Applied Radiology

Cystic adenomatoid malformation (CAM) is a congenital cystic abnormality of the lung resulting from cessation of bronchial maturation between the fifth and seventh week of gestation; it is characterized by abnormal development of terminal respiratory structures and overgrowth of mesenchymal elements, which produce the typical adenomatoid or gland-like appearance of the lesion.1-6 The older term, hamartoma, should be avoided.2 CAM accounts for approximately 25% of all congenital lung disorders4,7 and 95% of congenital cystic lung lesions.8 A slight male predominance has been reported; there is no racial predilection.4,9 CAM may involve any lobe, but the left lower lobe is the lobe most frequently affected.1

Histology

CAM consists of multiple cysts that are lined by cuboidal-to-ciliated, pseudostratified columnar (respiratory) epithelium.1-4 Cartilage generally is absent, reflecting the bronchial maldevelopment.1-5 Typically, the cysts remain connected to the tracheobronchial tree via small malformed bronchi.5,10 These abnormal communicating bronchi tend to collapse on expiration, resulting in delayed clearance of fetal lung fluid,3 air-trapping with progressive distention and enlargement of the cysts,5,11 and chronic, recurrent infection.5,10

Occasionally, small amounts of cartilage may be present in the malformed bronchi; rarely, abundant amounts of cartilage may be present.12 In addition, smooth muscle and mucin-producing cells may be present within the lesion.8

Stocker et al13 divide CAM into three types, based on gross and microscopic criteria. Type I lesions typically involve only part of a lobe and contain one or more large cysts, usually ranging from 2 cm to 10 cm in size, which are surrounded by many smaller cysts.4,13 The larger cysts are lined with ciliated, pseudostratified columnar epithelium. Cartilage plates may be present in the walls of the larger cysts, and clusters of mucogenic cells may be found in the lining of the large cysts or in alveoli adjacent to the cysts in about one-third of cases.4,13

Type II lesions also typically involve only part of a lobe and contain multiple smaller cysts that rarely exceed 2 cm in diameter.4,13 The cysts are lined by cuboidal or columnar epithelium and resemble dilated terminal and respiratory bronchioles arranged in a back-to-back configuration.7 Cartilage generally is absent, and mucogenic cells do not occur.4,13

Type III (microcystic) lesions tend to be large and bulky; they usually involve an entire lobe and occasionally may involve a whole lung.4,13 The cysts are lined by simple cuboidal epithelium and usually are microscopic, rarely exceeding 0.3 cm to 0.5 cm in diameter.4,13 Cartilage and mucogenic cells are absent.4,13 Type I CAM accounts for approximately 50% of cases; type II accounts for 40%; and type III accounts for 10%.13

Prenatal presentation

Clinical features-Maternal polyhydramnios and nonimmune fetal hydrops are clinical findings that may lead to a prenatal diagnosis of CAM in some cases.14,15 CAM may be discovered during the ultrasonographic (US) evaluation of polyhydramnios, or it may be found incidentally when routine prenatal US shows fetal hydrops or a cystic or solid fetal lung mass.14,15 Polyhydramnios and hydrops may occur together or alone; polyhydramnios has been reported in 25% to 75% of cases, and fetal hydrops with generalized edema and/or anasarca may occur in 33% to 81%.9,14,16,17

CAM may be quite large and cause compression of the ipsilateral fetal lung, compression and displacement of the heart and mediastinum, and, occasionally, compression of the contralateral lung.14 Compression of the fetal lung in utero interferes with the development of the lung and may cause pulmonary hypoplasia, which can be severe.4,14 Compression of the fetal esophagus impairs fetal swallowing and results in maternal polyhydramnios; hydrops is thought to occur from compression of the fetal heart or vena cava.14,15

The combination of maternal polyhydramnios, fetal hydrops, and pulmonary hypoplasia is almost always fatal.4,14 In cases in which large CAM lesions are associated with hydrops and polyhydramnios, the family may choose pregnancy termination or, possibly, fetal surgical intervention.14

Thoracentesis with drainage of fluid from the larger cysts has been done, but it offers little lasting effect because the cyst fluid rapidly reaccumulates.14 It is possible that in utero surgical resection of CAM may allow adequate lung growth to permit survival, but the overall risks and benefits of this procedure remain unknown.14 Fetuses with CAM who do not have hydrops have a good chance of survival in the setting of maternal transport, planned delivery, and immediate resuscitation and surgery.14

