38 William T.N. Culp and Geraldine B. Hunt The mediastinum is the space between the right and left pleural sacs that encloses a number of organs, vessels, and nerves.1 In some species (e.g., humans), the mediastinum is more fibrous and isolates the pleural space on one side from the other, allowing collapse of one lung or unilateral pleural effusion. In dogs and cats, the tissue in the mediastinum is very fine and easily perforated. Although it is unusual to diagnose a completely unilateral pleural effusion, it is possible, especially in chronic disease processes that cause pleural fibrosis. The cranial mediastinum contains the esophagus, the trachea, the vagus and phrenic nerves, the cranial vena cava and the large arteries to the head and forequarters (left subclavian and brachiocephalic), the thymus, and the thoracic duct (Figure 38.1). The vagus nerves tend to follow the course of the esophagus, and the phrenic nerves lie close to the cranial vena cava and then diverge onto the dorsolateral aspects of the pericardium. Figure 38.1 Schematic transverse section of the thorax through the heart and lungs. (Source: Evans & de Lahunta [1]. Reproduced with permission from Elsevier.) The paired internal thoracic arteries arise from the subclavian arteries and run caudoventrally in a fold of pleura from the precardial mediastinum to the craniomedial border of the transversus thoracis muscle, where they pass deep to that muscle (Figure 38.2). The internal thoracic arteries provide branches to the pericardium and phrenic nerve (pericardiophrenic arteries), the bronchi, the thymus, and the ventral intercostal muscles.1 Figure 38.2 Thoracoscopic image demonstrating the relationship between a thymoma and the internal thoracic artery (a) as well as the cranial vena cava (CVC) and the phrenic nerve. There are two lymphocenters (sternal lymph nodes) on either side of the ventral cranial mediastinum. They lie immediately cranial to the transversus thoracis muscle and cranioventral to the internal thoracic blood vessels. Typically, the nodes are ellipsoid in shape and 2 to 20 mm in length. The node may either be dumbbell shaped or consist of two separate nodes. The sternal lymph nodes appear to be a major lymphocenter for drainage of the peritoneal cavity (via the diaphragm). There are also a number of mediastinal lymph nodes situated more dorsally within the dorsal mediastinum and associated with the large vessels. In contrast to the sternal lymph nodes, they are rarely visible during open surgery or minimally invasive surgery (MIS). Three lymphocenters are found at the bifurcation of the trachea, the left, middle, and right tracheobronchial lymph nodes. These lymph nodes are commonly excised or biopsied as part of the staging process during lung lobectomy. Thoracoscopic mediastinal lymph node removal has been used in humans for some time,2 and techniques are developing in veterinary patients as well.3 The mediastinum is described as being divided into dorsal and ventral portions by a frontal plane passing through the roots of the lungs.1 In dogs, the ventral surface of the heart is attached to the sternum by the folds of pleura that extend from the thoracic inlet to the diaphragm. The caudal mediastinum is occupied by the esophagus, aorta, and thoracic duct. It is accessed thoracoscopically during thoracic duct ligation for treatment of chylothorax.1 All patients being considered for thoracoscopic resection of cranial mediastinal masses should have a complete blood count and serum biochemistry (including ionized calcium if total calcium levels are increased) obtained. A coagulation profile is desirable and is essential if a coagulopathy is suspected based on breed or the disease process present. Three-view thoracic radiographs are also recommended to evaluate the size of the mass, pulmonary or lymph node metastasis, comorbidities such as megaesophagus or aspiration pneumonia, or any pulmonary or pleural condition that might preclude one-lung ventilation (OLV). Abdominal sonography is generally performed as part of a normal staging plan for potentially malignant neoplasias. Confirmation of the presence and internal characteristics of the cranial mediastinal mass by sonography or computed tomography (CT) is important. Sonography is an excellent way of distinguishing a mass from mediastinal fat or evaluating the internal characteristics of a mass if a good sonographic window can be achieved.4 Contrast-enhanced CT provides important information about the vascular supply of the mass and evidence of local invasion into surrounding structures such as blood vessels (Figure 38.3). Because of its close association with the internal thoracic vein, this is the usual route of vascular invasion and the route by which invasive tumors enter the cranial vena cava. CT does not provide reliable distinction among different tumor types at this stage.5,6 Magnetic resonance imaging has not been thoroughly evaluated for characterization of cranial mediastinal masses but has a role for evaluation of patients with iodine allergy or renal disease, in which administration of iodinated contrast agents are contraindicated because identification of vascular involvement is crucial to surgical decision making.7 Figure 38.3 Computed tomography angiogram of a dog with a thymoma. A, The proximity to the internal thoracic artery and vein (a. and v.) is shown. B, More caudally, the tumor resides in close proximity to the cranial vena cava (CVC). Identification of the tumor type before surgery is highly desirable, although not always possible. Ultrasound- or CT-guided fine-needle aspiration (FNA) yields an