General Principles

Nephrolithiasis is more common in men than women by a 2:1 ratio, with a peak age at the third to fourth decade. There are certain medical conditions that predispose patients to kidney stones, including diabetes mellitus, hypertension, metabolic syndrome, distal renal tubular acidosis, gout, and ADPKD.

• Calcium-based stones are the most common type of kidney stones (80%). Among these, the most common type is mixed calcium oxalate and calcium phosphate followed by calcium oxalate alone, and then calcium phosphate alone. These stones are radiopaque. Calcium oxalate stones can be found in acidic or alkaline urine and can be dumbbell shaped or appear as paired pyramids (giving them an envelope appearance when viewed on end). Calcium phosphate stones can appear as elongated, blunt crystals and form in alkaline urine.
• Uric acid stones (10%) develop in conditions that promote an acidic urine, such as what is observed in patients with the metabolic syndrome. Hyperuricosuric states such as gout and myeloproliferative disorders are also associated with uric acid stones, though the predominant risk factor for their precipitation is an acidic environment. These stones are radiolucent, and the crystals can exhibit a variety of shapes, with needles and rhomboid forms being the most common.
• Struvite stones (10%) are also known as “triple phosphate” stones, with phosphate being present in its trivalent form and combining with three cations, ammonium, magnesium, and calcium. They are radiopaque and can extend to fill the renal pelvis, taking on a staghorn configuration. On microscopy, struvite crystals have a characteristic coffin-lid shape. They develop in alkaline urine associated with urea-splitting organisms (e.g., Proteus, Klebsiella, Serratia, Haemophilus, Pseudomonas) and are more commonly seen in patients with anatomic abnormalities (e.g., vesicoureteral reflux, obstruction of the pelviureteric junction, ureteral stricture).
• Cystine stones (<1%) are uncommon and form as a result of an autosomal recessive disorder, in which the renal epithelium has a decreased ability to reabsorb the dibasic amino acids cystine, ornithine, lysine, and arginine. Among these, only cystine is highly insoluble and precipitates to form stones in acidic urine. These stones have an intermediate radiolucency and appear as hexagonal crystals in the urine.