Presumed mediastinal cysts have a low prevalence in canine thoracic computed tomographic studies.

Mediastinal cysts (MCs) are rare lesions that can arise from embryonic remnants of various mediastinal structures. MCs usually are incidental findings in dogs and cats; however, they can reach a mass-like appearance. The description of MCs on CT in dogs is limited. This retrospective, single-center, descriptive, prevalence study aimed to determine the prevalence of presumed mediastinal cysts (PMCs) in dogs and assess their CT characteristics. Dogs that underwent a thoracic CT scan from January 2019 to August 2021 were included. CT images were evaluated for the presence of PMCs by two diagnostic imaging interns, two last year diagnostic imaging residents, and a board-certified veterinary radiologist. Number, location, margins, shape, volume, size, mass effect, and attenuation values of PMCs were assessed. A total of 866 CT scans were reviewed, and 49 PMCs were identified. The prevalence of PMCs in dogs was 5.66%. English Bulldog and mixed-breed dogs were subjectively overrepresented; however, the possibility of population bias could not be excluded. PMCs were subjectively more frequently observed in male dogs. The PMCs were predominantly round, small, solitary fluid-filled findings localized in the cranioventral mediastinum, with well-defined margins, homogeneous attenuation, and no contrast enhancement. The median attenuation value was 6.32 HU (range: -20.16 to 23.45 HU) precontrast and 7.58 HU (range: -2.45 to 20.79 HU) postcontrast, and the median volume was 1.19 cm3 (range: 0.02-45.32 cm3). Although the prevalence of PMCs was low in our sample population, findings supported prioritizing a differential diagnosis of incidental PMC for dogs with the above imaging characteristics.

Spindle-cell thymoma colliding with a bronchogenic cyst in a Yorkshire terrier.

An 8-year-old male Yorkshire terrier was presented to the Tufts Veterinary Hospital for evaluation of increased respiratory effort. A mediastinal mass composed of a spindle-cell thymoma within a bronchogenic cyst was diagnosed with computed tomography thoracic imaging, ultrasound-guided fine-needle aspirate biopsy, and histopathologic evaluation after surgical removal. Histologic evaluation showed a multilocular cyst structure as well as a mass characterized by spindle to polygonal thymic epithelial cells. The cyst was characterized by a lining of ciliated pseudostratified respiratory epithelium. To the authors' knowledge, this is the first report of a spindle-cell thymoma being associated with a mediastinal bronchogenic cyst in a dog.

Mediastinal basaloid carcinoma arising from thymic cysts in two dogs.

Mediastinal masses in dogs were diagnosed as basaloid carcinoma associated with multiple thymic cysts (MTCs). The masses were composed of MTCs and proliferating intracystic neoplastic basaloid cells, which immunohistochemically diffusely expressed p63 and cytokeratin 19. A gradual transition from the basal cell layers lining the cysts walls to the neoplastic cells was seen, and it was indicated that the neoplastic cells had originated from the basal cell layers of the cysts. To the best of our knowledge, this is the first report of basaloid carcinoma occurring in the mediastinal cavity in dogs. Although these tumors were demonstrated to be rare origins, basaloid carcinoma should be included in the differential diagnoses for canine mediastinal tumors.

A rhabdomyoma within a multilocular thymic cyst in a p53-null mouse.

A 9-month-old p53-null female mouse was found dead in its cage. At necropsy, a large thymic mass encompassed the heart. Microscopically, the mass was composed of numerous varying-sized cysts lined with simple squamous epithelial cells to columnar ciliated cells. Also present within this mass was a large aggregate of loosely arranged fusiform-shaped cells. These cells also were found in smaller numbers in the connective tissue surrounding the cysts. The larger aggregate of fusiform cells was positive for desmin and S-100 and negative for smooth muscle actin. Electron microscopy revealed well-formed Z lines and I bands of skeletal muscle phenotype. A diagnosis of rhabdomyoma within a congenital multilocular thymic cyst was made. The thymus contains a small population of myoid cells, which should be taken in consideration when evaluating thymic tumors.

