Disorders Of The Medias Tin Um

Vol. 19, No. 1 January 1997

V

Continuing Education Article

FOCAL POINT

Disorders of the Mediastinum

★The location and complex anatomy of the mediastinum make diagnosis and treatment of mediastinal disorders challenging.

KEY FACTS ■ The mediastinum is frequently incomplete in dogs and cats, so diseases often spread from one hemithorax to another. ■ Direct aspiration of a mediastinal mass can be performed with ultrasonographic guidance or by triangulation (based on orthogonal thoracic radiographs). ■ When the mediastinal mass is small or a limited amount of fluid is present, it can be helpful to place the ultrasound transducer on the dependent side. ■ Pneumomediastinum seldom causes dyspnea unless the patient also has pneumothorax or hydrothorax. ■ Immediate surgery is indicated in cases of esophageal perforation and in some cases of tracheal perforation.

Texas A&M University

Kenita S. Rogers, DVM, MS Michael A. Walker, DVM

T

he mediastinum is the central space between the pleural cavities and is covered by reflections of the parietal pleura. It physically separates the two hemithoraces and contains the heart and all intrathoracic structures except the lungs1,2 (Figure 1). It consists primarily of loose areolar tissue that extends to specified boundaries. The mediastinum is bounded cranially and ventrally by the thoracic inlet and sternum, laterally by the parietal pleura, caudally by the diaphragm, and dorsally by the thoracic vertebrae. It is not a closed cavity but communicates cranially with fascial planes of the neck through the thoracic inlet and caudally with the retroperitoneum through the aortic hiatus.

MEDIASTINAL COMPARTMENTS The anatomy of the mediastinum can be divided into three compartments: cranial, medial, and dorsal2 (Table I). The cranial mediastinal compartment is bounded by the sternum and thoracic inlet and follows the dorsal pericardial surfaces to the cardiophrenic ligament. It contains the heart, ascending segments of the great vessels, cranial vena cava, thymus, and sternal and cranial mediastinal lymphatics. The medial compartment of the mediastinum extends from the dorsal surface of the cranial compartment to the dorsal surface of the esophagus. It contains the trachea, esophagus, aortic arch, main pulmonary arteries, caudal vena cava, and tracheobronchial lymphatics. The dorsal compartment continues from the dorsal esophageal margin to the ventral surface of the thoracic vertebrae. It contains the descending aorta and paravertebral tissues. A mediastinal disease process may primarily involve a particular anatomic structure, or it may extend to another compartment.2 The delicate mediastinal tissue offers little resistance to the spread of disease processes to other compartments, although inflammatory diseases and neoplasia tend to extend preferentially within a given compartment.1 The mediastinum is frequently incomplete in dogs and cats: A disease process that begins in one hemithorax may extend to the other.

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Vertebral body Rib Costal parietal pleura (adherent to body wall) Azygos vein Aorta Esophagus

Body wall Lung Mediastinal space

syndrome, esophageal dysphagia, regurgitation, or noncompressible cranial thorax—particularly in cats. Mediastinal disorders, such as lymphoma, thymoma, and fungal infection, can cause extrathoracic abnormalities that prompt the pet owner to seek veterinary attention.

