Do you see what I see?
A diagnostic imaging approach to feline urethral obstruction.
Hanna Murray, DVM, Residency Trained
VSEC Radiology Department
Feline urethral obstruction (UO) is a common medical emergency encountered in small animal practice.¬† This problem affects more males than females given the length and narrowness of their urethra.¬† Urethral obstruction is a medical emergency which can potentially be life-threatening if unrecognized or left untreated due to severe changes in electrolytes or azotemia which develops as a sequela of the obstruction. If identified and treated early many of these changes can be corrected with supportive medical care.
Recent research shows that the majority of cases with UO are idiopathic in nature (53%). Other common causes include calculi (29%) and urethral plugs (18%)1,2. Less common causes in the feline can include urethral stricture or neoplasia.
Clinical signs of UO include stranguria, dysuria, and hematuria.¬† Other clinical signs related to the secondary effects of UO (azotemia/uremia) can include anorexia, vomiting, lethargy, or collapse.¬† Physical exam findings often reveal a large turgid bladder which cannot be expressed with gentle pressure.¬† Examination of the penis may reveal reddening, swelling, or even bleeding associated with trauma. Some cats may also exhibit visible urethral spasm. In some cases, a mucous plug is identified at the tip of the penis which may be manually removed to relieve the obstruction.¬† Diagnostics should include a minimum database (PCV/TS, BUN, creatinine, sodium, potassium, chloride, phosphorus, and calcium), urinalysis, and abdominal radiographs (ideally prior to catheterization).
Abdominal radiographs are often the first step in trying to identify an underlying cause for UO.¬† The radiographic study should include orthogonal projections (lateral and VD) and include the diaphragm through the penile urethra. Ideally survey radiographs are obtained prior to catheterization as the presence of the catheter and decreased urinary bladder size may make interpretation of the urinary tract difficult. Repeat radiographs (at least a lateral) following catheterization should be obtained to ensure proper placement of the urinary catheter.
When evaluating the abdomen, take care to evaluate the degree of detail within the abdomen, specifically surrounding the bladder. Generally, cats with UO will have an enlarged bladder which often can displace the intestines and colon (Figure 1).
Figure 1: Right lateral projection of a feline with urethral obstruction. Note the marked distention of the urinary bladder. The abdominal contrast and serosal detail are normal.
Loss of this detail may be an indicator of free fluid within the abdomen and urinary leakage. As mentioned previously, some felines have UO secondary to calculi formation, pay attention to changes in opacity along the ventral bladder wall and into the urethra all the way through the penis. This mineralized material can form distinct round calculi, however often is more amorphous and diffuse due to the fine sandy nature of the material (Figure 2).
Figure 2: Right lateral abdomen following urinary catheter placement. Note the irregular mineral opacity within the apex of the urinary bladder, consistent with urinary calculi (grit/sand).
If you have a digital system, window and leveling may help to highlight the subtle presence of mineral. Felines with UO may also have mineralization or calculi at the level of the kidneys and may be predisposed to repeat UO (Figure 3).
Figure 3: Abdominal ultrasound of the urinary bladder. Note the bladder wall thickening between the markers, consistent with cystitis. A urinary catheter is present within the lumen of the bladder.
For patients with clinical signs of UO without radiographic cause; advanced imaging such as abdominal ultrasound can be considered.¬† Abdominal ultrasound is a minimally invasive test which allows visualization of the urinary bladder, urinary bladder wall, as well as the kidneys.¬† Changes identified on ultrasound can include urinary bladder wall thickening sediment within the urinary bladder, mineralization within the kidneys or calculi formation within the urinary tract (kidneys, ureters, and bladder).¬† These calculi are not always radiographically visible (Figure 4).
Figure 4: Right lateral radiograph showing distention of the urinary bladder with urinary catheter placement and no identifiable mineral within the urinary bladder (A). Mineralization is present within the kidneys (black arrow). Ultrasound image of the same urinary bladder note the hyperechoic material within the gravity dependent portion of the bladder, consistent with urinary sediment or small calculi (red circle). At surgery small round red colored stones were removed.
