Hydronephrosis & Obstruction
- Hydronephrosis is just plumbing backup: urine can't get out, so it piles up and stretches the kidney's collecting system.
- It is a finding, not a diagnosis. Your real job is to hunt for the blockage and say where and why.
- Ultrasound is the cheap, radiation-free first look; CT is the workhorse for finding the actual obstruction (usually a stone).
- Adults get a loose mild/moderate/severe call; the fetus and infant get formal grades. Postnatally, a renal pelvis of 10 to under 15 mm across is UTD P1 (low risk) and 15 mm or more is UTD P2; any ureteral, parenchymal, or bladder abnormality makes it UTD P3 whatever the pelvis measures.
- Hydronephrosis without obstruction is a real and common trap: a dilated system isn't always a blocked one.
Picture a garden hose with a kink halfway down. You turn the tap on, the water keeps coming, but it has nowhere to go, so the hose upstream of the kink swells and gets fat. Your kidney runs the same way. Urine is made continuously, drains down through the collecting system and ureter into the bladder, and if anything pinches that path shut, the system upstream balloons. That swelling is hydronephrosis ("hydro" = water, "nephros" = kidney), literally water on the kidney. That's the whole idea in one sentence. The rest is just figuring out where the kink is and how angry to be about it.
A finding, not a final answer
Here's the thing I wish someone had drilled into me earlier: hydronephrosis is a description, not a disease. When I say a kidney is hydronephrotic, I've told you the pipes are dilated. I have not told you why. Saying "hydronephrosis" and stopping is like a mechanic saying "your car is making a noise" and handing you the keys. The diagnosis is the cause: a stone, a tumor, a stricture, an enlarged prostate squeezing the bladder outlet, a pregnant uterus leaning on a ureter, or in a newborn, a narrow ureteropelvic junction that was built that way.
So whenever you spot it, the next move is automatic: follow the dilated system downstream until you find the spot where it suddenly goes back to normal. That transition point is where the trouble lives.
Always describe hydronephrosis by its cause and level. "Right hydronephrosis to the level of a distal ureteral stone" is a diagnosis. "Hydronephrosis" alone is half a sentence.
Findings by modality
Ultrasound
Ultrasound is the usual first stop because it's fast, free of radiation, and great at spotting fluid. The dilated system shows up as branching anechoic (black) spaces in the center of the kidney, where urine has pooled in the calyces and renal pelvis, and the branches connect to each other, which is how you tell them from a cluster of cysts. As the pressure climbs, the calyces go from slightly separated to rounded to frankly ballooned, and eventually the cortex over them thins to a rind. Color Doppler helps twice over: it separates dilated calyces (no flow) from the prominent hilar vessels that can mimic them, and the ureteral jet, the little puff of color as urine squirts into the bladder, tells you that side is still draining at least partly. The renal resistive index on spectral Doppler rises in acute obstruction, though it is a supportive sign rather than a threshold test.
CT
Ultrasound tells you the system is dilated but is often lousy at showing why, especially down in the pelvis where bowel gas hides the ureter. That's where CT earns its keep. An unenhanced CT is the go-to for someone with flank pain, because the overwhelmingly common culprit is a ureteral stone, and stones light up bright white. You trace the dilated ureter down until it abruptly narrows, and right at that transition point sits your stone, tumor, or stricture.
CT also shows the supporting cast of an acute obstruction: a swollen kidney, smudgy stranding of the fat around it, and fluid tracking around the kidney, signs the system is under pressure rather than just casually roomy. If contrast is given, the obstructed kidney shows a delayed nephrogram (it enhances later and holds onto contrast longer than the other side) and delayed excretion into the collecting system; the excretory phase of a CT urogram then outlines the exact level and the shape of the blockage, which is the way to unmask a soft-tissue cause such as a urothelial tumor.
MRI
MR urography does the same job without ionizing radiation, which makes it the choice in pregnancy and in children when ultrasound has not answered the question. Heavily T2-weighted sequences show static fluid in the dilated system like a water map (and are the sequences relied on in pregnancy, where gadolinium is generally avoided), and contrast-enhanced excretory sequences show the level of obstruction the same way CT does. Its weakness is the stone itself, which is a signal void on MRI and easy to miss.
Nuclear medicine
When the anatomy says "dilated" but nobody can tell whether urine is actually draining, diuretic renography settles it. The tracer washes promptly out of a baggy-but-open system after the diuretic and stays stuck in a truly obstructed one, and the study also gives you each kidney's share of function, which is what decides whether a chronically obstructed kidney is worth rescuing.
