Prone to both uric acid and calcium oxalate stones. Mechanisms for CaOx formation:
- Calcium oxalate stones form through heterogenous nucleation (epitaxy)
- Urate can adsorb stone inhibitors in urine
- Solubility of calcium oxalate decreases with increasing concentrations of uric acid
Uric acid stones from in acidic urine (solubility of uric acid reduces with lower pH).
Requirements for uric acid stone formation:
- Low urine pH / acidic urine (pH < 5.5)
- Low urine volume
- Hyperuricosuria
Causes of hyperuricosuria:
- Dietary excess of purines (protein gluttony)
- Enzyme defects / endogenous overproduction
- Myeloproliferative disorders
- Chemotherapy and tumour lysis
- Catabolism
- Hyperuricaemia (associated with gout – up to 20 % patients with gout develop uric acid stones)
Urinary acidity causes:
- Insulin resistance – causes reduced urinary excretion of ammonium and increased endogenous acid production
- Acid intake (high PRAL)
- Loss of base (diarrhoea)
- T2DM, obesity and metabolic syndrome all linked to acidic urine and uric acid stones
Treatment:
- Low purine and low PRAL diets
- Overhydration
- Identify and stop offending medications
- Weight loss and diabetes management
- Allopurinol (xanthine oxidase inhibitor)
- Febuxostat
- Alkalinisation
- Sodium bicarbonate (potentially high sodium load may precipitate calcium stones)
- Potassium citrate
- For prevention, aiming for pH 6.2 – 6.8
- For dissolution, aiming for pH 6.5 – 7.2 (EAU)