In some cases, hydronephrosis will reach a stage of equilibrium and not progress; and in other cases, it may even improve spontaneously.

CLASSIFICATION AND CAUSES

• Variation of normal

• Reflux

• Non-obstructed hydronephrosis; e.g., diabetes insipidus

• Obstructed hydronephrosis: Infravesical obstruction; e.g., posterior urethral valves, supravesical obstruction, extrinsic, intrinsic, luminal; e.g. ureteropelvic junction obstruction, primary megaureter

• Others, such as prune-belly syndrome
  

PATHOLOGY

Impairment of all aspects of renal function except urinary dilution has been demonstrated in the hydronephrotic kidneys of patients studied after release of ureteral obstruction.

In hydronephrosis, fluid exits from the renal pelvis through perirenal extravasation, pyelovenous backflow and pyelolymphatic backflow.

Post-obstructive diuresis is rare and usually occurs after release of bilateral ureteral obstruction or obstruction of a solitary kidney.

Loss of renal tissue causes compensatory growth by hypertrophy of the remaining tissue.

The potential for progression or equilibration in hydronephrosis appears to be determined by several physiologic factors:

1. Urinary output and flow rates during diuresis

2. The type and degree of obstruction

3. Glomerular and renal tubular function

4. Renal pelvic compliance

Hydronephrosis can reasonably be viewed as a beneficial compensatory mechanism that actually protects the kidney against high intrapelvic pressures and further renal damage.

CLINICAL FINDINGS

Symptoms and signs depend upon the etiology of the hydronephrosis, whether it is acute or chronic, and/or whether there are any secondary complications, such as pain, infection, hypertension or anemia.

Infravesical obstructions causing hydronephrosis may present with various symptoms of voiding dysfunction and/or urinary tract infection (UTI), while supravesical causes of hydronephrosis may be totally asymptomatic or present with one of the manifestations just outlined.

EVALUATION

Urinalysis, microscopy and urine culture are important to rule out an infection and also to detect proteinuria and hematuria. A blood count may detect anemia and/or a leukocytosis, and blood chemistry can determine the level of the BUN and creatinine.

RADIOLOGIC EVALUATION

Hydronephrosis is often diagnosed in utero through maternal ultrasound or during evaluation of children with hematuria or a urinary tract infection by ultrasound and/or IVP.

Reflux must be excluded as a cause for hydronephrosis with a VCUG.

All young children must undergo a VCUG after one documented UTI.

The diagnosis of whether obstruction is significant in infancy involves either the pressure flow study, the Doppler study, or the diuretic renal scan. However, none these tests defines obstruction.

1. The diuretic renal scan (Lasix renal scan) involves injection of a radioisotope linked to a molecule that is concentrated and excreted by the kidney and then monitoring its passage through the upper urinary tract with a gamma camera and computer system after diuresis (with Lasix) is induced. The diuretic injection must be given only after the tracer has accumulated immediately above the site of the suspected obstruction.
   The factors that influence the rate of nucleotide washout and the T halftime include: renal function, the volume and contractility of the renal pelvis, and the severity of the outflow obstruction. Therefore, the diagnostic accuracy of this test and its application to the neonate may be somewhat limited because of renal immaturity. 
   A bladder catheter is also necessary to prevent bladder over-distention that might cause the bladder to prevent tracer washout from the kidney.

2. Diuretic Doppler Study.

3. The pressure flow study involves a percutaneous nephrostomy and unfortunately does not measure obstruction nor renal pelvic compliance. It measures distention and distensibility of the renal pelvis. Pressure measurements are made in response to a sustained constant but un-physiologically high flow rate. Some of these obstructions are volume-dependent, while others are pressure-dependent.
   

COMMON CAUSES OF HYDRONEPHROSIS

1. Reflux 

see vesicoureteral reflux.

2. Ureteropelvic junction obstruction (UPJ obstruction).

This is the most common site of obstruction in the upper urinary tract, commonly on the left. In children, the cause of the obstruction is usually intrinsic or extrinsic and due to an accessory polar vessel.

When the obstruction is deemed to be significant according to the studies outlined above, the UPJ obstruction will require surgical repair.

Repair is usually undertaken through a small rib-tip retroperitoneal approach to the kidney, renal pelvis and ureter to excise the area of obstruction and reconnect the renal pelvis to the ureter. A temporary stent is left down the repaired ureter for a few days and is removed in the office. Most children only need to be hospitalized for 2-3 days.

3. Primary megaureter.

This condition is due to an obstruction to the drainage of urine because of an intrinsic problem at the end of the ureter. If the tests outlined above indicate the need for surgery, a bladder approach is made. Through a small "bikini incision," the bladder is exposed and opened and the involved ureter dissected free. The terminal end responsible for the megaureter is excised, and the ureter is then reimplanted back into the bladder in a non-refluxing manner. In order for this to be achieved, a severely dilated ureter may also need to be tapered by excising a strip of the involved ureter or by narrowing it via a folding technique. A stent will be left in the repaired ureter and a catheter left in the bladder. Most children will require only 2-3 days in the hospital, and the bladder catheter will be removed before they leave. The stent will be removed some days later in the office.

SUMMARY

A combination of uroradiologic studies may determine the etiology of the hydronephrosis in the pediatric patient, but the big dilemma is in identifying obstructive causes and determining at what point intervention and possible reconstructive surgery are necessary to correct the problem.