CRANIAL MEDIASTINAL CYSTS IN NINE CATS LISAJ. ZEKAS,DVM, WILLIAM M. ADAMS,DVM
Nine cats, from 11 to 17 years of age (mean 13.6 years of age), were diagnosed with a cranial mediastinal cyst. Thoracic radiographs in all cats were characterized by an increased soft tissue opacity in the cranial mediastinum confirmed to he a cyst by ultrasonography or necropsy. Ultrasonographicallycysts appeared as an anechoic mass. A low-cellularity clear fluid was obtained on aspiration. The majority of the cats (n = 8) presented for unrelated conditions with no signs of respiratory distress. No treatment for the cyst was pursued except for drainage during ultrasonographic-guidedaspiration in several cats. On follow-up of eight cats, none were symptomatic for the cyst from 3-45 months after diagnosis. Mediastinal cyst should be considered when a cranial mediastinal mass is evident radiographically in an older cat. The majority of feline cranial mediastinal cysts are benign with no need for treatment. Veterinary Radiology & Ultrasound, Vol. 43, No. 5, 2002, p p 413418. Key words: cyst, mediastinum, feline, radiography.
Introduction
C
mediastinum have been reported in dogs, cats, and humans.'-7 Various cystic conditions have been reported, including branchial, parathyroid, thyroglossal duct, and pleural. The origin of these cysts is determined using histologic e ~ a l u a t i o n Cranial .~ mediastinal cysts have been reported in eight cats in single reports and retrospective Cysts in two of these cats were described histologically as branchial cysts of the thymus. Both affected cats presented with dyspnea and pleural effusion.' A thymic branchial cyst was also dia,Onosed in one cat who developed respiratory problems during ane~thesia.~ Idiopathic mediastinal cysts have been described based on cytologic evaluation of cystic fluid in four cats, none of whom had respiratory signs.233All were considered incidental findings. In another report, a mediastinal mass was removed surgically and diagnosed as a mediastinal parathyroid The purpose of this study is to describe radiographic and ultrasonographic findings and the clinical course in nine cats with mediastinal cysts and to compare these presentations with previous reports. YSTS LOCATED IN THE
Materials and Methods Medical records of cats diagnosed with cranial mediastinal cysts at the University of Wisconsin Veterinary Medical Teaching Hospital (UWVMTH) between November 1996
From the Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 201 5 Linden Dr., Madison, WI 53706. Address correspondence and reprint requests to Dr. Lisa J. Zekas. Received October 31, 2001; accepted for publication January 19, 2002.
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and February 2001 were reviewed. Data regarding signalment, presenting complaint, physical examination, fluid analysis, histology, and necropsy results, treatment, final diagnoses and clinical outcome were recorded. Thoracic radiographs, performed in all cases, and ultrasound examinations, performed in eight cats, were also reviewed. Cats were included if the diagnosis of cranial mediastinal cyst(s) was confirmed by ultrasound guided fine-needle aspirate or necropsy.
Results Nine cats were identified with a diagnosis of a mediastinal cyst. The cats ranged in age from 1 1-17 years old (yo; mean 13.6 years, median 14 years). Five were neutered females and four were neutered males. The weight ranged from 2.8-7 kg (mean 5.0 kg). Breeds included domestic short-hair (DSH; n = 6), domestic medium-hair (DMH; n = I ) , domestic long-hair (DLH; n = I), and Maine Coon (n = 1 ) . The cats were presented to the UWVMTH for various reasons. Only one cat had dyspnea, in addition to edema of the head and neck. Other cats were presented for evaluation of hyperthyroidism (n = 2), heart murmur (n = 2), neoplasia (n = 2; nasal and hind limb) or vomiting/diarrhea (n = 1). Subsequent diagnoses included hyperthyroidism (n = 2 ) , squamous cell carcinoma, lymphoma, ventricular septa1 defect, hypertrophic cardiomyopathy, and inflammatory bowel disease. One cat had clinic signs attributable to multiple system failure and was euthanized. An incidental mediastinal cyst was confirmed at necropsy. Thoracic radiographs were made in eight cats for evaluation for metastases or a suspected cardiac problem and in
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one cat for respiratory signs. The radiographic examinations included ventrodorsal and right and left lateral views. In all cats, the radiographs were interpreted as having or being suspicious for a cranial mediastinal mass. The radiographic appearance ranged from a discrete well-marginated soft tissue opacity cranial to the heart to an ill-defined increase of soft tissue opacity in the cranial thoraxlmediastinum (Figs. 1 4 ) . The most common shape was ovoid (n = 5), but others were irregular (n = l), lobular (n = I), or diffuse (n
FIG.2. (A) Right and (B) left lateral thoracic radiographs of an 1 I-yearold neutered female DSH cat. A soft tissue opacity is present in the cranioventral thorax. It appears more defined on the right lateral view. The opacity appeared ill-defined on the VD view.
