Introduction

Interventional radiology (IR) involves the use of contemporary imaging modalities such as fluoroscopy to gain access to specific structures in order to deliver different materials for therapeutic purposes. These techniques are routinely used in human medicine; however, despite potential applications these procedures have not yet been widely adopted in veterinary medicine. The relatively high morbidity and mortality rates associated with surgery of the cervical and intrathoracic trachea, as well as the difficulty in accessing certain areas of the nasopharynx and lungs, make IR techniques particularly well suited for this location. This section will present the advantages offered by IR techniques, some of the equipment involved and some of the techniques currently being used in veterinary patients with conditions affecting the respiratory tract.

Nasopharyngeal Stenosis

In contrast to the dynamic obstruction often identified with tracheal collapse, animals can develop fixed obstructions from congenital or acquired causes (e.g., nasopharyngeal stenosis). Historically, stenting has been avoided in benign human diseases because of the risk of potential long-term complications such as stricture and granulation tissue formation or stent fracture. The introduction of removable (and more recently bioabsorbable) stents helps to avoid these potential long-term complications. These technological advancements have led to re-evaluation of these devices for use in benign disease. Initial balloon dilation under endoscopic or fluoroscopic guidance remains the standard of care for veterinary patients with benign strictures in most locations, particularly those not amenable to easy surgical excision or repair. However, certain cases either fail repeated balloon dilation therapy or the repeated procedures may be declined by pet owners. IR techniques have been used to perform palliative stenting for benign strictures in the airway (nasopharynx and trachea) when conventional therapies have failed or were declined.

Tracheal Collapse

Tracheal collapse is a progressive, degenerative disease of the tracheal cartilage rings predominantly affecting small and toy-breed dogs. Hypocellularity and decreased glycosaminoglycan and calcium contents result in subsequent dynamic tracheal collapse during respiration. Clinical signs range from a mild, intermittent 'honking' cough to severe respiratory distress and are usually controlled with medications including anti-inflammatories, cough suppressants, sedatives/tranquillisers and bronchodilators. In addition, weight loss, restricted exercise and removal of secondhand smoke or inhaled allergens can further palliate symptoms. Those patients that have failed aggressive medical and environmental management, and have had other causes of respiratory disease either treated or ruled out, become candidates for surgical or interventional treatment.

Various surgical techniques have been described; however, the currently recommended surgical therapy for patients with extrathoracic tracheal collapse is extraluminal polypropylene ring prostheses. This technique involves placing extraluminal support rings around the trachea during an open cervical approach. Although one study reports a 75–85% overall success rate in 90 dogs for reducing clinical signs, this technique is not without complications. The same study reports that 5% of animals died perioperatively, 11% developed laryngeal paralysis from the surgery, 19% required permanent tracheostomies (half within 24 hours) and ~23% died of respiratory problems with a median survival of 25 months. In addition, while all dogs had extrathoracic tracheal collapse, only 11% of the dogs in this study had intrathoracic tracheal collapse. Upon review of their cases, the authors subsequently concluded that the use of this technique in animals with intrathoracic tracheal collapse cannot be recommended as the associated morbidity was unacceptably high.

The absence of low morbidity surgical options for animals with tracheal collapse has led to the investigation of human-designed stents for veterinary tracheal use. The advantages of intraluminal tracheal stenting include minimal invasiveness, avoiding dissection around the peritracheal neurovascular structures, shorter anaesthesia times and access to the entire intrathoracic trachea. A number of stents have been evaluated in the canine trachea, primarily stents made of stainless steel or nitinol (laser-cut, knitted or mesh). Clinical improvement rates in 75–90% of animals treated with self-expanding, intraluminal stainless steel stents have been reported. Immediate complications were mostly minor, although there was a perioperative mortality rate of approximately 10%. Reported late complications have included stent shortening, excessive granulation tissue, progressive tracheal collapse and stent fracture.

