Editing Pathophysiology of Urinary Tract Obstruction (section)

== Diagnosis and evaluation ==

=== History and physical exam ===

* '''Most common symptom with acute obstruction: flank pain;''' secondary to stretching of the collecting system.
** '''In contrast, chronic obstruction of the urinary tract is usually painless''' and patients may be entirely asymptomatic
* '''Obstructive uropathy should always be considered in patients with (3):'''
*# '''New-onset hypertension'''
*# '''Renal failure''' without a history of renal disease, diabetes, or hypertension
*# '''Recurrent UTIs'''

=== Laboratory ===

* '''Fractional excretion of Sodium (FENa)'''
** Often used to differentiate among the 3 types of acute renal injury: pre-renal, intrinsic, and post-renal
** '''(FENa = (PCr ×UNa)/(PNa ×UCr ))'''
*** '''FENa <1% suggests a pre-renal cause of acute renal failure'''
*** '''FENa >4% suggests a post-renal cause of acute renal failure'''

=== Imaging ===

==== Ultrasound ====

* '''Hydronephrosis'''
** '''The term hydronephrosis implies dilatation of the renal pelvis and calyces and can occur without obstruction.'''
*** '''Significant hydronephrosis can be present in the absence of obstruction (e.g., vesicoureteral reflux)'''
** '''The term obstructive nephropathy should be reserved for the damage to the renal parenchyma that results from an obstruction to the flow of urine anywhere along the urinary tract.'''
*** '''Significant obstruction can be present in the absence of severe hydronephrosis (e.g., very early in the course of acute renal obstruction)'''
**** Standard renal US may appear normal in 50% of patients with acute urinary obstruction
* '''Resistive index (RI)'''
** '''Definition of RI: (peak systolic velocity–end diastolic velocity)/peak systolic velocity'''
** Changes in intrarenal arterial waveforms have been shown to be associated with urinary obstruction
** '''RI >0.70 has been suggested as a technique to improve detection of urinary obstruction during US.'''
*** However, studies investigating RI in the detection of renal obstruction have not found it useful.
* '''Ureteric jets'''
** '''Color Doppler US can reliably identify ureteric jet dynamics in the bladder'''
** Requires good hydration of the patient
** Limited by the requirement of a normal contralateral collecting system for comparison.

==== CT ====

* '''Can detect most radiolucent stones with the exception of protease inhibitor stones (i.e., indinavir sulfate) and mucoid matrix stones'''
** '''Test characteristics for stone detection:'''
*** '''Sensitivity: 96%'''
*** '''Specificity and positive predictive value: 100%'''
* '''Low-dose CT may be limited in:'''
*# '''Stones < 3 mm'''
*# '''Patient obesity'''
*# '''Impaction at the ureterovesical junction'''
* '''Traditional CT urography involves 3 phases using a single IV bolus injection of contrast:'''
*# '''Unenhanced''' (initial)
*# '''Nephrogenic/nephrographic'''
*#* '''Obtained ≈100-120 seconds after contrast injection'''
*# '''Excretory'''
*#* '''Obtained ≈3-5 mins''' '''after contrast injection''' to evaluate the urothelium
** '''CT urogram/triphasic does not include corticomedullary''' (30-70 sec) '''phase'''
*** '''Corticomedullary best at looking at veins (renal vein involvement)'''
*** '''Nephrogenic best for parenchymal lesions'''
** '''Note: this is different than 2016 AUA Asymptomatic Microscopic Hematuria Guidelines which describe 4 phases: unenhanced, arterial, corticomedullary, and excretory'''

==== MRI ====

* '''Poor detection of renal and ureteral stones in comparison to CT because stones appear as signal voids on T1- and T2-weighted images'''
* '''The MRU measurement of contrast excretion is the renal transit time''', which is defined as the time it takes for contrast to pass from the renal cortex to the proximal ureters
** Interpretation of contrast excretion time
*** '''≤4 minutes: normal'''
*** > 4 and < 8 minutes: equivocal
*** '''≥ 8 minutes: obstructed'''
* MRU has been demonstrated to have an excellent correlation with the renal isotope GFR in the adult and pediatric patients with obstructed kidneys.
* '''IV gadopentetate-DTPA allows a dynamic, functional assessment of the collecting system that correlates well with diuretic renal scintigraphy, yet provides far greater anatomic detail than nuclear studies.'''
** Differential GFR can be assessed with post-imaging processing, and contrast washout can be measured to calculate renal clearance, differentiating dilated systems from obstructed systems

'''Excretory urography depends on glomerular filtration and renal excretion of iodinated contrast medium; therefore, the utility of excretory urography is limited in patients with renal insufficiency.'''

* '''The risk for contrast nephropathy increases with worsening renal function.'''
* '''Excretory urography should not be performed in patients with a history of contrast allergy or those in whom radiation exposure is a concern (i.e., pregnancy).'''

