Teaching Case

Female, 31 years old, 25 weeks pregnant in her first pregnancy. She has no significant clinical history. An ultrasound was performed by a general practitioner raising the suspicion for a diaphragmatic hernia in the left fetal hemithorax. The patient was forwarded to a specialized centre to perform a fetal MRI.

MRI revealed a mass with T2-weighted hypersignal occupying the left hemithorax (Figure1), promoting dislocation of the mediastinum to the right side – configuring heart dextroposition – compressing both lungs, defined in MRI by usual signal intensity. The left lung is more compressed than the right one, in the superior region (Figure2). The left diaphragm is flattened, which confirms the mass effect caused by the abnormal tissue, but no discontinuity or hernia was associated (Figure3).

Two structures compatible with vessels were seen inside this tissue, one originating from the descending thoracic aorta, presumably systemic arteries (Figure4-A), and the second oriented to the heart (most probably the left atrium), suggesting pulmonary vein (Figure5-A).

MRI features are consistent with supra-diaphragmatic bronchopulmonary sequestration. The distinction between intralobar and extralobar types is not accurate in this case as the own pleura covering could not be defined and the extralobar type can also have systemic venous drainage. Although this case presents hypertensive signals, like diaphragm flattening and lung compression, no fetal hydrops was evident, usually related to esophagus and thoracic vein compressions.

These findings are also schematically presented in Figure6.

Background 

Bronchopulmonary sequestration is the presence of lung tissue that does not present communication with the bronchial tree by a normally located bronchus and receives its blood supply from anomalous systemic arteries [1-3].Most cases are described in the left lower lobe, and about 90% of them are supradiaphragmatic [1].

It’s divided in two types: Intralobar pulmonary sequestrations (IPS) do not present pleural covering, are contiguous with the normal lung and their drainage is usually through the pulmonary veins [1,2]. In extralobar pulmonary sequestration (EPS), the abnormal lung tissue is completely separated from the normal lung by its distinct pleural covering [2]. Unlike in IPS, its drainage is usually through systemic drainage [1], although in 25% of the cases it may also drain to pulmonary veins [4].

The differences between IPS and EPS are illustrated in Figure7.

Clinical Perspective 

Most patients with ILS present in adolescence or early adulthood with recurrent bacterial pneumonia and rarely with hemothorax and high-output heart failure. On the other hand, ELS is often diagnosed on prenatal ultrasound or MRI and is frequently associated with other congenital anomalies. Infants with ELS may be asymptomatic or may suffer from respiratory distress owing to lung hypoplasia or mass effect. ELS rarely becomes infected, as its pleural investment prevents contact with inhaled air [5].

Imaging Perspective 

CT and MRI are the reliable imaging modalities for demonstrating the anomalous artery supplying, with CT allowing better evaluation of the parenchyma changes [6]. However, in fetal evaluation, ultrasonography and MRI are the methods of choice. Although fetal ultrasound is the primary modality for imaging the chest and detecting masses [7], MRI can be helpful in limiting conditions such as maternal obesity and oligohydramnios. 

BPS appears as a solid hyperintense lesion on T2-weighted images, frequently in the left lower lobe. Identification of the systemic vascular supply, especially with balanced sequences, facilitates the diagnosis [1]. These sequences are ultrafast and have robust imagery with a liquid/tissue contrast and excellent signal-to-noise ratio.

Outcome 

When no other malformations are associated, it has an excellent prognosis – most of it will disappear before birth. However, larger lesions can compress mediastinum structures, causing hydrops. In those cases, fetal intervention and early delivery can be indicated [1]. When complications occur in the postnatal period, some of the treatment options include embolization and/or surgical resection [1]. Since MRI can detect associated anomalies and complications, it is of great value in therapy planning.

Take-Home Message/Teaching Points

In fetal MRI, when facing a T2-weighted hyperintense mass in the fetal thorax - especially in the lower left pulmonary lobe - the diagnosis of BPS must be remembered, and an active search for anomalous vessels may support this diagnosis. 

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