The ureteropelvic junction (UPJ) is the narrow, funnel-shaped area connecting the kidney’s urine-collecting chamber, the renal pelvis, to the ureter, which transports urine down to the bladder. UPJ obstruction refers to a blockage at this point that slows or stops the free flow of urine. In many adult cases, the cause of this drainage impairment is an aberrant blood vessel, often called a “crossing vessel.” This article focuses on the unique mechanism by which this vessel causes obstruction.
Understanding Ureteropelvic Junction Obstruction
The primary function of the kidney is to filter waste from the blood, producing urine that collects in the central renal pelvis. From the renal pelvis, urine must pass unimpeded into the ureter and continue its descent to the bladder. This movement relies on regular, wave-like muscular contractions, known as peristalsis, along the length of the ureter.
When a blockage occurs at the UPJ, the downward flow of urine is interrupted, causing it to back up. This accumulation of urine causes the renal pelvis and the kidney’s collecting system to swell, a condition called hydronephrosis. Hydronephrosis is the physical hallmark of any UPJ obstruction.
Sustained pressure from hydronephrosis can lead to progressive damage to the kidney tissue, potentially reducing its ability to filter blood effectively. While congenital factors are common causes in children, extrinsic compression by a crossing vessel is frequently identified in adults. The severity of the obstruction dictates the potential for kidney damage and the urgency of intervention.
The Mechanism of Crossing Vessel Compression
A crossing vessel is an accessory renal artery or vein that supplies the lower pole of the kidney. These vessels are considered aberrant because they originate from the main renal artery, aorta, or iliac vessels and follow an unusual path across the UPJ. The presence of such a vessel near the junction is a normal anatomical variation.
The vessel itself does not typically cause a blockage until a secondary process occurs, such as kinking or tethering of the ureter. The ureter, a soft, flexible structure, becomes trapped between the rigid crossing vessel and the expanding renal pelvis. As the kidney produces urine, the ureter is compressed externally, similar to a garden hose pinched over a rock.
This mechanical compression causes the ureter to become acutely angled or tethered, significantly impairing the natural peristaltic waves needed to propel urine. The lower pole of the kidney often receives its entire blood supply from this accessory vessel. Therefore, the vessel cannot simply be cut to relieve the pressure, making blood flow preservation a defining factor in surgical planning.
Specialized Diagnostic Imaging
Diagnosis begins with standard imaging, such as an ultrasound, which identifies hydronephrosis. However, a standard ultrasound cannot reliably differentiate between a mild narrowing and a crossing vessel, or assess the functional impact of the blockage. Identifying the presence and exact location of the aberrant blood vessel requires more sophisticated imaging techniques.
Computed Tomography Angiography (CTA) or Magnetic Resonance Urography (MRU) are often employed to visualize the complex anatomy of the renal vasculature and the urinary tract simultaneously. These studies provide detailed, three-dimensional maps that show the aberrant vessel crossing the ureter and the resulting compression. This visualization guides the surgeon on how to manage the vessel during pre-operative planning.
Functional studies, such as a Diuretic Renal Scan, are also used to measure the severity of the obstruction. During this test, a radioactive tracer is injected, and a diuretic is administered to stimulate urine production. The scan measures the time it takes for the tracer to be washed out of the kidney, providing a quantitative assessment of the drainage impairment. This combination of structural and functional imaging determines if the obstruction warrants surgical intervention.
Surgical Correction and Resolution
The standard surgical procedure to correct UPJ obstruction is a pyeloplasty, which aims to re-establish free-flowing drainage of urine from the kidney. The most common approach involves excising the obstructed segment and re-connecting the healthy renal pelvis and ureter. This is often performed using minimally invasive techniques, such as laparoscopic or robotic-assisted surgery, which allow for high precision and faster recovery times.
When a crossing vessel is present, the pyeloplasty technique is modified to manage the vessel without compromising blood supply to the lower pole of the kidney. The surgeon creates a new connection, or anastomosis, between the ureter and the renal pelvis that is positioned anterior to the crossing vessel. This maneuver, known as transposition, effectively moves the ureter’s drainage point away from the compressive force.
The goal is to create a wide, funnel-shaped junction that allows urine to drain freely, placing the reconstructed ureter in a tension-free path over the blood vessel. A temporary stent is usually placed internally, extending from the kidney to the bladder, to support the new connection as it heals. This meticulous surgical approach achieves a high success rate in resolving the obstruction and preserving kidney function.