The urinary tract may be obstructed at any point between the kidney and the urethral meatus. This results in dilatation of the tract above the obstruction. Dilatation of the renal pelvis is known as hydronephrosis.
Obstructing lesions may lie within the lumen, or in the wall of the urinary tract, or outside the wall, causing obstruction by external pressure. The major causes of obstruction are shown. Overall the frequency is the same in men and women. However, in the elderly, urinary tract obstruction is more common in men owing to the frequency of bladder outflow obstruction.
Obstruction with continuing urine formation results in:
1 Progressive rise in intraluminal pressure
2 Dilatation proximal to the site of obstruction
3 Compression and thinning of the renal parenchyma, eventually reducing it to a thin rim and resulting in a decrease in the size of the kidney.
UPPER TRACT OBSTRUCTION. Loin pain occurs which can be dull or sharp, constant or intermittent. It may be provoked by measures that increase urine volume and hence distension of the collecting system, such as a high fluid intake or diuretics, including alcohol. Complete anuria is strongly suggestive of complete bilateral obstruction or complete obstruction of a single kidney.
Conversely, polyuria may occur in partial obstruction owing to impairment of renal tubular concentrating capacity. Intermittent anuria and polyuria indicates intermittent complete obstruction. Infection complicating the obstruction may give rise to
malaise, fever and septicaemia.
BLADDER OUTFLOW OBSTRUCTION. Symptoms may be minimal. Hesitancy, narrowing and diminished force of the urinary stream, terminal dribbling and a sense of incomplete bladder emptying are typical features. The frequent passage of small volumes of urine occurs if a large volume of residual urine remains in the bladder after urination. Incontinence of such small volumes of urine is known as ‘overflow incontinence’ or ‘retention with overflow’. Infection commonly occurs, causing increased frequency, urgency, urge incontinence, dysuria and the passage of cloudy smelly urine. It may precipitate acute retention.
Loin tenderness may be present. An enlarged hydronephrotic kidney may be palpable. In acute or chronic retention the enlarged bladder may be felt or percussed.
Examination of the genitalia, rectum and vagina are essential, since prostatic obstruction and pelvic malignancy are common causes of urinary tract obstruction. However, the apparent size of the prostate on digital examination is a poor guide to the presence of prostatic obstruction.
Routine blood and biochemical investigations may be abnormal, e.g. there may be a raised blood urea or serum creatinine, hyperkalaemia, anaemia of chronic disease or blood in the urine, but the diagnosis of obstruction cannot be made on these tests alone and further investigations must be performed.
This is a reliable means of ruling out upper urinary tract dilatation. Ultrasound cannot distinguish a baggy, lowpressure unobstructed system from a tense, high-pressure obstructed one, so that false-positive scans are seen. However, a normal scan does rule out urinary tract obstruction.
In obstructive nephropathy, the relative uptake may benormal or reduced on the side of obstruction, peak activity may be delayed and parenchymal (as distinct from pelvic) transit time prolonged. If doubt exists as to whether obstruction at the pelviureteric junction is present, frusemide may be administered; satisfactory ‘washout’ of a radionuclide rules out obstruction and vice versa. In general, absence of uptake of radiopharmaceutical indicates renal damage sufficiently severe to render correction of obstruction unprofitable.
This is the most widely used investigation. Urography canusually exclude obstruction even in the presence of severe renal failure, provided that a high dose of contrast medium, renal tomography and, if necessary, delayed films are employed. A plain film is necessary to detect calcification. However, calculi overlying bone are easily missed. In recent unilateral obstruction, the affected kidney is enlarged and smooth in outline. The nephrogram is delayed due to a reduction in the GFR. The calyces and pelvis fill with contrast medium later than on the normal side.
In time the nephrogram on the affected side becomes denser than normal, owing to the prolonged nephron transit time, which allows greater than normal concentration of contrast medium within the tubules. Later, the site of obstruction may be seen, with dilatation of the system proximal to the level of the block .
A full-length film should be taken after an attempt at bladder emptying by the patient. Complete emptying indicates either that no obstruction to bladder outflow exists or that intravesicular pressure can be raised sufficiently to overcome it. Apparent bladder outflow impairment may be the result of nervousness or embarrassment on the part of the patient or failure to carry out the X-ray before the bladder has refilled with contrast medium from above, or may be due to an atonic but non-obstructed bladder. Vesicoureteric reflux can result in contrast medium returning to the bladder from above, giving the appearance of a partially full bladder.