Long-term survival in six cats with mediastinal cysts.

CASE SERIES SUMMARY: Mediastinal cysts were diagnosed as incidental findings in six cats evaluated for non-thoracic disease, including staging for historical bladder leiomyosarcoma, flea dermatitis and hairballs, and hyperthyroidism. Radiographically, the cysts appeared as soft tissue opacities cranial to the heart. Ultrasound revealed the masses to be thin-walled, single lumen, anechoic, fluid-filled structures. One cat also had thoracic and abdominal CT performed for cancer staging; the CT revealed a well-defined, fluid-attenuating mass without peripheral contrast enhancement in the cranial mediastinum. Fine-needle aspiration confirmed acellular fluid consistent with a cyst in five cases; in one case the cyst ruptured during aspiration and no fluid was obtained. Post-aspiration, all masses were no longer visible with ultrasound or radiographs. No treatment was recommended for the cysts. Long-term follow-up (2-9 years post-diagnosis) was available in all six cats. The cysts recurred in five cats but were never associated with clinical signs. RELEVANCE AND NOVEL INFORMATION: Mediastinal cysts are an important benign differential for cranial mediastinal masses in cats. Treatment for the cysts does not appear to be indicated. This series also includes the first CT description of this clinical entity.

Cholecystopexy and Pericardial Pseudocyst Removal in a Dog with a Congenital Peritoneopericardial Diaphragmatic Hernia.

A 4 mo old spayed female golden retriever was presented with a peritoneopericardial diaphragmatic hernia (PPDH) that was diagnosed during neutering. Echocardiography revealed a fluid-filled structure and parts of the liver in the pericardial cavity. Computed tomography confirmed the existence of the PPDH and the herniation of the right medial liver lobe and the gallbladder. Cystic masses were observed in the pericardial and the peritoneal cavities, possibly communicating through the PPDH. A median laparotomy revealed a single lobulated cystic lesion extending into both the pericardial and peritoneal cavities through the PPDH. Because of the nonviable aspect of some parts of the liver parenchyma, the gallbladder was dissected from the fossa, and the central division of the liver was resected. A cholecystopexy was performed on the diaphragm to limit gallbladder mobility. The PPDH was closed in the standard fashion. Histopathology of the cystic structure was compatible with a pericardial pseudocyst. Two months postoperatively, the dog was healthy, and the results of blood biochemistry and abdominal ultrasonography were normal. A pericardial pseudocyst can be associated with a PPDH in young dogs. Moreover, cholecystopexy appears to be a safe and effective method of limiting gallbladder mobility after resection of the central hepatic division.

Concurrent spindle-cell thymoma and thymic cysts in a Barbary sheep (Ammotragus lervia): case report and review of the literature.

An ~21-year-old female Barbary sheep (Ammotragus lervia) died spontaneously following a lengthy episode of difficulty in walking. An ~6 × 3 × 3 cm, unilocular cystic growth was found in the cranioventral thorax. The fibrotic cystic wall, lined by a single layer of flattened to cuboidal epithelial cells, was invaginated and partially encircled solid masses of fusiform neoplastic cells with multiple intratumoral cystic structures. The fusiform neoplastic cells were intensely positive for cytokeratin (CK) and partially positive for α-smooth muscle actin and vimentin, but negative for thyroid transcription factor-1 (TTF-1) and neuron-specific enolase (NSE). The intratumoral cysts were lined by CK-positive but TTF-1- negative, NSE-negative, flattened, cuboidal to columnar epithelial cells, suggestive of cystically dilated medullary duct epithelium-derived structures. Based on the location and histopathologic findings of the growth, concurrent spindle-cell thymoma and thymic cysts was diagnosed. We also discuss the correlation between thymic cysts and thymoma and review the literature of thymomas in ovine and wildlife species.

Pericardial cyst in a 2-year-old Maine Coon cat following peritoneopericardial diaphragmatic hernia repair.

