Meldrum KK. Pathophysiology of Urinary Tract Obstruction. In: Wein AJ, Kavoussi LR, Partin AW, Peters CA, eds. Campbell-Walsh Urology. 11th ed. Philadelphia, PA: Elsevier; 2016.
Definition, Etiology, PathogenesisTop
Obstructive nephropathy refers to a group of anatomic and functional abnormalities of the urinary tract that cause impaired urine flow due to a partial or total blockade of the urinary tract.
1) Mechanical: Benign prostate hyperplasia or prostate cancer, bladder neck stenosis, other pelvic tumors (uterine, ovarian, colorectal cancers; retroperitoneal tumors), uterine prolapse, retroperitoneal fibrosis, stenosis of the ureteropelvic junction (acquired or congenital subpelvic ureteral stenosis) or ureterovesical junction (acquired or congenital ureteral stenosis), posterior urethral valve, ureterocele, nephrolithiasis.
2) Functional (neurologic): Spinal cord injury, neurogenic bladder (bladder neck spasm), congenital spinal malformations.
A special form of obstructive nephropathy is reflux nephropathy, a complication of vesicoureteral reflux.
Functional abnormalities include impaired transport of hydrogen and potassium ions, impaired ability to concentrate urine, contraction of blood vessels, reduced renal blood flow, and reduced glomerular filtration rate. Chronic obstruction leads to distention of the collecting system, tubulointerstitial fibrosis, and loss of the renal parenchyma. A typical sign is enlargement of the renal pyelocalyceal system, referred to as hydronephrosis. In patients with a coexisting urinary tract infection, the presence of bacteria and their endotoxins in the renal parenchyma aggravates the existing damage.
Clinical Features and Natural HistoryTop
Clinical manifestations are not characteristic and can be highly varied, depending on the site of obstruction and rate and degree of hydronephrosis progression. Slowly progressive hydronephrosis may be painless, although pain may be associated with an underlying condition or infection. A rapidly developing urinary tract obstruction may cause renal colic. Urine output may be normal, increased, or decreased. Conditions with bilateral obstruction or obstruction in a solitary kidney can result in anuria. In patients with partial obstruction, polyuria may alternate with oliguria. Once the obstruction has been eliminated, patients often develop polyuria, which can be due to osmotic diuresis and a decreased response to vasopressin. This “postobstructive” diuresis needs close monitoring until it resolves because of the possibility of profound serum electrolyte abnormalities that may result (mostly potassium and sodium abnormalities). In patients with hydronephrosis, an epigastric mass (unilateral or bilateral) may be palpable. Hydronephrosis may sometimes also cause local flank pain on percussion. In patients with distension of the bladder, a suprapubic mass may be palpable.
1. Urinalysis: Abnormalities depend on the cause of urinary tract obstruction. Decreased urine specific gravity, microscopic or gross hematuria, and leukocyturia are often observed. Mild proteinuria (<1.5 g/d) may occur.
2. Blood tests: Increased serum urea and creatinine levels (in renal failure); acidosis and hypokalemia (in distal renal tubular acidosis).
3. Imaging studies: Ultrasonography reveals the presence of hydronephrosis and sometimes also location of the obstruction. The absence of hydronephrosis does not always exclude urinary tract obstruction or even a complete blockade. This is particularly true in patients who are dehydrated or have poor kidney function (ie, small kidney or advanced chronic kidney disease) on the affected side. In such individuals a computed tomography (CT) scan without contrast may be considered if the clinical suspicion for obstruction remains high (ie, flank pain, history of nephrolithiasis, history of retroperitoneal lymphoma or radiation). Other studies (urography, voiding cystourethrography, ascending pyelography) are useful in locating the obstruction and determining its nature. Radionuclide renography (with or without furosemide) is sometimes helpful in differentiating functional dilation of the pyelocalyceal system from hydronephrosis caused by an anatomic obstruction.
Treatment depends on the location of obstruction and the cause and degree of renal impairment. A total obstruction of the urinary tract causing acute kidney injury requires urgent intervention. Urologic consultation is advised to bypass the obstruction for treatment. This may involve placement of a Foley or suprapubic catheter for lower urinary tract obstructions and possible stenting or nephrostomy tube placement for upper urinary tract obstructions. Serum chemistry abnormalities as a consequence of acute kidney injury due to obstruction should be monitored closely and treated appropriately (see Acute Kidney Injury).