Interventional radiology in veterinary medicine: Now and in the future (Proceedings)

Interventional radiology involves the use of imaging modalities such as fluoroscopy or ultrasonography to gain access to different structures in order to deliver materials for therapeutic purposes. The use of interventional techniques in veterinary patients offers a number of advantages compared to more traditional therapies. These procedures are minimally invasive and can therefore lead to reduced perioperative morbidity and mortality, shorter anesthesia times and shorter hospital stays. Some techniques such as chemoembolization of tumors or palliative stenting for malignant obstructions offer alternative treatment options for patients with conditions that may not be amenable to standard therapies. There are not necessarily disadvantages for the patient as much as the provider, in the initial cost of equipment and time to gain expertise. Radiation exposure to personnel can be substantial when using fluoroscopic guidance and full lead gowns, thyroid shields, and leaded glasses are recommended and may add to the initial purchase price of equipment.

Small and toy breed dogs are predisposed to tracheal collapse due to cartilage degeneration. The trachea collapses dynamically during respiration. These patients can present with mild "honking" to respiratory distress due to complete airway obstruction. Aggressive medical management is the treatment of choice in these patients. Combinations of anti-inflammatories, cough suppressants, sedatives, bronchodilators, antibiotics, weight loss, restricted exercise, and removal of inhaled allergens (such as smoke) can help to control clinical signs. Treatment of concomitant diseases is imperative as well. Small airway disease, pulmonary parenchymal disease, heart disease, and brachycephalic airway syndrome exacerbate tracheal collapse and should be addressed. Patients that fail this aggressive management and concurrent diseases have been adequated treated can be candidates for interventional or surgical treatments. Extraluminal tracheal ring prostheses has a reported efficacy of 75-85% for extrathoracic tracheal collapse, but significant morbidity is associated with surgery. Complications such as laryngeal paralysis, perioperative death, and permanent tracheotomy can be seen. If significant intrathoracic tracheal collapse is present surgery can have even higher morbidity. Intraluminal self-expanding metallic nitinol stents have excellent flexibility and can now be placed in the trachea (both cervical and intrathoracic) to treat tracheal collapse using a minimally invasive procedure. Another advantage of tracheal stent placement versus surgery is the short anesthesia time, which is beneficial in geriatric patients with concurrent cardiac or pulmonary disease. It is important to realize that medications will still be required in a majority of patients following stent placement however. When used appropriately with careful patient selection, significant improvement in the quality of life can be achieved with stent placement in addition to medical management, with a lower morbidity than surgery. There have however been complications associated with intraluminal tracheal stent placement which include stent shortening, excessive granulation tissue, progressive tracheal collapse and stent fracture. In young patients, stents should be carefully considered as the life span of these stents has not been determined.

Nasopharyngeal stenosis is a condition which there is narrowing of the nasopharynx caudal to choane resulting in stertorous breathing with an exaggerated inspiratory effort. These patients often have severe mucoid to mucopurulent nasal discharge for long periods of time (weeks to years). This can be congenital or acquired in etiology and is more common in cats than dogs. Balloon dilation has been used successfully as has numerous surgical procedures. Balloon dilation may allow for recurrent stenosis to occur. Surgery can require a long anesthetic procedure and is invasive. A reported alternative treatment is balloon expandable metallic nasopharyngeal stent placement. It was rapidly performed in an average of 38 minutes, and was effective in relieving clinical signs for 12-28 months.

Transitional cell carcinoma is a common neoplasia of the urinary bladder and urethra of dogs. Local disease results in approximately 10% of TCC cases developing complete urinary tract obstruction due to tumor progression. The cause of death is due to the primary tumor in up to 60% of cases. Clinical signs are most often associated with the primary tumor and include both dysuria and complete urinary tract obstruction. Prostatic neoplasia (most commonly adenocarcinoma) is another common tumor that affects the urinary tract in male dogs. Clinical signs in this disease are associated with the local affects of the tumor in up to 40% of these dogs. Minimally invasive techniques to palliate the clinical signs associated with local disease include ultrasound guided laser ablation and balloon expandable and self expanding metallic urethral stents. Stents can be placed under fluoroscopic guidance rapidly, safely, and effectively relieve urethral obstructions. In a small study involving urethral stent placement in 12 patients, seven dogs had good to excellent outcome, 3 had fair outcome and 2 had a poor outcome (severe incontinence and atonic bladder). Palliative stenting can also be useful in malignant obstructions in the gastrointestinal tract (due to leiomyosarcoma or adenocarcinoma) to relieve constipation/obstipation dyschezia and/or tenesmus.

Ureteral stents can also be placed for both benign (stricture or stone) obstructions as well as malignant obstructions (TCC occluding the ureterovesicular junction). The use of interventional and minimally invasive radiology techniques in the ureters are increasingly being studied and continue to evolve.

Portosystemic shunts are abnormal vascular communications between the portal venous system and the systemic circulation. Clinical signs include various degrees of neurologic dysfunction due to insufficient hepatic perfusion and diminished portal blood detoxification. Diagnosis can be via numerous techniques. Transcolonic nuclear scintigraphy and ultrasonography are widely accepted as non-invasive imaging modalities to evaluate PSS's. Ultrasonographic sensitivity was 81% and specificity was 67% for detection of PSS. Thus, failing to detect a shunt with ultrasonography does not exclude the possibility of PSS. Nuclear medicine studies have both higher sensitivity and specificity in detecting portosystemic shunts, but differentiation between intrahepatic and extrahepatic PSSs is not possible. Definitive identification and anatomic characterization of the PSS can be achieved minimally invasively with interventional diagnostic angiographic procedures, such as transvenous retrograde portography. This procedure does not require celiotomy as required with mesenteric portography. A balloon jugular catheter is passed into the caudal vena cava. As the cava is occluded, iodinated contrast is administered resulting in retrograde filling of the of the abdominal cava and any PSS. Medical treatment of PSSs can alleviate clinical signs, long-term management is rarely successful and surgical treatment is most often considered the definitive treatment. Most congenital portosystemic shunts are extrahepatic in origin. Success rate with surgical occlusion in dogs is high. Intrahepatic portosystemic shunts are less common and are surgically challenging for veterinary surgeons to treat. These surgical procedures have a high morbidity and mortality rate. Interventional radiology techniques for attenuation of PSS's are minimally invasive, allow for simultaneous angiography and pressure measurements, and avoids technically demanding surgical procedures. Thrombogenic coils can be used for staged embolization with or without stent placement. Complications can include coil migration into the heart and lungs.

Percutaneous arterial embolization with and without and chemoembolization techniques can be performed. Arterial embolization allows selective catherter directed delivery of particulate material in order to control hemorrhage, occlude vascular malformations, or reduce tumor growth. When used with chemoembolization chemotherapy is delivered directly to the tumor site which can result in a 10-50 times intra-tumoral drug concentration versus systemic chemotherapy. Subsequent particulate embolization results in tumor cell necrosis and greatly reduces the excretion of chemotherapy resulting in minimized systemic toxicity. This procedure may be of benefit in non-resectable hepatocellular carcinoma's in veterinary patients as the literature shows increased survival times in human patients treated in this fashion. Post embolization malalise in people is reported to last several days and responds to supportive care, fluids, anti-emetics, analgesics, and gastroprotectants.

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