Diagnostic Imaging Survey Radiography Survey radiographs of the Mediastinal pleura thorax are the most practical means of confirming a mePleural space diastinal disorder. RadiograHeart phy can show the location Visceral pleura and extent of some lesions; (adherent to lung) it can also suggest or rule out diagnostic differentials.2 Only a few mediastinal orSternum gans (heart, trachea, caudal vena cava, aorta) are visible Figure 1—Transverse section of the canine thorax. The mediastinum divides the thorax into in thoracic radiographs of right and left halves. Note that the mediastinal space does not communicate with the pleunormal patients.3 A normal ral space (From Thrall DE: The mediastinum, in Thrall DE (ed): Textbook of Veterinary esophagus is occasionally Diagnostic Radiology, ed 2. Philadelphia, WB Saunders Co, 1994, p 278). visible, and the thymus may be identified in animals younger than 1 year. Other DIAGNOSTIC mediastinal structures are EVALUATION too small to be visible, borHistory der on other structures with The history that is most the same radiopacity, or lack suggestive of a mediastinal sufficient adjacent fat to disorder is recent trauma or provide contrast. invasive diagnostic or surgiOn a lateral radiograph, a cal procedures involving the distinct opacity can be seen head, neck, or thorax. Travel ventral to the trachea in the history and geographic locacranial compartment; but tion could suggest particular individual structures (left infectious or parasitic dissubclavian artery, brachioeases. cephalic trunk, cranial vena Figure 2—Lateral view of the thorax of a dog with pneumo- cava, mediastinal lymph Physical Examination Many of the clinical signs mediastinum. The visualization of the outer surface of the nodes, thymus) usually canassociated with mediastinal trachea, esophagus, and great vessels is enhanced because not be discerned. In ventrodorsal or dorsoventral disorders are vague and are of contrast provided by free mediastinal gas. radiographs, most of the specifically related to the size, cranial mediastinum is sulocation, and pathologic conperimposed on the spine. The width of the medisequences of the lesions.2 Clinical signs that should astinum on the ventrodorsal view should be approxiprompt investigation of the mediastinum include abmately twice the width of the spine in a dog, whereas a normal respiratory effort, Horner’s syndrome, vena cava Vena cava

DIFFICULT RESPIRATION ■ HORNER’S SYNDROME ■ VENA CAVA SYNDROME ■ DYSPHAGIA

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TABLE I Mediastinal Components and Associated Diseases Compartment

Associated Diseases

Cranial Heart

Cardiomegaly, cardiac neoplasia

Ascending segments of great vessels

Vessel dilatation, vascular neoplasia

Cranial vena cava

Dilated cranial vena cava

Thymus

Thymoma, thymic hemorrhage

Lymphatics

Lymphoma, metastatic and infectious causes of lymphadenopathy

Miscellaneous

Fat pads, cysts, ectopic or neoplastic thyroid and parathyroid tissue, mediastinitis, pneumomediastinum, other tumors

Medial Trachea

Mediastinitis, pneumomediastinum

Esophagus

Mediastinitis, pneumomediastinum, megaesophagus or diverticulum, hiatal hernia, diaphragmatic-mediastinal hernia, gastroesophageal intussusception, mass disorders (neoplasia or granuloma)

Aortic arch

Aortic dilatation

Main pulmonary arteries

Pulmonary arterial dilatation

Caudal vena cava

Dilated caudal vena cava

Lymphatics

Lymphoma, metastatic or infectious lymphadenopathy

Miscellaneous

Ectopic or neoplastic thyroid or parathyroid tissue, chemodectoma, cysts

Dorsal Descending aorta

Aortic dilatation

Paravertebral tissues

Infiltrative or mass disorders

Miscellaneous

Cysts, mediastinitis, pneumomediastinum, tumors

normal feline mediastinum is rarely wider than the sternum.2,3 In obese patients, the mediastinum may be widened by fat accumulation, which may be mistaken for a mediastinal mass. Pneumomediastinum. In cases of pneumomediastinum, the free gas provides excellent contrast, thus enhancing visualization of intramediastinal structures, which are seen best on the lateral view3 (Figure 2). The mediastinum is not greatly widened in these cases, so

the abnormality is difficult to appreciate on the ventrodorsal or dorsoventral view. If sufficient pressure develops within the mediastinum, air may escape into the pleural cavity, thus producing pneumothorax (Figure 3). Survey radiographs may also reveal subcutaneous emphysema or gas within the retroperitoneal space. Mediastinal Mass. The radiographic appearance of most mediastinal masses is similar (Figure 4). The ventrodorsal projection is more helpful than the lateral

