History
A 6-year-old 35.2-kg (77.4-lb) sexually intact male Labrador Retriever was examined because of a 2-week history of coughing, dyspnea, regurgitation, dysphagia, and lethargy. The dog had been evaluated by its primary veterinarian for regurgitation and coughing 2 weeks earlier, at which time pneumonia was suspected and treatment with orbifloxacin, sulfamethoxazole, and gentamicin (unknown dosages) was initiated along with a canned-food diet. The dog's regurgitation resolved; however, abnormal respiratory signs worsened, and the dog became dyspneic and cyanotic. Results of thoracic radiography (not shown) at that point suggested a thoracic mass, and the dog was referred for further evaluation and management.
On referral examination, the dog was bright, alert, and responsive but became dyspneic and cyanotic during periods of excitement or nervousness. The dog's rectal temperature was within reference limits; however, the dog was azotemic (BUN concentration, 25 mg/dL [reference range, 8 to 24 mg/dL]; serum creatinine concentration, 1.5 mg/dL [reference range, 0.5 to 1.4 mg/dL]) and had hyperphosphatemia (5.4 mg/dL; reference range, 2.5 to 5 mg/dL), hypoalbuminemia (2.3 g/dL; reference range, 2.5 to 3.9 g/dL), hypocholesterolemia (106 mg/dL; reference range, 140 to 306 mg/dL), and high activity of alanine aminotransferase (174 U/L; reference range, 10 to 90 U/L). In addition, the dog had an Hct of 31.3% (reference range, 34.0% to 60.0%) combined with a low concentration of hemoglobin (10.7 g/dL; reference range, 11.0 to 19.0 g/dL), lymphopenia (620 lymphocytes/μL; reference range, 1,000 to 4,800 lymphocytes/μL), and neutrophilia (13,795 neutrophils/μL; reference range, 3,000 to 11,500 neutrophils/μL). The dog was hospitalized, and orthogonal thoracic radiographs were obtained the following day (Figure 1)
Left lateral (A) and a ventrodorsal (B) thoracic radiographic views of a 6-year-old 35.2-kg (77.4-lb) sexually intact male Labrador Retriever evaluated because of a 2-week history of coughing, dyspnea, regurgitation, dysphagia, and lethargy.
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.423
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Diagnostic Imaging Findings and Interpretation
Thoracic radiography revealed an ovoid, smoothly marginated, soft tissue opacity (12.6 × 12.6 × 8.8 cm) spanning the dorsal aspect of the cranial and middle regions of the mediastinum and causing severe ventral deviation and compression of the intrathoracic trachea (Figure 2) Given the dorsal location of the mass, primary considerations for potential tissues of origin included the esophagus, neuroendocrine tissue, or paraspinal musculature, and key differential diagnoses included neoplasia, granuloma, or less likely abscess or hematoma.
Same images as in Figure 1. A large, smoothly marginateci soft tissue opacity (12.6 × 12.6 × 8.8 cm; white arrows; A and B) in the cranial aspect of the mediastinum causes severe ventral deviation and compression of the intrathoracic trachea (black arrows; A).
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.423
Dynamic fluoroscopic barium esophagography (not shown), performed to assess the esophageal lumen in relation to the mediastinal mass, revealed dilation of the caudal cervical and cranial intrathoracic aspects of the esophagus, with a narrowing of the cranial intrathoracic aspect of the esophagus at the level of the mass. The contrast material in the esophageal lumen traveled along the ventral edge of the mass through a narrowed and undulating course. The findings were consistent with an extramural compressive esophageal mass, with a differential diagnosis list including neoplasia and granuloma.
To further characterize the mass, percutaneous thoracic ultrasonography (not shown) was performed and revealed a round and sharply demarcated 9.7-cm-diameter mass in the cranial aspect of mediastinum. Within the mass, numerous ill-defined, hypoechoic nodules were identified along with multiple, irregularly shaped, sharply marginated anechoic regions that had distal acoustic enhancement. No blood flow in the mass was detected with color flow Doppler ultrasonography. Ultrasonographically guided fine-needle aspirate samples of the mass were obtained and submitted for cytologic evaluation, bacterial culture, and susceptibility testing.
The following day, the dog underwent general anesthesia and thoracic CT for surgical planning. A sharply demarcated, ovoid, fluid-dense mass (9.4 × 7.6 × 10.8 cm) was present in the craniodorsal region of mediastinum (Figure 3) The mass had contrast enhancement of the outer rim and areas extending inward from it, but not in the central region. At the level of the mass, the esophagus and trachea were markedly compressed and deviated ventrally. Given this pattern of contrast enhancement, we primarily suspected central necrosis in either a tumor or granuloma; however, an abscess was also considered.
Transverse postcontrast CT images of the thorax of the dog in Figures 1 and 2 obtained at the levels of the second (A), third (B), and fifth (C) ribs. A large, peripherally contrast-enhancing, centrally fluid-dense mass (white arrows; A through C) is in the cranial region of the mediastinum. The esophagus (asterisk; A) and trachea (black arrows; A through C) are compressed and deviated ventrally by the mass. The dog's left is to the right of the images, and the images are displayed in a soft tissue window (window width, 400 HU; window level, 0 HU).
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.423
Treatment and Outcome
The dog was hospitalized, administered oxygen through a nasal cannula, and treated with acepromazine maleate (0.5 to 1.0 mg, IV, as needed) and butorphanol tartrate (0.5 mg, IV, once) for anxiety. Results of the cytologic evaluation were inconclusive but suggested an inflammatory lesion or a necrotic center of a tumor. Bacterial culture yielded faint growth of Nocardia sp after 48 hours of incubation.
