• Mild cardiomegaly
• Dilated, undulating pulmonary blood vessel extending caudodorsally, best seen on the RLAT
• Mediastinal shift to the left, accentuated by obliquity
• Overconditioned with large accumulations of mediastinal and subcutaneous fat
• Ovoid soft tissue opaque mass cranial to the heart, best seen on the RLAT

Radiographic Diagnosis:

• Mild cardiomegaly with pulmonary vessel congestion, likely related to some degree of decompensation
• Ovoid soft tissue opaque mass of the cranial mediastinum
• Overconditioned

Comments:  

Recommend focal ultrasound cranial to the heart to further investigate the mediastinal mass.  Mediastinal cyst is considered, though malignancy (thymoma, FeLV lymphoma, carcinoma) remains possible.

• The right cranial lung lobe is homogenously soft tissue attenuating with multiple air bronchograms within it and is reduced in size, consistent with collapse. Additional multifocal regions of linear oriented, soft tissue attenuations are present within the ventral right middle and right caudal and dorsal segments of the left cranial lung lobes, consistent with atelectasis.
• A mild cranial and leftward mediastinal shift is present. The pulmonary parenchyma is otherwise normal with no pulmonary nodules or masses identified
• The heart is generally enlarged.  Pulmonary blood vessels are normal
• A thin linear mineral attenuation is at the level of the aortic root, most likely incidental
• Cranial to the heart, an ovoid, smoothly marginated, mass has a noncontrast enhancing, fluid attenuating center. The rim surrounding this fluid is eccentric, thicker along its left craniolateral aspect. The cyst measures 2.0 cm in length and 2.2 cm in width at its maximal dimensions.
• The esophagus contains a mild amount of homogenous, noncontrast enhancing fluid intermittently. The esophagus is otherwise normal.

• Fluid filled mass of the cranial mediastinum
• Mild cardiomegaly without congestive heart failure

COMMENTS:

In contrast to the ultrasound, the rim of this mass is eccentric and thicker than expected for a mediastinal cyst.  This increases the possibility of a neoplastic process (eg cystic thymoma), potentially associated with the current clinical signs.

Unfortunately, the patient arrested following her CT. Her AchRAb titer results revealed a markedly positive serum titer of 10.92, diagnostic for acquired myasthenia gravis. See the gross necropsy findings below.

Feline patients with myasthenia gravis have a high incidence of a cranial mediastinal mass when compared to dogs. ^1,2 In one retrospective study 52% of cats diagnosed with myasthenia gravis had a cranial mediastinal mass diagnosed on thoracic radiographs. ¹ The most common clinical sign for myasthenia gravis is generalized weakness. ¹ In acute fulminant myasthenia gravis, patients can progress to severe respiratory distress secondary to intercostal and diaphragmatic muscle weakness. ² Megaesophagus is less commonly seen in cats when compared to dogs due to the presence of more smooth muscle in their esophagus.  The minimal esophageal fluid and gas on CT of this patient may simply be variant of normal or related to anesthesia. ^1,2

The most common mediastinal masses in cats are lymphomas and thymomas followed by idiopathic mediastinal cysts and others. ³ Due to the thin-walled appearance of the mediastinal mass in this case it was suspected to be a benign mediastinal cyst. Ultrasonographic appearance of mediastinal cysts tend to be thin walled, anechoic fluid filled structures, similar to the findings in this case.⁴ The utilization of a linear probe with higher frequency and better resolution may have been beneficial in identifying the eccentric rim of this mass. Reportedly, CT will reveal a well-defined fluid attenuating mass without peripheral contrast ring enhancement in cats with mediastinal cysts. ⁴ In a study assessing nine cats with mediastinal cysts majority of the patients did not have clinical signs such as dyspnea, pleural effusion, ect. ⁵ Our patient had a contrast enhancing rim, dyspnea, and pleural effusion which should prioritize cystic thymoma over a mediastinal cyst, although more research is needed. The eccentricity of the mass and cranial soft tissue attenuation may also be more indicative of thymic origin rather than a benign mediastinal cyst. Feline patients with a cranial mediastinal mass present on thoracic radiographs may benefit from early AchRAb titer testing, especially if weakness is noted. ^1,2