Additionally, excess tissue or masses within the proximal urethra and trigone region of the urinary bladder may also be identified via ultrasound.¬† Ultrasound guidance may also be used for therapeutic cystocentesis prior to relief of urethral obstruction.¬† In some patients the abdominal ultrasound may be normal; however, this finding does not exclude idiopathic cystitis or inflammation within the pelvic or distal urethra as an underlying cause.
Occasionally UO can have adverse effects which result in more aggressive treatment such as surgical intervention.¬† Due to the degree of urinary bladder distention and increased abdominal pressure when patients are straining to urinate, rupture of the urinary bladder wall becomes a concern.¬† The presence of free fluid within the abdomen does not indicate an overt rupture, but may be secondary to increased permeability of the bladder wall and mural leakage of urine.¬† In cases of suspected bladder rupture a retrograde contrast cystourethrogram should be performed to assess bladder wall integrity (Figure 5).
Figure 5: contrast cystogram. The urinary bladder is distended with contrast material, leakage of contrast material into the peritoneal space is not identified.
- For this procedure, patients should be placed in left lateral recumbency under heavy sedation or light anesthesia. Iodinated contrast is then instilled into the urinary bladder.¬† During this procedure the urinary bladder should be palpated to avoid overdistention of the bladder3.
- The feline bladder should whole a volume of 2-5 mL/kg3.
- Radiographs should be obtained in left lateral recumbency, oblique, and dorsal recumbency to assess for bladder leakage.
- Presence of contrast within the peritoneal space confirms the presence of a bladder rupture, indicating the need for surgical intervention.
- Take care to avoid leakage of the contrast material around the catheter or catheter tip as contrast material on the radiography table or hair coat can mimic leakage.
Treatment of UO often involves placement of a rigid urinary catheter and is often replaced by an indwelling urinary catheter.¬† This procedure often results in urethral trauma, swelling, and occasionally a tear within the urethra.¬† In some cases the presence of a urethral tear can lead to mild peritoneal effusion identified via abdominal radiographs or abdominal ultrasound. In these cases, contrast urethrography can help to evaluate the integrity of the urethra and position of the suspected tear (Figure 6).
Figure 6: Contrast urethrogram. Contrast is injected under pressure. Note the complete distention of the urethra at the level of the pelvis. Caudal to the pelvis contrast is identified, extending dorsal to the urethra, consistent with a urethral tear (red circle).
- For this procedure patients are placed in left lateral recumbency with a urinary catheter placed within the bladder. The urinary catheter is then retracted slowly into the urethra injecting iodinated contrast to evaluate the post pelvic, pelvic, and pre-pelvic urethra3.
- The volume of contrast is between 2-5ml
- Radiographs should be obtained while injecting under pressure to assess for leaks within the urethra.
- Treatment options very based on the location of the urethral tear.
In patients which are difficult to catheterize, contrast urethrography can be used to identify the presence of urethral stricture or radiolucent calculi within the urethra.
- For this procedure contrast is administered as the catheter is advanced into the urinary bladder.
- Take care to avoid leakage of the contrast material around the catheter or catheter tip as contrast material on the outside of the patient can make interpretation difficult.
Diagnostic imaging is a valuable tool in the diagnosis and management of the UO patient. Combining radiographs with abdominal ultrasound can help give a more complete picture of this disorder in the complicated patient and help steer treatment.
- Gerber B, Eichenberger S, Reusch CE. Guarded long-term prognosis in male cats with urethral obstruction.J Feline Med Surg¬†2008; 10:16-23.
- Cooper ES. Controversies in the management of feline urethral obstruction. J Vet Emerg Crit Care 2015:25(1):130-137.
- Wallack ST. Iodinated esophagram-dog. In: Wallack ST, editor.The Handbook of Veterinary Contrast Radiography.¬†Solano Beach, Calif, USA: San Diego Veterinary Imaging; 2003.2003¬†
- WALLACK, S. T. (Ed) (2003) The Handbook
- of Veterinary Contrast Radiography. San
- Diego, San Diego Veterinary Imagi