The numbers
Adults mostly get the eyeball grading below. The fetus and the infant get real numbers, because antenatal ultrasound turned "dilated renal pelvis" into one of the most common findings on the anatomy scan, and someone had to decide which babies need follow-up.
| What | Threshold / value | Why it matters |
|---|---|---|
| Antenatal APRPD, normal | <4 mm at 16–27 weeks; <7 mm at ≥28 weeks | Below this the pelvis is not "dilated" at all |
| Antenatal APRPD, UTD A1 (low risk) | 4 to <7 mm at 16–27 weeks, or 7 to <10 mm at ≥28 weeks, with or without central calyceal dilation | Mild; needs a follow-up scan, not a panic |
| Antenatal APRPD, UTD A2–3 (increased risk) | ≥7 mm at 16–27 weeks or ≥10 mm at ≥28 weeks, or peripheral calyceal dilation, ureteral dilation, parenchymal abnormality, or bladder abnormality | Earlier and more specialized follow-up |
| Postnatal UTD P1 (low risk) | APRPD 10 to <15 mm and/or central calyceal dilation | Mild; surveillance |
| Postnatal UTD P2 (intermediate) | APRPD ≥15 mm or peripheral calyceal dilation | Closer follow-up, further imaging often considered |
| Postnatal UTD P3 (high risk) | Ureteral dilation, parenchymal abnormality (thinning, abnormal echogenicity, cysts), or bladder abnormality, regardless of APRPD | The most concerning finding drives the category |
| Adult kidney length (sonography) | Median 11.2 cm left, 10.9 cm right; median volume 146 cm³ left, 134 cm³ right | A hydronephrotic kidney that is shrinking has been obstructed for a long time |
The anteroposterior renal pelvic diameter (APRPD), the front-to-back width of the renal pelvis on a transverse image, is the one measurement everything else hangs on. Note the pattern in the UTD tables: the pelvis diameter sets the floor, and any extra finding (calyces out to the periphery, a visible ureter, abnormal parenchyma, an abnormal bladder) bumps the category up.
How good is the test
Ultrasound and CT are both very good at showing a dilated collecting system, but no robust pooled sensitivity or specificity figure exists for either modality in detecting hydronephrosis or predicting obstruction, so I will not put a number on it. The honest framing is this: the imaging finding is easy, and the diagnostic work is in deciding whether "dilated" means "obstructed," which is where the transition point, the secondary signs, and, when needed, diuretic renography come in.
Grading it
Adults: the eyeball scale
| Grade | What you see | Hose analogy |
|---|---|---|
| Mild | Slightly plump renal pelvis, calyces barely dilated | A hose that's just a little tight |
| Moderate | Pelvis and rounded calyces clearly ballooned out | A noticeably swollen hose |
| Severe | Big urine-filled sacs, kidney tissue thinned to a rind | A water balloon with a peel left on it |
That last grade is the one that makes me wince: chronic, high-grade obstruction slowly squashes the working kidney tissue (the cortex) until there's barely any left. Time matters. A kidney can take a short blockage, but weeks to months of it does permanent damage.
Children: the Society for Fetal Urology (SFU) grades
The SFU scale (introduced in 1993) is a pure pattern grade, no ruler required, and it is still the language of most pediatric urologists.
| SFU grade | Findings on ultrasound |
|---|---|
| 1 | Slight separation of the central renal echo complex (pelvis only) |
| 2 | Pelvis further dilated, with a single or a few calyces seen |
| 3 | Pelvis dilated with fluid-filled calyces throughout the kidney; parenchyma of normal thickness |
| 4 | As grade 3, but the parenchyma over the calyces is thinned |
Fetus and infant: the UTD classification (2014 multidisciplinary consensus)
The urinary tract dilation (UTD) system was built to replace a tangle of competing scales with one shared language across obstetrics, radiology, and urology. It splits into an antenatal (A) and a postnatal (P) version, each driven by the APRPD plus a handful of "extra" findings.
| Category | Setting | Defining criteria |
|---|---|---|
| Normal | Antenatal | APRPD <4 mm at 16–27 weeks; <7 mm at ≥28 weeks |
| UTD A1 (low risk) | Antenatal | APRPD 4 to <7 mm at 16–27 weeks or 7 to <10 mm at ≥28 weeks, with or without central calyceal dilation |
| UTD A2–3 (increased risk) | Antenatal | APRPD ≥7 mm at 16–27 weeks or ≥10 mm at ≥28 weeks, or peripheral calyceal dilation, ureteral dilation, parenchymal abnormality, or bladder abnormality |
| UTD P1 (low risk) | Postnatal | APRPD 10 to <15 mm and/or central calyceal dilation |
| UTD P2 (intermediate risk) | Postnatal | APRPD ≥15 mm or peripheral calyceal dilation |
| UTD P3 (high risk) | Postnatal | Ureteral dilation, parenchymal abnormality (thinning, abnormal echogenicity, cysts), or bladder abnormality, regardless of APRPD |
Classification is always driven by the most concerning finding. The common pediatric causes behind these grades, ureteropelvic junction obstruction and vesicoureteral reflux, have their own pages at hydronephrosis and UPJ obstruction and vesicoureteral reflux and VCUG.