FAT ACCUMULATION ■ OBESE PATIENTS ■ SUBCUTANEOUS EMPHYSEMA

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projection in differentiating pendent portion of the dewhether a lesion is within pendent hemithorax. the mediastinum or in a Contrast Radiography lung. 3 A lesion might be Specific mediastinal disease within the mediastinum if processes (e.g., esophageal or the lesion is near or on the cranial mediastinal masses) midline or when it displaces may be located and outlined another mediastinal strucwith special contrast proture. Pulmonary masses are cedures. The most useful typically lateral to the medicontrast procedures include astinum. esophagograms, lymphanMasses in the cranial megiograms, and angiograms. diastinal compartment often elevate the trachea, as can Computed Tomography large volumes of pleural fluid.3 Fluid within the medi- Figure 3—Lateral view of the thorax of a dog with pneumo- and Magnetic astinum typically resembles mediastinum and pneumothorax. The heart appears dor- Resonance Imaging Computed tomography a mass lesion radiographical- sally displaced above the sternum in this view; a radio(CT) and magnetic resoly. When fluid is entrapped lucent space separates the two structures. nance imaging (MRI) are adjacent to the caudoventral routinely used to evaluate mediastinal reflection, it the mediastinum in humans. They are less commonly may appear as a small triangle with soft-tissue opacity used in veterinary medicine because of limited availabilnear the diaphragm on the dorsoventral projection. Lymphadenopathy. Lymphadenopathy often causes a ity, high cost, and anesthetic requirements.4 These promediastinal mass (Figure 5). The cranial mediastinal cedures can help the veterinarian identify the specific lymph nodes are located along the cranial vena cava just site of origin of many diseases, distinguish fat and fluidventral to the trachea. Their enlargement produces a visfilled cysts from other lesions, and evaluate thoracic arible mass that often elevates the trachea dorsally. Enlargement of the sternal lymph nodes usually Causes of Pneumomediastinum appears as an isolated soft-tissue opacity cranial ■ Esophageal rupture to the heart and dorsal to the sternebrae, seen ■ Penetrating wounds of the best on the lateral projection. head, neck, and cranial ■ Perforating The tracheobronchial or hilar lymph nodes lie thorax (air dissects into esophageal foreign on the proximal part of the bronchi at the tramediastinum via thoracic body cheal bifurcation and are in the medial compartinlet) ■ Esophageal ulceration ment of the mediastinum. Their enlargement is ■ Airway or alveolar with perforation usually more notable on the lateral view, alrupture ■ Therapeutic though divergence of the main-stem bronchi may be apparent on the ventrodorsal projection. The ■ Iatrogenic injury: procedures: ease of detecting these nodes depends on their traumatic intubation, ballooning or size and on the opacity of the surrounding lung.3 bronchoscopy, bougienage Mediastinal Position. The position of the mechanical ■ Esophageal neoplasia mediastinum can be shifted by a unilateral deventilation, ■ External trauma crease in lung volume (ipsilateral shift), a unilattranstracheal wash, ■ Abdominal surgery or eral increase in lung volume (contralateral shift), or the presence of a pulmonary or chest wall tracheostomy, rupture of a gas-filled mass (contralateral shift).3 Mediastinal position overinflation of viscus must be assessed on ventrodorsal or dorsoventral endotracheal cuff ■ Mediastinoscopy films. Improper patient positioning with rota■ External trauma ■ Mediastinal infection by tion can create a false impression of mediastinal ■ Underlying gas-producing organisms shift. Horizontal-beam radiography may help pulmonary pathology (rarely) differentiate pleural fluid from an intrathoracic mass; free pleural fluid gravitates into the deMEDIASTINAL SHIFT ■ PULMONARY MASSES ■ LYMPH NODES

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eas that are difficult to image with survey radiographs. However, computed tomography and magnetic resonance imaging have a poor ability to distinguish between mediastinal masses and adjacent collapsed or consolidated lung lobes and cannot reliably differentiate benign from malignant causes of lymphadenopathy.1

Thyroid Scintigraphy Thyroid scans using technetium-99m, or occasionally iodine-131, can detect ectopic or metastatic thyroid tissue. This tissue is most often present within the cranial mediastinum or mediastinal lymph nodes. Ultrasonography Ultrasonographic examination may delineate the extent of cardiac and extracardiac masses and can be used to guide fine-needle aspiration or needle core biopsy of mediastinal masses. Ultrasonography is most useful for identifying diseases in the cranial compartment and for determining whether a mass is cystic. Ultrasonography can provide satisfactory evaluation of mediastinal masses that are large and contiguous with the thoracic wall and can detect mediastinal or pleural fluid.5 Indeed, pleural fluid may facilitate visualization of mediastinal masses by acting as an acoustic window. When the mediastinal mass is small or a limited amount of fluid is present, it can be helpful to image the patient from the dependent side. This approach takes advantage of atelectasis in

the dependent lungs, flow of fluid toward the dependent side, and physical displacement of the mediastinal mass toward the dependent side as a result of gravity.

