Ureteral Stones & Complications
- A ureteral stone is a tiny rock wedged in a tiny tube, and the imaging test of choice is an unenhanced CT of the abdomen and pelvis (the "CT KUB"), which finds it with about 97% sensitivity and 96% specificity in acute flank pain.
- Almost all stones are dense (white) on CT, including uric acid stones, the ones famous for being invisible on a plain X-ray.
- Don't just find the stone: report its size and location, because those two facts drive whether it passes on its own. About 68% of stones 5 mm or smaller pass spontaneously; over 5 mm and up to 10 mm, only about 47% do. A stone at the ureterovesical junction passes about 79% of the time, one in the proximal ureter only about 48%.
- The real diagnosis is often the consequence: hydroureter, hydronephrosis, and fat stranding. These secondary signs tell you the stone actually matters.
- The scary complication is an obstructed, infected system. That is a urologic emergency, not a "drink water and wait" situation.
Imagine the world's worst pebble in the world's narrowest garden hose, except the hose is inside you and it really, really wants you to know about it. That's a ureteral stone. People describe the pain as worse than childbirth, which is a bold claim from a stone roughly the size of a sesame seed. Our job on imaging is calmer: find the rock, measure it, and figure out how angry the kidney upstream is about the traffic jam.
Why CT, and why no contrast
The go-to study is an unenhanced CT of the abdomen and pelvis, often called a CT KUB (kidneys, ureters, bladder). The trick is that stones are dense: they soak up X-rays like little chunks of bone, so they show up bright white against the darker soft tissue and fat around them. Adding IV contrast would be like turning on every light in the room when you're trying to spot a single firefly: the contrast lights up the kidneys and ureters white too, and your white stone vanishes into the glare. So we deliberately leave the lights off.
The stone itself began life in the kidney (that broader story lives on the urolithiasis page). This page is about what happens once it sets off down the ureter, gets stuck, and starts causing complications.
Nearly all stones are visible on CT, including uric acid stones. That surprises people, because uric acid stones are the classic "radiolucent" stones that hide on a plain abdominal X-ray. CT sees density directly, so it catches them anyway. The plain-film blind spot doesn't apply here.
Findings by modality
CT (unenhanced)
The stone is a dense focus lying within the ureter, which is the key phrase: you have to trace the ureter from the renal pelvis to the bladder on thin sections and prove the dot is inside the tube, not merely near it. Coronal reformats make the tracing far easier than axial images alone. Stones love to lodge at the three natural pinch-points where the ureter is narrowest: where the kidney funnels into the ureter (the ureteropelvic junction), where the ureter crosses over the iliac vessels at the pelvic brim, and where it tunnels through the bladder wall (the ureterovesical junction). That last one is the most common place to get stuck; it's the off-ramp with the worst congestion.
Two signs help settle "stone or not stone" when the dot is ambiguous. The soft-tissue rim sign is a thin halo of edematous ureteral wall wrapped around the calculus, which only a stone inside the ureter can produce. The comet-tail sign is a tapering soft-tissue tail trailing away from a calcification, the collapsed vein a phlebolith lives in, and it argues against a stone.
Then the secondary signs, which are really the whole point:
- Hydroureter: the ureter above the stone is dilated, like a hose swollen behind a kink. Follow it down and it should snap back to normal caliber exactly at the stone.
- Hydronephrosis: the collecting system inside the kidney balloons with backed-up urine. This earns its own deep dive at hydronephrosis and obstruction.
- Perinephric and periureteric fat stranding: the normally crisp black fat around the kidney and ureter turns hazy and dirty-looking, the imaging version of swelling and irritation.
- Renal enlargement and a slightly lower-attenuation kidney compared with the other side, because a kidney under pressure is congested and edematous.
- Perinephric fluid: a crescent of fluid around the kidney is the clue that a calyceal fornix has torn under pressure and urine is leaking (more on that below).
CT with contrast (CT urography)
You do not need contrast to find the stone, but the excretory phase of a CT urogram does answer two follow-up questions. A delayed nephrogram on the obstructed side (the kidney lights up later and stays lit longer than its neighbour) confirms the obstruction is functionally real. And excreted contrast escaping out of the collecting system into the perinephric space proves a forniceal rupture and urinoma rather than just reactive fluid.