The single most useful trick is finding the transition point: the exact spot where a fat, dilated ureter snaps back to normal caliber. Whatever is sitting at that point is your answer. No transition point, no clear mechanical obstruction, and you should start thinking about the non-obstructive causes below.
The trap: dilated does not always mean blocked
Now the pitfall that humbles everyone. A collecting system can be roomy without anything blocking it. A very full bladder backs pressure up the ureters; pregnancy dilates the right side as a matter of routine; some people just have a baggy, floppy system left over from an old, long-resolved problem. Reflux, urine sloshing backward, can dilate things too, without a true blockage.
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Extrarenal pelvis | A large fluid-filled pelvis outside the renal sinus | Calyces are normal, no cortical thinning, no transition point, stable over time |
| Parapelvic (renal sinus) cysts | Multiple anechoic spaces in the sinus | The spaces do not connect with each other or with the pelvis; the excretory phase shows contrast around, not in, them |
| Overfull bladder | Bilateral mild dilation | Rescan after voiding and it melts away |
| Pregnancy | Right-sided (and sometimes bilateral) dilation, mild, in a pregnant patient | Smooth tapering at the pelvic brim where the uterus leans on the ureter, no stone, no stranding |
| Vesicoureteral reflux | Dilated ureter and pelvis, especially in a child | No mechanical transition point; VCUG shows contrast refluxing up from the bladder |
| Post-obstructive residual dilation | The system stays baggy after the stone has passed or the stricture has been fixed | No transition point, no pressure signs; diuretic renography shows prompt washout |
| Prominent hilar vessels on ultrasound | Branching hypoechoic structures in the sinus | Color Doppler fills them with flow; calyces stay black |
| Megacalycosis and other congenital variants | Many dilated calyces with a normal pelvis | Normal-caliber pelvis and ureter, no functional obstruction on renography |
Don't reflexively equate hydronephrosis with obstruction. Check the bladder (is it overfull?), ask about pregnancy, and look hard for a transition point and pressure signs. If the system is dilated but the urine is clearly moving, it may not be obstructed at all. When it's genuinely unclear, diuretic renography settles whether urine is actually draining.
Reporting
- Side and severity: mild, moderate, or severe in adults; SFU grade or UTD category in the fetus and child, with the APRPD in millimetres.
- Level of transition and the cause if visible: stone (with size), soft-tissue mass, stricture, extrinsic compression, or none identified.
- Signs of acute pressure: perinephric stranding and fluid, renal swelling, delayed nephrogram.
- Signs of chronicity: cortical thinning, a small kidney compared with the normal length.
- The bladder and the other kidney: outlet obstruction, a distended bladder, bilateral disease, the contralateral kidney's health.
- Whether you think it is obstructed, and if you cannot tell, the suggestion of diuretic renography rather than a shrug.
Hydronephrosis is backed-up plumbing: easy to see, but only half the story. The skill isn't spotting the swollen system; it's chasing it downstream to the kink, naming the cause, grading it in the right language for the patient's age, and judging whether the kidney is merely roomy or genuinely strangling.
References
- Nguyen HT, Benson CB, Bromley B, et al. Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD classification system). J Pediatr Urol 2014;10(6):982–998. Used for: the antenatal APRPD thresholds (normal <4 mm at 16–27 weeks and <7 mm at ≥28 weeks; UTD A1 4 to <7 mm or 7 to <10 mm; UTD A2–3 ≥7 mm or ≥10 mm) and the postnatal UTD P1 (10 to <15 mm), P2 (≥15 mm), and P3 criteria, in Key Points, the Measurement chips, "The numbers", and "Grading it".
- Fernbach SK, Maizels M, Conway JJ. Ultrasound grading of hydronephrosis: introduction to the system used by the Society for Fetal Urology. Pediatr Radiol 1993;23(6):478–480. Used for: the SFU grade 1 to 4 definitions and the 1993 date of the system in "Grading it".
- Emamian SA, Nielsen MB, Pedersen JF, Ytte L. Kidney dimensions at sonography: correlation with age, sex, and habitus in 665 adult volunteers. AJR Am J Roentgenol 1993;160(1):83–86. Used for: the adult median kidney length (11.2 cm left, 10.9 cm right), median volume (146 cm³ left, 134 cm³ right), and the 665-volunteer cohort in the Measurement chip and "The numbers".
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