Practical tips and tricks for lower urinary tract obstruction

Introduction

Minimally invasive procedures for the treatment of urinary tract obstruction are increasingly available in veterinary medicine. These procedures have numerous advantages over traditional surgery and allow quick patient recovery, reduced hospitalization time and reduced morbidity. While advanced interventional radiology and endoscopic procedures require specific training, advanced equipment and referral to a specialist, many minimally invasive urinary tract procedures can be performed in general practice (1). This article will describe minimally invasive diagnostic and treatment options for the management of cats and dogs with urethral obstruction, with a focus on procedures that can be performed in general practice.

Presentation

Clinical signs related to lower urinary tract obstruction (LUTO) are a common reason for dogs and cats to present at the clinic. Prior to complete urethral obstruction, various signs, such as stranguria, dysuria, pollakiuria, periuria and a reduced or absent urine stream, may be observed. Straining to urinate is never considered normal and always merits investigation. Clinical signs may vary with the degree of urethral obstruction; animals may inappropriately urinate in the house, frequency of urination may increase, the urine flow/pressure may be reduced, hematuria may be observed, they may vocalize or vomit, and strain to urinate with little to no urine emission. Excessive licking of the genital region may be observed. On occasion, owners will have difficulty differentiating straining to urinate with constipation, especially in cats (Figure 1). Incontinence or urine dribbling may also be observed and can be associated with overflow due to a partial blockage, and is a common cause of incontinence in cats (2).

Male cats and dogs are more likely to develop urethral obstruction given their narrow penile urethra. The incidence of urethral obstruction in cats is variable between studies, with idiopathic obstruction (+/- 50%), urethral plugs (18-60%) and urolithiasis (20-30%) being commonly observed. Neoplasia and urethral stricture are less commonly encountered (5%) (3-6). Urethral obstruction due to urolithiasis is common in male dogs, with calcium oxalate stones occurring most frequently (7). Struvite-containing stones are more commonly observed in female dogs due to their predisposition to urinary tract infections with urease-producing bacteria (7-12). Urethral obstruction secondary to blood clots, urethral plugs, urethral strictures and neoplasia may also occur in dogs (13-15).

Figure 1. Cat owners may observe their pet in the litter tray straining, but may not be able to differentiate between difficulty urinating and constipation. © Shutterstock

Physical examination

Exam findings aid in the diagnosis of underlying disease and help in prioritizing diagnostic tests. The classic changes on physical examination of a patient with urethral obstruction are straining to urinate associated with abdominal discomfort and a distended/firm urinary bladder. Depending on whether urethral obstruction is partial or complete, the bladder may be difficult to manually void, or cannot be expressed in cases of complete obstruction. Rectal palpation may reveal a distended or turgid urethra, or a mass may be palpable in cases of proliferative urethritis or urethral neoplasia. Rectal examination allows palpation of the prostate in male dogs, and assessment of the pelvic urethra in both females and males. Stones may be palpated along the urethra, especially in male dogs, where nearly 80% of the urethra can be palpated on physical examination. Careful palpation of the penile urethra is indicated, as this is a common site for urethral obstruction due to the narrowing of the urethral lumen as it passes through the os penis. In female dogs a vaginal examination should be performed prior to rectal palpation. Rectal examination may be difficult in cats and necessitates anesthesia, but the prepuce and vulva can be examined. 

Equipment for minimally invasive procedures

The remainder of this article will cover some minimally invasive diagnostic and treatment options for the management of patients with challenging lower urinary tract obstruction. In order to perform these, the authors recommend the following items: hydrophilic guidewires, open-ended catheters, iodinated contrast agent, digital radiography and ultrasound. Radiopaque guidewires are available in different sizes and lengths; these should be at least twice the length of the catheter used to facilitate exchanges (Table 1). The authors prefer hydrophilic-coated angled guidewires for the urinary tract, as they will slide easily when wet due to their low friction coefficient, thus avoiding perforation of the urethra.

Table 1. Equipment for minimally invasive access to the lower urinary tract.

Urethral obstruction diagnosis

Abdominal radiographs are recommended whenever urethral obstruction is suspected. These may show bladder overdistension and will identify any radiopaque stones, as well as determining their size, number, contour and location (Figure 2). In male dogs and cats with urethral obstruction, stones are more likely to be observed in the distal urethra where the urethral lumen narrows. In order to minimize the risk of missing a lesion/urolith in the penile urethra due to femur superimposition, two sets of lateral radiographs are recommended, one with the pelvic limbs flexed and the other with the pelvic limbs extended. The pelvic urethra is poorly visible on standard radiographs, and ultrasound only allows evaluation of the proximal portion. Oblique radiographs, with the patient placed in lateral recumbency and a hind leg elevated at a 45^o angle, helps visualize the pelvic urethra (Figure 3). Cystourethrography can be performed in practice and allows evaluation of the entire urethra (see below). Bladder ultrasound is useful in identifying small and/or radiolucent stones and soft tissue masses. Ultrasound evaluation of the penile urethra, especially proximal to the os penis, is sometimes overlooked, but is warranted where urethral obstruction is suspected. 

