Left Flank Pain, an Empty Bladder, and a Questionable Sinus
Mr. K, 44, presents at 02:00 with 6 hours of severe left flank pain radiating to the groin, one episode of vomiting, and gross hematuria. He weighs 130 kg, received morphine 20 minutes ago, and cannot tolerate lying on his right side or holding still through a breath-hold. You find a left kidney with a 1.5 cm anechoic area in the central sinus and a rounded cystic structure medial to the sinus; the right kidney appears normal. The bladder is nearly empty because he voided just before scanning. He asks repeatedly whether he can go home.
1. Is this hydronephrosis, an extrarenal pelvis, or a parapelvic cyst? What features distinguish them, and what does your finding represent?
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Distinguishing features:
- Hydronephrosis: branching, communicating anechoic spaces that split the echogenic sinus, with the calyces connecting to the pelvis. Even at grade 1 the fluid is within the sinus and follows its architecture.
- Extrarenal pelvis: a single anechoic collection positioned medial to the sinus and outside the renal parenchyma. It does not branch, does not extend between sinus echoes, and does not communicate with calyces - it is a normal variant, appears identical in every plane, and does not change with hydration.
- Parapelvic cyst: round, well-defined, thin-walled, no internal flow, and, critically, it does not communicate with the collecting system. Color Doppler may show displaced sinus vessels around it rather than within it.
Applying this to Mr. K: a 1.5 cm single central collection plus a separate rounded medial structure without demonstrable communication is at most grade 1 (mild) hydronephrosis, and possibly normal variant anatomy. It is not the classic bear-paw pattern of established obstruction.
The judgment point: the history is compelling, but the sonographic finding is equivocal. Do not upgrade the report to "moderate hydronephrosis" to match the clinical story, and do not dismiss the finding as normal variant without attempting to trace communication in at least two planes.
2. You attempt a resistive index but the waveform varies with every breath and the patient will not hold still. How much weight should the RI carry in this report?
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Little to none - and probably omit the number entirely.
- Diagnostic performance. An RI is neither sensitive nor specific for ureteral obstruction. Early or partial obstruction frequently produces a normal RI, and an elevated RI occurs with dehydration, NSAID use, advanced age, intrinsic renal disease, and elevated intra-abdominal pressure. A normal RI does not exclude a stone; an elevated one does not confirm obstruction.
- Technical validity in this patient. A reliable RI requires a consistent waveform over at least three cardiac cycles from a segmental or interlobar artery, ideally with the patient in suspended respiration. A tracing that varies breath to breath is measuring motion and phase, not renal vascular resistance.
- Comparison with the other side is the only meaningful internal control, and that requires the same quality on both kidneys.
What to do instead: attempt the Doppler in suspended expiration with a small sample volume and a low wall filter, and if the tracing remains inconsistent, state that Doppler evaluation was technically limited by patient motion and body habitus. An unreliable RI is worse than no RI because it invites a false sense of exclusion. Let the gray-scale finding and the clinical picture drive management, and let CT answer the question.
3. The bladder is empty and you cannot identify a stone in the distal ureter. He is asking to go home. What does the report say and what do you recommend?
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Report content:
- Degree and laterality of hydronephrosis, using explicit grading rather than adjectives alone, and note the possibility of an extrarenal pelvis or parapelvic cyst as an alternative explanation.
- State clearly that the distal ureter and ureterovesical junction were not visualized because the bladder was nearly empty, and that the obstructing level therefore cannot be localized.
- Note the technically limited Doppler and the reason.
- Describe the right kidney as normal, since symmetry is diagnostically useful.
Recommendation: non-contrast CT of the abdomen and pelvis is the reference standard for stone disease and will localize the stone and identify obstruction, and it should be considered if the clinical suspicion warrants it. Alternatives include re-scanning after oral hydration if the bladder can be filled, which is reasonable only if the delay is clinically acceptable. Do not attempt an endocavitary approach in a male patient; that is not indicated and does not improve distal ureteral visualization.
Answering his question: disposition is not the sonographer's decision, but his question deserves a clear response - the imaging does not show dangerous complications such as a large obstructed kidney or free fluid, and that information goes to the treating clinician now. If he develops fever, worsening pain, or inability to urinate, he needs to be seen urgently.