accurate result in a high proportion of cases, but the results of cytology do not correlate with histopathologic diagnosis in a subset of cases, possibly because lymphocytes exfoliate more readily than epithelial cells.8 Cytology may also be confounded by the presence of necrotic or cystic components within the mass and the presence of lymphocytes and mast cell tumors in normal thymus.9 Flow cytometry has been demonstrated to be useful in distinguishing types of cranial mediastinal masses.10 Needle-core biopsy may be required if the results of FNA cytology are inconclusive. However, the mass may not be definitively identified in some patients until a larger sample, representative of the entire mass can be submitted. Thymoma and lymphoma are the two most common cranial mediastinal tumors in dogs and cats.11 Sarcomas and ectopic thyroid carcinomas have also been reported, but these are very rare. A series of dogs with cranial mediastinal carcinoma was also reported.12 Benign conditions such as branchial cyst have also been reported in dogs and cats.4 To maximize surgical success, tumor size and local invasiveness should be taken into account. Tumors that are invading the local vasculature; are not clearly encapsulated; or have dense adhesions to other thoracic structures, such as the lungs, pericardium, or thoracic wall, may not be good candidates for thoracoscopic removal. Tumor size is also an important variable and should be assessed relative to patient size. Tumors should not be so large as to preclude the ability to achieve good working space for visualization and instrument manipulation, and the tumor should be small enough to be removed through a limited intercostal incision (either before or after morcellation). Currently, the authors consider thoracoscopic removal of masses up to 8 cm in dogs weighing greater than 30 kg and approximately 5 cm in dogs weighing 15 to 30 kg; however, these guidelines may be adjusted as future techniques develop or experience is gained. The size of an anterior mediastinal mass in a human patient is a major factor that is considered when making the decision to go forward with a minimally invasive or an open surgical approach. In one study, tumors that measured greater than 8 cm in size were determined to be too large to be removed via a thoracoscopic technique in humans,13 although other authors have a lower cutoff size of 3 to 5 cm.14-16 Additionally, when vascular invasion is noted on preoperative imaging, an open approach is elected.13,17 One of the main factors dictating the success of mass removal and the need for conversion from a minimally invasive procedure to an open one is visualization and effective working space. Development of small trocar–cannula assemblies and telescopes allows for thoracic access to be achieved in most canine or feline patients.3,18 However, more room is sometimes required for other instrumentation such as vessel-sealing devices (VSDs), surgical staplers, and surgical clips. There has been a tendency for surgeons to avoid video-assisted thoracic procedures in patients weighing less than 10 kg, but recent work suggests that thoracoscopy is also feasible in small dogs and cats. Body condition score may have an impact on accessibility, with obese animals posing particular problems because of fat deposits in the thoracic wall and mediastinum. Age, per se, is not an important criterion for selection. However, it seems intuitively reasonable to expect that geriatric patients may benefit particularly from a minimally invasive approach with its reduced morbidity and quicker recovery compared with open techniques. The diagnosis of a mediastinal mass in a child is not a contraindication for thoracoscopic removal, and this procedure has been described in several pediatric studies.14,19 The same surgical technique that is used in adults can be used in children, and complications rates are similarly low.14,19 Some authors do not recommend thoracoscopic resection of mediastinal masses in very young children because of the small size of their airways being unable to accommodate the smallest double-lumen tube.17 Different species have different anatomic characteristics that affect the ease of thoracoscopy. To date, most clinical experience in small animals has been with dogs. With a relatively large thorax and noncompliant thoracic wall, dogs have been thought of as better candidates than cats, which have compliant (collapsible) thoracic walls and limited intrathoracic working space. However, thoracoscopy has been reported in cats,20 and our emerging experience is that feline thoracoscopy is feasible using pediatric equipment and creative methods for achieving sufficient working space. Myasthenia gravis is a known paraneoplastic syndrome of thymoma in cats and dogs, and the resultant megaesophagus may predispose the patient to perioperative regurgitation and aspiration. This is not a contraindication to a minimally invasive approach, although the potential for increased anesthesia time in surgeons’ hands who are still developing their skills should be weighed against the likelihood of a more rapid postoperative recovery. Aspiration pneumonia or other pulmonary disorders may cause abnormalities of ventilation and gas exchange that might preclude safe OLV or relative lung collapse necessary for visualization during thoracoscopy.
Thoracoscopic Mediastinal Mass Resection
Preoperative Considerations
Surgical Anatomy
Preoperative Diagnostics
Patient Selection
Tumor Types
Tumor Characteristics
Age
Species
Comorbidities
Patient Preparation
Surgical Preparation
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