Antegrade pyelography and ureterography
This defines the site and cause of obstruction. It can be combined with drainage of the collecting system by percutaneous needle nephrostomy.
This is indicated if antegrade examination cannot be carried out or if there is the possibility of dealing with ureteric obstruction from below at the time of examination. The technique carries the risk of introducing infection into an obstructed urinary tract. In obstruction due to neuromuscular dysfunction at the pelviureteric junction or retroperitoneal fibrosis, the collecting system may fill normally from below. Cystoscopy, urethroscopy and urethrography Obstructing lesions within the bladder and urethra can be seen directly by endoscopic examination .
Urethrography involves introducing contrast medium into the bladder by catheterization or suprapubic bladder puncture, and taking X-ray films during voiding to show obstructing lesions in the urethra. It is of particular value in the diagnosis of urethral valves and strictures.
Pressure changes within the bladder during filling and emptying can be recorded. Demonstration that a high voiding pressure is required to maintain urine flow is indicative of bladder outflow obstruction. This may be combined with video cystography and urethrography to define the site of obstruction. Normally, while the bladder is being filled there is only a small pressure rise before the voluntary initiation of urination. Uninhibited contractions of the detrusor muscle during filling may be seen in upper motor neurone bladder neuropathy, such as occurs in multiple sclerosis. Less commonly, a neuropathic bladder may be ‘hypotonic’, readily accepting large volumes of fluid before the initiation of weak contractions at a low intravesical pressure. A common cause of such lower motor neurone bladder neuropathy is diabetes mellitus.
Pressure-flow and video studies may enable a logical decision to be taken as to whether surgery to relieve bladder outflow impairment should be carried out.
• Relieving the obstruction
• Treating the underlying cause
• Preventing and treating infection
The ultimate aim of treatment is to relieve symptoms and to preserve renal function. Temporary external drainage of urine by nephrostomy may be valuable, as this allows time for further investigation when the site and nature of the obstructing lesion is uncertain and doubt exists as to the viability of the obstructed kidney, or when immediate definitive surgery would be hazardous. Recent, complete upper urinary tract obstruction demands urgent relief to preserve kidney function, particularly if infection is present.
In contrast, with partial urinary tract obstruction, particularly if spontaneous relief is expected, e.g. by passage of a calculus, there is no immediate urgency. In recent years, increasing use has been made of the insertion of stents to relieve obstruction, either temporarily or on a long-term basis.
This depends on the cause of the obstruction. Dialysis may be required in the ill patient prior to surgery. Nephrectomy or nephroureterectomy is justified when obstruction is due to malignant disease or when it is judged that no worthwhile amount of renal excretory function will be conserved by, or will return after, relief of obstruction.
Permanent urinary diversion is required when the obstruction cannot be relieved; in such cases malignant disease is usually present. Ureteric anastomosis to an ileal conduit opening on to the abdominal wall is often a satisfactory method of diversion. In some patients, obstruction is best relieved by the insertion of indwelling catheters or stents into the ureter. An obstruction high in the urinary tract may require a permanent nephrostomy. In obstruction due to untreatable malignant disease it is wise to consider carefully whether urinary diversion or stent insertion is justified, since this may exchange a painfree death from renal failure for a painful one with malignant invasion of bones or nerves.
Diuresis usually follows relief of obstruction at any site in the urinary tract. Massive diuresis may occur following relief of bilateral obstruction owing to previous sodium and water overload and the osmotic effect of retained solutes combined with a defective renal tubular reabsorptive capacity (as in the diuretic phase of recovering acute tubular necrosis). This diuresis is associated with increased blood volume and high levels of atrionatriuretic peptide (ANP). Defective renal tubular reabsorptive capacity cannot be the sole mechanism of severe diuresis since this phenomenon is not observed following relief of unilateral obstruction. The diuresis is usually self-limiting, but a minority of patients will develop severe sodium, water and potassium depletion requiring appropriate intravenous replacement. In milder cases oral salt and potassium supplements together with a high water intake are sufficient.