Urinary Tract Obstruction

Urinary tract obstruction is any impediment to the flow of urine between the renal calyces and the urethral meatus. Two variables define it clinically and determine everything about presentation and outcome: whether it is acute or chronic, and whether it is unilateral or bilateral.

That second distinction is the one most often missed. A unilateral obstruction with a healthy contralateral kidney produces pain but a normal creatinine, because the other kidney compensates entirely. Renal function only deteriorates when obstruction is bilateral, or affects a solitary functioning kidney, or sits below the bladder. A normal creatinine therefore does not exclude a kidney being destroyed.

Obstruction raises pressure upstream, which is transmitted back to the nephron. Glomerular filtration falls, and sustained pressure causes tubular atrophy, interstitial fibrosis and irreversible loss of parenchyma — with the kidney thinning to a rim around a dilated collecting system. Time matters: function is largely recoverable if relieved within a week or two, and progressively less so thereafter.

Causes are best organized by level:

• Upper tract (kidney and ureter) — calculi, the commonest acute cause; ureteric or renal pelvis tumour; blood clot; pelviureteric junction obstruction; extrinsic compression by pelvic or retroperitoneal malignancy, lymphadenopathy, or retroperitoneal fibrosis; ureteric stricture after surgery, radiotherapy or tuberculosis; and pregnancy, in which the gravid uterus causes usually benign right-sided hydronephrosis.
• Bladder outlet and urethra — benign prostatic hyperplasia, much the commonest cause in older men; prostate carcinoma; urethral stricture; bladder neck stenosis; and blood clot or stone within the bladder.
• Functional rather than mechanical — neurogenic bladder from spinal cord injury, multiple sclerosis, diabetic autonomic neuropathy or cauda equina syndrome; and drugs with anticholinergic or sympathomimetic activity, a very common and reversible precipitant of acute retention.

Presentation depends almost entirely on the speed of onset.

Acute upper tract obstruction — classically a ureteric stone — causes severe colicky loin-to-groin pain, with the patient characteristically restless and unable to find a comfortable position, in contrast to peritonitis where they lie still. Nausea, vomiting and haematuria are usual.

Acute lower tract obstruction — acute urinary retention — causes an inability to pass urine with suprapubic pain and a tender, palpable, dull bladder. It is intensely uncomfortable and requires prompt catheterization.

Chronic obstruction is frequently silent, which is why it is dangerous. It may present only as a vague loin ache, nocturia, or the non-specific symptoms of renal impairment. Chronic retention is classically painless, with a grossly distended bladder that the patient has accommodated to, and it may present as overflow incontinence — often misinterpreted as simple incontinence when the bladder is in fact full.

Lower urinary tract symptoms in men — hesitancy, poor stream, terminal dribbling, incomplete emptying, frequency and nocturia — suggest gradual bladder outlet obstruction. Fever with loin pain and obstruction indicates an infected obstructed system, which is a urological emergency requiring drainage, not simply antibiotics.

Post-obstructive diuresis follows relief of a chronic bilateral obstruction: a large diuresis from accumulated solute, impaired tubular concentrating ability and retained urea, which can cause dehydration and dangerous electrolyte loss. Urine output must be monitored and replaced for the first 24–48 hours.

Other consequences are irreversible renal parenchymal loss, recurrent infection and pyonephrosis, stone formation from stasis, bladder hypertrophy with diverticulum formation, and hyperkalaemic or acidotic complications of renal failure.

Ultrasound is the first-line investigation — it is quick, safe and identifies hydronephrosis without contrast or radiation. It has two important limitations: dilatation may be absent in the first hours of an acute obstruction, and in retroperitoneal fibrosis the ureters can be encased without dilating.

Non-contrast CT is the investigation of choice for suspected ureteric stone, identifying nearly all calculi and their site. A bladder scan before and after voiding quantifies residual volume and is the simplest way to identify chronic retention.

Blood tests should include creatinine, eGFR and electrolytes, with particular attention to potassium. Urinalysis and culture identify infection. In men, a PSA is reasonable but should be interpreted cautiously in the presence of retention or catheterization, both of which raise it. Where obstruction is confirmed, the priority is drainage — urethral or suprapubic catheter for lower tract, nephrostomy or ureteric stent for upper tract — and the cause is investigated afterward.

Chronic obstruction is a reversible cause of chronic kidney disease, and should be excluded in any unexplained renal impairment — an ultrasound is cheap and the diagnosis is otherwise easily missed. Recurrent urinary infection and stone disease both perpetuate and result from obstruction. In men, benign prostatic hyperplasia carries its own burden of lower urinary tract symptoms and sexual dysfunction. Long-term catheterization brings recurrent infection, bladder stones and, over many years, an increased risk of squamous cell carcinoma of the bladder.