Mediastinum: The Mediastinum
The mediastinum is the central space of the thorax located between the two pleuropulmonary cavities to the right and left, the cervicothoracic inlet above, and the interdiaphragmatic thoracoabdominal outlet inferiorly. It contains the heart and great vessels, the thymus, the esophagus, the trachea and main bronchi, lymph nodes, and mediastinal pleural reflections, as well as the vagus and phrenic nerves.
On the lateral thoracic radiographs, the mediastinum is divided into the anterior, middle, and posterior compartments ( Fig. 1.161a ). The anterior mediastinum is located between the sternum and the pulmonary root, and the posterior mediastinal space extends posteriorly from a line connecting the anterior surfaces of the thoracic vertebral bodies. The middle mediastinum is the remaining portion.
On the frontal thoracic radiograph, the mediastinum is divided in the superior, middle, and inferior mediastinum ( Fig. 1.161b ). The superior mediastinum extends from the thoracic inlet to the superior aspect of the hila. The middle mediastinum comprises the hilar region. The inferior mediastinum is bordered by the lower aspect of the hila and the diaphragm.
This division of the mediastinal space is used for putting together a differential diagnosis. Keep in mind, however, that lesions arising in one mediastinal compartment often involve an adjacent one, which makes it sometimes difficult to define the originating site.
Neonates and Infants
The superior and often the middle mediastinum are physiologically widened because of the presence of the thymus. When the thymus shrinks or disappears, as a consequence of stress, the mediastinum will appear narrow. The thymus rebounds after relief of the stress. The shape and size of both the normal as well as the rebounding thymus is highly variable from patient to patient.
The thymus gradually involutes. By the age of 3 years, the thymus is usually not border-forming on the frontal chest radiograph.
The mediastinum assumes the size and contours as in adults.
In early childhood, the mediastinum does lend itself to US examination. There are three main approaches to the mediastinum: (1) suprasternal approach allows examination of the superior mediastinum; (2) right and left parasternal approach, with the patient in lateral decubitus, explores the anterior mediastinal compartment and the heart; (3) subcostal abdominal approach allows transdiaphragmatic investigation, which is useful for masses of the cardiophrenic angles.
MRI is superior to CT for evaluating extension of lesions, in particular of tumors invading the spinal canal and those in contact with the heart or the cervicothoracic or thoracoabdominal junction.
Extremely variable in its radiographic appearance. May mimic cardiomegaly, upper lobe consolidation. Its outline can be convex, concave, wavy, or notched (indentation from anterior ribs).
Similar to cardiac silhouette in density. On a lateral view, retrosternal clear space is filled in.
US, CT, MRI.
Normal finding. Prominent soft-tissue density until the age of 3 y. A normal thymus has no mass effect on the trachea.
Atrophies with stress of acute illness and on steroid therapy. Regenerates (rebounds) after the illness has cleared or the steroid therapy is terminated. Shape and size varies patient to patient, both normally and after rebound.
Commonly unilateral upper mediastinal mass. Calcifications and cysts may be detected in the thymoma. Can cause displacement or compression of the trachea.
Very rarely occurs in children.
Encapsulated, noninvasive, or invasive. Round or ovoid epithelial cell mass may be lobulated. Symptoms present late.
T-cell leukemia/NHL (with thymic infiltration)
Lymphoma cells infiltrate the thymus, causing enlargement and lobular lateral borders. Paratracheal, parabronchial, perihilar neoplastic lymphadenopathy. CXR usually shows a widening of the anterior mediastinum that may extend into the middle or posterior mediastinum. Contrast-enhanced CT or MRI defines the extent of the disease and will also detect lung parenchymal involvement, pleural effusions, chest wall disease, and complications caused by impingement on vital structures, most commonly the airway. The lesion is often heterogeneous and the low attenuation areas represent necrosis. Rebound enlargement of the thymus often occurs following treatment. The thymus is then usually homogeneously enlarged without associated lymphadenopathy.
Very high peripheral white-cell count. Most common cause of a mediastinal mass in the pediatric age group, accounting for 46%–56% of all mediastinal masses and over 80% of the malignant ones. Frequently affects children under the age of 5 y. Boys are more often affected than girls.
Gradual onset of cough, respiratory distress, dysphagia, and even cardiac failure may be presenting symptoms. Central nervous system and gonadal involvement can also occur.