Two days after the referral examination, a left lateral thoracotomy was performed. A cranial mediastinal mass circumferentially surrounded the esophagus. Before resection, 10 mL of red to brown-tinged fluid was drained from the mass. Owing to the intricate contact with surrounding major blood vessels and the deep extension into the chest, the mass was incompletely removed. A thoracostomy tube was placed, and bupivacaine hydrochloride (7.5 mg/kg [3.5 mg/lb] infused through the thoracostomy tube followed by 0.9% saline [NaCl] solution flush, q 8 h) and buprenorphine hydrochloride (0.2 mg/kg [0.1 mg/lb], IV, q 6 to 8 h) were administered. The dog recovered from anesthesia without complications, and the mass was submitted for histologic evaluation.
The day after surgery, treatment was initiated with tramadol (5.7 mg/kg [2.6 mg/lb], PO, q 8 h for 7 days) and carprofen (2.1 mg/kg [1.0 mg/lb], PO, q 12 h for 3 days) for analgesia and trimethoprim-sulfamethoxazole (34.1 mg/kg [15.5 mg/lb], PO, q 8 h for 30 days) for the suspected infection with Nocardia sp. Administration of buprenorphine was discontinued 3 days after surgery because the dog no longer showed signs of pain on palpation. The dog continued to do well and ate without evidence of dysphagia or regurgitation. Supplemental oxygen was discontinued, and the nasal canula was removed. Through the thoracostomy tube, 152 mL of serosanguinous pleural effusion was removed, and over the next 2 days, the volume of pleural effusion decreased. Therefore, the thoracostomy tube was removed.
Histologic evaluation of the mass revealed no evidence of a nocardial infection, unlike the findings on bacterial culture of the fine-needle aspirate sample of the mass. Instead, results of histologic evaluation indicated an anaplastic sarcoma with extensive necrosis. The mass was unencapsulated and largely composed of sheets of densely packed polygonal cells intermixed with scattered multinucleated cells in an abundant eosinophilic-to-amphophilic matrix that also contained scattered small stellate-shaped cells. Individual polygonal cells had marked anisocytosis and anisokaryosis. Mitotic figures were numerous and frequently atypical. The neoplastic stroma had multifocal expansive zones of amorphous necrotic debris, and in 1 section, a single island of immature cartilage was noted. Results of immunohistochemical staining were diffusely and strongly positive for vimentin but negative for cytokeratin, consistent with an anaplastic sarcoma.1
The dog continued to improve, and 5 days after surgery was discharged with its remaining prescriptions of tramadol and trimethoprim-sulfamethoxazole. Unfortunately, the dog was lost to follow-up.
Comments
Although mediastinal masses are common in dogs and cats, dorsally located mediastinal masses occur much less commonly than those in cranioventral or hilar regions.2 Commonly, dorsally located mediastinal masses originate from enlargement of the esophagus; however, it is often difficult to determine tissue of origin radiographically, especially for masses with homogeneous opacity.2 Additionally, different tumor types of cranially located mediastinal masses have substantial overlap in size and shape and in contrast enhancement pattern; therefore, histologic evaluation is typically required for a definitive diagnosis.3 As was the case in the dog of the present report, positive contrast esophagography is often a useful diagnostic tool when the origin of the mass is suspected to be esophageal.2
Esophageal neoplasia is rare in dogs, accounting for only 0.5% of reported tumors.4,5 Of 8 dogs with esophageal neoplasia in a study,4 2 had primary tumors of esophageal origin and 6 had metastases from other origins (thyroid carcinoma most commonly) affecting the esophagus. The most commonly diagnosed primary esophageal neoplasms are squamous cell carcinoma and fibrosarcoma, followed by leiomyosarcoma and osteosarcoma.5 Although rare, esophageal neoplasms tend to be malignant and aggressive.4 Similar to the dog in the present report, surgical resection can be difficult owing to limited surgical access. In addition, excessive tension on excision sites and esophageal motion contribute to high rates of postoperative complications.5 Chemotherapy protocols have also been described in conjunction with surgical treatment of esophageal soft tissue sarcomas in dogs.5
In the dog of the present report, cytologic evaluation did not provide a conclusive diagnosis; however, Nocardia sp was cultured from the fine-needle aspirate sample. Nocardiosis is uncommon in dogs, and although infections have been reported in immunocompetent patients, infections concurrent with distemper, leukemia, and other conditions contributing to immunodeficiencies are more common.6,7 Pulmonary nocardiosis is the most common form in humans, accounting for approximately 70% of all reported cases.6 With pulmonary nocardiosis, clinical signs in humans include a cough, fever, dyspnea, weight loss, chest pain, and hemoptysis,7 and radiographic findings are nonspecific, mainly multiple irregular nodules.6 Trimethoprim-sulfamethoxazole is the treatment of choice for most forms of nocardiosis, and gentamicin, which the referring veterinarian prescribed to the dog in the present report, has been shown to be effective in vitro.7 Prognosis depends on the tissues affected, with disseminated, pulmonary, and neurologic infections having poor prognoses.7
Diagnostic imaging was critical in treating the dog of the present report because it not only helped us locate and sample the lesion, but it also provided us with structural detail essential in deliberating and planning surgical treatment. The underlying cause of the lesion, however, could not be identified with diagnostic imaging, and on the basis of additional diagnostic procedures performed, mediastinal anaplastic sarcoma with cultured nocardiosis was diagnosed in the dog. Other findings in the dog did not provide supportive evidence of typical nocardiosis; however, it was unclear whether the dog had been pyrexic before receiving antimicrobial treatment prescribed by the referring veterinarian. Findings in the dog of the present report served as an excellent reminder that nocardiosis may be associated with a multitude of clinical signs and that the diagnosis or exclusion of it may be difficult.
Acknowledgments
The authors declare that there were no conflicts of interest.
References
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