Figure 4A

Figure 4B Figure 4—(A) Lateral and (B) ventrodorsal thoracic radio-

graphs of a dog with a cranial mediastinal mass (thymoma). The mass elevates the trachea and displaces the cranial lung lobes. There is a silhouette sign with the cranial margin of the heart.

Pathology and Microbiology Thoracocentesis Collection of pleural fluid can be useful in cases of mediastinal disease with concurrent pleural effusion. If pleural fluid obscures the mediastinum and a mass lesion is suspected, removing the fluid and repeating thoracic radiographs may be helpful. Fluid samples should be collected for cytology as well as culture if the patient’s clinical signs and the analysis of the fluid suggest bacterial mediastinitis. Diseases that can be diagnosed in this manner include large-cell lymphoma as well as bacterial infection leading to suppurative mediastinitis and empyema. Percutaneous Aspiration Direct aspiration of a mediastinal mass can be accomplished with ultrasonographic guidance or by triangulation (based on assessment of the position of the mass on orthogonal thoracic radiographs). Diseases that can be diagnosed by this technique include lymphoma, thymoma, bacterial or fungal mediastinitis, periesophageal masses, and cysts. Contraindications for this technique include severe dyspnea, coagulopathy, and difficulties in restraining the patient. Before aspiration, the skin site should be clipped and

CYSTS ■ MEDIASTINAL OR PLEURAL FLUID ■ ACOUSTIC WINDOW

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prepared for surgery. Twenty-five–gauge needles are generally adequate for aspiration and are available in various lengths. The needle is inserted between the ribs and into the mass, with no redirection once the needle has been advanced. After aspiration and needle withdrawal, material within the needle is ejected onto a clean glass slide. If sufficient material is available, horizontal smears as well as vertical pull-apart slides are prepared because of the fragility of cells important in some disease processes, particularly lymphoma. If fluid is withdrawn, samples are collected for fluid analysis and culture.

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Exploratory Thoracotomy An invasive procedure may be required for diagnostic evaluation as well as for therapeutic intervention. The surgical approach to the thoracic cavity depends on the location of the lesion, but median sternotomy is appropriate for most mediastinal disorders. The goals of the surgical procedure are to obtain an incisional or excisional biopsy for histopathology and culture, assess resectFigure 5—Lateral view of the thorax of a dog with lymph- ability of the lesion, and esadenopathy due to coccidioidomycosis. An enlarged ster- tablish adequate drainage (if nal lymph node is visible as an isolated soft-tissue opacity appropriate). cranial to the heart and dorsal to the sternebrae. Enlarged cranial mediastinal and hilar lymph nodes are also evident. There is a poorly circumscribed pulmonary mass in the DISEASES Pneumomediastinum midcaudal lung fields.

Causes of Mediastinitis ■ Esophageal perforation or rupture ■ Ingestion of foreign bodies (e.g., bones, fishhooks, sticks) or caustic agents ■ Iatrogenic rupture during endoscopy or biopsy, or during therapeutic procedures (e.g., ballooning or bougienage) ■ Esophageal neoplasm ■ Trauma ■ Tracheal perforation or rupture ■ Traumatic intubation ■ Traumatic bronchoscopy ■ Other kinds of trauma ■ Penetrating thoracic trauma (bite wounds,

■

■

■ ■

■

projectiles, migrating foreign bodies [e.g., grass awn, sewing needle]) Direct extension of infection from adjacent tissues (e.g., head, neck, or axillary region) via fascial planes Direct extension of infection from intrathoracic tissue (e.g., lung, pericardium, pleura, or lymph nodes) Complication of tube thoracostomy Complication of thoracic surgery, particularly of mediastinal structures Secondary to bacteremia (rarely)

