Urolithiasis. Etiology & pathogenesis. Types of stones. Clinico-morphological characteristics. Complications
Urolithiasis is calculus formation within the urinary tract, most commonly in the kidney. There are three main types of stones:
- 80% are calcium oxalate (with or without calcium phosphate)
- 10% are magnesium ammonium phosphate
- 6% are uric acid/cystine stones
Staghorn-like calculus from the renal pelvis – the stone is a cast of the pelvis, with branches that grew into the calyces.
Image: “Renal stone” by Dr. Roshan Nasimudeen, CC BY-SA 3.0, via Wikimedia Commons.
Example: magnesium ammonium phosphate (struvite) stones form in alkaline urine infected by urease-producing bacteria such as Proteus, which split urea into ammonia. They can grow into large staghorn calculi that fill the renal pelvis and calyces.
The most important cause of urolithiasis is increased urinary concentration of the stone’s constituents so it exceeds solubility, e.g. many patients with calcium stones have hypercalciuria, and many with uric acid stones have hyperuricemia or hyperuricosuria. Other causes include changes in urine pH and bacterial infection.
Stones are generally unilateral and form in renal pelvis and calyces, often with many stones in one calyx. Clinical presentation is with or without the intense pain known as renal colic (characteristic pain of the flank that radiates towards groin) there may be hematuria as well.
The main complication is urinary flow obstruction that causes hydronephrosis, that is to say dilation of the renal pelvis due to a buildup of fluid. There are two categories:
- Congenital - due to atresia of the urethra, valve formations or other malformations that may cause build up
- Acquired - FBs, proliferative lesions, inflammatory, neurogenic or normal pregnancy
Hydronephrosis can lead to kidney failure as we have a buildup of fluid pressure that prevents glomerular filtration.