Congenital Pulmonary Airway Malformation (CPAM, or Congenital Cystic Adenomatoid Malformation, CCAM)

General: Classified into five distinct types. Ultrasound is considered very useful for in utero diagnosis of these malformations. It is unilateral in ~95% of cases, and involves only a single lobe in 80~90% of cases.

Associated anomalies are found primarily with Type 2 and include, but are not limited to:

• Type 0: (acinar dysplasia, acinar agenesis) Rare (1~3%) and considered incompatible with life. May be seen in premature infants cyanotic at birth but survive for only a few hours. It is generally associated with other congenital malformations, especially cardiovascular malformations and dermal hypoplasia. Grossly, both lungs are small and firm with a diffusely granular surface. Microscopically, shows bronchus-like structures with glands, muscle, and cartilage. A prominent mesenchymal component separates them, with extramedullary hematopoiesis, large thin-walled vascular channels, and amorphic basophilic debris.

• Type 1: (large or predominant cyst type) Most common (60~70%), usually presenting in the first month of life, though occasionally not until young adulthood, and has a good prognosis due to resectability. Grossly, consists of large (3~10cm) cysts surrounded by smaller cysts and compressed but essentially normal parenchyma. Microscopically, the larger cysts are lined by ciliated, pseudostratified columnar epithelium which may appear to spread in a lepidic fashion, and the smaller cysts are lined by cuboidal to columnar epithelium. The cyst walls are comprised of elastic tissue overlying fibromuscular tissue and possibly cartilage plates. Approximately half have mucous producing cells among the epithelium. There are a few reports of Type 1 cysts associated with bronchioalveolar carcinoma; those may be confusing noise more than a useful association at this time.

• Type 2: (medium cyst type) Uncommon (10~15%), consisting of small (<= 2cm) cysts and presents exclusively within the first year of life. Present in ~50% of extralobar pulmonary sequestrations. Has a poorer prognosis than Type 1 due to increased association with other anomalies, (see above). Grossly, the cysts are evenly distributed and blend with normal parenchyma. Microscopically, there are typical bronchiole-like structures lined by cuboidal to columnar epithelium with a thin underlying fibromuscular layer. Mucous cells and cartilage plates are absent, except as components of trapped adjacent normal parenchyma.
  • Rhabdomyomatous dysplasia: Considered a variant or subgroup of Type 2. Contains ribbons of striated muscle fibers diffusely throughout. There are a few reports of rhabdomyosarcoma arising from this, though they may have instead arisen from a form of pleuropulmonary blastoma.

• Type 3: (small cystic or solid type) Rare (~5%), consisting of small (<~0.2cm) cysts, and is seen exclusively in the first month of life. Significant male predominance. Poor prognosis due to association with polyhydramnios and anasarca. Grossly, consists of a large, bulky parenchymal mass involving an entire lobe or lung. Mass effect typically shifts the mediastinum and the contralateral lung is usually hypoplastic. Microscopically, resembles immature lung devoid of bronchi but contains irregular stellate-shaped bronchiole-like structures lined with cuboidal epithelium and surrounded by alveolar ductules and saccules also with cuboidal epithelium — this confers an "adenomatoid" appearance. Mucous cells, cartilage, and rhabdomyomatous cells are absent, and vessels are sparse.

• Type 4: (peripheral cyst type) Uncommon (~15%), consisting of large (up to ~10cm) air-filled cysts, and has a good prognosis following resection. Previously many of these were classified as Type 1. No sex prediliction and has a clinical presentation ranging from asymptomatic to mild respiratory distress to acute distress from pneumothorax, often presenting at up to 4 years of age. May be a hamartomatous malformation of the distal acinus. Radiographically, displays large air-filled cysts with mediastinal shift and possible pneumothorax. Involves single lobe in ~80% of cases. Grossly, large smooth and thin-walled cysts present at periphery of lobe. Microscopically, lined by flattened epithelial cells (type I and II alveolar lining cells), with occasional areas of low cuboidal epithelium. The wall is composed of loose mesenchymal tissue, prominant vasculature, and possibly dense connective tissue in older patients; the mesenchymal tissue may be confused with pleuropulmonary blastoma.

Stains:

• Positive:
• Negative:
• Suggested, focused panel:

((())) (PathologyOutlines, Stocker's Pediatric Pathology 2001)