Usually presents as abdominal, thoracic, or head and neck mass. Lymphoblastic or T-cell NHL is the most common type and can be indistinguishable from acute lymphoblastic leukemia. Over one-third of NHLs have their primary site in the mediastinum.
Mediastinal adenopathy, as part of generalized lymphadenopathy.
Involvement of the thymus may be seen on CT, showing a low-attenuation masslike enlargement of the thymus with enhancing septa and presence of thymic calcifications. Cannot be differentiated on a chest film from the normal thymus until they have become very large or if the trachea is displaced.
Group of idiopathic disorders characterized by the proliferation of specialized bone marrow–derived Langerhans cells and mature eosinophils. The pathogenesis is unknown.
Often combined with T-cell immunodeficiency. Fever, hepatosplenomegaly, generalized lymphadenopathy are present.
A rare disease.
Thyroid, retrosternal goiter, thyroid carcinoma
Retrosternal soft-tissue density that often projects above the clavicles and moves with swallowing. Larger masses often displace and may narrow the esophagus and trachea.
Can best be diagnosed by radionuclide thyroid scan but can also be recognized on US, CT, and MRI.
Ectopic thyroid tissue, may occur anywhere in the midline from the base of the tongue to the mediastinum. Locations outside the midline (e.g., neck) have been described.
Predominantly in prepubertal girls.
A mass lesion of which 97% occur in the anterior mediastinum, either within or near the thymus gland. It can extend from the anterior to the middle mediastinum. Only 3% occur in the posterior mediastinum. Larger tumors are often bilateral and asymmetric. Can “splay” the carina Contains soft tissue, fat, fluid, or calcium; the latter two findings are pathognomonic. Could be predominantly cystic.
Their outlines are sharp, wavy, or rounded.
Pleural fluid may occur.
CXR may show calcifications.
CT and MRI can define the extent and internal characteristics of the mass (es) better than CXR.
Derive from multipotential cells that arise from an early event in embryogenesis. Because of this multipotential nature, different tissues may be found within the mass, and these may be well differentiated or immature. They comprise 10%–25% of anterior mediastinal tumors in children, and teratoma is the most common mediastinal GCT.
Teratomas can be mature (benign), immature, and mixed malignant types.
Mature (benign) teratomas account for approximately 75% of all mediastinal GCTs. They can occur at any age but there are two peaks of incidence: at 2 y of age and in adolescence.
Large tumors tend to produce respiratory symptoms due to tracheobroncheal compression. In older children, often very large asymptomatic masses are found.
Lymphoma (NHL), (T-cell) leukemia
Paratracheal, parabronchial, and perihilar lymphadenopathy may extend into the anterior mediastinum. Lymphoma cells may infiltrate and enlarge the thymus. Pleural effusion and tracheal narrowing may also occur.
CXR usually shows lymph node enlargement and a widened, dense, and sharply defined anterior mediastinal lesion. Contrast-enhanced CT or MRI defines the extent of the lesion better.
NHL frequently affects children under the age of 5 y. Hodgkin lymphoma is rare in children < 5 y but its incidence increases in later childhood and in the teenage years.
Pediatric NHL usually presents as abdominal, thoracic, or head and neck masses. Lymphoblastic or T-cell NHL is the most common type and can be indistinguishable from acute lymphoblastic leukemia. Over one-third of NHLs have their primary site in the mediastinum.
Pediatric Hodgkin disease is more likely to present as asymptomatic cervical or supraclavicular lymph-adenopathy; systemic symptoms such as fever being more common with Hodgkin disease. Two-thirds of patients with Hodgkin disease will have mediastinal lymphadenopathy.
Mediastinitis, mediastinal effusion
Localized (abscess) or diffuse (cellulitis) pus. CXR shows superior mediastinal widening and obliteration of normal mediastinal contours. Trachea may be displaced or narrowed. Pleural effusion often present. CT shows obliterated mediastinal fat, low attenuation mediastinal fluid collection, air collections, abscess, pleural and pericardial fluid, and adenopathy. Early postoperative changes and mediastinitis are difficult to differentiate. Contrast study with a water-soluble contrast medium is indicated to check for perforation, stenosis, or displacement.
Mostly results from perforation of the esophagus or trachea; spontaneously or secondary to malposition of catheters or tubes, foreign body aspiration or ingestion, after open chest surgery, leakage at sites of surgical anastomoses, tumor, and infection.