Causes Pneumomediastinum is the presence of free gas within the mediastinal space. Fascial planes separating the soft-tissue compartments of the neck, thorax, and abdomen serve as routes of spread for air.6 The extent of spread depends on the quantity of air and rate of dissection as well as the integrity of the various layers of the mediastinum, pericardium, pleura, and other fascial membranes. Gas within the mediastinum can originate from five sites: lung, mediastinal airway (trachea, proximal main-stem bronchi), esophagus, abdominal cavity, and the head and neck region.7 Pneumomediastinum has many possible causes (see Causes of Pneumodiastinum), but it may also arise spontaneously. Although pneumomediastinum is often a benign condition with no overt clinical signs, it can also be associated with subcutaneous emphysema or pneumothorax. Pneumothorax can result when excessive air in the mediastinal space ruptures the mediastinal pleura. Most patients with pneumomediastinum and dyspnea also have concurrent pneumothorax.2 Clinical Signs Clinical signs related to pneumomediastinum depend on the underlying cause, the volume and pressure of the mediastinal air, and the presence or absence of concurrent pneumothorax and infection.2 Many patients are asymptomatic, and

ASPIRATION TECHNIQUE ■ SMEAR PREPARATION ■ PNEUMOTHORAX

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the diagnosis is made only because routine radiographs are performed after specific high-risk diagnostic or therapeutic procedures. For example, thoracic radiographs are often made to assess for development of pneumomediastinum or pneumothorax after an extensive balloon dilatation procedure is performed to relieve an esophageal stricture. If subcutaneous emphysema develops, it can be mild to severe. Concurrent tracheal injury and pneumothorax lead to dyspnea and coughing, whereas esophageal rupture and concurrent mediastinitis typically result in thoracic pain, fever, and dysphagia. When air does not freely escape from the mediastinum into the neck, pressures within the cranial thorax can rise markedly, thus leading to such sequelae as engorged neck veins, hypotension, ventilatory failure, and diminished venous return.

Diagnosis A compatible history can support a diagnosis of pneumomediastinum, but assessment of survey radiographs is required for confirmation. In cases of pneumomediastinum, thoracic radiographs characteristically show structures that are not normally identifiable, such as the cranial vena cava, azygos vein, brachiocephalic trunk, aorta, esophagus, and tracheal wall (Figure 2). If subcutaneous emphysema is present, fascial planes of the neck and front limbs may be recognized. Radiographs may also be helpful in determining whether concurrent pneumothorax, pneumoretroperitoneum, hydrothorax, or a radiopaque esophageal foreign body is present.8

Causes of Mediastinal Mass Lesions ■ Abscess or granuloma (local manifestation of a systemic infection or secondary to contamination from a bite wound or esophageal perforation ■ Cysts (pleural, lymphatic, bronchogenic, thymic) ■ Neoplasia ■ Primary and spontaneous from a mediastinal structure (lymph nodes, thymus, trachea, esophagus, chemoreceptors, paravertebral tissue) ■ Extension from adjacent tissue (lung, thoracic inlet) ■ Ectopic thyroid and parathyroid tissue ■ Component of a multicentric or diffuse tumor (lymphoma, histiocytic disorders) ■ Metastatic disease from a distant site

Treatment Most cases of pneumomediastinum are self-limiting and require no therapy. Patients with substantial dyspnea usually have concurrent pneumo- or hydrothorax, which will require either simple thoracocentesis or tube thoracostomy with frequent evacuation. Placement of a thoracic tube should be considered when concurrent pneumothorax or hydrothorax does not resolve with repeated simple thoracocentesis. Neck wounds should be treated with primary wound closure and a neck bandage to help limit the spread of subcutaneous air. Esophageal perforations and large tracheal lacerations require immediate surgical intervention, but small tracheal injuries require surgery only if the site does not quickly heal spontaneously. If there are no signs of respiratory distress and the associated em-

■ Mediastinal lymphadenopathy ■ Bacterial infection ■ Fungal infection ■ Mycobacterial disorders ■ Neoplastic disorders ■ Lymphoma ■ Metastatic neoplasia (carcinoma, mast cell tumor, histiocytic disorders, lymphomatoid granulomatosis) ■ Inflammatory disorders (eosinophilic granulomatosis) ■ Miscellaneous ■ Hemorrhage ■ Edema ■ Fat pads or lipoma ■ Esophageal diverticula or megaesophagus ■ Granuloma due to Spirocerca lupi ■ Gastroesophageal intussusception ■ Hiatal or diaphragmaticmediastinal hernia

physema is regressing, cage rest is usually adequate therapy for small tracheal injuries. Resolution of pneumomediastinum may take as long as 2 weeks after the primary disease has been effectively treated, whereas subcutaneous emphysema usually dissipates within a week. Exacerbation of the disease process may result in pneumothorax, pneumoretroperitoneum, or expansion of subcutaneous emphysema on serial radiographs.

