Chapter 40. Nephrolithiasis

• Definitive diagnosis is via passage or removal of stone material; material passed should be retrieved and analyzed to determine composition.
• Diagnosis is also made by radiologic visualization of stones; composition may be inferred from Hounsfield units, but only analysis is definitive.
• Typical history of renal colic is suggestive but not diagnostic in the absence of one of the above.
• Underlying causes of stone formation can usually be determined by an organized evaluation of urine and serum chemistries.

Stones affect males about twice as often as females; the lifetime prevalence of stones in males is 12% and in females 6%, and appears to be increasing in the United States. Calcium oxalate and calcium phosphate stones account for about 80% of all stones passed, uric acid and struvite about 5–10% each, and cystine about 2%. Recurrence rates of common calcium oxalate stones are about 50% at 5–10 years; recurrence of cystine, uric acid, or struvite stones is higher, without treatment. Stones smaller than 5 mm in diameter are usually able to pass spontaneously; stones above that size are less likely to do so, and will often require urologic procedures for removal, such as extracorporeal shock wave lithotripsy (ESWL), or cystoscopic removal.

The final common pathway to stone formation for any stone type is supersaturation of the urine with respect to the components making up the stone. Supersaturation means that the ambient concentrations of the stone materials (for example, calcium oxalate) exceed their solubility, and are therefore high enough to permit crystals to form and grow. Moderate levels of supersaturation, particularly with respect to calcium oxalate, may be tolerated because of the presence of substances that retard crystal formation and growth; inhibitors include glycoproteins, such as inter-α-trypsin inhibitor and osteopontin, glycosaminoglycans, and small molecules such as citrate. Stone formers may lack adequate levels of inhibitors; alternatively, persistently high levels of supersaturation may overwhelm such protection. Supersaturation may result from increased excretion of poorly soluble substances such as calcium, oxalate, or cystine, in addition to other confounding factors, for example, from low urine volume that raises the concentrations of such substances, or in some cases from persistently low or high urine pH, which decreases the solubility of uric acid or calcium phosphate, respectively. The type of stone formed correlates with the supersaturations found in the urine. In general, the urine of stone formers is more supersaturated with stone minerals than is the urine of non-stone formers, and prevention is aimed at lowering supersaturation for the relevant stone components.

Both heredity and environmental factors play a role in stone formation. Cystinuria is an example of a monogenic disorder resulting in stone formation, in which an abnormal dibasic amino acid transporter in the proximal tubule results in stone formation because of decreased renal reabsorption of cystine. There ...