A pericardial cyst developed in a 2-year-old male neutered Maine Coon cat following surgery for an incidentally diagnosed congenital peritoneopericardial diaphragmatic hernia. The cyst caused no clinical signs in the cat, although clinical findings included positional right-sided cardiac tamponade and compression of thoracic structures, associated with a cardiac arrhythmia and axis deviation on electrocardiography. Extensive assessment of the cyst included radiography, echocardiography, computed tomography, exploratory thoracotomy, electrocardiography, histopathology and fluid analysis. Surgical removal of the cyst was curative, and the arrhythmia and axis deviation resolved. This report details case management from initial diagnosis to long-term follow-up, adding to the limited body of literature available on feline pericardial cysts. This is also the first report to associate cardiac arrhythmia with a pericardial cyst.

Meningoencephalitis associated with an unidentified apicomplexan protozoan in a Pacific harbor seal.

A Pacific harbor seal (Phoca vitulina richardsii) was found on the central California coast with neurologic signs and labored breathing, which were unresponsive to treatment. Necropsy revealed a nonsuppurative necrotizing meningoencephalitis, a multilocular thymic cyst, and nonsuppurative cystitis and renal pyelitis. Microscopic examination revealed protozoans in the brain, thymic cyst, and bladder mucosa. Ultrastructurally, the protozoal tachyzoites were different from those of Neospora caninum, Toxoplasma gondii, and Sarcocystis neurona; the rhoptries were small and had electron-dense contents, and the organism divided by endodyogeny. Specific antibodies were not detected in serum using agglutination (N. caninum, T. gondii) and immunoblot assays (S. neurona). Immunohistochemistry for these organisms was negative. Polymerase chain reaction on brain tissue using specific primers did not amplify T. gondii deoxyribonucleic acid. The meningoencephalitis in this seal thus appears to have been caused by a novel protozoan.

Thymic epithelial cysts in the pigeon.

Medullary epithelial cells forming cysts in the thymus of pigeon chicks were examined in order to clarify these morphological characteristics by immunohistochemical and electron microscopic techniques. Light microscopically, cysts were divided into intracellular and intercellular types. Intracellular cysts were positive for the periodic-acid Schiff reaction, while the intercellular type was not. With immunohistochemical staining for keratin, the edges of the intercellular cysts were moderately stained, whereas the cytoplasm of epithelial cells was weakly positive. Heterophils, eosinophils and lymphocytes were included in most of the intercellular cysts. Ultrastructurally, there were two types of epithelial cells which contained intracellular cysts densely packed with microvilli. They differed with regard to the shape of the rough endoplasmic reticulum and bundles of keratin filaments, and in the presence of large vacuoles. The walls of the intercellular cysts were composed of a single type of epithelial cell with a pale nucleus and bundles of keratin filaments. Small vesicles measuring 150 to 200 nm in diameter were numerous in the apical cytoplasm beneath the microvilli. In intercellular cysts, there were abundant heterophils, eosinophils, lymphocytes and macrophages which mostly appeared to be living cells. This evidence suggests that the intercellular cysts might provide a specific microenvironment for leukocytes in the pigeon thymus.

Thymoma and multiple thymic cysts in a dog with acquired myasthenia gravis.

An anterior mediastinal cystic lesion in an 11-year-old mongrel dog was examined. The dog showed dysbasia and vomiting due to megaoesophagus, and anterior mediastinal round mass lesion, approximately 35 mm in diameter, was found by X ray. Based on clinical examinations, the dog was diagnosed as acquired myasthenia gravis and was successfully controlled by anticholinesterase treatment for approximately 4 months. The dog died of thermic stroke and was necropsied. Grossly, fatty tissues with cysts containing yellowish fluid and white nodules were found in the anterior mediastinal area. Histopathologically, multiple cysts, neoplastic tissues, and atrophic thymus were found within the examined tissues. The cysts were lined by thin wall consisting of ciliated long cuboidal and non-ciliated round cells and were filled with eosinophilic colloidal fluid. Some extended cysts contained neoplastic foci within their lumen and walls. The neoplastic tissues consisted of mixed population of large epithelial cells with abundant clear cytoplasm and large oval nuclei, and lymphocytes. Immunohistochemically, proliferating epithelial cells were intensely positive for keratin and cytokeratin, and more than half number of infiltrating lymphocytes were intensely positive for CD3 suggesting T cells. All these findings indicate the neoplastic lesion is thymoma and multiple cysts are considered as thymic or brachial cleft cysts.

