Pulmonary Lobar Collapse

Mechanisms and Causes of Lobar Collapse

• Lobar collapse: this is divided into those causes due to an endobronchial obstruction (intrinsic or extrinsic) or those causes without obstruction

  • ▪

    A segment of atelectatic lung pivots around the central hilum via its attaching bronchovascular structures – any fissures will also influence the direction of collapse

• Common causes differ between adults and children:

  • ▪

    Adults: intrinsic obstruction is commonly due to a bronchogenic tumour (especially in middle-aged or elderly smokers) or mucous plugs

    • –

      Rarer causes: foreign bodies ▸ broncholiths ▸ focal bronchostenosis due to inflammation or trauma

  • ▪

    Children: intrinsic obstruction is commonly due to an inhaled foreign body or mucous plugs ▸ tumours are very rare

• Subsegmental atelectasis (linear or plate atelectasis): this describes atelectasis involving less than a whole segment ▸ it is usually a thin horizontal linear opacity abutting the pleura

  • ▪

    It is commonly located within the mid or lower lung and can cross segmental boundaries

Radiographic Considerations

• Increased opacity of the affected lobe: this is due to retained secretions and a reduction in lobe aeration

• Silhouette sign: this describes loss of an air–soft tissue interface when a segment of collapsed lung abuts an adjacent soft tissue structure (e.g. a heart border)

• Direct signs of volume loss: fissure displacement ▸ displacement and crowding of the pulmonary vessels and bronchi

  • ▪

    Upper lobe collapse: hilar elevation ▸ the ipsilateral main bronchus becomes more horizontally orientated

  • ▪

    Lower lobe collapse: a ‘small hilum’ – the collapsed lower lobe obscures the lower lobe artery and therefore smaller vascular structures are seen at the expected hilar position ▸ the ipsilateral main bronchus becomes more vertically orientated

• Indirect signs of volume loss: compensatory shifts of adjacent structures (e.g. hyperinflation of the remaining lobes in proportion to the degree of volume loss) ▸ the contralateral lung may extend across the midline ▸ the anterior junctional line is displaced to the contralateral side ▸ displacement of the azygo-oesophageal or posterior junctional lines on a PA CXR ▸ rib crowding

  • ▪

    A juxtaphrenic diaphragmatic peak: this describes a small triangular density at the highest point of the hemidiaphragm dome with upper lobe (± middle lobe) collapse – this is a useful ancillary sign

  • ▪

    ‘Shifting granuloma’ sign: a change in position of a granuloma with hyperexpansion

  • ▪

    Mediastinal shift: this is greatest with a lower lobe collapse, and least with middle lobe collapse ▸ little mediastinal shift is seen with an acute upper lobe collapse but there is a greater shift with chronic fibrotic upper lobe volume loss

  • ▪

    Luftsichel sign : paramediastinal translucency secondary to an overinflated superior segment of the ipsilateral lower lobe occupying the space between the mediastinum and medial aspect of the collapsed upper lobe ▸ L > R ▸ typical of left upper lobe collapse

  • ▪

    An elevated hemidiaphragm (particularly with left upper lobe collapse) is of limited value

• Ancillary features: absence of air bronchograms within an affected lobe should raise the possibility of a central obstructing lesion

  • ▪

    ‘Golden's S’ sign: this describes the S shape (reverse S on the right) of a major fissure due to a combination of collapse and a central mass ▸ it is commonly seen in the right upper lobe (but can be seen in any lobe)

    • –

      Focal central convexity: collapse around the central mass

    • –

      Concave outline peripherally: peripheral collapse not occurring around a central mass

Computed Tomography of Lobar Collapse

• Accurate delineation of a tumour mass from surrounding collapsed lung can be difficult but collapsed lung will usually enhance more than a tumour (the maximal difference is seen between 40 s and 2 min post IV injection) ▸ increased uptake on PET-CT within tumour vs lung collapse

• CT mucous bronchogram sign: tubular, low attenuation branching airways that are dilated with inspissated secretions and seen within the enhancing collapsed lobe (post IV contrast administration) – this is highly suggestive of an obstructing lesion causing lobar collapse

  • ▪

    However this can also be caused by excessive mucus production combined with decreased mucociliary function (e.g. ABPA, asthma, cystic fibrosis)

• Potential pitfalls: air bronchograms may be seen in peripheral collapsed lobes (due to collateral air drift or tumour necrosis) ▸ a proximal obstructing lesion may not cause complete lobar collapse (if a fissure is incomplete and allows collateral air drift) ▸ occasionally the parenchyma and airways can become fluid filled due to a central obstructing lesion with little or no volume loss giving rise to the appearance of a ‘drowned lobe’ ▸ false-positive diagnoses may be due to bronchial strictures, mucous plugs, or compression by a large pleural effusion

Right Upper Lobe Collapse