Ultrasound
Ultrasound is the other player, especially when you want to avoid radiation: pregnant patients, children, or someone who has had ten of these CTs already. It's great at showing a swollen kidney (dilated anechoic calyces and pelvis) and can catch stones at the very top of the ureter (near the kidney) and the very bottom (behind the bladder, best seen with a full bladder as an acoustic window), where they appear as echogenic foci with posterior shadowing and, on color Doppler, a twinkling artifact. The long middle stretch of ureter is hidden behind bowel gas, so it misses stones in that no-man's-land. Absence of the normal ureteral jet on the painful side supports obstruction; a jet that is present argues the obstruction is at most partial.
Radiograph (KUB)
A plain film is mostly used to follow a stone that has already been proven radiopaque on CT, so the urologist can track it down the ureter without a CT each time. Calcium stones show; uric acid stones do not; and pelvic phleboliths and bowel content make the distal ureter a minefield.
The numbers
Size and location are the two facts that decide the stone's fate, and they come straight off the CT.
| What | Threshold / value | Why it matters |
|---|---|---|
| Spontaneous passage, stone ≤5 mm | 68% (95% CI 46–85%) | The everyday cutoff for a trial of passage with analgesia and follow-up |
| Spontaneous passage, stone >5 to ≤10 mm | 47% | About half will still need help; urology follow-up is not optional |
| Passage by CT-measured size | 1 mm 87%; 2–4 mm 76%; 5–7 mm 60%; 7–9 mm 48%; >9 mm 25% | Every millimetre counts, so measure on the plane that shows the stone largest |
| Passage by location | Proximal ureter 48%; mid ureter 60%; distal ureter 75%; ureterovesical junction 79% | Distal stones have a short trip and the best odds; proximal stones the worst |
| Unenhanced CT in acute flank pain | Sensitivity 97%, specificity 96%, accuracy 97% | Why CT is the test of choice and the intravenous urogram is history |
The passage figures come from two different sources and they tell the same story: a small stone near the bladder is very likely to leave on its own, a large stone high in the ureter is not, and everything in between is a probability, not a promise.
The "soft signs" can clinch the diagnosis when the stone itself is tiny or already passing. An asymmetrically enlarged, edematous kidney with stranding on one side, even with only a whisper of a stone, tells you which side hurts and that the obstruction is real and recent. If you find hydroureter and stranding but no stone anywhere along the ureter, check the bladder: the stone may have just dropped in.
How good is the test
Unenhanced helical CT, in patients presenting with acute flank pain, carries a sensitivity of 97%, a specificity of 96%, and an accuracy of 97% for ureteral stone. That is the setting the figure was measured in, and it is the setting you will use it in, which is a rare and happy alignment.
Ultrasound and the plain radiograph are both less sensitive, especially for small stones and the mid ureter, but published accuracy figures for both vary too much to quote one here, so I will not put a number on it.
What could it be instead
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Phlebolith | Small round calcification in the pelvis, right along the course of the distal ureter | Lucent center, comet-tail sign, no soft-tissue rim, lies outside the traced ureter, nothing dilated above it |
| Calcified iliac artery or vessel wall | Dense focus at the pelvic brim, exactly where the ureter crosses | Curvilinear and continuous with the vessel on adjacent slices |
| Calcified lymph node, appendicolith, or bowel content | Bright dot near the ureter's course | Located in a node, the appendix, or the bowel lumen on coronal images while the ureter runs past untouched |
| Pyelonephritis | Swollen kidney with perinephric stranding and flank pain | No stone, no hydronephrosis, fever, and a striated nephrogram if contrast is given (see pyelonephritis) |
| Recently passed stone | Hydroureter and stranding with no stone in the ureter | The stone is in the bladder, or the patient reports the pain has just eased |
| Ureteral tumor or stricture | Hydroureter with a transition point | The transition is a soft-tissue mass or a smooth narrowing rather than a calculus; needs CT urography (see urothelial cancer) |
A phlebolith, a small calcified clot in a pelvic vein, looks almost exactly like a ureteral stone: a tiny white dot in the pelvis. The tell is the soft-tissue rim sign, where a true ureteral stone is wrapped in a thin rim of swollen ureter wall, while phleboliths often have a little comet tail of vessel trailing off them. When stuck, trace the ureter: a real stone sits in the ureter and has dilation above it.
