Introduction

Interventional radiology (IR) involves the use of contemporary imaging techniques such as fluoroscopy and ultrasonography to selectively access vessels and other structures in order to deliver different materials for therapeutic reasons. Currently, the investigation and application of these techniques is expanding in veterinary medicine, particularly applications pertaining to the urinary tract. The relatively common incidence of urinary tract obstruction, neoplasia and trauma, combined with the morbidity associated with traditional surgical techniques, makes the use of less invasive procedures such as laparoscopy, endoscopy and IR highly appealing. This lecture will include an introduction to veterinary IR and briefly present a number of procedures including anterograde urethral catheterisation, percutaneous cystostomy tube placement, palliative urethral stenting for malignant obstructions and intra-arterial chemotherapy and arterial embolisation of urogenital tumours.

Advantages

The use of IR and endosurgical techniques in veterinary patients offers a number of advantages compared to more traditional therapies. These procedures are minimally invasive and lead to reduced perioperative morbidity and mortality, shorter anaesthesia times and shorter hospital stays. The less equipment-intensive procedures can result in reduced cost as well. In addition, some techniques, such as palliative stenting for malignant obstructions, offer alternative treatment options for conditions that may not be amenable to standard surgical or medical approaches. The primary disadvantages of IR include the required technical expertise, the specialised equipment necessary (fluoroscopy with or without digital subtraction capabilities) and the large initial capital investment necessary to provide a suitable inventory of catheters, guidewires, balloons, stents and coils.

Equipment

As most of these procedures are minimally invasive (performed through catheters or small holes in the skin), traditional sterile operating rooms are not required, but recommended. The entry sites receive a traditional sterile scrub, and operators wear full lead gowns, lead thyroid shields, caps, gowns and masks. The radiation exposure during conventional or C-arm fluoroscopy can be substantial. The operator should review radiation safety guidelines, minimise exposure time and beam size, and maximise shielding and distance from the beam.

For many of the more commonly performed IR procedures, a traditional fluoroscopy unit is sufficient. A C-arm fluoroscopy unit has the advantage of mobility of the image intensifier, permitting multiple tangential views without moving the patient. Digital subtraction angiography (DSA) and 'road-mapping' allow high-resolution images to be obtained with minimal use of contrast agent which is often a concern in our relatively small veterinary patients. DSA is required for super-selective angiograms of small calibre vessels and those vessels in the head (or where there is substantial bone which makes angiogram visualisation difficult). Occasionally, ultrasonography is useful for percutaneous needle access into vessels or other structures (urinary bladder, renal pelvis, etc.). Commonly used equipment includes guidewires, introducer sheaths and dilators, catheters, balloons, stents and embolic agents. Each will be discussed briefly as they pertain to the subjects covered.

Techniques

Percutaneous Anterograde Urethral Catheterisation

Urethral catheterisation is typically a simple and routine procedure in veterinary patients in order to monitor urine output, establish urine drainage in patients that are recumbent or have mechanical/functional urethral obstructions or to ensure patency following surgical procedures. Occasionally, retrograde catheterisation can be difficult in very small (female) patients or in feline patients with significant urethral trauma/tears (e.g., following manipulations to relieve urethral obstruction). Anterograde urethral catheterisation performed under direct fluoroscopic visualisation can be performed rapidly, easily and safely in patients in whom attempts at routine retrograde catheterisation have failed.

Under general anaesthesia (recommended) or heavy sedation, the patient is placed in lateral recumbency and the flank and caudal ventral abdomen are clipped and scrubbed. Cystocentesis is performed and contrast medium is injected through the cystocentesis needle to define the urinary bladder and urethra. Under fluoroscopic guidance, a guidewire is advanced anterograde into the bladder and through the urethra until exiting the penis or vulva. A urinary catheter is then advanced over the wire in a retrograde fashion into the urinary bladder and the guidewire is removed. The urinary catheter is secured in place in a routine fashion.

Percutaneous Cystostomy Tubes

Cystostomy tubes are regularly placed during surgery to manage veterinary patients with urinary obstructions or to divert urine away from a traumatised urethra. Occasionally, these patients are severely debilitated and metabolically unstable, such that even a short period of general anaesthesia would be dangerous. A variety of cystostomy tubes and techniques is available for percutaneous placement in order to quickly and safely establish urine drainage and/or diversion. Locking-loop drainage catheters have been used for such purposes in veterinary patients. These tubes can be placed directly into the urinary bladder via palpation alone, or with fluoroscopic or ultrasound guidance.

Ureteral Stenting

Ureteral stenting for malignant obstructions, strictures or stones is performed for a variety of disorders to divert urine from the renal pelvis into the urinary bladder. This technique can be useful in patients with ureteral obstruction and due to ureteral stones, ureteral or trigonal obstructive cancer, following ureteroscopy, for postoperative ureteral anastomosis, ureteral tears, ureteral spasm or ureteral inflammation. In addition, the presence of the ureteral stent results in subsequent passive ureteral dilation to permit passage of previously obstructive ureteroliths, or allow passage of the flexible ureteroscope for appropriate ureteral intervention. This technique is currently under investigation for use in veterinary patients with ureterolith-induced obstructions, particularly in cats. Ureteral stenting is also ideal in patients with nephroliths or ureteroliths that are undergoing extracorporeal shock-wave lithotripsy to aid in fragment passage following treatment. Stent placement in the urethra for bladder tumours causing obstruction is now being performed routinely.

Urethral Stenting for Malignant Obstructions, Strictures and Detrusor-Urethral Dyssynergia

Malignant obstruction of the urethra can cause severe dysuria and life-threatening azotaemia in some patients. Transitional cell carcinoma of the urethra and/or prostate is the most common lower urinary tract neoplasia encountered in small animal patients, with greater than 80% of dogs experiencing dysuria and approximately 10% developing complete urinary tract obstruction. Chemotherapy has been successful in slowing tumour growth but complete cures are uncommon. When signs of obstruction occur, more aggressive therapy is indicated. Cystostomy tube placement, transurethral resection and surgical diversion have been described but are either invasive or associated with significant morbidity (complications of manual urine drainage, frequent urination, urinary tract infection). Placement of self-expanding metallic stents using fluoroscopic guidance through a transurethral approach can provide a fast, reliable and safe alternative to establish urethral patency in both males and females. We have experienced good to excellent palliative outcomes in greater than 75% of cases. Urethral stenting may also be useful in patients with benign urethral strictures when traditional therapies have failed or when surgery is refused or not indicated.

A contrast cystourethrogram is performed and trans-urethral retrograde or anterograde guidewire access across the malignant narrowing is obtained. Measurements of the normal urethral diameter and the length of obstruction are obtained and an appropriately sized self-expanding metallic urethral stent is chosen (approximately 10–15% greater than the normal urethral diameter and 1 cm longer than the obstruction on both the cranial and caudal ends). The stent is deployed under fluoroscopic guidance and a repeat contrast cystourethrogram is performed to document restored urethral patency.

These techniques have also been used for urethral strictures and detrusor-urethral dyssynergia.

References

References are available upon request.