• Occupying the majority of the cranial thorax, there is a soft tissue attenuating and heterogeneously contrast-enhancing mass that results in dorsal and rightward displacement of the cardiac silhouette and is, at times, difficult to differentiate from the underlying myocardium. There are punctate foci of mineralization within this mass.
• The cardiac silhouette is mildly enlarged. The combination of the mass and the cardiac silhouette results in dorsal displacement of the trachea and carina and attentuation of the carina as well as the caudal lobar bronchi.
• There is collapse of the right cranial lung lobe and near complete collapse of the right middle lung lobe (not highlighted in available images, a lung window was included in the original study).
• Best appreciated in some of the reformatted MIP images, there is the impression of distention and tortuosity of the caudal lobar pulmonary arteries, particularly on the left

• Large, partially cavitated, cranial mediastinal mass. Given the signalment, the primary consideration is a thymoma, though other neoplasia (lymphoma, carcinoma) cannot be excluded. The cavitation most likely represents intralesional cavitation. Acute hemorrhage (as could occur iatrogenically) would be expected to be more hyperattenuating on CT
• Mild generalized cardiomegaly consistent with the clinical history of heart disease. No tomographic evidence of left-sided cardiac decompensation, though this may be secondary to the reported diuretic administration as this can resolve the imaging findings associated with pulmonary edema quite rapidly while the clinical improvement lags
• Subjective pulmonary artery dilation/tortuousity may still be within normal limits for this species (see below). True pulmonary arteritis/hypertension cannot be excluded

Unfortunately, aside from reportedly hemorrhagic/non-diagnostic cytologic results, no additional information regarding patient outcome was available.

It is important to recognize that rabbits normally have a large and persistent thymus that can be associated with benign hyperplasia alone.  Generally slow-growing with rare metastases, thymomas are the most commonly diagnosed cranial mediastinal mass in rabbits (over thymic lymphoma/carcinoma) but are still uncommon with no apparent sex predilection.  Although mediastinal masses may be identified incidentally on screening radiographs as in other species, this case highlights some common presenting signs for mediastinal disease in rabbits. In addition to respiratory distress, patients may present with bilateral exophthalmos +/- third eyelid prolapse with retrobulbar venous plexus congestion secondary to cranial vena cava compression. Similarly, vascular compression can result in edema of the head, neck, and forelimbs.

As in other species, cytology alone can be difficult when differentiating thymic lesion origins, and aspirates of cavitated regions may yield non-diagnostic results due to necrosis and hemorrhage. Treatment options for mediastinal masses in rabbits can include chemotherapy, surgery, or radiation therapy.

Cardiac disease is commonly seen in rabbits, most commonly chronic atrioventricular valvular insufficiency, though hypertrophic cardiomyopathy, dilated cardiomyopathy, and infectious myocarditis are also documented. Evaluation of cardiovascular structures in this patient was challenging due to the sheer size of the mediastinal lesion. The vertebral heart score of New Zealand white rabbits has been investigated (7.6 +/- 0.39 on the right lateral and left lateral projection was 7.94 +/- 0.54 vertebrae in a left lateral view), and an evaluation of CT thoracic features of normal New Zealand white rabbits suggests a normal cardiac silhouette of ~4 intercostal spaces. Although there certainly could be breed variations, these published radiographic and CT parameters can aid subjective assessment of cardiovascular features of rabbits across multiple imaging modalities when echocardiographic is not readily available. Measurements regarding normal vascular diameter are limited and I could find limited information regarding naturally occurring pulmonary hypertension in rabbits. In the CT study of normal rabbits, mean cross sectional area of caudal lobar arteries was ~14-15 mm³. In this patient, the left caudal lobar artery area is ~11mm³, suggesting normalcy. Careful correlation with the history is therefore essential when considering pulmonary vessel change in species with limited normal published values, and further work is necessary to investigate pulmonary vessel change in small mammals.

• Giannico, Amália Turner, et al. “Determination of normal echocardiographic, electrocardiographic, and radiographic cardiac parameters in the conscious New Zealand white rabbit.” Journal of Exotic Pet Medicine 24.2 (2015): 223-234.
• Huston, S. M., Lee, P. M. S., Quesenberry, K. E., & Pilny, A. A. (2012). Cardiovascular disease, lymphoproliferative disorders, and thymomas. Ferrets, Rabbits, and Rodents, 257.
• Müllhaupt, Désirée, et al. “Computed tomography of the thorax in rabbits: a prospective study in ten clinically healthy New Zealand White rabbits.” Acta Veterinaria Scandinavica 59.1 (2017): 1-9.