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Flashcards in Mediastinum Deck (17):
1

Mediastinal anatomy

  • Consists of 2 layers of mediastinal pleura and the space between them
  • Fenestrated; unilateral disease is uncommon
  • Communicates with the neck cranially and retroperitoneal space caudally
  • 3 mediastinal reflections
    • Cranioventral
    • Caudoventral
    • Plica vena cava

2

Radiographic anatomy

  • Of the many structures present in the mediastinum of the normal thorax, only a few structures are seen normally
    • Heart
    • Caudal vena cava
    • Aorta 
    • Trachea
    • Thymus (young animals)
    • Occasionally esophagus (left lateral)

3

Structures that normally aren't visible?

Why?

  • Cranial vena cava
  • Azygos vein
  • Lymph nodes
  • Nerves
  • Why are other structures not seen?
    • Silhouetting
    • Insufficient thickness or size to absorb the x-rays
    • Scant amount of fat to provide contrast

4

Radiographic anatomy: DV or VD view

  • Width of mediastinum depends on fat accumulation (or disease)
  • In the dog, should be less than 2x the width of the thoracic body vertebral bodies
  • In the cat, should be less than 1x the width of the thoracic body vertebral bodies
  • Often in obese patients width of mediastinum exceeds these cut-offs

5

Caudoventral mediastinal reflection

  • Between accessory and left caudal lobes
  • On the left, since accessory lobe extends to the left
  • Only seen on DV/VD views

6

Thymus: radiographic anatomy

  • Normally seen in young animals
  • Resides in the cranial MR
  • Best seen as "sail" sign in a VD or DV view
  • Usually not seen in the lateral view
  • If detected, it may silhouette with the cranioventral margin of the heart

7

Mediastinal disease: 4 general classifications

  • Mediastinal masses
  • Mediastinal fluid
  • Pneumomediastinum
  • Mediastinal shift
    • Lung- or pleural space-related disease

8

Mediastinal masses

  • Common disease entity
  • Radiographic appearance of mediastinal masses have a similar appearance
  • DDx:
    • Neoplasia, abscessation, cysts, lymphadenopathy, granulomas (parasit. or fungal), hernias, hematomas, congenital
  • Clinical signs may vary due to size, location, and complicating/confounding factors (e.g. pleural fluid)
    • Pyrexia, muffled heart sounds, cough, palpable mass, malaise, regurg, exc. int.
  • DV or VD best views for addressing mass location

9

Mediastinal masses: displacement

  • Cranial masses
    • May result in displacement of heart and lungs caudally
    • May deviate trachea dorsally (note: pleural fluid)
    • May compress trachea

10

Mediastinal masses: location

  • DDx will depend on lesion location
  • Important to know location of lymph nodes, as well as afferent and efferent lymph vessels
    • Sternal
    • Cranial mediastinal
    • Tracheobronchial

11

Examples of cranioventral diseases:

  • Sternal lymphadenopathy
  • Mediastinal lymphadenopathy
  • Vascular ring anomaly --> megaesophagus
  • Abscessation/esophageal perforation secondary to foreign body
  • Thymoma 
  • Hemorrhage

12

Examples of dorsal diseases:

  • Neurogenic and paraspinal tumors
  • Tracheobronchial (perihilar) lymphadenopathy
  • Vascular ring anomaly --> megaesophagus

13

Tracheobronchial lymph nodes

  • "Bow-legged cowboy sign" on DV view
  • Enlargement of the lymph nodes results in the displacement of the principle bronchi laterally and/or a curved appearance
  • On lateral view may obscure the ventral margin of aorta
  • DDx: left atrial enlargement

14

Diseases not typically assoc. w/ mediastinal lymphadenopathy

  • Mammary adenocarcinomas
  • Metastatic lung tumors
  • Bacterial pneumonia
  • Pyothorax
  • Thoracic wall tumors

15

Pitfalls for mediastinal masses? Additional diagnostics?

  • Pleural effusion can 'hide' them
  • Additional diagnostics
    • US first
    • Then consider draining and re-radiographing only

16

Pneumomediastinum

  • Free gas in the mediastinum
  • Remember connection btn neck and retroperitoneal space
    • Results in pneumoretroperitoneum and/or subcutaneous emphysema (or vice versa)
  • May progress to pneumothorax (but not the reverse)--must tear the pleura
  • Does NOT result in dyspnea

17

Pneumomediastinum: causes? Radiographic signs?

  • Causes
    • Air escaping from lung along pleural planes (barotrauma)
    • Trauma in neck, facial planes, or to back and retroperitoneal space
    • Rupture or trauma of cervical trachea or esophagus--iatrogenic, neoplasia, necrosis, inflammation
    • Gas-producing organism (unlikely)
  • Signs
    • Gas is a contrast agent and allows for the visualization of structures previously undetected
      • Cranial vena cava, brachiocephalic trunk, left subclavian, esophagus, trachea, etc.