The decision to perform surgery or stenting is a complicated and unresolved one. Regardless, all animals should receive aggressive medical and environmental management before considering either of these treatment options. Failure to administer medications is not a valid reason to pursue these techniques as the majority of these animals will require additional medical management in the future. Patients must be treated on an individual case basis; however, some basic guidelines can be used. In the author's opinion, if only cervical tracheal collapse is present, then surgical rings should be considered. An exception may be in a geriatric patient or one with excessive comorbidities (extensive cardiac or pulmonary disease, endocrinopathies, etc.) in which prolonged anaesthesia or healing could preclude more invasive surgery. In addition, the author would prefer to avoid intraluminal stent placement in younger animals, as long-term follow-up (> 5 years) in tracheal stented animals has not yet been performed. If only intrathoracic tracheal collapse is present then surgery is unlikely to resolve the problem and will probably be associated with excessive morbidity; therefore an intraluminal stent should be considered. Patients with both cervical and intrathoracic tracheal collapse can be managed with either a single long stent spanning both segments, or with surgical rings in the cervical trachea and a stent for the intrathoracic trachea.

The benefit of intraluminal tracheal stents in patients with bronchial collapse is currently debatable. Unfortunately, there are no data available to either support or refute the routine use of tracheal stenting in these patients. The author currently avoids placement of stents within collapsing mainstem bronchi as the benefit achieved will probably be minimal and temporary, due to continued collapse of more distal bronchi. However, it seems that certain patients can benefit from tracheal stenting, even when concurrent bronchial collapse is present and remains untreated. These clients are warned that continued coughing should be anticipated as the bronchial collapse will remain following placement of the tracheal stent.

Malignant Airway Obstructions and Benign Tracheal Stenoses

Veterinary patients can present with advanced stages of malignancy in which traditional therapies such as surgery, chemotherapy, or radiation therapy are associated with excessive morbidity, cost or poor outcome. Presenting clinical signs may be associated with the tumour location and subsequent local effects rather than the systemic effects of the tumour burden; for example, a tracheal tumour resulting in subsequent airway obstruction. Benign conditions can also result in subsequent tracheal stricture formation leading to progressive airway obstruction. The same techniques described above have been used for treatment of tracheal malignancies or stenoses secondary to trauma when surgical procedures were declined, not indicated or expected to result in excessive morbidity or mortality. The author has placed stents for intrinsic malignant tracheal obstructions; however, tracheal stents could also provide palliation for extrinsic compression of the airways.

Tracheal Foreign Bodies

IR techniques are useful for retrieval of obstructive tracheal or bronchial foreign bodies, particularly in very small patients in which surgery or endoscopy would be either dangerous or impossible. Occasionally, tracheal diameter restrictions require extubation before tracheoscopy/bronchoscopy can be performed. The endoscopes used are often occlusive in these small patients, impairing adequate ventilation. The techniques described above allow passage of very narrow snares or stone baskets through a bronchoscope adapter and endotracheal tube while a complete anaesthetic circuit is maintained.

Chylothorax

Idiopathic chylothorax is a frustrating disease to manage in which the pleural space fills with a chylous effusion resulting in dyspnoea and sclerosing pleuritis over time if left untreated. Numerous surgical techniques have been described with varying success and invasiveness. Recently, thoracoscopic treatment has been described. Another less commonly considered technique is that used in humans which involves glue embolisation of the thoracic duct using cyanoacrylate. This technique has been evaluated experimentally in animals as well as in a small number of clinical patients. This technique is performed under fluoroscopic guidance through an abdominal approach and involves performing lymphangiography with subsequent glue embolisation of the cisterna chyli and thoracic duct. Glue embolisation may be a viable primary therapy or follow-up treatment if a previous surgical attempt has failed.

Vascular (Intraluminal) Foreign Body Retrieval

Image-guided removal of vascular foreign bodies from the lungs (catheter fragments, etc.) can be performed relatively rapidly and minimally invasively using IR techniques in order to avoid more aggressive surgical techniques or the risk associated with leaving these foreign materials in vivo. Similar to the human experience, the increased use of vascular access and implantable devices in veterinary patients will probably result in an increased frequency of dislodged or migrated vascular foreign bodies encountered and diagnosed by veterinarians.

References

References are available upon request.