kidney stone
Meaning of kidney stone: Pain comes from the blocked tube stretching above the stone rather than from the stone scraping its way along.
Definition of kidney stone
A kidney stone is a hard crystalline mass forming inside the kidney when minerals dissolved in the urine become concentrated enough to settle out and grow. Small stones pass unnoticed. A stone lodging in the ureter, the tube carrying urine from kidney to bladder, causes severe waves of flank pain, nausea and blood in the urine.
Why does a small stone hurt so much?
The pain does not come from the stone scraping along the tube. It comes from what happens behind it. A stone lodged in the ureter blocks the flow of urine, which continues to be made, so pressure rises in the tube and then in the collecting system of the kidney itself. The stretched wall of the ureter contracts hard against the obstruction in waves, and stretch receptors in the swollen kidney fire. The result is severe pain arriving in surges, felt in the flank and travelling toward the groin as the stone moves lower. Sustained back pressure swells the kidney, a state called hydronephrosis, and prolonged obstruction damages kidney tissue.
What are stones made of?
| Type | Formed when | Note |
|---|---|---|
| Calcium oxalate | Urine is concentrated and oxalate is high | Much the commonest |
| Calcium phosphate | Urine is persistently alkaline | Often with a kidney tubule disorder |
| Uric acid | Urine is persistently acidic | Does not appear on a plain X-ray |
| Struvite | Bacteria split urea, raising alkalinity | Can fill the kidney as a branched stone |
| Cystine | An inherited transport defect | Begins in childhood and recurs |
Composition matters because it points to the underlying cause and to what makes another stone likely. Low urine volume is the factor common to every type, since a stone forms only where the substances dissolved in urine exceed what that volume can hold.
What else produces the same pain?
Hematuria, blood in the urine, accompanies most obstructing stones and is frequently invisible, showing only on testing. Neither that finding nor the pattern of pain is unique to stones. A leaking swelling of the main abdominal artery can present as sudden flank or back pain with collapse, which is why a first episode of such pain in an older adult is not assumed to be a stone. Infection above an obstructing stone adds fever and shaking chills and behaves as an emergency, because pus is trapped under pressure. Imaging settles the question, and a low-dose scan without contrast has largely replaced intravenous pyelography for the purpose.
Also known as
Used in a sentence
A scan without contrast demonstrated an obstructing kidney stone in the lower ureter.
Good to know
Severe one-sided flank pain with blood in the urine is not specific to stones, and other causes including a leaking swelling of the abdominal aorta produce a similar picture, particularly in older adults with no previous stone. Fever alongside an obstructing stone indicates infection trapped above it.