Mediastinal fat deposition
CT: exquisite delineation of the excess fat, more common in anterior and posterior mediastinum.
Most often seen as a complication of steroid therapy. Seen in obesity, Cushing syndrome.
(see Table 1.11 )
(see Table 1.85 )
Symmetric widening of the middle mediastinum, enlarged cardiac silhouette with a normal lateral view.
Due to fluid retention in the mediastinum.
Traumatic aortic dissection
Widening of the middle mediastinum as a result of mediastinal hemorrhage.
Rare in children.
Paravertebral soft-tissue mass. CT demonstrates vertebral destruction. MRI better for the soft-tissue component and extent.
(see Table 4.116 )
Neurogenic tumor (neuroblastoma, ganglioneuroma)
Well-circumscribed paraspinal soft-tissue mass, round to ovoid, in the posterior mediastinum that contains calcifications in 50%–75%. Can be bilateral and asymmetric. Often widening of intercostal spaces. Erosion or destruction of ribs is possible. CT and MRI to delineate the extent of the mass. MRI is excellent to assess extension into the vertebral canal.
Originates from the sympathetic ganglia. Neuroblastoma is the most common posterior mediastinal mass in young children. Approximately 16% of all neuroblastomas are of mediastinal origin. Neuroblastoma is a highly malignant tumor. It occurs mostly in children < 3 y. Children may present with a variety of signs and symptoms (fever, malaise, bony metastasis, cord compression, paraneoplastic syndromes, skin metastases, opsomyoclonus, etc.). Ganglioneuromas are benign tumors that may represent matured neuroblastoma. They are often an incidental finding.
Well-defined mass, tubular or rounded, in the posterior mediastinum, preferentially on the right side, may be bilateral.
Frequently associated with vertebral dysraphism including hemivertebrae and anterior spina bifida. Lower cervical and upper thoracic vertebrae are usually affected. Frequently attached to the vertebra by a fibrous band and may extend intraspinally through the spinal defect. MRI is imaging modality of choice.
Cysts lined with GI mucosa, most are mucus-filled. Sixty-five percent are diagnosed in the first year of life. Represent a failure of complete separation of the notochord from the foregut during the third week of embryogenesis.
In a third of the patients, these cysts are associated with malformations of central nervous system (tethering, syringohydromyelia, or an intradural cyst) and/or GI tract.
Hematoma associated with vertebral fracture
Widened mediastinum and flattened, fractured vertebral body on CXR and CT. CT will demonstrate the posterior mediastinal hematoma.
(see Table 4.70 )
A smooth, round, dense opacity on CXR. CT and MRI may show cystic masses, their exact location, and may demonstrate if there are daughter cysts. The germinative membrane may be visible. Calcifications are well visualized on CT and sometimes on CXR.
Mediastinal localization is rare.
Paraspinal abscess (so-called migrating abscess)
Typical radiographic findings are not detectable until 2–4 wk after the onset of symptoms. CXR shows a paravertebral density; more common bilateral (TB), collapse of the intervertebral disk and destruction of the adjacent vertebral bodies. MRI is the investigational tool of choice in diagnosing spondylodiscitis, particularly in the early stages of the disease when other investigations still yield negative results.
Possible clinical signs are limping, refusal to sit or walk, increased irritability, neurologic or abdominal symptoms.
Etiology in children includes both infectious and inflammatory causes. S. aureus is the most common bacterium isolated.
Normal finding. Most common cause in infancy, less common in older children (> 3 y) (see Table 1.59 ).
Thymoma: (see Table 1.59 ).
Thymolipoma: Superior and middle compartments of the anterior mediastinum, substernal in location. Linear calcification occurs in 7.5%–20% of cases. Often grows to a very large size and may cause displacement of the trachea.
On CXR, occasionally the diagnosis of a fatty lesion can be entertained when the periphery of the mass appears more radiolucent than its bulky center. On CT and MRI, the mass is sharply defined and predominantly fatty.
Thymolipomas are benign hamartomas containing fat. Lobulated and encapsulated. Half of the cases are asymptomatic and discovered incidentally.