Mediastinitis Causes Inflammation within the mediastinal space (mediastinitis) may result from acute, subacute, or chronic processes (see Causes of Mediastinitis).2,7 Most forms of mediastinitis result from infection but vary widely in clinical presen-

SUBCUTANEOUS EMPHYSEMA ■ VENTILATORY FAILURE ■ THORACIC TUBE

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can be displaced by abscesstation.9 The mediastinum is es or granulomas. uniquely vulnerable to the An esophagogram may be spread of infection and inuseful for evaluating the flammation because of the structural integrity of the continuous and closed nature esophagus. Oral aqueous ioof the space. dine solutions are considered Acute or suppurative menontoxic when accidentally diastinitis is usually bacterial spilled into body cavities, so and typically follows perfotheir use is indicated when ration or rupture of the esophesophageal perforation is susagus or trachea. 7,10,11 Head pected. Computed tomograand neck infections can also phy or magnetic resonance spread to the mediastinum imaging may help outline the via fascial planes, and mediextent of disease. Although astinitis occasionally results fine-needle aspiration may be from sepsis, pneumonia, or pericarditis. Anaerobic bac- Figure 6—Lateral radiograph of a dog with megaesophagus. useful for collecting speciteria are often an important The enlargement of the esophagus within the medial com- mens for culture and cytolocomponent if the infection partment of the mediastinum has caused ventral deviation gy, surgical biopsy via thoraresults from esophageal per- of normal intrathoracic structures, such as the trachea and cotomy may be necessary for heart. definitive diagnosis and to faforation. The infection can cilitate chest tube placement. be diffuse but may be localDiagnostic differentials ized to single or multiple abscesses. for radiographic evidence of mediastinitis include meChronic mediastinitis usually results from fungal indiastinal masses (e.g., neoplasia or lymphadenopathy, fection, especially histoplasmosis, coccidioidomycosis, esophageal diverticula, mediastinal hemorrhage secblastomycosis, or cryptococcosis.2,7,12 Such bacteria as ondary to trauma, thoracic surgery, coagulopathy, or Actinomyces, Nocardia, and Corynebacterium species thymic vascular disruption); mediastinal fat in an obese may also cause chronic mediastinitis—usually producpatient; and mediastinal edema secondary to infection, ing discrete abscesses or granulomas. In humans, trauma, heart failure, or lymphangiectasia. chronic granulomatous inflammation produces varying amounts of host reaction; in some cases, it eventually Treatment progresses to a largely acellular fibrosis.9 Specific treatment for mediastinitis is based on the nature and severity of the underlying disorder. AntibiClinical Signs otics are indicated for suspected bacterial infection. A The clinical signs associated with acute mediastinitis broad-spectrum antibiotic (including activity against are usually rapid in onset and catastrophic. Common anaerobes) should be chosen, and therapy should be clinical manifestations include dyspnea, dysphagia, thocontinued for at least 6 weeks. When a fungal disease is racic pain, fever, concurrent pneumothorax or hydiagnosed, antifungal therapy is continued for at least 3 drothorax, and edema of the neck, head, and/or to 6 months and is best guided by repeat thoracic radiforelegs (vena cava syndrome). Chronic mediastinitis is ology and (in some cases) serial serology. often insidious, with few distinct clinical signs. Immediate surgery is indicated in cases of esophageal perforation and some cases of tracheal perforation. SurDiagnosis gery may also be necessary to collect diagnostic culture The diagnosis of mediastinitis is based on a compatiand biopsy specimens or to establish pleural or mediastible history, physical examination findings, and diagnosnal drainage (particularly after conservative methods have tic testing. The initial radiographic examination may failed) and for any large abscess or granuloma causing an not be diagnostic, although the mediastinum may be obstruction.10 Segmental pulmonary resection may be rewidened with loss of structural detail. In cases of quired when mediastinitis involves adjacent lung tissue.2 esophageal or tracheal rupture, air may be seen within Thoracostomy tube drainage is usually required for severthe mediastinum or soft tissue of the neck, and radioal days after surgery to treat concurrent pyothorax. Failure paque esophageal foreign bodies or tracheal damage to treat mediastinitis may result in abscess formation and may be recognized. Concurrent pneumothorax or hyempyema.2 drothorax may be present, and the trachea or esophagus ESOPHAGEAL OR TRACHEAL RUPTURE ■ ANAEROBIC BACTERIA ■ FUNGI