FIG. 1 . Ventrodorsal (A) and right lateral (B) thoracic radiographs of a 14-year-old neutered female DMH cat. A well-marginated soft tissue opacity is present in the cranial mediastinum noted on the VD view. An illdefined increased soft tissue opacity is seen in the cranioventral thorax on the lateral view.
= 2). The cyst was well marginated and seen equally well on all views in two cats. The cyst was well marginated on at least one view in five other cats. In these cats it was best seen on the following views; ventrodorsal (n = 3), right lateral (n = l), and both VD and right lateral (n = 1). In two cases, the cyst was ill defined on all views. The cyst silhouetted with the cranial cardiac margin on at least one view in seven cats. It was noted to cause slight caudal displacement of the cardiac silhouette in two cats. Other findings included static partial collapse of the right middle lung lobe in one cat and multiple (static) pulmonary nodules in another. Cysts were diagnosed in eight cases by ultrasound and ultrasound-guided fine-needle aspiration of a clear, colorless fluid. In one cat, ultrasound-guided aspiration of an anechoic structure was attempted, but no fluid was obtained. The structure was no longer visible on post-aspirate ultraimages Or radiographs. Cyst rupture was presumed, although no pleural l-luid was noted on post-aspirate radio-
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FIG.3. Ventrodorsal (A) and left lateral (B) thoracic radiographs of a 13-year-old neutered obese male DLH cat. There is a generalized increased soft tissue opacity in the cranial thorax, which is slightly right-sided on the VD view. Also noted is an alveolar pattern in right middle lung lobe, interpreted as partial collapse with fibrosis. (It was persistent on all subsequent studies up to 35 months).
graphic or ultrasonographic examinations. The ultrasonographic appearance of the cysts was that of an anechoic, ovoid-to-bilobed thin-walled structure with distal enhancement and a decrease in size after aspiration. Ultrasonographic measurement of the widest diameter of the cyst(s) was available in five cats and ranged in size from 10.4-51.2 mni (mean 14.2 mm). The amount of fluid that could be aspirated was recorded in three cats, 4, 6, and 20 ml, respectively. In five cats, when aspirations were performed, complete drainage was attempted. In one cat, this had to be discontinued due to poor patient compliance. Aspirated
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FIG.4. Ventrodorsal (A) and right lateral (B) thoracic radiographs of a 14-year-old neutered male DSH cat. A well-marginated ovoid soft tissue opacity is seen in the cranial thorax near the thoracic inlet. An incidental elongated aortic arch is also noted.
fluid was analyzed in seven cats. The fluid was classified as acellular to minimally cellular. When cells were present they included small numbers of neutrophils, red blood cells, platelets, small lymphocytes, and/or macrophages in varying combinations. In one cat, the fluid was interpreted as a very mild mononuclear inflammation. No tissue or neoplastic cells were seen in any cats. Follow-up radiographic and/or ultrasonographic examinations were available on four cats from 1 to 40 months after initial diagnosis. The cyst in a hyperthyroid cat, which
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was aspirated but not drained, was static in appearance on radiographs 3 months later. Another cat treated for squamous cell carcinoma of a rear limb was re-radiographed for metastasis at 13, 18, and 40 months post-diagnosis. The mediastinal cyst, which was drained at initial presentation, was not visible on these images. Another cat with hypertrophic cardiomyopathy and pulmonary nodules (presumed granulomas) had a static radiographic appearance to the cyst at a recheck 11 months post-diagnosis. In one cat with a ventricular septa1 defect, the mediastinal cyst was drained initially, with a decrease in size confirmed on immediate post-aspirate radiographs. Four months later, the cyst had recurred. On thoracic radiographs and ultrasound examination performed at 35 months, the cyst had continued to enlarge (from 2 cm to 5 cm). While the cyst had recurred and progressed, the cat had no respiratory signs. Three patients did not return to the VMTH for follow-up imaging. A hyperthyroid cat whose cyst was drained was reported well at 15 months post-examination according to the owner. The cat with dyspnea and cervical edema had a small amount of fluid aspirated from the cyst (approx. 4 ml - presumed to drain structure), and no respiratory signs or edema were present at 3 months. The cause of the dyspnea and cervical edema was not positively determined. The cat, with vomiting and diarrhea due to inflammatory bowel disease, was re-radiographed by the referring veterinarian 4 1 months later. The cyst was reported to have enlarged, but the cat had no clinical signs referable to the cyst. Two cats had evidence of other cystic disease. One cat had multiple hepatic cysts and another had numerous small
cysts in the liver consistent with biliary cystadenoma. None of the five cats with ultrasound exams of the abdomen or the cat that underwent necropsy was reported to have renal cysts.
