Right middle lobe collapse

Right middle lobe collapse (or simply termed middle lobe collapse) has distinctive features, but can be subtle on frontal chest radiographs. 

For a general discussion please refer to the article on lobar collapse.  

It is important to note that of all the lobes, the right middle lobe is the most likely to be chronically collapsed. When this is the case it is referred to as right middle lobe syndrome 1-3

Plain radiograph

On lateral projection, right middle lobe collapse is usually relatively easy to identify, appearing as a triangular opacity in the anterior aspect of the chest overlying the cardiac shadow. The horizontal fissure is displaced inferiorly and the inferior part of the oblique fissure, displaced anterosuperiorly.

On frontal radiographs however the findings are more subtle 1-2:

  • the normal horizontal fissure is no longer visible (as it rotates down)
  • blurring of the right heart border
  • increased opacity adjacent to the right heart border requires a degree of consolidation as well as atelectasis

Non-specific signs indicating right sided atelectasis may also be present although due to the small size of the right middle lobe they may well be subtle or absent 2:

  • elevation of the hemidiaphragm
  • crowding of the right sided ribs
  • shift of the mediastinum to the right

Presence of linear opacities in the collapsed right middle lobe should suggest that the collapse is chronic (right middle lobe syndrome), with associated bronchiectasis 3.

Frontal projection

On frontal (PA or AP) projection, right middle lobe collapse should be distinguished from:

  • consolidation of the right middle lobe
  • pectus excavatum: downward sloping ribs and shift of the heart away from the right are clues; lateral projections make the distinction easy
Lateral projection

On the lateral projection, right middle lobe collapse should be distinguished from:

  • fluid within the oblique fissure (pseudotumour): horizontal fissure should be visible as separate to the opacity 2
  • fat within the oblique fissure
  • consolidation of the right middle lobe
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Article information

rID: 15345
System: Chest
Section: Gamuts
Synonyms or Alternate Spellings:
  • Right middle lobe atelectasis
  • Atelectasis of the right middle lobe
  • Collapse of the right middle lobe
  • Middle lobe collapse
  • RML collpase

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Cases and figures

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    Case 1: annotated
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    Case 1: annotated
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    Case 2: paediatric
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    Case 3
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    Case 4: with concurrent RLL pneumonia
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