Epidemiology

Urinary stones are the third most common affliction of the urinary tract, superseded only by infection and prostatic pathologies. The incidence of calculi is increasing, with prevalence rates in countries such as the United States, Sweden, and the UK more than 9%. Men have an increased risk of urolithiasis compared to women; however, this difference in incidence is reducing. Stones can occur in all ages; however, the peak age is approximately 45. The risk of stone formation has shown correlation with body mass index and with certain diseases including diabetes mellitus and cardiovascular disease.

Aetiology

Urinary stones have been affecting humans, and dogs, for civilisations. Despite this, much of the aetiology of urolithiasis is unknown. Stone formation comprises a complex cascade. Urine becomes supersaturated with stone forming salts, with a resultant precipitation out of solution, forming crystals or nuclei. These crystals can be retained within the kidney at anchoring sites that promote growth and aggression and resultant stone formation.

Stone formation is related to supersaturation of urine. The solubility product is the concentration product a solution reaches where no further added salt crystals will dissolve. Below the solubility product, urine is undersaturated and crystals do not form. Above the solubility product, crystals should form, but don’t because of inhibitors in urine. Above a certain concentration, inhibitors become ineffective, urine is supersaturated, and the concentration of solute at which this is reached (crystallisation starts) is the formation product. Urine is metastable between the solubility product and formation product.

Supersaturation explains the formation of crystals in static solutions, but it cannot explain crystal formation in urine as it traverses through the nephron. Crystals form (nucleation) and if they aggregate together, they may retain in the lumen and grow (free particle theory). Alternatively, they may attach to damaged tubular surfaces (fixed particle theory).

Symptoms

Ureteric stones commonly present with sudden onset severe flank pain. The pain is commonly colicky (waves of increasing severity followed by reduced severity pain) and may radiate from the loin to the groin.

Signs