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Functional Urology Unit, Casa Madre Fortunata Toniolo, Bologna, Italy
Urodynamics of upper urinary tract is often synonymous with the assessment of upper urinary tract dilatation without apparent cause of obstruction.
Hydronephrosis and hydroureter are quite common clinical conditions.
Hydronephrosis is defined as distention of the renal calyces and pelvis as a result of obstruction of the outflow of urine distal to the renal pelvis. Analogously, hydroureter is defined as a dilation of the ureter.
The etiology and presentation of hydronephrosis and/or hydroureter in adults differ from that in neonates and children. Anatomic abnormalities (including urethral valves or stricture at the ureterovesical or ureteropelvic junction) account for the majority of cases in children. In comparison, calculi are most common in adults, while prostatic hypertrophy or carcinoma, retroperitoneal or pelvic, are the primary causes in older patients. Hydronephrosis or hydroureter is a normal finding in pregnant women. The renal pelvises and calyceal systems may be dilated as a result of progesterone effects and mechanical compression of the ureters at the pelvic brim.
Early diagnosis of obstruction is important because most cases can be corrected and a delay in therapy can lead to irreversible renal injury.
However, dilatation does not always equate with obstruction since poor drainage of the urine may be a consequence of an intrinsic problem of the pyeloureteral wall.
12.1 Physiology of Urine Transport
Ureters actively convey urine from the kidney to the bladder. This process is performed by two major mechanisms: the active one (physiologic) resulting from contractile activity of the smooth muscles in the wall of pyelo-ureteral system, and passive flow driven by hydrostatic pressure (pathologic) (Fig. 12.1).
Figure 12.1
The transport of urine in the ureter is achieved in two ways: (a) peristaltic activity in which the bolus of urine is pushed ahead of a contractile wave which almost completely obliterates the ureteral lumen; (b) pressure gradient (dilated system) in which the pressure head of bolus is not generated by a contraction of ureteral smooth muscle fibers but caused by the weight of urine
The coordinated muscular contractions propagating along the ureter and providing the active mechanism of urine transport have been generally termed as “ureteral peristalsis.” Contractions of smooth muscles within upper urinary tract are evoked by action potential activity in atypical smooth muscle cells termed pacemaker cells or ICC-like cells (interstitial cells of Cajal). Produced urine is arranged into a bolus that is propelled to the bladder by peristaltic waves occurring from two to six times per minute. Baseline or resting ureteral pressure ranges from 0 to 5 cm H2O, and superimposed ureteral contractions range from 20 to 80 cmH2O.
With increasing urine flow rate, the initial response of the ureter is to increase peristaltic frequency. After the maximal frequency is achieved, further increases in urine transport occur by increase in bolus volume. As the flow rate continues to increase, several of the bolus coalesce and finally the ureter becomes filled with a column of fluid and dilates.
In addition to an increase in fluid input, ureteral dilatation can occur also from a decrease in fluid output secondary to an obstruction where due to the abnormally high resistance to flow, the bolus is pushed back by the contraction wave.
Finally, the relation between ureteral intraluminal pressure and intravesical pressure plays a pivotal role in determining the efficacy of urine passage across the UVJ into the bladder. During filling the normal bladder maintains a relatively low intravesical pressure that facilitates the transport of urine across the UVJ and prevents ureteral dilatation. The ureter has been shown to decompensate when sustained intravesical pressure due to poor bladder compliance approaches 40 cmH2O.
12.2 Evaluation
Imaging studies are generally used to exclude obstruction at the level of the ureters or above by detecting dilatation of the collecting system.
Renal ultrasound, CT scanning, and IVP are the techniques commonly used to establish the cause of dilatation with a clear preference for renal ultrasonography to avoid the potential risks of allergic and toxic complications by contrast media.
Footnote
In presence of dilated ureters a voiding cystourethrography (VCUG) should be performed in boys to detect a vesico-ureteral reflux (VUR) and to evaluate the posterior urethra for the presence of urethral valves.
When the above radiologic tests demonstrate dilatation without apparent obstruction or in cases of asymptomatic dilatation, more specific investigation should be employed in an attempt to clarify the condition. These include:
Diuretic renography or diuretic IVP
Doppler sonography
Pressure-flow study (Whitaker test)
12.2.1 Diuretic Renography & Diuretic IVP
Diuretic renography or diuretic IVP involves the administration of a loop diuretic (e.g., 0.5 mg/kg of furosemide) prior to radionuclide renal scanning or during IVP. The marked increase in urine flow should, if obstruction is present, slow the rate of washout of the radioisotope during renal scanning or further increase the size of the collecting system on IVP.
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