AssessmentsKidney stones: Clinical practice
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 38-year-old woman comes to the office because of ongoing urinary frequency, urgency, and dysuria. Patient’s medical history includes recurrent urinary tract infections, with about four to six each year for the last three years. She says that her symptoms typically resolve with antibiotic use, but will return once she stops using the antibiotics. Urinalysis is performed and shows the following:
Which of the following is the most likely underlying cause of this patient's recurrent urinary tract infections?
Content Reviewers:Rishi Desai, MD, MPH
They appear when solutes in the urine precipitate and crystalize. Depending on which solute precipitates to form the stone, there can be calcium oxalate, calcium phosphate, uric acid, cystine, and struvite stones.
Risk factors for developing kidney stones depend on their composition.
Risk factors for calcium oxalate stones include high urine calcium, high urine oxalate, low urine citrate, and dietary factors include low calcium, low potassium, and low fluid intake, as well as a high oxalate, and a high animal protein intake.
Calcium phosphate stones usually develop in individuals with renal tubular acidosis type I and II.
Cystine stones occur in the setting of cystinuria - a genetic condition where too much cystine is excreted.
Finally, struvite stones, also called staghorn calculi, are made up of magnesium ammonium phosphate, and the main risk factor is a urinary tract infection with a bacterium that produces urease - like Proteus and Klebsiella. These bacteria increase urine pH, making it a favorable environment for magnesium ammonium phosphate to precipitate.
Sometimes, kidney stones can be asymptomatic and discovered incidentally during an ultrasound or a CT-scan.
Other times a kidney stone can cause symptoms due to urinary obstruction and renal distention.
It can cause renal colic, which is when there’s acute pain that’s so intense that it requires IV pain medication.
Passing a large stone through a narrow ureter has been compared to passing a baby’s head through the vaginal canal! The difference is that you don’t have to raise and nurture the stone once it passes.
In renal colic, the location of the pain depends on where the stone is located.
A stone in the renal pelvis or the proximal ureter causes unilateral flank pain and tenderness, whereas a stone that’s located lower in the ureter causes unilateral flank pain that radiates to the testicle or labia on the affected side.
In addition, there can also be macroscopic hematuria, nausea, and vomiting, and if the stone is in the distal ureter, there may be dysuria and urgency as well.
If the blockage is severe, when there’s bilateral obstruction- like when there are stones in both kidneys and they completely obstruct the flow of urine or when a person has only a single functioning kidney - a solitary kidney - that gets obstructed, it can lead to signs of acute kidney injury, like anuria- which is when there’s less than 100 milliliters of urine per day.
Lab studies include a basic metabolic panel, where blood urea nitrogen or BUN and serum creatinine are done in order to check the renal function.
With acute kidney injury, serum creatinine is elevated.
Additionally, bloodwork and urinalysis may give clues as to the composition of the stone - low serum bicarbonate and potassium levels may suggest an underlying renal tubular acidosis and a calcium phosphate stone.
A urinalysis may show microscopic hematuria or, especially with struvite stones, there may be signs of a urinary tract infection - like bacteriuria and positive leukocyte esterase.
On microscopy, if there are calcium oxalate crystals in the urinary sediment, this may point towards a calcium oxalate stone.
Alternatively, with uric acid stones, uric acid crystals may be present.
Finally, hexagonal cystine crystals are diagnostic for cystine stones.
The best way to see a kidney stone is by doing a non-contrast abdominal CT-scan, but in pregnancy, an ultrasound is done instead.
Now, on a CT scan, the location and density of a stone can suggest its composition.
Calcium oxalate and calcium phosphate stones are radiopaque.
A struvite stone is also radiopaque, but not quite as dense as calcium stones and is usually large and located in the renal pelvis.
Uric acid stones are lower density than both calcium and struvite stones.
Finally, a cystine stone is lucent and it’s barely seen on the CT-scan.
When an ultrasound is done, stones that are located in the renal pelvis and proximal ureter can be seen. The stones are easily identified on the ultrasound, because they cast an acoustic shadow.
Additionally, indirect signs of obstruction may be seen on ultrasound - like hydronephrosis, which is when the kidney are filled up with urine and swell up.
Sometimes, a kidney-ureter-bladder radiography or KUB radiography can be done and radiopaque stones- like calcium oxalate and calcium phosphate stones and struvite stones that are larger than 5 millimeters can be seen, but stones smaller than 5 millimeters or uric acid stones and cystine stones are often missed.
Now treatment mostly depends on the location- specifically, whether the stone is in the renal pelvis or in the ureter, size, and composition of the stones.
In some cases, urgent decompression of the collecting system may also be necessary following analgesia.
For example, when there’s bilateral obstruction caused by stones in both kidneys and acute kidney injury, or when there’s obstruction of a solitary kidney, or if the individual presents with signs of sepsis- like with a struvite stone that’s associated with a UTI or when there’s hydronephrosis and a secondary bacterial kidney infection.
Decompression can be done with percutaneous nephrostomy - which is when a catheter is inserted through the back or flank into the kidney in order to drain the urine.