(see Table 1.59 )
Cervicomediastinal lymphangioma or hemangioma
Fig. 1.179, p. 102
The diagnostic modalities of choice are US and MRI. On US, macrocystic lesions appear as a multiloculated or septated cystic mass, sometimes with fluid–fluid levels. On Doppler US, flow can be demonstrated only within the septa. Microcystic lesions are hyperechoic (“bright”) without any flow on Doppler.
On MRI, lymphatic malformations are septated masses with a low signal intensity on T1-weighted and a high signal intensity on T2-weighted sequences. The presence of proteinaceous fluid or hemorrhage within the lesion can cause variable signal intensity on both T1- and T2-weighted sequences.
The majority of vascular anomalies involving the mediastinum are actually lymphatic malformations. Typically, they involve the anterior mediastinum but are not restricted to any particular mediastinal compartment; other locations include the axilla, superior mediastinum, mesentery, retroperitoneum, and lower limbs.
Often discovered at birth and usually asymptomatic. Spontaneous shrinkage can occur but sudden enlargement is an indication of bleeding or inflammation. In symptomatic patients, sclerosing therapy with the aid of interventional radiology can be useful.
Fig. 1.180, p. 102
Soft-tissue masses, which may be unilateral or bilateral, if border forming, on CXR. Calcifications are rare.
DD: cystic teratoma.
One percent of all mediastinal masses. Girls/boys: 2/1. Two-thirds diagnosed in the first year of life. Large cysts in neonates may cause respiratory distress, recurrent respiratory tract infections, cough, dyspnea, dysphagia.
Increased distance between the right mediastinal border and the lucent stripe of the trachea. Hilar lymph nodes may or may not be enlarged.
To visualize lymph nodes on CT, intravenous contrast should be used.
TB and in older school-aged children Hodgkin disease.
On CXR, a mass or curvilinear density in the right upper lungfield or paramediastinally, ending with a teardrop density, the azygos vein.
An accessory fissure caused by the azygos vein being “caught” by the budding lung tissue of the RUL.
Anomalies of the aortic arch, (e.g., high right aorta), poststenotic dilatation in aortic stenosis, or coarctation of the aorta
(see Table 1.83 )
Achalasia (so-called megaesophagus)
The flattened lateral border of the dilated esophagus overlaps the right mediastinum, causing widening of the middle mediastinum. Air-fluid level in the esophagus, or heterogeneous density caused by food content, or only air. Small or absent stomach bubble. Confirmation with esophagram.
(see Table 2.31 )
Soft-tissue density, sharply demarcated at the level of the aortic arch. Projected over the great vessels on the lateral CXR. CT.
Most are tuberculous in origin.
High-riding aortic arch
(see Table 1.83 )
(see Table 1.83 )
Usually unilocular, round or oval, fluid-filled density. May be air-filled when there is communication with the tracheobronchial tree (rare).
CXR may demonstrate a discrete rounded mass near to the carina (often subcarinal) or in the paratracheal region, sometimes displacing or compressing the trachea, bronchi, or esophagus. There may also be hyperinflation, atelectasis, or consolidation associated.
CT and MRI are helpful to determine nature and extent. Enhancement on CT is minimal and peripheral, on T2 sequences the cyst “lights up like a lightbulb.”
Congenital abnormality of division of the embryonic primitive foregut. Five percent of all pediatric mediastinal masses.
The most common of the bronchopulmonary malformations. Most occur in the middle mediastinum (85%), at or near the carina, but localization may be in the posterior mediastinum, intrapulmonary, or more rarely in the neck, pericardium, or abdominal cavity.
There is usually no communication with the airways. Most commonly mediastinal (85%). Patients are asymptomatic or cyst can cause persistent cough, progressive dyspnea, wheeze, stridor, and cyanosis.
Ectopic thymus (retrocaval thymus)
Thymic tissue posterior to the superior vena cava. May extend into the posterior mediastinum. Characteristic imaging features on CT and MRI.
In normal infants. Most cases in children 2 y of age or younger.
Mostly an incidental finding, no biopsy is needed.
Increase in the extrathoracic soft tissues, ipsilateral skeletal enlargement.
Apparent external differences in size between the two halves of the body.
Diaphragmatic hernias and defects
(see Table 1.86 )
Mediastinal pancreatic pseudocyst
Air-fluid or fluid-containing mass in the middle to lower mediastinum. Convex expansion of the affected side of the mediastinum or of the heart border. Pleural effusion is present in majority of cases. CT demonstrates the presence of thin- or thick-walled cystic lesion.
