Pathogenesis

Stone formation is due to supersaturation, that is, an urinary concentration of the stone's constituent in excess of the solubility in urine.

Predisposing factors include:

• excess of normal constituents in blood and urine:
  • increased serum calcium in hyperparathyroidism
  • increased serum uric acid in gout or following chemotherapy for leukaemia
  • decreased urinary volume in hot climates
    
    
• impaired drainage:
  • chronic urinary obstruction due to an enlarged prostate
  • hydronephrosis
    
    
• presence of abnormal constituents:
  • urinary infection producing epithelial sloughs upon which calculi may deposit
  • foreign bodies such as a urinary catheter

Which of these elements is involved may be suggested by the composition of the stone, but calculi often form in the absence of obvious precipitating factors. In fact many patients with increased urinary excretion of calcium, oxalates or uric acid do not form stones and furthermore, most calculi are unilateral.

Alternative theories suggest:

• abnormal urinary tract mucoproteins may predispose to stone formation
• postulated inhibitors of stone formation may be deficient