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The patient should be monitored daily while it is febrile, dyspneic, or showing signs of pain. Radiographs are taken at weekly intervals to evaluate bacterial disease and every 2 to 3 weeks for fungal disease. After clinical improvement is noted, radiographs should be performed at 3- to 6-week intervals. If medical management fails to resolve the clinical signs, surgical exploration of the thoracic cavity must be considered. After surgery, thoracic fluid can be managed by a thoracostomy tube; the tube is removed once residual thoracic fluid is minimal.

Mass Lesions Causes Mass lesions of the mediastinum include space-occupying lesions within the mediastinal pleura as well as enlargement of lymph nodes within the mediastinal compartments (see Causes of Mediastinal Mass Lesions). Conditions that produce space-occupying lesions within the mediastinum include infectious abscesses or granulomas, cysts, neoplasia, lymphadenopathy, and a variety of miscellaneous processes including hemorrhage and edema. The important causes of infectious abscesses and granulomas are discussed in the section on Mediastinitis. Inflammatory but noninfectious causes of granulomas (e.g., eosinophilic granulomatosis) can also result in lymphadenopathy.13,14 Mediastinal cysts are uncommon, usually benign, and most frequently located in the cranial compartment. Cysts may arise from various cell lines, including pleural, lymphatic, bronchogenic, and thymic.15–17 The most common types of neoplasia involving the mediastinum spontaneously arise from a mediastinal structure, such as lymphatics (lymphoma, lymphangiosarcoma, lymphangioma), thymus (thymoma, squamous cell carcinoma), trachea, esophagus, chemoreceptors (chemodectoma), and paravertebral tissue. 18–26 Tumors can also extend into the mediastinum from adjacent tissue, such as the lung, mesothelium (mesothelioma), or thoracic inlet. Neoplasia can also arise from ectopic thyroid or parathyroid tissue, represent metastasis from a distant primary site, or be a component of a multicentric process (e.g., lymphoma or malignant histiocytosis).27–32 Lymphadenopathy of the sternal, cranial mediastinal, or tracheobronchial lymph nodes can be infectious or inflammatory or result from metastatic neoplasia. Infectious organisms include bacteria associated with acute mediastinitis, fungal organisms, and (rarely) such organisms as mycobacteria. Lymphoma is the most common neoplasm involving mediastinal lymph nodes. A cranial mediastinal mass may suggest a poor progno-

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sis for hypercalcemic dogs with multicentric lymphoma.33 The cranial mediastinal form of lymphoma can be solitary in cats; it is more frequently diagnosed in young cats with feline leukemia virus infection.21 Metastasis to mediastinal nodes is typically due to carcinoma (pulmonary, thyroid, mammary, or head and neck), but the site of origin can be distant (e.g., urogenital or gastrointestinal). Other tumor types that spread to these nodes include mast cell tumor, lymphomatoid granulomatosis, and histiocytic disorders.27–32 Various disorders have reportedly caused a masslike lesion in the mediastinum. These disorders include hemorrhage associated with thymic involution, coagulopathy, edema due to heart failure, trauma, infection, lymphatic obstruction, exudative fluid due to feline infectious peritonitis, excessive fat associated with obesity, and esophageal diverticula or megaesophagus34-37 (Figure 6).

