Outlet Obstruction

Causes/Differential Dx of urinary obstruction

Congenital (UPJO, PUV), stones, ureteral strictures, extrinsic compression (tumor, pregnancy), retroperitoneal fibrosis, BPH, urethral strictures, dysfunctional bladder

Classic presentation of obstruction

Flank pain, suprapubic pain, UTI, overflow incontinence, anuria, incidental hydronephrosis on imaging.

Upper vs lower tract obstruction clues

Hydronephrosis on US/CT = upper

Distended bladder= lower

Key anatomic choke points

UPJ, pelvic inlet crossing over iliac vessels, UVJ.

Ureteral “water under the bridge” concept

Ureter passes UNDER uterine artery/vas deferens → injury risk during gynecologic surgery.

Unilateral Urethral Obstruction (UUO) early physiologic response

↑RBF via afferent vasodilation (PGE2, NO) + ↑tubular pressure → slight GFR drop. (open faucet, plug the drain (aff/eff)).

Unilateral Urethral Obstruction (UUO) second/third physiologic response

↓RBF from afferent vasoconstriction (RAAS/Ang II) →tubular pressure decreases (shut off sink)→ progressive GFR decline

Bilateral Urethral Obstruction (BUO) physiologic response

Minimal early vasodilation, profound later vasoconstriction, persistent ↑ureteral pressure, blood flow shifts inner → outer cortex (BUOyant boat where water goes from IN boat to OUT boat)

Why BUO causes worse physiology

No “escape route” for vasoactive mediators (because both ureters are obstructed)→ sustained pressure + severe post‑obstructive diuresis.

Post‑obstructive diuresis definition

Large urine output (>200 mL/hr) after relief of BUO or solitary‑kidney UUO→leads to profound loss of Na and water which can be pathologic

Mechanisms of post‑obstructive diuresis

↑ANP, urea washout, ↓Na transporters in TAL, ↓aquaporins(these all occur during obstruction to reduce water) → obstruction relieved→salt wasting then water diuresis without proper homeostasis and correction from body

Risk factors for POD

BUO, solitary kidney obstruction, bladder retention >1 L.

Management of PostObstructive Diuresis

Monitor vitals, BMP, urine output→give fluids, replace electrolytes if depleted

Male bladder outlet obstruction causes

BPH, urethral stricture, median lobe enlargement.

Female obstruction considerations

Ureter injury during pelvic surgery

Medications: alpha blockers

Tamsulosin, doxazosin, terazosin, prazosin, alfuzosin relax prostate smooth muscle → improve flow.

Medications contraindicated in outlet obstruction

Anticholinergics (oxybutynin) worsen retention → NOT first‑line in BPH with high PVR.

Most common cause of prenatal hydronephrosis

UretoPelvic Junction obstruction.

Posterior urethral valves

Congenital cause of severe male infant obstruction → bladder thickening + hydronephrosis.

Overflow incontinence mechanism

Chronic retention → bladder overdistension → dribbling despite obstruction.

UUO vs BUO Differences

UUO: blood flow out to in, early vasodilation via PGs and NO followed by prolonged vasoconstriction
BUO: blood flows from in to out (BUOyant boat), little early vasodilation, more profound vasoconstriction and Post Obstructive Diuresis (POD)