Imaging findings-On US, type III (microcystic) CAM appears as a uniformly echogenic lung mass that is typically more echogenic than fetal liver.14,15 Types I and II CAM also are densely echogenic, but contains one or more fluid-filled cysts of varying size (figure 1).14,15 There often is extensive mass effect and mediastinal shift; associated fetal hydrops and maternal polyhydramnios also can be seen.14,15

Differential diagnosis-The US differential diagnosis of fetal thoracic masses includes diaphragmatic hernia, pulmonary sequestration, bronchopulmonary foregut duplication cysts, and mediastinal cystic teratomas.14 Diaphragmatic hernia can be distinguished either by careful US assessment demonstrating normal bowel in the fetal abdomen or an amniogram with or without computed tomography (CT).14

Bronchopulmonary foregut duplication cysts typically are single, fluid-filled cysts with little or no appreciable solid component, and they should be easily distinguishable from CAM.2,10 Sequestration and mediastinal cystic teratomas may have both cystic and solid components and, thus, may be difficult to distinguish from CAM; however, the findings of a solid or mixed cystic and solid fetal lung mass, fetal hydrops, and/or maternal polyhydramnios should strongly suggest CAM.14

Neonatal presentation

Clinical features-Approximately 90% of cases of CAM present in the first year of life, with most presenting in the first six months.4,8,9,11 The majority present in the immediate neonatal period and affect both premature and full-term infants.4 The most common clinical presentation is acute progressive respiratory distress beginning at or shortly after birth, with cyanosis, grunting, retractions, and tachypnea.4,5

On physical examination, there may be prominence of the affected side of the chest, ipsilateral hyperresonance and decreased breath sounds, distant and shifted heart sounds, and apparent hepatosplenomegaly from hyperexpansion of the thorax.4,18 Progressive expansion of CAM can result in severe respiratory distress with contralateral shift of the mediastinum, cardiac compression, and compromise of venous return, and it can require emergency surgical resection.10,18 Superimposed infection is an uncommon early presenting feature.5

Associated congenital anomalies have been reported in about one-fifth of patients with CAM.8 Associated congenital anomalies may occur with any of the three types of CAM, but are more common in types II and III.4,6-9,15 Reported extrapulmonary anomalies include cardiac malformations, renal agenesis, jejunal atresia, prune-belly syndrome, and pectus excavatum.8,16,19-21

Pulmonary anomalies associated with CAM include pulmonary hypoplasia, pulmonary sequestration, and systemic arterial supply to the lung without sequestration.4,6,8,16,22-24 Pulmonary hypoplasia results from compression of the surrounding ipsilateral or contralateral lung by a large CAM lesion and the displaced heart and mediastinum, and it may be severe, resulting in death.8,16

Rosado-de-Christenson and Stocker4 have reported that 15% to 25% of extralobar pulmonary sequestrations contain small foci of type II CAM. Similarly, there have been several reports of CAM containing foci of pulmonary sequestration.6,22 In these patients, anomalous systemic arteries arising from the aorta or its branches supply the malformed lung.6,22 Occasionally, anomalous systemic arteries may supply CAM without the histologic changes of sequestration.23,24

Imaging findings-On chest radio-graphs, CAM is seen most often as either a predominantly cystic or a mixed cystic and solid mass containing multiple cysts of varying size.1,4 Overall, the size of the cysts correlates well with the type of CAM.1,4 In type I CAM, the cysts are often large and range from 2 cm to 10 cm or more in size (figures 2 and 3).1,4 In type II CAM, the cysts are smaller and more uniform in size and seldom exceed 2 cm in diameter (figures 4 and 5).1,4 Occasionally, air-fluid levels may be present in the cysts from retained fetal lung fluid.4

Typically, CAM is an expansile lesion that causes compression and displacement of surrounding intrathoracic structures.1,4,5,11 Compression of the adjacent lobe or lobes and contralateral shift of the heart and mediastinum are common (figures 3 and 5).1,4,5,11 Occasionally, progressive expansion of the lesion from air-trapping can be observed on serial radiographs (figure 6).5,11

Sometimes, CAM may have a solid appearance on chest radiographs. This can be due to the presence of a type III (microcystic) lesion.4 However, types I and II CAM initially may appear solid on plain films due to retained fetal lung fluid within the lesion (figure 6).3,25 Approximately 30% of fetal lung fluid is expressed through the tracheobronchial tree at birth; the remainder is removed via pulmonary lymphatics and the vascular capillary system.3

The abnormal bronchial connections of CAM may cause poor fetal lung fluid mobilization at birth, resulting in an initial solid (fluid-filled) appearance.3 US may be helpful in showing the true cystic nature of the mass.25 Over the next few hours following birth, slow clearance of fetal lung fluid from CAM will result in the typical appearance of air-filled cysts within the lesion (figure 6).3

