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Rhabdomyomatous dysplasia in pulmonary sequestration with type 2 CPAM

Chest CT showed a hypodense mass (green arrow) along the medial aspect of the left lower lobe adjacent to the distal descending thoracic aorta (pink arrow) (Figure 1, axial image). The maximum intensity projection (MIP) image shows a small systemic arterial branch (orange arrow) arising from the celiac artery (blue arrow) supplying the mass (green arrow) compatible with pulmonary sequestration (Figure 2, coronal reformatted image). Corresponding histologic sections show bronchiole-like structures lined by columnar epithelial cells with a thin underlying fibromuscular layer (Figures 3-5). The absence of accompanying pulmonary artery branches and the lack of a smooth muscle layer (as would be expected in a true bronchiole) facilitates distinction of these bronchiole-like strcutures from true bronchioles. In addition, there were striated muscle fibers throughout the lesion, representing rhabdomyomatous dysplasia (Figure 6).

Congenital pulmonary airway malformation (CPAM) is the most common type of congenital lung lesion, frequently diagnosed antenatally. Bronchopulmonary sequestration is the most common lesion in the differential diagnosis of CPAM, the former being defined by the absence of a connection to the tracheobronchial tree. In contrast to CPAM, which derives its blood supply from the pulmonary artery, the blood supply of sequestrations is derived from an anomalous systemic artery rather than the pulmonary circulation.

CPAM needs to be distinguished from pleuropulmonary blastoma and may rarely give rise to a mucinous adenocarcinoma (especially type 1 CPAM). The presence of non-neoplastic striated muscle fibers, described as “rhabdomyomatous dysplasia”, is extremely rarely seen in the lung. Associated cardiovascular and/or lung abnormalities have been described. Fraggetta reported striated muscle proliferation in type 2 CPAM and pulmonary sequestration. A rare case of pulmonary rhabdomyomatous dysplasia in a newborn with neurofibromatosis type 1 with no other pulmonary complications has also been described. The origin of striated muscle cells has been proposed as misplaced striated muscle from the pharynx, esophagus or diaphragm (i.e., a developmental error). Others favor the hypothesis of myoblastic differentiation/metaplastic transformation of primitive mesenchymal cells. In our case, neither lethal congenital malformations nor features of neurofibromatosis type 1 were present.

Take home message

Bronchopulmonary sequestrations are defined by systemic artery supply; they may harbor type 2 CPAM and skeletal muscle fibers.

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Raghavendra Pillappa, M.D
Assistant Professor of Pathology
Pulmonary Pathologist
Director, Immunohistochemistry
Department of Pathology
Virginia Commonwealth University Health System
Richmond, VA, USA

Shaimaa A Fadl, MBChB
Assistant Clinical Professor
Cardiothoracic Imaging and Emergency Radiology
Department of Radiology
Virginia Commonwealth University Health System
Richmond, VA, USA