The complications
Most stones are a painful inconvenience that pass with time and fluids. Here is the list of ways they stop being an inconvenience.
Obstruction with forniceal rupture. When pressure in a blocked collecting system climbs high enough, the weakest point, the fornix of a calyx, tears, and urine leaks into the perinephric space. On unenhanced CT you see a crescent of perinephric fluid; on the excretory phase of a contrast study you see contrast frankly leaking out of the collecting system. It sounds dramatic, but it is actually a pressure-release valve: the leak takes pressure off the kidney, and most urinomas resolve once the obstruction does.
Infection behind the obstruction. Urine trapped behind a stone is a warm, stagnant pond, and bacteria throw a party. A blocked, infected kidney (pyonephrosis) can tip someone into sepsis fast. On imaging, worry when an obstructed system contains layering debris or gas, or the kidney and perinephric fat look inflamed, in a febrile patient.
An obstructing stone plus signs of infection (fever, pus, a sick patient) is a urologic emergency. The pressure has to come off the kidney, typically by draining it with a stent or a nephrostomy tube, and that can't wait for the stone to pass on its own.
Loss of kidney function. A short obstruction is survivable; weeks to months of it quietly thins the renal cortex and can leave a permanently damaged kidney. If the CT shows a thinned cortex wrapped around a chronically dilated system, that stone has been there a while, and the "wait and see" clock has already run out.
Steinstrasse. After shock-wave lithotripsy, the fragments of a shattered stone can pile up in a line down the ureter, a "stone street," and cause a fresh obstruction of their own. It looks exactly like it sounds: a column of small dense fragments in a dilated ureter.
The solitary kidney. An obstructing stone in a patient's only functioning kidney obstructs their entire urine output. Always look at the other kidney and say so if it is absent, atrophic, or blocked too.
Reporting
- Stone: present or absent, side, exact location (ureteropelvic junction, proximal, mid, distal, ureterovesical junction), and size in millimetres on the plane that shows it largest.
- Obstruction: hydroureter and hydronephrosis and how marked, perinephric and periureteric stranding, renal enlargement, delayed nephrogram if contrast was given.
- Complications: perinephric fluid or contrast extravasation (forniceal rupture), gas or debris in the collecting system (concern for pyonephrosis), cortical thinning (chronic obstruction).
- Context: the stone burden in both kidneys, the state of the contralateral kidney, any stent or nephrostomy already in place, and any non-urinary cause for the pain.
- The sentence that matters: "obstructing" or "non-obstructing," stated plainly, because that single word decides whether the urologist is reading your report tonight or tomorrow.
Find the bright stone on the unenhanced CT, measure it, locate it at one of the pinch-points, but spend equal energy on the upstream story. Hydronephrosis, hydroureter, and fat stranding turn "there's a stone" into "there's an obstructing stone," and the moment infection joins that picture, the clock starts ticking.
References
- Smith RC, Verga M, McCarthy S, Rosenfield AT. Diagnosis of acute flank pain: value of unenhanced helical CT. AJR Am J Roentgenol 1996;166(1):97–101. Used for: the 97% sensitivity, 96% specificity, and 97% accuracy of unenhanced helical CT for ureteral stone in acute flank pain, in Key Points, "The numbers", and "How good is the test".
- Preminger GM, Tiselius HG, Assimos DG, et al. 2007 Guideline for the management of ureteral calculi. J Urol 2007;178(6):2418–2434. Used for: the spontaneous passage estimates of 68% (95% CI 46–85%) for stones ≤5 mm and 47% for stones >5 to ≤10 mm, in Key Points and "The numbers".
- Coll DM, Varanelli MJ, Smith RC. Relationship of spontaneous passage of ureteral calculi to stone size and location as revealed by unenhanced helical CT. AJR Am J Roentgenol 2002;178(1):101–103. Used for: the passage rates by CT-measured size (1 mm 87%; 2–4 mm 76%; 5–7 mm 60%; 7–9 mm 48%; >9 mm 25%) and by location (proximal 48%, mid 60%, distal 75%, ureterovesical junction 79%) in Key Points and "The numbers".
Sign in to take the quiz and track your progress