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Pulmonary sequestration

Pulmonary sequestration (also called accessory lung) refers to aberrant formation of segmental lung tissue that has no connection with the bronchial tree. It is a bronchopulmonary foregut malformation (BPFM).

Epidemiology and clinical presentation

The estimated incidence is at 0.1%. Some authors propose a greater male prevalence (this may be the case for the extralobar type) ref. The age of presentation is dependent on the type of sequestration and this, in turn, determines the clinical presentation. ELS more commonly presents in newborns, whereas ILS presents in late childhood or adolescence with recurrent pulmonary infections.

Pathophysiology

Pulmonary sequestration can be divided into two distinct groups based on the relationship of the aberrant segmental lung tissue to the pleura:

  • intralobar sequestration (ILS)
    • accounts for the majority (75-85% of all sequestrations 4-5,7)
    • present later in childhood with recurrent infections
  • extralobar sequestration (ELS)
    • less common (15-25% of all sequestrations 4-5,7)
    • usually present in the neonatal period with respiratory distress, cyanosis and / or infection
    • recognized male predilection M:F ratio ~ 4:1
    • can be infra diaphragmatic in ~ 10 % of cases

The two types of sequestration are similar in their relationship to the bronchial tree and arterial supply but differ in their venous drainage and the relationship to the pleura.

By definition, there is no communication with the tracheobronchial tree. In the vast majority of cases, the anomalous lung tissue has a systemic arterial supply which is usually a branch of the aorta. Venous supply is variable and dependant on the type of sequestration:

  • intra-lobar sequestrations
    • venous drainage commonly occurs via the pulmonary veins, but can occur through the azygous / hemi azygous system, portal vein, right atrium or the IVC
    • closely connected to the adjacent normal lung and do not have a separate pleura
  • extra-lobar sequestrations
    • venous drainage through the systemic veins into the right atrium
    • separate from any surrounding lung with its own pleura
Genetics

Almost all cases occur sporadically.

Location

Overall, sequestration preferentially affects the lower lobes. 60% of intralobar sequestrations affect the left lower lobe, and 40% the right lower lobe. Extra-lobar sequestrations almost always affect the left lower lobe, however approximately 10% of extra-lobar sequestrations can be sub-diaphragmatic8.

Associations

Are common with the extra-lobar type (~50-60%) and can include 

Radiographic features

Plain film - chest radiograph
  • radiographs will often show an opacity in the affected segment
  • may show cystic spaces if infected
  • both ILS and ELS can rarely have air bronchograms as they may be connected with the gastrointestinal tract
Ultrasound

The sequestrated portion of lung is usually more echogenic than the rest of the lung. On antenatal ultrasound, an extra-lobar sequestration may be seen as early as 16 weeks gestation and typically appears as a solid well-defined triangular echogenic mass 8. Colour Doppler may identify a feeding vessel (in-utero cases) from the aorta. If the sequestration is sub diaphragmatic, it may appear as an echogenic intra abdominal mass.

CT
  • cross sectional imaging frequently demonstrates the arterial supply by the descending aorta
  • they may arise below the diaphragm in 20% of patients
  • usually doesn't contain air unless infected
  • 3D reconstructions can be particularly helpful in detecting 7
    • anomalous arterial vessels
    • concurrent anomalous veins 
    • differentiating between intra-lobar and extra-lobar sequestrations
Angiography

Not part of routine investigation but is the gold standard in determining arterial supply.

MRI
  • T1 - the sequestrated segment tends to be of comparatively high signal to normal lung tissue14
  • T2 - also tends to be of comparatively high signal 14
MR angiography

Can be helpful in demonstrating anomalous arterial supply

Complications

Treatment and prognosis

Traditionally treatment has been a surgical resection. Extra-lobar sequestrations with their separate pleural investments can usually be removed sparing normal lung, tissue, although with an intra-lobar type, segmental resection or even lobectomy will be necessary 

Coil embolisation has also been successfully trialled in selected cases 4.

Spontanenous involution has been reported in occasional cases 10.

Differential diagnosis

General imaging differential considerations include

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