Figure 2. Plain (a) and contrast (b) abdominal radiographs from a cat that presented with hematuria and discomfort urinating; whilst bladder stones may be easy to visualize on radiographs, urethral calculi can be much more difficult to identify. © E. McNeill 

Figure 3. Urethral calculi can be difficult to identify, as they may be superimposed over the pelvic bones on lateral radiographs; oblique radiographs may help in this situation. © Dr. C. Vachon

Contrast cystourethrography

Contrast cystourethrography is indicated as a diagnostic imaging modality for the investigation of any patient with suspected urethral obstruction or voiding anomalies. This allows for complete evaluation of structural and functional urethral obstructions, and is especially useful for the detection of radiolucent stones, urethral stricture, spasm, urethral tears, reflex dyssynergia, functional urethral outlet obstruction (FUOO), and extra- and intra-luminal masses. When investigating a functional obstruction, consideration should be given to the sedative/anesthetic agents used, as these can result in muscle relaxation and will alleviate a dynamic (i.e., urethral spasm) obstruction. Cystourethrography is generally safe, with few reported complications (e.g., bacterial contamination, iatrogenic damage to the urethral/bladder wall, anecdotal air embolism) (16,17).

A fluoroscopic cystourethrogram is ideal for real time evaluation of the urinary tract, especially for dynamic pathologies such as urethral spasm, reflex dyssynergia or FUOO. Imaging the voiding phase is essential (as opposed to retrograde filling through a urinary catheter) in order to obtain a complete and diagnostic examination.

The patient is deeply sedated or anesthetized and aseptically prepared for retrograde catheterization. A marker catheter positioned within the colon may be useful when specific measurements are required (e.g., for urethral stent placement), and lateral and dorsoventral views will ensure complete evaluation of the lower urinary tract. A guidewire is gently advanced retrograde through the urethra (preferably under fluoroscopic guidance) until it curls in the bladder; ultrasound guidance can be beneficial to confirm positioning if fluoroscopy is not available. Over the guidewire, a premeasured (length between the distal urethra and the bladder apex) open-ended urinary catheter is advanced until it reaches the bladder, and the guidewire removed. A non-ionic iodine water-soluble contrast agent (usually mixed with saline in a 50:50 ratio) is infused in the bladder until it is firm on palpation and the trigone and urethra are well delineated on fluoroscopy/radiography. In order to obtain a diagnostic voiding phase, the bladder must be well distended; underfilling is a common reason for non-diagnostic studies.

Obtaining maximal distension of the urinary bladder and urethra is achieved by manually voiding the previously distended urinary bladder while simultaneously injecting contrast through a urinary catheter positioned at the mid-distal end of the urethra (Figure 4). The urinary catheter is withdrawn while continuing to inject contrast to allow evaluation of the distal urethra. If the cystourethrogram is performed with radiography, the timing of the radiograph is crucial, and must be taken during manual voiding.

If urethral catheterization is unsuccessful (due to a tear or complete/distal urethral obstruction) cystourethrography can still be performed. Here contrast may be injected through a needle inserted into the urinary bladder (reverse cystocentesis); if necessary, partial decompression via cystocentesis may be beneficial if the bladder is overdistended, with the amount of urine removed replaced by the same amount of contrast agent, until sufficient bladder filling is obtained. The needle is withdrawn from the bladder and contrast cystourethrography performed as described above.

In order to achieve diagnostic imaging while maintaining optimal radiation protection, it is important to perform the study with lead gloves and gown, which may partially impair the visualization of the bladder during the study. However, the most important phase of a voiding cystourethrogram is the urethrogram phase; the bladder can be evaluated by ultrasound once fully distended with contrast prior to voiding.

