Who gets kidney stones?
Urinary stones have afflicted humans for centuries; the first reported cases are of bladder and renal calculi found in Egyptian mummies dated 4800BC. The prevalence of the disease is between 2 and 3% and it is estimated that the likelihood of a Caucasian developing a stone by the age of 70 is approximately 1 in 8. Acute renal colic is a common, often recurrent condition with an annual incidence of 1-2 cases per 1000 and a lifetime risk of 10-20% for men and 3-5% for women. The following factors are known to play a role in kidney stone formation:
Approximately 25% of patients with renal stones have a family history of stone disease and it is possible that urolithiasis is a polygenic defect with partial penetrance. Several disorders that have a clear genetic basis do predispose to stone formation (renal tubular acidosis, cystinuria, xanthinuria).
Age and sex
Men suffer more stones than females (3:1). The peak incidence is between 20 and 40 years of age. Stones in childhood are usually predisposed to by urinary infections, a metabolic defect or an anatomical abnormality.
The prevalence of stone disease is highest among those living in mountainous regions, deserts and tropical areas. However the capability of individuals to carry their stone risk from area to area suggests that the major tendency to stone formation resides in the individual. Geography represents just one aspect of the other environmental factors - such as dietary habits, temperature and humidity - superimposed on the intrinsic factors that predispose to stone formation.
Climactic and seasonal factors
There is an association between stone formation and environmental temperature (this may partly explain the geographical variations) with stone formation being more common in the summer months. This tendency may be due to relative dehydration and the subsequent production of concentrated, acidic urine. Alternatively some workers suggest the increased exposure to sunshine leads to increased urinary calcium excretion via the increased production of vitamin D.
The dietary intake of certain food and fluid substances that lead to a higher urinary excretion of substrates that form stones increases the incidence of calculi. A high intake of protein, oxalates, calcium, phosphates and other elements often lead to an excess excretion of them in urine. However, a reduced calcium diet can increase the risks of further stone formation. It is therefore important that the diet is balanced and a relative excess or total abstinence of these constituents is avoided.
Urinary stones occur when offending salt crystallises in the urine and this occurs when the urine is supersaturated with the substance. Calcium and oxalate crystallisation can be modified by the presence of so-called urinary inhibitors (i.e. citrate, magnesium). However, the majority of patients have crystals in their urine but rarely do these form calculi and therefore other factors are important in stone formation. These include other inhibitors of crystallisation, and aggregation namely pyrophosphate, glyosaminoglycans, nephrocalcin and Tamm Horsfall proteins. Crystal aggregation and retention are necessary to lead to stone formation and therefore anatomical abnormalities (medullary sponge kidney, pelviureteric junction obstruction, calyceal diverticulae) increase the risk of stone formation. Bacterial infection is well known to predispose to stone formation by alteration of urinary PH, production of urease and perhaps by increasing urinary matrix production. Finally, abnormal intra and inter cellular calcium transport may lead to crystal retention in the tubules acting as foci for crystal aggregation, retention and eventual stone formation.