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 dependant on the type of sequestration and this, in turn, determines the clinical presentation.

Pathophysiology

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

• intra-lobar sequestration (ILS)
  
  • accounts for the majority (75-85% of all sequestrations ^4-5,7)
  • present later in childhood with recurrent infections
• extra-lobar 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 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

Preferentially affects the lower lobes. Usually the left lower lobe followed in frequency by the right lower lobe. Approximately 10% of extra-lobar sequestrations can be sub-diaphragmatic (also tends to be left sided) ⁸.

Associations

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

• congenital pulmonary airway malformation (CPAM) : which is then sometimes termed a hybrid lesion
• congenital heart disease
• congenital diaphragmatic hernia

Radiographic features

Plain film - chest radiograph

• radiographs will often show an opacity in the affected segment
• may show cystic spaces if infected

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

• frequent respiratory tract infection
• In neonates can be complicated by high output cardiac failure

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

• persistent pneumonia
• congenital pulmonary airway malformation (CPAM) - can also be an association
• bronchogenic cyst
• pulmonary arteriovenous malformation
• scimitar syndrome