Figure 4. A fluoroscopic cystourethrogram allows real time evaluation of the urinary tract. With the dog under general anesthetic and the bladder filled with a contrast agent, the bladder may be manually emptied (e.g., via pressure applied over the bladder area with a wooden implement) while simultaneously injecting contrast through a catheter positioned at the mid-distal end of the urethra. Inset; the large arrows show the direction of urine flow, while the small arrows denote the distension of the urethra and bladder neck. Care must be taken to ensure the operator’s hands remain outside the primary radiation beam whilst performing fluoroscopy. © Drawn by Sandrine Fontègne

Retrograde catheterization with angle tip guidewire

An angle tip hydrophilic guidewire can be easily passed through the urethra when urethral access is difficult. The guidewire, leading with the angle or floppy tip, is gently advanced along the urethra to the bladder; it should be able to pass along the urethral lumen through a stricture and alongside stones, although gentle redirection of the wire may be necessary to move around an obstruction. Fluoroscopy, radiography or ultrasound may be used to confirm proper positioning in the urinary bladder. An open-ended urinary catheter can be subsequently advanced over the guidewire into the bladder before removing the guidewire and flushing the bladder. Positioning is once again confirmed using radiography or ultrasound, and the catheter can then be sutured in place.

Antegrade catheterization

Where retrograde catheterization is unsuccessful, but urethral catheter placement is necessary (e.g., torn urethra, bladder rupture) anterior catheterization can be performed, preferably with ultrasound or fluoroscopic guidance, although this is not mandatory. The patient is anesthetized, aseptically prepared and placed in dorsal recumbency. An 18G-22G intravenous catheter is first inserted into the apex/body of the distended urinary bladder (avoiding the bladder trigone) (Figure 5a). The stylet is withdrawn, and an angle tip hydrophilic guidewire (for sizes see Table 1) is passed through the catheter towards the trigone. The guidewire is advanced until it exits through the prepuce/vulva (Figure 5b), achieving through and through access (flossed). The intravenous catheter is then removed from the bladder lumen (Figure 5c) and an open-ended urinary catheter passed over the guidewire in a retrograde manner until positioned in the bladder lumen. The guidewire is then removed and the urinary catheter sutured in place.

Figure 5. Antegrade catheterization. (a) An 18G-22G intravenous catheter is inserted into the distended urinary bladder, using ultrasound for guidance if desired. (b) An angle tip hydrophilic guidewire is passed through the catheter towards the trigone and advanced until it exits through the prepuce/vulva. (c) The intravenous catheter is removed from the bladder and an open-ended urinary catheter passed over the guidewire in a retrograde manner until positioned in the bladder lumen; the guidewire is then removed. © Drawn by Sandrine Fontègne

Urohydropulsion

Apart from medical dissolution, urohydropulsion is the least invasive procedure for the removal of bladder stones, and minimal equipment is required to perform it successfully; a red rubber urinary catheter and sterile saline are sufficient. Female dogs and female cats benefit most from this procedure; it is not indicated in male cats due to the high risk of urethral obstruction. Urohydropulsion can be attempted in male dogs, but tends to be less successful given the long and tortuous urethral anatomy. Small stones (2-3 mm) can usually be voided using this technique; larger stones may be voided depending on the size of the patient. The urethral diameter can be estimated by passing a urinary catheter; the largest size that passes through the penile urethra will determine the approximate size of stone that may be voided using this technique.

The patient is anesthetized and aseptically prepared for retrograde urethral catheterization. The urinary catheter is used to fill the bladder, while avoiding overfilling (bladder capacity is approximately 10-15 mL/kg). If catheterization is unsuccessful, passive urethral/bladder filling may be achieved by infusing saline in the vestibule. Once the bladder is firm, the catheter is removed and the patient placed vertically. The urinary bladder is palpated, gently pulled cranially and shaken to move the stones towards the bladder trigone and to straighten the urethra. Gentle manual pressure is applied to the bladder to promote voiding. The bladder should be maintained with gentle cranial traction during voiding to prevent kinking of the urethra. If urethral obstruction occurs from passage of the stones, the stones can be retropulsed using a urinary catheter and the procedure aborted or repeated. The above technique can be repeated until all stones have been voided; it can also be used to obtain stones for mineral analysis.

Mild hematuria may occasionally be seen afterwards, but antibiotic administration following the procedure is controversial; 3-5 days of antibiotics may be administered if there is severe hematuria or a break in sepsis. If a lower urinary tract infection is suspected prior to the procedure, antibiosis treatment is recommended prior to performing urohydropulsion to reduce the risk of iatrogenic damage (i.e., a tear to the bladder or urethral wall). If urohydropulsion is not successful, cystoscopy (+/- laser lithotripsy or basket removal) and percutaneous cystolithotomy are alternatives that usually require referral. Medical dissolution should always be considered prior to any stone removal.

Conclusion

Minimally invasive interventional therapies are the new standard of care in veterinary medicine and are increasingly available and requested by pet owners. These procedures are associated with minimal tissue injury, shorter recovery times and decreased morbidity. Proper materials/equipment and careful case selection are essential for successful management. For further understanding of the usefulness of these techniques, the reader is referred to the associated case-based article entitled “Lower urinary tract obstructions – a case-based approach” which describes how these procedures can be used to treat patients.

References

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