Kidney stones (nephrolithiasis)

Definition

Nephrolithiasis (urolithiasis) is the formation of solid concretions (urinary calculi) in the renal collecting system or urinary tract. They range from microscopic crystals to staghorn calculi several centimeters in size that fill the entire calyceal system. When a stone passes into the ureter, it typically causes ureteral colic.

Classification

Stone types

• Calcium stones (about 85%; calcium oxalate 40–80%, calcium phosphate 18–24% of all stones): calcium-containing and therefore radiopaque
• Uric acid stones (5–10%): radiolucent, mainly with acidic urine (pH < 5.5)
• Struvite stones (magnesium ammonium phosphate, 5–15%): infection stones due to urease-producing bacteria such as Proteus or Klebsiella
• Cystine stones (1–2%): only in cystinuria
• About 40% of stones are of mixed composition.

Occurrence & epidemiology

In the United States, about 1–2% of adults develop kidney stones each year; by age 70, up to 19% of men and 10% of women have formed at least one urinary calculus.

Aetiopathogenesis

Causes and pathophysiology

• Hypercalciuria: the main risk factor for calcium stones (in 50% of men and 75% of women with calcium stones), often familial with normal serum calcium; urinary calcium > 6.2 mmol/day in men or > 5.0 mmol/day in women
• Hypocitraturia in about 40–50% of calcium stone formers; citrate binds urinary calcium and normally impairs crystallization
• Other causes of calcium stones: renal tubular acidosis (more than 10%), primary hyperparathyroidism (up to 5%), hyperoxaluria (primary, dietary or enteric in Crohn disease, ulcerative colitis or malabsorption), sarcoidosis
• Uric acid stones: mainly with acidic urine (pH < 5.5), gout, type 2 diabetes
• Struvite stones (infection stones): urinary tract infection with urease-producing bacteria such as Proteus or Klebsiella; three times more common in women
• General: low fluid intake with concentrated urine, high sodium intake, family history; cystine stones only in cystinuria

Stones tend to lodge at physiologic narrowings: the ureteropelvic junction, the crossing of the iliac vessels and the ureterovesical junction. Stones larger than 5 mm are more likely to lodge, smaller ones more often pass spontaneously. Even partial obstruction reduces glomerular filtration; without infection, permanent damage usually occurs only after about 28 days of complete obstruction.

Clinical features

Stones in the renal parenchyma or calyces often remain asymptomatic. When a stone enters the ureter, renal colic develops: excruciating, wave-like pain usually lasting 20–60 minutes, often with nausea and vomiting. Patients are restless, pace or constantly change position, and appear pale and sweaty. There is costovertebral angle tenderness, but no signs of peritonitis.

• flank pain spreading to the abdomen: upper ureter or renal pelvis
• pain spreading along the ureter to the genital region: lower ureter
• suprapubic pain with urgency and frequency: distal ureteral or bladder stone

Gross or microscopic hematuria is common, but not present in all patients. Fever, dysuria and cloudy urine indicate concomitant infection; an infected obstructed kidney can rapidly lead to urosepsis.

Diagnosis

• Urinalysis: hematuria, possibly pyuria and bacteria as signs of infection, crystals; hexagonal cystine crystals confirm cystinuria. A normal urinalysis does not exclude stones.
• Noncontrast CT (ideally with reduced exposure): the preferred initial study in most patients; shows location, size and degree of obstruction, also nonradiopaque stones and other causes of pain
• Ultrasound (hydronephrosis, kidney stones) and abdominal radiography: with typical symptoms, with little or no ionizing exposure; ultrasound is less sensitive for small ureteral stones without hydronephrosis.
• Stone analysis: collection of the stone by straining the urine and analysis of its composition
• Search for the cause: serum calcium on two occasions (hyperparathyroidism), two 24-hour urine collections in a strong family history or predisposing conditions

24-hour urine collection

• Parameters: volume, calcium, oxalate, citrate, uric acid, cystine, sodium, pH.
• Classic: hypocitraturia as common cause.

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Further reading (open access)

1. MSD Manual Professional: Urinary Calculi
2. AWMF-Leitlinienregister 043-025: Urolithiasis (S2k, Diagnostik und Metaphylaxe)

Cross-references