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Stent placement is a novel technique used for obstructive diseases in the respiratory tract (severe tracheal collapse), and urinary tract [ureter (ureteral obstruction from stones and cancer) and urethra (cancer, severe inflammation, or spasm)].
These procedures can be life-saving in cases of severe tracheal collapse where medications are not powerful enough to control clinical signs. This can be very useful in cats with unilateral or bilateral kidney obstruction (ureteral obstruction). The stent provides a direct communication between the bladder and kidney and avoids the need to surgically removing the stones as surgery can lead to significant complications.
There are several indications for stent placement.
Tracheal stent:
1) Persistent cough due to tracheal collapse (when unable to manage medically)
2) Respiratory distress, exercise or heat intolerance due to tracheal collapse (when unable to manage medically)
Urethra stent:
1) Urethral stricture due to calculi or inflammation
2) Urethral stricture due to urethral or prostatic cancer
Ureteral stent:
1) Congenital ureteral stricture
2) Ureteral obstruction due to bladder cancer (transitional cell carcinoma)
3) Ureteral calculi (treatment and prevention of future obstruction)
Stents in veterinary medicine - What can it offer our patients?
Joao Felipe de Brito Galvao, MV, MS, DACVIM (SAIM)
Interventional radiology has advanced significantly in veterinary medicine over the past decade. Nowadays, we are able to offer a variety of palliative procedures that can considerably improve the quality of life and longevity of our patients. Current applications and highlights of stent use in veterinary medicine are described.
Tracheal stenting
Refractory tracheal collapse dogs benefit from placement of a tracheal stent. Even though highly successful (70-90% show clinical improvement)1, this procedure is recommended after patients failed to respond to anti-tussive and anti-inflammatory therapy. Tracheal stenting is the best therapy available for severe tracheal collapse, nowadays rarely leads to complications such as stent migration, fracture, formation of excessive granulation tissue. Ideally, measurement and stent placement should be done at the same time because most patients that require tracheal stent placement are in respiratory crisis and may be dependent on mechanical support to keep the tracheal lumen open. Most complications from stent placement are secondary to underestimation of the stent diameter required.2 More recently, thoracic CT has been recommended instead of plain radiology for measurement of tracheal diameter. Cross-sectional area of the tracheal varies by approximately 20% when comparing inspiration and expiration in normal dogs.
Nasopharyngeal stenting
Nasopharygeal stenosis can be either congenital or acquired (e.g. chronic rhinitis, foreign body). Balloon dilation under endoscopic and/or fluoroscopic guidance remains the standard of care in veterinary medicine. However, nasopharyngeal stenting may be beneficial for maintaining an open lumen, especially in refractory cases. Covered stents should be considered initially if stenosis completely occludes airflow.4 Possible complications include stent migration and excess granulation tissue formation. Mitomycin C, an antineoplastic agent that inhibits fibroblast proliferation and reduces collagen cross-linking, has been used to reduce scar tissue formation and prevent re-stricture. Despite possible complications, the combination of balloon dilation and stent placement offers the best treatment option.
Esophageal stenting
Esophageal stricture formation can occur secondary to many conditions including foreign body, cancer, and gastroesophageal reflux, among others. Benign esophageal stricture typically requires 1-5 balloon dilation procedures.5 However, some cases may require many more depending on the patients ability to form scar tissue, location, duration and extent of the stricture, as well as perpetuating factors such as gastroesophageal reflux. In order to help decrease scar tissue formation intralesional steroid (i.e. triamcinolone) and, more recently, mitomycin C, have been used. Esophageal stents offer the opportunity to help decrease restricture in cases of benign stricture, subsequently requiring less balloon dilation procedures. Additionally, esophageal stenting offers palliative care in cases of malignant esophageal strictures.6 Stent migration is the main complication of esophageal stent placement. Special suturing techniques are available to minimize this risk, if indicated.
Colorectal stenting
Benign rectal strictures are typically successfully treated via balloon dilation. However, balloon dilation is not effective in management of a colorectal neoplastic obstruction. Surgery is typically the treatment of choice for colorectal cancer. Unfortunately, invasive surgery such as splitting the pelvis is often required for removal of a colorectal mass. Dehiscence is also possible, especially when dealing with the colonic surgery. Colorectal stent has the potential of improving quality of life as well as longevity. For instance, a dog with a carcinoma in situ had significant improvement of his clinical signs for 212 days after colorectal stent placement.
