Renal Calculus
Approximately 2% of the population in the UK have a urinary tract stone at any given time. A much higher prevalence of stone disease has been recorded elsewhere, notably in the Middle East. In the West, most stones occur in the upper urinary tract. The incidence of bladder stones has declined in the UK since the eighteenth and nineteenth centuries, whereas in some developing countries they are still common.
Most stones are composed of calcium oxalate and phosphate; these are more common in men. Mixed infective stones, which account for about 15% of all calculi, are twice as common in women as in men. The overall male:female ratio of stone disease is 2:1.
Stone disease is frequently a recurrent problem. More than 50% of patients with a calculus will have formed a further stone or stones within 10 years. The risk of recurrence increases if a metabolic or other abnormality predisposing to stone formation is present and is not modified by treatment.
Causes of Renal Calculus
It is in a sense surprising that stones are not universal, since some constituents of urine are at times present in concentrations that exceed their maximum solubility in water. The presence of inhibitors of crystal formation in normal urine appears to be of importance in preventing stones.
Many stone-formers have no detectable metabolic defect, although microscopy of warm, freshly passed urine reveals both more and larger calcium oxalate crystals than are found in normal subjects. Factors predisposing to stone formation in these so-called 'idiopathic stone-formers' are:
- chemical composition of urine that favours stone crystallization
- production of a concentrated urine as a consequence of dehydration associated with life in a hot climate or work in a hot environment
- impairment of inhibitors that prevent crystallization in normal urine. Postulated inhibitors include inorganic magnesium, pyrophosphate and citrate. Organic inhibitors include glycosaminoglycans and nephrocalcin (an acidic protein of tubular origin). Tamm-Horsfall protein may have a dual role in both inhibiting and promoting stone formation.
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