Discussion Cysts located in the cranial mediastinum can originate from various sources.* Parathyroid cysts develop when there is dilation and persistence of the duct that connects the parathyroid and thymic primordia during embryonic development. It becomes displaced with the caudal migration of the thymus. Parathyroid cysts are characterized histologically by pseudostratified columnar epithelium and contain a proteinaceous material. Thyroglossal cysts are derived from remnants of the thyroglossal duct. These cysts are lined by multilayered thyroidogenic epithelium, which often has colloid-containing follicles. Thymic duct epithelium originates from the middle series of branchial pouches, which are displaced caudally into the mediastinum. These ducts are the source of cysts lined by ciliated epithelium found in the thymus, referred to as thymic branchial cysts. Cysts originating from the pleura are also noted. Histological examination would be required to definitively differentiate between these cysts. Previously reported mediastinal cysts in cats have been associated with varying clinical signdpresenting complaints and cyst origins (Table 1). Several authors consider cysts to be incidental findings because the patients had no signs referable to the cysts.24 The majority of cats in this study (8 of 9) had no signs potentially attributable to the mediastinal
TABLE1. Previously Reported Feline Mediastinal Cysts Ref. No. of No. Cases
Signalment
Type of cysts Thymic branchial
3
6 yo and 14 yo M-American, Shorthair: F-Siamese Not given
3
1
13 yo: FS: DSH
4
I
10 yo; FS: DLH
Idiopathic: (mediastinal pleura) Parathyroid
5
1
10 yo; castrated; Thymic
1
2
2
DSH
Idiopathic
branchial
Presenting Complaint/ History
Radiograph/ Ultrasound Findings”
Respiratory distress Pleural effusion and (pleural effusion caudal displacement of tracheal bifurcation and dyspnea) (R) Unknown Thin walled; single: anechoic fluid (US) Oral squamous cell carcinomametastasis check Geriatric health check
Well circumscribed mass with deviation of trachea and heart (R) Well-circumscribed (R)
Nasal squamous cell carcinoma; cyanosis during anesthesia
Homogeneous mass displacing heart (R); numerous small anechoic cavitations (US)
“Imaging study performed: (R) radiography or (US) ultrasonography. hFluid acquired by (H) tissue histology or (A) aspirate. ‘Tissues obtained at (S) surgery or (N) necropsy.
Fluidh Mucin-filled (H)
Histology/Cyst Lining‘ Multilobulated: single-layer ciliated columnar epithelium; peripheral infiltration of lymphocytes (N) NA
Clear, acellular. low protein and cellularity (A) Clear, low Non-ciliated simple to pseudo stratified, low cuboidal or protein, acellular (A) squamou5 cells (N) Clear, acellular or Multicystic; Cuboidal to eosinophilic columnar ciliated epithelium proteinaceous (S) (HI Numerous small cysts: unlined Clear, low cellularity or lined by squamous cells: pericyst connective tissue, (various types) lymphocytes and (A) macrophages.
