Kidney stones grow like crystals in an oversaturated solution
A kidney stone begins much like rock candy: urine carrying more dissolved minerals than it can hold starts seeding tiny crystals, which clump into hard masses. Surgeons in ancient India described removing them around 600 BC, and between 1 and 15 percent of people worldwide develop one at some point.
The medical vocabulary is a Latin and Greek mix: renal and nephro both mean kidney, and calculus and lithiasis both mean stone, hence renal calculi, nephroliths and nephrolithiasis. The condition is old. Sushruta wrote of surgery to extract stones as early as 600 BC. In 2015 there were roughly 22.1 million cases and about 16,100 deaths worldwide.
Chemistry drives the process. Calcium oxalate makes up 70 to 80 percent of stones, followed by calcium phosphate and uric acid. When urine becomes supersaturated, seed crystals form by nucleation, faster when there is a surface to grow on. Acidity matters enormously for uric acid: at pH 7.0 about 158 mg dissolves in 100 mL of urine, but at pH 5.0 less than 8 mg does. Not every crystal becomes a stone, since it must grow big enough to lodge. In 1937 Alexander Randall identified calcium phosphate deposits in kidney tissue, now called Randall's plaques, that are thought to act as anchors.
Genes and surroundings share the blame. Twin studies in 2005 put heritability at 56 percent, and people in hot climates face higher rates because they lose more fluid. Underlying conditions play a part too; 3 to 20 percent of stone formers have medullary sponge kidney, and Crohn's disease raises the odds. Calcium has a paradoxical record: a Women's Health Initiative study found postmenopausal women taking calcium and vitamin D supplements for seven years had a 17 percent higher risk than those on placebo, while calcium eaten in food appears not to raise risk, perhaps because it binds oxalate in the gut.
The pain is notorious. A stone blocking the ureter causes intense, waxing pain spreading from the flank to the groin or inner thigh, because nerve signals get referred as it travels down. Shared embryonic origins of the urinary, genital and digestive systems explain why nausea and vomiting often come too.
Source: Kidney stone disease