Rare complication of acute or chronic pancreatitis. May have no specific symptoms or may be associated with back pain, dysphagia, or esophageal reflux.
Neurogenic tumors (neuroblastoma, ganglioneuroma, ganglioneuroblastoma, neurofibroma)
(see Table 1.61 )
(see Table 1.61 )
Esophageal duplications, tumors, and cysts
On chest radiographs, benign mediastinal cysts may appear as a sharply marginated, round or oval area of increased opacity.
UGI contrast examination will show extrinsic or intramural mass effect.
Their appearance at CT or MRI imaging mimics that of bronchogenic cysts.
The CT features of such a benign mediastinal cystic structure ranges from (a) a smooth, oval, or tubular mass with a well-defined thin wall that usually enhances after intravenous contrast administration; (b) homogeneous low attenuation (0–20 HU); (c) no enhancement of cyst contents; and (d) no infiltration of adjacent mediastinal structures. Masses that show most or all of these features are invariably benign. MRI can be useful in showing the cystic nature of these masses because these cysts “light up” when imaged with T2-weighted sequences.
Developmental in origin; may result from failure of the solid esophageal tube to vacuolate completely to form a hollow tube or from abnormal budding of the dorsal foregut.
Many are asymptomatic, but they may cause dysphagia, pain, or other symptoms owing to compression of adjacent structures.
The majority are detected in infants or children, usually adjacent to or within the esophageal wall. Ectopic gastric mucosa in the cyst may cause hemorrhage or perforation of the cyst or infection.
Fig. 1.186, a–c, p. 106
(see Table 1.66 )
(see Table 4.116 )
Superiorly located paravertebral meningocele
Round, smooth, or lobulated homogeneous mass in the upper, posterior mediastinum sometimes associated with multiple vertebral segmentation anomalies and widening of the spinal canal.
CT and MRI are essential, not only for the diagnosis, but also for the depiction of relationship to surrounding structures and the exclusion of other possible accompanying lesion such as neuroma in the setting of neurofibromatosis type I.
Herniation of leptomeninges through an intervertebral foramen or a defect in the vertebral body to form a cerebrospinal fluid–filled sac.
Intrathoracic meningocele is rare and is usually associated with neurofibromatosis type I or Marfan syndrome. Rarely as an isolated defect.
Most of the reported thoracic meningoceles are not strictly anterior in location, but also lateral or anterolateral.
Clinical manifestations are closely related with its size and its relationship to surrounding structures. Back pain, paraparesis from insult to the spinal cord, shortness of breath, coughing, and palpitation by compression of the lung and mediastinal structures but also progressive hydrothorax caused by rupture of meningoceles all has been reported in the literature. Small meningoceles may be incidentally diagnosed on a routine chest radiograph.
(see Table 1.66 )
Paraspinal abscess (so-called migrating abscess)
(see Table 1.61 )
(see Table 1.61 )
Inferior pulmonary ligament/accessory lung
Radiographically similar to extralobar pulmonary sequestration; however, there is a connection between the esophagus or bronchi by a supernumerary bronchus.
Accessory lung buds or extensions. Clinically and histologically identical to pulmonary sequestration. Is found incidentally.
Chylothorax and other mediastinal effusions
Unilateral, very rare (< 10%) bilateral. Paramediastinal effusion, often in combination with a pleural effusion.
Lymphatic fluid in the pleural space secondary to leakage from the thoracic duct or one of its main tributaries. Results from birth trauma or is “idiopathic” in neonates; traumatic in older children. May also occur with lymphangectasia and after cardiac surgery. Milky fluid after eating.
Pulmonary sequestration (intralobar or extralobar)
(see Table 1.21 )
(see Table 1.61 )
Very rare tumors
CT and MRI to determine location and extent of the disease.
Low attenuation of fat content on CT.
High signal on T1-spin echo weighted imaging.
Very rare tumors.
Chest film shows a rounded or oval density. The salt-and-pepper appearance on MRI is characteristic, with the pepper representing flow voids of vessels and the salt representing the T1-bright tumor parenchyma. MIBG scintigraphy.
Vascularized chromaffin cell neoplasms that secrete catecholamines and, in some cases, other active peptides. Classic symptoms are hypertension, headache, palpitation, and excessive sweating.
Paravertebral, adjacent to the mediastinum; rounded or ovoid densities. Mostly occurring with a known primary tumor.