Urethral stenting
Urethral stenting has been commonly used for urethral transitional cell carcinoma (TCC) or prostatic carcinoma. Average survival time post stent placement in dogs with TCC is approximately 78 days.8, 9 Therefore, stent placement should be considered palliative treatment in cases of urinary obstruction. Nevertheless, treatment with NSAIDs before and chemotherapy after stent placement increased survival time to 251 days in a previous study.9 Incontinence occurred in approximately 30% of females and 20% of males in one report.8 Incontinence should be carefully discussed with the owner prior to stent placement even though most owners in a previous report did not feel it affected their pets quality of life. Urethral stenting can also be considered for benign urethral obstructions such as granulomatous urethritis, if refractory to medical therapy (Figure 1). Covered stent is preferred since granulation tissue can easily grow through the nitinol stent (Figure 2). Nevertheless, stenting in these circumstances should be considered as a last resort. Lastly, urethral stenting can be considered in cases of stricture secondary to trauma due to calculi.
Ureteral stenting
Ureteral stenting has been commonly used in human medicine for many years and it is becoming more common in veterinary medicine. Previously, cats with bilateral ureteral obstruction and kidney failure were doomed unless surgery was an option. Surgical intervention can be curative, but can be associated with significant complications including uroabdomen, urosepsis, ureteral stricture and re-obstruction due to continued calculi formation. Ureteral stenting offers significant advantages compared to surgery besides being less invasive. Its main advantage is due to the fact the stent causes passive dilation of the ureter. Therefore, urine flows within and around the stent. This is very important in cats that are considered œstone formers (i.e. cats that are prone to forming calcium oxalate nephroliths). Ureteral stents in these cats will lead to a remarkable decrease in the likelihood of re-obstruction. Previously, cats that were severely azotemic because of bilateral ureteral obstruction were considered to have a very low chance of long-term survival. Stent placement has shown that this is not always the case. A previous case series reported a cat with a creatinine of 24 mg/dl decreased to 6.1 mg/dl after 72 hours of stent placement and 5.5 mg/dl 3 months later.10 In a different report, 5 of 6 cats with ureteral stenting had improvement of their azotemia.11 Therefore, initial creatinine should not be considered of prognostic value. Possible complications of ureteral stenting in cats include predisposition to ascending kidney infections due to the direct communication between the kidney and the bladder. This is especially important in the presence of underlying kidney disease. Ureteral stenting in cats is normally placed under surgical guidance. Occasional hematuria and dysuria can be seen in some cats with bilateral stents. It is very important to use double pig-tail stents to prevent stent migration. Lastly, these stents can be used for less common diseases, such as congenital bilateral ureteral stenosis.
In summary, nowadays there are various stenting procedures available for our patients that may not only prolong their survival time, but do so with quality.
References:
1. Moritz A, et al. Management of advanced tracheal collapse in dogs using intraluminal self-expanding biliary wallstents. JVIM. 2004; 18(1): 31-42.
2. Woo HM, et al. Intraluminal tracheal stent fracture in a Yorkshire terrier. Can Vet J. 2007; 48(10): 1063-6.
3. Leonard CD, et al. Changes in tracheal dimensions during inspiration and expiration in healthy dogs as detected via computed tomography. AJVR 2009; 70(8): 986-91.
4. Berent AC, et al. Use of a balloon-expandable metallic stent for treatment of nasopharyngeal stenosis in dogs and cats: six cases (2005-2007). JAVMA 2008; 233(9): 1432-40.
5. Leib MS, et al. Endoscopic balloon dilation of benign esophageal strictures in dogs and cats. JVIM 2001; 15(6): 547-52.
6. Hansen KS, et al. Use of a self-expanding metallic stent to palliate esophageal neoplastic obstruction in a dog. JAVMA 2012; 240(10): 1202-7.
7. Culp WT, et al. Use of a nitinol stent to palliate a colorectal neoplastic obstruction in a dog. JAVMA 2011; 239(2): 222-7.
8. McMillan SK, et al. Outcome of urethral stent placement for management of urethral obstruction secondary to transitional cell carcinoma in dogs: 19 cases (2007-2010). JAVMA 2012; 241(12): 1627-32.
9. Blackburn AL, et al. Evaluation of outcome following urethral stent placement for the treatment of obstructive carcinoma of the urethra in dogs: 42 cases (2004-2008). JAVMA 2013; 242(1): 59-68.
10. Nicoli S, et al. Double-J ureteral stenting in nine cats with ureteral obstruction. Vet J. 2012; 194(1): 60-5.
11. Zaid MS, et al. Feline ureteral strictures: 10 cases (2007-2009). JVIM 2011; 25(2): 222-9.
12. Lam NK, et al. Endoscopic placement of ureteral stents for treatment of congenital bilateral ureteral stenosis in a dog. JAVMA 2012; 240(8): 983-90.