Introduction

Caudal mediastinal masses in small animals are infrequently documented. Possible differential diagnoses for a caudal mediastinal mass include hiatal hernia, esophageal foreign bodies, paraesophageal abscess/empyema, and neoplasia such as mesothelioma, lipoma, and granuloma resulting from parasitic or bacterial agents.^1–7 Limited reports exist on the surgical management of caudal mediastinal granulomas (CMGs). For instance, Franklin et al. successfully managed a bacterial-caused caudal mediastinal granuloma through surgical debridement and omentalization.⁸ Sivacolundhu et al. documented the treatment of three caudal mediastinal granulomas caused by Nocardia and Actinomyces spp., with two dogs surviving after surgery and antibiotic treatment, whereas one experienced cardiopulmonary arrest because of intraoperative blood loss.⁷ Additionally, Gremillion et al. described a paraesophageal granuloma caused by Pseudomonas spp., but the outcome of this case was not provided.⁹ This current case report details the successful resolution of a nonresectable paraesophageal granuloma through antibiotic therapy alone, with the use of serial computed tomographic (CT) imaging to monitor treatment response.

Case Report

A 3 yr old intact female springer spaniel, weighing 13.3 kg, was brought to a university veterinary teaching hospital exhibiting symptoms of lethargy, hyporexia, pyrexia, and a mild cough persisting for 3 days. Prior thoracic radiographs conducted by the referring veterinarian showed a soft tissue opacity in the caudo-dorsal lung fields. The patient was treated with amoxicillin-clavulanic acid and nonsteroidal anti-inflammatories for 3 days with no response. A year before presentation, the dog had undergone surgical removal of a grass seed from her sublumbar muscles. On presentation, the dog was quiet but alert. The heart rate was 108 beats per minute with sinus rhythm and no audible heart murmur. The respiratory rate was 28 breaths per minute, and the rectal temperature was 38.8°C. Mucous membranes were pink and moist, and capillary refill time was less than 2 s. Oxygen saturation was 98%, and systolic blood pressure, measured using the oscillometric method, was 155/92 mm Hg. Hematocrit (packed cell volume) measured at 47% (reference range 35–55%), and total solids were 7.9 g/dL (reference range 5.5–7.5 g/dL). Blood gas analysis was within normal range, and blood smear evaluation revealed a subjectively increased white blood cell count with a left shift and an adequate number of platelets (>15 per high-power field).

Imaging Findings, Diagnosis, Treatment, and Outcome

The patient was sedated, and a CT scan of the thorax was performed. Images were acquired using a 64-row multidetector computed tomography scanner^a. For postcontrast series of images, a standardized dose of 2 mL/kg iodinated contrast medium, ioversol^b, was administered after a fixed- duration delay of 55 s. The CT study revealed a rounded mass measuring ∼7 cm × 5 cm just cranial to the diaphragm, bordering the esophagus to the left. The mass exhibited diffuse, heterogeneous contrast enhancement. The margins of the mass along the esophagus were poorly distinguishable. Within the mass, there was a small, well-defined mineralized body (Figure 1A). The cranial sternal, splenic, and jejunal lymph nodes were moderately enlarged, up to 1 cm. The tracheobronchial and mediastinal lymph nodes were rounded and conspicuous, measuring up to 0.7 cm. A mass effect was associated with the lesion, with compression and atelectasis of the adjacent lung parenchyma most pronounced in the accessory lung lobe. There were patchy areas of pulmonary hyperattenuation throughout all lung lobes, and although these were suspected to represent mild areas of atelectasis as the lung volume was generally decreased, septic foci could not be definitively excluded. A small amount of fluid-attenuating, nonenhancing material was present in the ventral aspect of each pleural space, representing effusion. The ventral pleura was mildly thickened, likely representing pleuritis. The patient underwent surgical exploration of the thorax through median sternotomy. The caudal golf ball–sized esophageal mass was identified dorsal to the esophagus. A stomach tube was passed, but it did not facilitate differentiation of the esophageal margin to which the mass was adhered. The mass was very hard and well circumscribed, being adherent ventrally to the cauda vena cava and to the left caudal lung lobe. Cranial celiotomy was performed, and the mass was found to be adherent to the diaphragm. Owing to the mass extent and severity of adherence, the mass was considered unresectable. An incisional biopsy of the mass was performed, and a thoracic drain^c and wound-soaking catheter^d were placed. The median sternotomy was closed routinely, and the patient recovered uneventfully from anesthesia. The patient was hospitalized in the intensive care unit for 5 days and was discharged on day 6 with oral administration of acetaminophen/codeine^e (10 mg/kg twice daily for 7 days), amoxicillin/clavulanic acid^f (20 mg/kg three times daily for 4 wk), and meloxicam^g (0.1 mg/kg once daily for 4 wk). Histopathological analysis revealed moderate, diffuse, chronic active inflammation and fibrosis/fibroplasia, indicative of an infectious origin, with no evidence of neoplasia. Culture and sensitivity testing identified Escherichia coli, which was sensitive to amoxicillin and clavulanic acid. At the 4 wk follow-up, the patient exhibited no symptoms, and the clinical examination revealed no abnormalities. A repeat CT scan demonstrated a substantial reduction in size of the paraesophageal mass (Figure 1B). The mineral-attenuating focus within the mass remained unchanged compared with the previous study. The mass exerted a diminished mass effect on adjacent structures, evidenced by reduced ventral deviation of the esophagus and resolution of left lateral displacement of the aorta. The previously noted patchy areas of pulmonary hyperattenuation and small pleural effusion had resolved, with no evidence of atelectasis or fluid-attenuating material in the pleural space. All previously enlarged lymph nodes reduced to normal size. Meloxicam was discontinued, and the antibiotic course was extended for an additional 4 wk. At the 8 wk follow-up, the patient remained clinically healthy, and the repeated CT scan indicated that the mass size remained static (Figure 1C). Antibiotics were discontinued at this point. During the 12 wk reevaluation, another CT scan revealed a further reduction in the paraesophageal lesion’s size (height 0.45 cm × width 0.9 cm) (Figure 1D). The lesion appeared rounded with diffuse, heterogeneous contrast enhancement, similar to its previous appearance. Three months later (6 mo after surgery), another follow-up CT scan showed the mass to have significantly decreased in size, almost completely resolved (width 0.1 cm) (Figure 1E). The patient remained clinically normal without the need for medications, and there was no recurrence of clinical signs. In a follow-up call with the owner 1 yr after the last CT scan, the patient was reported to be clinically asymptomatic.