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cysts. Cats were presented for unrelated complaints. Thoracic radiographs were made to evaluate for metastases or cardiovascular abnormalities. The cysts were considered incidental findings and at least three cats remained clinically unaffected by the cysts 3 years later. This correlates well with the reports of idiopathic mediastinal cysts and one parathyroid cyst.24 It is in contrast to thymic branchial cysts where two cats and eleven of fifteen dogs had dyspnea. Pleural effusion was confirmed in one cat and nine dogs.' Three of the canine cysts in that study were found to have ruptured, resulting in severe chronic inflammatory reaction, hemorrhage and edema, whereas other dogs also had pericyst hemorrhage, edema, and cellular infiltrates. Twelve of the 15 dogs with thymic branchial cysts died or were euthanized as a consequence of the cyst. Only one of the nine cats in this study had a primary complaint of dyspnea, with inspiratory/expiratory noise and edema of the head and neck regions. Thymic branchial cysts have been reported to cause swelling of the head and neck and forequarters in dogs, likely a result of vascular compression.' Vascular compression has also been reported in a human secondary to a thymic cyst.' Cervical branchial cysts in a cat and horse have been reported to contribute to unusual gasping sounds and dyspnea when stressed.','" A mediastinal parathyroid cyst was noted to cause tracheal compression and exertional dyspnea in humans. I The specific cause of dyspnea and edema in one cat reported here is confused by a history of prior cervical mass from which 12 ml of brown fluid was drained by the referring veterinarian and the ultrasonographic appearance of an irregular thyroid gland surrounded by fluid. Cytologically, this fluid was interpreted as hemorrhagic and neutrophilic. There was no radiographic evidence of tracheal compression or attenuation, although the radiographs were made with endotracheal intubation for general anesthesia. Although only a small amount of fluid was removed from the mediastinal cyst (approx. 4 ml) and the cat only received a histamineblocker, all clinical signs resolved in this animal and it was doing well three months later. Cats in this study were geriatric, similar to previous reports, but somewhat older than dogs with thymic branchial cysts (mean age 7.5 The significance of this difference is unknown due to varying lifespans. Cysts of branchial, parathyroid, and thyroglossal origins are considered congenital, but rate of enlargement is unknown. One multicystic structure drained in this study recurred to its original size by 4 months. In five cats, complete drainage was attempted when aspirations were performed. Of those cats with significant drainage, defined as visualization of cyst collapse during ultrasound and/or absence of mass on postaspirate radiographs, one had not recurred at 40 months after, three recurred, and two were not re-imaged. Of those not re-imaged, both were reported to have no clinical signs. Cysts described as idiopathic or pleural are of unknown
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origin and may originate later in life. Because no thoracic radiographs were available at earlier ages for the cats in this study, it is unknown whether the cysts were present when the cats were younger. Differential diagnoses for cranial mediastinal masses include abscess, granuloma, lymphadenopathy, hematoma, ectopic thyroid tissue, cyst, or neoplasia, including thymic lymphosarcoma, and thymoma." Signalment, history, and physical examination findings may help prioritize or exclude some of these causes. Because radiographic identification of the cyst was variable between views from case to case, including all three radiographic views is advised. The cyst was best seen on the ventrodorsal view in three of the cases, therefore attempts to take a well positioned ventrodorsal view should be made. Although the cranial mediastinal mass was initially identified on radiographs, ultrasonographic evaluation was instrumental in the diagnosis of a cyst versus other abnormalities. Alternatively, aspiration guided by other methods, i.e., fluoroscopy, could also be useful. The diagnosis may be critical to treatment considerations and prognosis. Seven of these cats had no clinical signs referable to the mediastinal cyst. The primary diagnosis (unrelated neoplasia or systemic disease) resulted in the other two cats being euthanized. In the absence of respiratory signs, intervention does not appear to be warranted when a cystic structure is confirmed. Surgery to remove the cysts has been reported and recommended for functional cysts (i.e., parathyroid) or space-occupying disease causing tracheal or vascular compres~ion.'.~,' Based on the poor outcome of the two reported thymic branchial cysts, surgery may also be indicated for that condition. Although histologic evaluation is needed to confirm the origin of a cyst, clinical findings may be helpful. Dyspnea, pleural effusion, and aspiration of mucinous fluid with or without inflammatory cells have been reported with thymic branchial cysts and are not consistently reported with other types. Multilobulated appearance has also been reported for thymic branchial cysts, although there was an asymptomatic cat in the group reported here that had a bilobed cyst on ultrasound examination. A multicystic appearance has also been reported for lymphangiomas and cystic thymomas.s In summary, opacities in the cranial mediastinum on thoracic radiographs should be investigated. Ultrasonographic identification of an anechoic mass which contains clear fluid supports a diagnosis of mediastinal cyst. Of the 17 cats described in previous reports and this study, 13 (76%) were considered asymptomatic for their cyst. For the 15 cats with cysts containing thin, low cellular fluid, 13 (87%) had no clinical signs referable to their cyst. The majority of feline cranial mediastinal cysts are benign with no need for treatment. This is in contrast to mediastinal thymic branchial cysts in dogs, which are reported to have a high incidence of morbidity and mortality.
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