Computed tomography (CT) has been used recently to evaluate CAM.7,10,26,27 CT is superior to plain films in demonstrating subtle CAM lesions (figures 4 and 7), in showing the cystic nature of CAM, in determining the anatomical distribution of the lesion, in showing compression of the surrounding lobe or lobes, and in determining the severity of mediastinal displacement.7,10,26,27

Currently, we routinely use a 5-mm slice thickness in CT of the infant chest. Thinner sections of 2 mm to 3 mm can be done through abnormal areas, if needed. The shortest scan time possible should be employed; spiral CT is preferable because of the reduced scan time and decreased need for sedation. Standard CT technique (120 kv and 200 mAs) is similar for both conventional and spiral CT. A pitch of 1:1 is recommended for spiral CT. Reduction in mAs (low-dose chest CT) can be done at the expense of greater artifact production and reduced resolution.

Differential diagnosis-The differential diagnosis of CAM in the neonatal period is limited and consists primarily of diaphragmatic hernia, congenital lobar emphysema, bronchopulmonary foregut duplication cysts, and pulmonary sequestration.2,5,10,26 Diaphragmatic hernia can be excluded via either a contrast study or the presence of bowel loops in the abdomen on the plain radiograph.2

Congenital lobar emphysema may present as a hyperlucent lobar lesion with mass effect; however, congenital lobar emphysema is not a cystic lesion, a feature that distinguishes it from CAM.28 Bronchopulmonary foregut duplication cysts usually are single, fluid-filled mediastinal masses that rarely cause symptoms in the neonate. Of these, bronchogenic cysts may occur rarely as solitary, air-filled lung cysts. The presence of multiple air-filled cysts is unusual in bronchogenic cyst and favors CAM.2,10,26,27

Most extralobar sequestrations are solid and should be distinguishable

from types I and II CAM, which are multicystic; however, it may be difficult to distinguish between extralobar sequestration and type III CAM, which are both solid. Intralobar sequestration is a hyperlucent lesion of the lung that is occasionally cystic and may be difficult to distinguish from types I and II CAM.

The presence of anomalous systemic arteries arising from the aorta or its branches favors sequestration; however, this can occur in some cases in which both CAM and sequestration coexist, and it can be seen in some cases of CAM without associated sequestration.6,10,22-24 Therefore, it is not always possible to distinguish between CAM and sequestration prior to surgical resection. However, since CAM accounts for approximately 95% of all congenital cystic pulmonary lesions,8 the diagnosis of CAM can be made with considerable confidence when an expansile, multicystic lung mass is encountered in a neonate.

Presentation in later life

Clinical features-Approximately 10% of cases of CAM present after the first year of life.11 In older children and adults, the most common clinical pres-

entation is chronic or recurrent pulmonary infection.7,10,29,30 Spontaneous pneumothorax also has been described as a late presenting sign of CAM, presumably caused by rupture of a thin-walled cyst.31

Imaging findings-The findings of CAM in older patients are similar to those in neonates, with a multicystic, expansile mass being the most common finding.4,7,10,11,29,30 Superimposed infection is common and usually is manifested by the presence of air-fluid levels within the cysts (figure 8).7,11,29 Occasionally, spontaneous pneumothorax may be seen.31

Differential diagnosis-In older children and adults who present with chronic or recurrent pulmonary infection, the main differential diagnosis of CAM consists of lung abscess or cavitary pneumonia.10 The presence of multiple thin-walled cysts within the lesion favors CAM;7,29 however, the cyst walls in infected CAM may be thick, resembling those in a lung abscess.10 Clues that suggest infected CAM include a history of recurrent disease in the same location, increased volume of the affected lobe, and the absence of air bronchograms or pleural disease.10

Treatment and outcome

The treatment of choice for CAM is surgical resection; lobectomy is preferred to segmental resection. Lobectomy usually is well-tolerated, whereas major complications, including persistent air leaks, have been reported with segmental resection.2,4,5,18,26,32

The prognosis of CAM is variable and depends on the size of the lesion, the degree of associated pulmonary hypoplasia, and the severity of other congenital abnormalities. In general, type I lesions have a better prognosis than types II or III CAM.4,8,9,14,33 Fetal hydrops carries a poor prognosis, and the combination of maternal polyhydramnios, fetal hydrops, and pulmonary hypoplasia is almost always fatal.4,14 Infants without significant pulmonary hypoplasia or severe associated congenital anomalies and older patients with CAM usually do well following surgical resection of the affected lobe.18,32 AR

References

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