What’s Your Interpretation?

By Jason Crawford, DVM, DACVR

“Nube” is a 16 year old spayed female DSH that presented to the emergency department after the owners were out of town for four days, and returned home to find “Nube” hunched over with difficulty using her rear limbs.  On physical examination, there was lumbar spinal pain and decreased rear limb reflexes.  There were no significant biochemical abnormalities.   There is a history of hyperthyroidism which has been well controlled.  Screening thoracic radiographs were performed.  List any abnormal findings with differential diagnoses, along with further recommendations.

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On thoracic radiographs, there is a round soft tissue opacity in the cranioventral thorax, just cranial to the heart (between black arrows on the lateral view). On the VD view, the cranial mediastinum is widend (between black arrows), which may be partially due to fat deposition, but there is also an asymmetric bulge on the left side of the mediastinum, which is not typical of fat deposition.  The heart and pulmonary vessels are normal in size and shape.  The lungs are normal and there is no pleural effusion present.  Differential diagnoses for cranial mediastinal masses include lymphadenopathy, neoplasia (including lymphoma and thymoma), ectopic thyroid tissue, abscess, granuloma or mediastinal cyst.  Further investigation of cranial mediastinal masses is warranted due to the potential of neoplastic disease, and ultrasound examination is typically sufficient for evaluation of a mediastinal mass and sampling via guided fine needle aspiration.  Occasionally, computed tomography is needed due to the position of a mass and the surrounding soft tissues.

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A thoracic ultrasound exam was performed, and there was a 2.5 x 1.9 cm thin-walled, anechoic fluid-filled mass in the cranial mediastinum (white calipers). Aspiration of the mass yielded approximately 10 cc of a clear, colorless fluid, typical of a mediastinal cyst.  This was considered to be a benign lesion and unrelated to the presenting complaint of pain, which was presumed to be related to trauma, IVDD, or spinal cord disease.  No further testing was completed.

Cranial mediastinal cysts are typically congenital in nature, although usually are detected in older animals, and the rate of fluid accumulation is unknown.  They typicaly originate from embryonic parathyroid, thyroglossal, or thymic duct epithelium.  The majority of cats in which mediastinal cysts occur are asymptomatic, with no clinical signs attributed to the cyst.  In the absence of clinical signs, the cysts are considered to be incidental findings and intervention is not typically warranted.  Occasionally, with very large cysts, respiratory distress or edema of the head and neck are present.  In these cases drainage of the cysts are recommended.  However, most mediastinal cysts will refill with fluid following drainage, so surgical removal of the cyst may be necessary in cases of a severe mass effect or in functional cysts (i.e. hyperparathyroidism).

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Mediastinal cysts appear to be less commonly reported in dogs.  However, thymic branchial cysts in dogs are reported to have a higher morbidity and mortality rate as a direct result severe, chronic inflammatory reaction, hemorrhage, or edema caused by the cyst.



Zekas, LJ, Adams WM.  Cranial mediastinal cysts in nine cats. Vet Radiol Ultrasound  2002; 43(6):  413-418.



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