RENAL CONSULTS
NEPHROLITHIASIS 80% are Ca stones, usually CaOx or, less often, CaP, so reasonable to assume Ca-containing. Rarer types: uric acid (<10%), struvite (Mg-NH4-PO4), cystine (cystinuria: hexagonal crystals and urinary cystine >250 mg/l).
RISK FACTORS FOR CALCIUM-CONTAINING KIDNEY STONES Etiology Urinary
Specific Cause Lower volume Higher Ca Higher oxalate (CaOx stones)
Lower citrate
Higher pH (CaP stones) Hyperuricosuria
Comments
Increases calcium and oxalate concentration. Def: >0.1 mmol/kg/day. Seen in 50% of Ca stone formers. Usually idiopathic, consider primary hyperPTH or chronic acidaemia. Causes haematuria in the absence of stone. Def: > 0.5 mmol/day. Mainly results from reduced intestinal binding by Ca. Seen in enteric oxaluria (malabsorption of fatty acids and bile salts from malabsorption syndromes, Crohn's, bowel resection or diversion, jejunoileal bypass, bariatric surgery, CF). Also low dietary Ca or increased intestinal Ca absorption. Markedly increased levels in primary hyperoxaluria and Vit C toxicity. Def: <2.5 mmol/day. Citrate inhibits stone formation. Consider chronic acidosis from diarrhoea, RTA, carbonic anhydrase inhibitors (topiramate), ureteral diversion. Hypokalemia can trigger hypocitraturia. Alkaline urine (UTI, RTA, high dietary alkali intake) promotes CaP. CaOx stones not pH-dependent in physiologic range. Hyperuricosuria not a risk factor for Ca stones; consider urate stones.
Anatomic
Medullary sponge kidney Marked increase in Ca stones, esp. if hypercalciuria or hypocitraturia. Present in 12-20 % of calcium stone formers, and 20-30 % of women or < 20 years.
Diet
Lower fluid intake
Lower dietary Ca Higher oxalate Lower potassium High animal protein Higher sodium Higher sucrose Lower phytate Medical
Grapefruit juice is associated with an increased risk of stones. Coffee and tea: lower risk by 10 % per cup per day. Beer and wine: lower risk of stone formation. Increases risk, possibly by reduced binding of dietary Ox. Contribution of dietary intake is disputed. Effect of GI disease and intestinal Ca binding may be more NB. Response to dietary restriction variable. Higher dietary K reduces risk by reducing urinary Ca and increasing urinary citrate (K-rich foods often high alkali). DASH diet reduces risk. Increased stone risk in men via hypercalciuria and decreased urine citrate. Enhances excretion of urinary Ca. Increased stone risk in women. Increased dietary phytate (cereal, dark bread, beans) lowers risk.
Primary hyperPTH HyperCa often mild and intermittent. Pts prone to CaP stones. Distal (type 1) renal CaP stones secondary to hypercalciuria, hypocitraturia and high urine pH (>6.0) tubular acidosis Incomplete distal RTA causes hypocitraturia without overt metabolic acidosis. Suspect if urine pH > 5.3 plus low/normal serum tCO2 or nephrocalcinosis. Family history Positive FHx = RR of 2.6 for stones, heritability 50%. Dent's disease: X-linked recessive Cl channel mutation causing hypophosphataemia and hypercalciuria. RENAL HANDBOOK 51