Clinical Signs The clinical signs of a mediastinal mass reflect the underlying disease. Small, slowly growing tumors or lymph nodes or small walled-off abscesses or granulomas may produce no clinical signs. Coughing and dyspnea may result from pleural effusion (including chylothorax) or compression of the trachea or segmental bronchi. Esophageal compression or megaesophagus can produce dysphagia or drooling. Edema of the face, neck, and forelimbs may be secondary to compression of the vena cava. Peripheral nerve entrapment can cause laryngeal paralysis with resultant upper airway obstruction, changes in vocalization, stridor, and Horner’s syndrome.2,12,38,39 Thymoma has been associated with paraneoplastic disorders, including myasthenia gravis, hypogammaglobulinemia, hypercalcemia, and aplastic anemia. Lymphoma patients may have systemic signs associated with hypercalcemia, peripheral lymphadenopathy, myelosuppression, or coagulopathy.18–21,40,41 Diagnosis Diagnosis is based on compatible history, physical examination findings, and results of diagnostic testing. A noncompressible cranial thorax in a cat may suggest a mass in the cranial compartment of the mediastinum. Thoracic radiography may show an abnormal opacity in any mediastinal compartment as well as compression and displacement of the trachea, heart, or esophagus. Diagnostic differentials include any extramediastinal abnormality that would increase soft-tissue radiopacity in or near the mediastinum, including lung consolidation, lung masses, and diaphragmatic hernia. An esoph-

NEOPLASIA ■ PARANEOPLASTIC DISORDERS ■ SYSTEMIC SIGNS

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agogram may demonstrate esophageal pathology or extrinsic displacement or compression. Ultrasonography may be useful in detecting abscesses, cysts, or neoplasms as well as pericardial effusion. Radioisotope studies are useful to diagnose ectopic thyroid tissue. Computed tomography and magnetic resonance imaging can help define the intrathoracic tissues involved with a mediastinal mass. Cytology of lung and lymph node aspirates or pleural fluid may be diagnostic. Aspirates and biopsy specimens should be cultured, and serology may be indicated for specific infectious diseases (e.g., coccidioidomycosis). Mediastinoscopy may be indicated for masses in the cranial mediastinal compartment, but an open biopsy via thoracotomy may be necessary for definitive diagnosis. Impression smears should be made during surgery if lymphoma is suspected because chemotherapy and/or radiation therapy is the treatment of choice.20,21,42 Excisional biopsy is attempted for other mediastinal masses.

Treatment The recommended treatment for abscess or granuloma includes specific antibacterial or antifungal agents with surgical drainage or removal when appropriate. Cysts may require extrathoracic fine-needle drainage. The cystic structure should shrink or disappear after this procedure; if it recurs, surgical removal can be considered. Treatment for neoplasia varies with the tumor type. Surgical excision is indicated for selected cases of thymoma, chemodectoma, teratoma, and thyroid or parathyroid tumors. Chemotherapy is the treatment of choice for lymphoma and some metastatic tumors. Radiation therapy may be indicated as adjunctive therapy for lymphoma and thymoma. Follow-up care can be varied. After surgery, an indwelling chest tube may be indicated for intermittent evacuation of fluid or air or for thoracic lavage. The thorax should be radiographed 24 to 72 hours after surgery to check for persistent pneumothorax or effusion. If a mediastinal abscess has been diagnosed, body temperature and complete blood counts should be monitored frequently to search for evidence of persistent infection. CONCLUSION A great diversity of disease processes can affect the mediastinum. The complex anatomy of the structures within this potential space, as well as its location within the body, make diagnostic evaluation of mediastinal disorders challenging. Survey radiography can be valuable in isolating the most likely location of a disease

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process within the mediastinum and helping to direct a logical clinical workup. Treatment of mediastinal disorders can be complex, with medical and surgical methods being beneficial in individual cases.

About the Authors Dr. Rogers is affiliated with the Department of Small Animal Medicine and Surgery, and Dr. Walker is with the Department of Large Animal Medicine and Surgery, College of Veterinary Medicine, Texas A&M University, College Station, Texas. Dr. Rogers is a Diplomate of the American College of Veterinary Internal Medicine (Internal Medicine and Oncology), and Dr. Walker is a Diplomate of the American College of Veterinary Radiology.

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