestinal mucosal cell membranes.

Pathogenesis: cystine is the least soluble naturally-occurring amino acid; its excessive excretion in cystinuria directly produces crystal and stone formation.

Morphology: small, rounded, smooth, often multiple, yellowish, waxy.

Other Rare Types (<2%)

Inherited amino acid metabolism disorders, e.g. hereditary xanthinuria producing xanthine stones.

Clinical Relevance

• The four stone types map cleanly onto four distinct upstream metabolic or infective processes (calcium metabolism/idiopathic hypercalciuria, urease-producing infection, purine metabolism, and a specific amino acid transport defect) — recognising a stone’s composition on analysis or imaging is diagnostically informative about the underlying systemic process, not just a description of the stone itself.
• Struvite stones being infection-induced (and specifically Proteus-associated, not E. coli) is a clinically actionable fact — these stones will not resolve or be prevented by addressing metabolic factors alone, since ongoing infection with a urease-producing organism is the actual driver, making eradication of the causative organism central to management.
• Uric acid stones’ radiolucency, in contrast to calcium stones’ radio-opacity, is a practical imaging point with real diagnostic consequences — a patient with classic renal colic and a normal plain X-ray should not be assumed stone-free, since a uric acid stone is invisible on that specific modality.
• The pH-dependence of uric acid solubility (falling roughly 13-fold from pH 7 to pH 5) is the direct mechanistic basis for urinary alkalinisation as a treatment/prevention strategy in uric acid stone formers — this is a case where understanding the underlying physical chemistry translates directly into a rational, mechanistically-grounded clinical intervention.

• Four calculus types compared: incidence, etiology, pathogenesis, and morphology
• Uric acid solubility falling with urinary pH, driving stone formation and its treatment