Immune deficiency states (DiGeorge syndrome, Bruton agammaglobulinemia, congenital hypogammaglobulinemia), lymphopenic agammaglobulinemia, Nezelof syndrome
A very narrow superior mediastinum due to absence of the thymus on CXR.
MRI to confirm absence of thymus.
Transposition of the great vessels
(see Table 1.78 )
Thymic stress atrophy
Fig. 1.188, p. 108
For example, due to infection, infant respiratory distress syndrome, steroids.
Coincidental timing of the radiograph during burping.
Normal finding, especially in infants.
Incompetent lower esophageal sphincter
Air outlines the esophagus, bowed slightly convex to the right.
(see Table 2.31 )
(see Table 2.31 )
Esophageal stenosis or stricture
Air or heterogeneous lucency in the dilated portion proximal to the narrowing.
(see Table 2.31 )
Hiatus hernia and paraesophageal hernia
Fig. 1.187, p. 107
Cystic lucency in the cardiophrenic angle of the mediastinum or overlying the cardiac silhouette. On the lateral view in the middle mediastinum.
(see Table 2.31 )
Passage of air into the esophagus via the fistula.
(see Table 2.33 )
RDS and birth trauma
The lucency of the wide air-filled trachea and the main bronchi is enhanced by the surrounding density of the air space disease (HMD, RSD).
In neonates with dyspnea, tachypnea, cyanosis.
Pericardial cyst, bronchogenic cyst.
Irregular air collection scattered throughout the mediastinum; small cysts or “string of pearls” appearance.
(see Table 1.60 )
Pneumomediastinum (mediastinal emphysema)
Linear or bubbly air collections in the mediastinum. In infants, the air can outline both lobes of the thymus, “butterfly” sign. Large collections of air may elevate the thymus on both the frontal and lateral radiographs to produce a “spinnaker sail” sign. Air outlines the aortic arch and trachea and extends into neck and subcutaneous tissues. Occasionally causes a pneumothorax or even pneumoperitoneum.
In infants on mechanical ventilation; in neonates may be spontaneous in origin, a complication of increased intra-alveolar pressure (even coughing, vomiting) causing alveolar rupture. Air then passes into the interstitium of the lung and travels along the perivascular space to the hilum and into the mediastinum. In older children most commonly seen in asthma, unless there is history of trauma.
Rarely the consequence of aspirated foreign body or rupture of the trachea/bronchus.
The “sharp mediastinum” sign on a supine view; an apical lucency on an upright view.
To differentiate from pneumomediastinum by a lateral decubitus (horizontal beam) film. The air in the pleural space “rises,” where mediastinal air does not move.
Mach effect, mimicking pneumomediastinum
A thin band of lucency immediately adjacent to the mediastinum.
“Artifact”; the appearance is caused by the transition between two very different densities (air in lung, mediastinal soft tissue).
When the lateral chest radiograph is slightly rotated in infants, a retrosternal thin band of lucency is seen.
Projectional: represents the anterior lung on the side away from the table.
Due to volume loss: unilateral pulmonary atelectasis, pulmonary hypoplasia, pulmonary agenesis, postlobectomy and pneumonectomy
Mediastinal shift to the abnormal side. Contra-lateral compensatory overinflation. Elevation of the hemidiaphragm of the involved side with often contralateral diaphragmatic depression. Narrowed intercostal spaces on the abnormal side.
Due to overexpansion: congenital lobar emphysema, tension pneumothorax, airtrapping with foreign body aspiration, cysts, bullae, unilateral pleural effusion, pectus excavatum, tumor, lung sequester
Mediastinal shift away from the lesion. Depression of the diaphragm of the hyperinflated side.
Diaphragmatic causes: eventration of the diaphragm
(see Table 1.46 )
Congenital diaphragmatic hernia
Foreign body aspiration
In inspiration, both lungs may be equally aerated, the mediastinum is central. In expiration, only the uninvolved healthy side will decrease in size; the involved side retains more air due to air trapping. The mediastinum shifts to the healthy side.
Air trapping in expiration due to bronchial narrowing or check-valve mechanism. Dynamic mediastinal motion.
Common cause of respiratory distress in children between 6 mo and 3 y of age.
Especially seen in patients with pectus excavatum in whom the mediastinum is shifted (in nearly all cases to the left side).