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Discussion

The mediastinal serous cavity is a potential space located in the caudal part of the mediastinum, on the right of the esophagus and between the heart base and diaphragm, and it originates from the greater omentum.¹⁰ The exact pathogenesis of CMG remains incompletely understood. In human medicine, CMG is most associated with tuberculosis or histoplasmosis. In dogs, it is speculated that it may arise from inhaled foreign bodies leading to bronchopneumonia in the caudal lung lobes, subsequently causing local extension of the infection into the mediastinal serous cavity or foreign bodies that perforate the esophagus.¹⁰ In this patient, the presumed cause of the granuloma is suspected to be a penetrating esophageal foreign body, given its close adherence to the esophagus. Alternatively, a migrating plant seed or other inhaled foreign body leading to bronchopneumonia and subsequent extension into the caudal mediastinal serous cavity cannot be ruled out. It is worth noting that esophageal granulomas on the caudal thoracic esophagus can be caused by the parasite Spirocerca lupi.⁶ However, given that this disease is not endemic in the United Kingdom, it was considered unlikely in this instance, although a fecal flotation test was not performed. The preferred diagnostic modality for caudal mediastinal pathology is a CT scan. It can also provide information for surgical planning and concurrent intrathoracic pathology. In human medicine, CT is used for serial follow-up monitoring of cases with mediastinitis treated conservatively, as well as to track early recurrence.¹¹ In this article, we report the sequential CT monitoring of a conservatively managed case with no recurrence in 6 mo. In human medicine, the treatment for mediastinal granuloma depends on the severity of clinical signs. Asymptomatic patients may undergo conservative management involving serial CT monitoring, as spontaneous resolution of granulomas has been observed. Conversely, for symptomatic and severe cases, early, aggressive intervention is recommended, consisting of surgical debridement and continuous drainage through thoracostomy tubes.¹² In dogs, surgical resection and antibiotic treatment is the treatment of choice for caudal mediastinal abscess/empyema. In a limited case series involving seven dogs with paraesophageal abscesses, surgical debridement with or without omentalization via thoracotomy or sternotomy, along with thoracic drainage and antibiotic administration, resulted in complete recovery for all dogs.² In cases of CMG, another small case series detailed three dogs with CMG attributed to Actinomyces and Nocardia. One dog achieved full recovery after complete resection and antibiotic treatment, whereas another recovered after partial resection and antibiotic therapy. The third dog succumbed intraoperatively as a result of acute hemorrhage during the attempt to remove the granuloma.⁷ A separate case report documented successful management of CMG in a dog through surgical debridement, omentalization, and antibiotic treatment. Omentalization, however, remains controversial because of the associated risk of the granuloma with underlying neoplasia and the potential for seeding cancer cells into the abdomen.⁸ These case reports demonstrate the potential for a successful outcome after surgical treatment. However, complete resection is not always possible, and attempts to achieve this can lead to severe and life-threatening complications. Nonsurgical approaches for managing caudal mediastinal empyema have been documented in a dog and two cats. In a single case report, a dog with a paraesophageal abscess underwent treatment through repeated drainage guided by ultrasound and antimicrobial therapy. A follow-up CT scan at 14 days after discharge revealed a reduction in abscess size, and antibiotic administration continued for an additional 2 wk. On reevaluation 8 mo later, there was no recurrence of clinical signs.¹³ Two reports in the literature describe the medical treatment of cats with paraesophageal abscesses. In the first report, a cat underwent thoracic drainage through bilaterally placed thoracostomy tubes along with antibiotic therapy, achieving complete resolution as evidenced by a follow-up CT scan 6 wk after discharge.¹⁴ The second cat was solely treated with antibiotics, with the authors reporting no recurrence of clinical signs 21 days after discharge, although no repeated CT or radiographs were performed at that stage.¹⁵

Conclusion

This case report represents the first documented instance of successful nonsurgical management of a paraesophageal granuloma in a dog. In this case, drainage was not necessary as there was no cavitated abscess, and surgical excision or debridement of the granuloma was not feasible owing to the circumferential encroachment of the diaphragm and the esophagus.

Informed consent was obtained from the pet owner prior to submitting this case report. The patient described in this study was clinically managed in accordance with contemporary standards of care, as outlined in the American Animal Hospital Association guidelines and the JAAHA Instructions for Authors.