Use of nuclear scintigraphy to characterize in intrathoracic mass in a foal.

A 2-month-old thoroughbred colt with noisy and difficult respiration was diagnosed as having an intrathoracic mass, using conventional radiographic techniques. Nuclear scintigraphy was then used to characterize the mass, leading to successful surgical removal.

Benign cranial mediastinal lesions in three cats.

Cranial mediastinal lesions were detected in three cats, associated with respiratory impairment (case one), spontaneous pneumothorax (case two) and myasthenia gravis (case three), respectively. On gross and histological examination, the first case was considered either a lymphangioma or a branchial cystic mass of the thymic region of the mediastinum; a cystic lesion was suggested by sonographic detection of multiple anechoic cavitations within a circumscribed mass, while fine needle aspiration cytology excluded lymphosarcoma. The second case was diagnosed histologically as a cystic thymoma, but the third case was not examined microscopically. The masses were amenable to surgical excision in the first two cats, while this proved unnecessary in the third case because of resolution following treatment with dexamethasone. Corticosteroid responsiveness was unhelpful in distinguishing between these benign lesions and lymphosarcoma, as in two cases there was a partial or complete response to dosing with prednisolone or dexamethasone. These cases are presented to emphasise that conditions other than lymphosarcoma can produce cranial mediastinal lesions in cats, and that the prognosis for surgical treatment of lymphangiomas, multilocular thymic cysts and cystic thymomas can be excellent.

Benign masses in the pericardium of two dogs.

Two miniature Schnauzer dogs were treated for pericardial masses. In one dog the mass consisted of necrotic fat that was attached to the apex of the pericardium by a pedicle. No obvious communication with the abdomen was present. The second dog had a peritoneopericardial hernia associated with a chronic cystic haematoma. In each case the mass was presumed to have arisen following congenital displacement of the omentum into the pericardium. The pericardial mass was removed and subtotal pericardectomy performed in both dogs. Recovery was complete in each case.

Review of thymic pathology in 30 cats and 36 dogs.

Data are presented from 30 cats and 36 dogs in which thymic disease was recognised clinically or on postmortem examination. The diagnoses included thymic lymphoma (19 cats, 12 dogs), thymoma (five cats, 18 dogs), thymic branchial cyst formation or cystic change (one cat, four dogs), thymic hyperplasia (two cats), congenital hypoplasia (one cat, one dog), thymic haemorrhage (one cat, one dog) and thymic amyloidosis (one cat). Thymic lymphoma occurred in younger dogs and cats, and was recorded equally among domestic shorthaired and purebred (especially Siamese) cats. Eight cats with thymic lymphoma were tested for feline leukaemia virus and four were positive. Thymoma occurred more frequently in older cats and dogs, and in Labradors and German shepherd dogs. Thymic tumours were associated with paraneoplastic hypercalcaemia (six dogs), megaoesophagus (two dogs) or interface dermatitis with basement membrane immune complex deposition (one cat). Non-neoplastic thymic diseases were associated with myasthenia gravis (one cat), pemphigus foliaceus (one cat) and superficial necrolytic dermatitis (one cat).

Mediastinal cyst in a dog.

An eleven-month-old long-haired German pointer with a cyst on the medial side of the left lung is described. The content was a mucous liquid in a quantity of 400 ml. The cyst was the cause of a heavy dyspnoca. After removal of the cyst, the animal improved quickly. The origin of the cyst could not be determined definitely probably it was a congenital bronchogenic cyst.