Exercise 6 discussion

Clinical history and questions

Nine year-old female spayed cattle-dog crossbred: presented with worsening dyspnoea and exercise intolerance, and pitting oedema of the head and neck.  There was no fever.  After radiography of the thorax, a gloomy prognosis, euthanasia and necropsy were offered (in that order).

This view is at the standard display stage (carcass on its left side), with rt. cranial lung lobe reflected dorsocaudally.

In addition to the changes shown here, some thoracic lymph nodes were enlarged, but none elsewhere in the body.

  1. Describe the abnormalities.
  2. What underlying disease process/es could produce these changes?
  3. Make a differential diagnostic list, in order of probability.
  4. With your preferred diagnosis in mind, what is the pathogenesis of these macroscopicchanges?
  5. Relate the pathology to the clinical signs.

An interpretation of Diagnostic Exercise No. 6 – Roger Kelly

Dyspnoea and oedema of head and neck of a 9 year-old cattle-dog


In this view of the opened thorax (rhs) there is an irregular somewhat nodular thickening of the anterior mediastinum, and there are several abnormally-located interanastomosing, thin-walled, tortuous blood vessels coursing over the mass and over the pericardial sac.


New blood vessels can appear as part of the inflammatory response, but that is usually at the microvascular level.  The mass in the mediastinum likewise could be inflammatory in origin, but there was no history of fever clinically, and there is nothing in the view that suggests either deposition of inflammatory exudate or scarring.

Blood vessels as thin-walled as these, relative to their size, have to be veins.  This, and their abnormal location and tortuosity, reliably indicate that they are venous collaterals, which in turn informs us that there has been a significant obstruction to veins somewhere in the area, which has stimulated the formation of these collateral veins, which never seem to take a direct route.

Oedema can be due to increased capillary permeability (most often acute inflammation), or to reduction in arterio-venous pressure differential, or to lowered plasma oncotic pressure (hypoproteinaemia).  The presence of the venous collaterals strongly favours the probability that venous obstruction and elevation of venous pressure is the cause of the oedema in this case.  This in turn suggests that the mass in the thoracic inlet has restricted venous return to the anterior vena cava.  A space-occupying mass here could also compress the trachea and cause the dyspnoea noted in the history.  The lungs seem not to have been compressed much by the growth, so restriction of tidal volume is not likely to have caused the dyspnoea.

Basic processes:

Neoplasia seems more likely than proliferative inflammation, for the reasons given above, while anomalous development is unlikely on grounds of age, if nothing else, and degeneration of itself does not produce masses.  So this is most likely to be a neoplasm of the anterior mediastinum.

Differential diagnosis:

One of the most common neoplasms in this site is thymoma, whether this be epithelial, lympho-epithelial, or purely lymphocytic (thymic lymphoma).  This was a lympho-epithelial thymoma.  With the exception of one or two local lymph nodes, no other lymphoid tissue in the animal was involved.