KIDNEY STONES

Nephrolithiasis

Kidney stones are crystals that form in the urine. They usually precipitate in the renal pelvis and then get trapped in the ureters. Since kidney stones are sharp and jagged, getting lodged into the ureteral wall is intensely painful and can cause some light hematuria. Every time a peristaltic wave is generated in the ureters, the unilateral flank pain flares up. That’s why the pain is described as colicky (comes and goes). Patients are often described as squirmy and unable to get comfortable. Over the course of several (excruciating) hours and days, the stone tumbles down towards the bladder and urethra (the urethra is usually a little wider than the ureters, so passage through it is less painful). The ureteral lumens are pretty small (roughly 4mm). If a stone is larger than the lumen, then it will be trapped forever! It turns into a very painful hydraulic dam. Urine made by the kidney will continue to build up behind the stone, dilating the proximal ureter and renal pelvis (this is called hydronephrosis, or “water kidney”). Sometimes hydronephrosis provides the extra burst of pressure needed to push the stone out. Treatment depends on the stone. Small stones will pass on their own after a few hours or days. Selective alpha-1 agonists (Tamsulosin) can dilate the ureter to facilitate passage. Larger stones require surgery or lithotripsy (low frequency ultrasonic pulverization of the stone). Staghorn stones must be treated surgically.  

Kidney stones are crystals. Crystals are created (or precipitated) when the concentration of a mineral in the urine gets high enough. Keep in mind that the concentration depends on two variables, the amount of the solute and the amount of water. That’s why dehydration always promotes crystal formation. 

All patients with a kidney stone will complain of severe unilateral flank pain. Begin their workup by getting a UA to screen for hematuria (1+ hematuria is 90% sensitive). The next step is radiology. There are a few options. The most definitive imaging is with a CT scan. You’ll be looking for a tiny bright white blip in the abdomen. Make sure to get a non-contrast CT scan, because the bright contrast could obscure the white stone. Another semi-reasonable option is with an ultrasound, looking for hydronephrosis. It’s good at finding big and proximal stones. In the olden days, the only choice was with an abdominal x-ray (known as a KUB scan or kidneys, ureter and bladder). The sensitivity was crummy because a lot of stones are radiolucent, or invisible on an x-ray. 

I’ll introduce four kinds of stones

Struvite

Struvite stones are made up of Ammonium, Magnesium and Phosphate. They are the second most common stone (15%). They are a consequence of UTIs (the letters UTI are found in Struvite) caused by a Urease (+) organism (Proteus, Klebsiella). These bacteria use Urease to turn urea into ammonia, which makes the urine less acidic. Struvite stones can grow into what is called a Staghorn calculi, which is a massive stone that fills the entire renal pelvis (the branching calyces look like a stag’s antlers). Staghorns are a bacterial reservoir, and will cause recurrent pyelonephritis unless surgical excision is performed.