==== Nuclear renography ====

* '''<span style="color:#ff0000">Tracers commonly used in urology (3): MAG3, DTPA, DMSA'''
*# '''MAG3'''
*#* '''Tracer properties'''
*#** High extraction by the kidneys
*#*** Renal uptake is 55% compared with 20% uptake by DTPA
*#** Rapid clearance
*#** Low radiation dose
*#** '''Tubular secretion'''
*#* '''Preferred radiopharmaceutical in the evaluation of the obstructed collecting system'''
*# '''DTPA'''
*#* '''Tracer properties'''
*#** '''Removed almost exclusively by glomerular filtration'''
*#*** MAG3 actively secreted by the tubules
*#* '''Preferred radiopharmaceutical in the evaluation of GFR'''
*#** '''Adequate imaging of the collecting system, however, is GFR-dependent with DTPA and is therefore limited in patients with renal insufficiency and those <6 months of age because of the immaturity of renal function'''
*# '''DMSA'''
*#* Tracer properties
*#** Binds to the proximal convoluted tubules in kidney so the excretion pattern of the kidneys cannot be assessed§
*#** '''Best tissue to background activity ratio'''
*#* '''Preferred radiopharmaceutical in the evaluation of renal Scarring and prediction of renal recovery (superior to DTPA and MAG-3)'''

{| class="wikitable"
|'''<span style="color:#ff0000">Tracer'''
|'''<span style="color:#ff0000">Clinical question'''
|'''<span style="color:#ff0000">Clearance'''
|'''<span style="color:#ff0000">Useful in renal failure'''
|-
|'''<span style="color:#ff0000">MAG3'''
|'''<span style="color:#ff0000">Obstruction</span>, differential renal function, perfusion, effective renal plasma flow'''
|'''95% secretion, <5% glomerular filtration'''
|'''<span style="color:#ff0000">Yes'''
|-
|'''<span style="color:#ff0000">DTPA'''
|'''<span style="color:#ff0000">Obstruction</span>, differential renal function, perfusion, filtration (GFR)'''
|'''>95% glomerular filtration'''
|'''<span style="color:#ff0000">No</span> (since tracer has to be filtered)'''
|-
|'''<span style="color:#ff0000">DMSA'''
|'''<span style="color:#ff0000">Morphology</span> (cortical defects, ectopic or aberrant kidneys) and differential renal function'''
|'''60% tubular filtration, some glomerular filtration'''
| 
|}
§§

* '''Phases of a nuclear scan (3):'''
*# '''Flow (initial) phase'''
*#* '''Characterized by rapid renal uptake''' of the radiopharmaceutical, reflecting renal perfusion
*#* '''Shows renal uptake, background clearance, and abnormal vascular lesions'''
*# '''Renal phase'''
*#* '''Characterized by a more gradual rise in uptake over time, usually peaking after 2-5 minutes'''
*#* '''Primarily evaluates renal function'''
*#** '''Most sensitive indicator of renal dysfunction'''
*#* '''Urinary obstruction can diminish the rate of uptake of the radiotracer during the second phase and can therefore alter the assessment of differential renal function'''
*# '''Excretory phase'''
*#* '''Characterized by a gradual decrease in renal counts''' over time
*#* Often augmented by the administration of a diuretic (diuretic renogram) to induce high urine flow and prevent the false positive results that can be caused by urine stasis in a dilated collecting system
*#** The diuretic (usually furosemide 0.5 mg/kg) is administered when maximum collecting system activity is visualized.
*#* '''<span style="color:#ff0000">The T1/2 is the time it takes for collecting system activity to decrease by 50% from that at the time of diuretic administration'''
*#** '''<span style="color:#ff0000">T1/2 < 10 minutes: normal, non-obstructed collecting system'''
*#** T1/2 10-20 minutes: mild to moderate delay, may be a mechanical obstruction
*#** '''<span style="color:#ff0000">T1/2 > 20 minutes: high-grade obstruction'''
*#*** '''The level of obstruction can usually be determined, as can abnormalities such as ureteral duplication'''
*#* '''Causes of false-positive results (6): High Definition Nuclear Renography Can Deceive'''
*#*# '''Hepatobiliary excretion''' if the area of intestinal activity or gallbladder activity is included in the area of study
*#*# '''Dehydration''' because of the suboptimal response to a diuretic agent
*#*# '''Neonates''' because of renal immaturity
*#*# '''High-grade Reflux'''
*#*# '''CKD (poor renal function)'''
*#*# '''Presence of massive collecting system Dilation with urinary stasis'''
* Measurement of differential renal function and tracer washout will vary depending on the protocol and radiopharmaceutical used, and care should be taken when interpreting results if comparative studies have been performed using different protocols or radiopharmaceuticals.

==== Whitaker test ====

* Involves placement of a percutaneous needle in the collecting system of the kidney and the infusion of contrast at a rate of 10 mL/min. A urodynamic catheter is also placed in the bladder, and intravesical pressures are monitored and subtracted from measured intrapelvic pressures during the infusion. Intrapelvic pressures are noted at the time that contrast is first seen extending passed the ureteropelvic junction and passed the ureterovesical junction.
* '''Interpretation:'''
** Pressure <15 cm H2O considered normal
** Pressure 15-22cm considered indeterminate
** '''Pressure >22 cm H2O suggestive of obstruction'''