Problems with the airway are much more common in children than in adults. It has been said that one of the differentiating features between a pediatric and general radiologist is that a pediatric radiologist remembers to look at the airway. For practical purposes, abnormalities of the airway can be divided into acute upper airway obstruction, lower airway obstruction (extrinsic compression, intrinsic obstruction), obstructive sleep apnea (OSA), and congenital high airway obstruction syndrome (CHAOS).
Clinically, children with acute upper airway obstruction (above the thoracic inlet) tend to present with inspiratory stridor, whereas children with lower airway obstruction (below the thoracic inlet) are more likely to present with expiratory wheezing. However, the categorization of a child with noisy breathing into one of these two groups can be very difficult. The primary imaging evaluation of the pediatric airway for acute conditions should include frontal and lateral high-kilovolt radiography of the airway and frontal and lateral views of the chest.
▪ Acute Upper Airway Obstruction
Acute stridor in a young child is the most common indication for imaging the pediatric airway. The most common causes of acute upper airway obstruction in children include inflammatory disorders and foreign bodies. The most common inflammatory disorders include croup, epiglottitis, exudative tracheitis, and retropharyngeal cellulitis and abscess. Anatomic structures that are especially important to evaluate on radiographs of children with acute upper airway obstruction include the epiglottis, aryepiglottic folds, subglottic trachea, and retropharyngeal soft tissues.
Croup (acute laryngotracheobronchitis) is the most common cause of acute upper airway obstruction in young children. The peak incidence occurs between 6 months and 3 years of age. The mean age at presentation of croup is 1 year of age. In children older than 3 years, other causes of airway obstruction should be suspected. Croup is viral in cause and is usually a benign, self-limited disease. Redundant mucosa in the subglottic region becomes inflamed, swells, and encroaches upon the airway. The children present with a barky (“croupy”) cough and intermittent inspiratory stridor. It usually occurs following or during other symptoms of lower respiratory tract infection. Most children with croup are managed supportively as outpatients, and the parents are managed by reassurance. Inhaled corticosteroids are becoming a popular therapy in children with croup. They have been shown to reduce the length and severity of illness.
The purpose of obtaining radiographs in a patient with suspected croup is not so much to confirm the diagnosis but rather to exclude other, more serious causes of upper airway obstruction that require intervention. However, characteristic radiographic findings that indicate croup are best seen on frontal radiographs. With croup, there is loss of the normal shoulders (lateral convexities) of the subglottic trachea secondary to symmetric subglottic edema ( Fig. 2-1 ). Normally, the subglottic trachea appears rounded, with “shoulders” that are convex outward ( Fig. 2-2 ). In croup, the subglottic trachea becomes long and thin, with the narrow portion extending more inferiorly than the level of the pyriform sinuses. The appearance has been likened to an inverted V or a church steeple (see Fig. 2-1 ). The term church steeple can be confusing because some steeples look like croup and some are shaped like the normal subglottic airway ( Fig. 2-3 ). Lateral radiographs may demonstrate a narrowing or loss of definition of the lumen of the subglottic trachea (see Fig. 2-1 ) or hypopharyngeal overdistention. With croup, the epiglottis and aryepiglottic folds appear normal.
In contrast to croup, epiglottitis is a life-threatening disease that can potentially require emergent intubation. The possibility that a child with epiglottitis might arrive in a deserted radiology department was once a constant source of anxiety for on-call radiology residents. However, most cases of epiglottitis are caused by Haemophilus influenzae and are now preventable by immunization (HiB vaccine), so the incidence of epiglottitis has dramatically decreased. The causes of epiglottitis are now also more heterogeneous. Related to this, care of children with epiglottitis is now more of a challenge because health care workers are less used to recognizing and treating patients with this disorder. Children with epiglottitis are usually toxic appearing and present with an abrupt onset of stridor, dysphagia, fever, restlessness, and an increase in respiratory distress when recumbent. In the pre-HiB vaccine era, the classically described peak age of incidence was 3.5 years. However, since the introduction of the HiB vaccine, there has been a marked increase in the mean age of presentation to 14.6 years. Because of the risk for complete airway obstruction and respiratory failure, no maneuvers should be performed that make the patient uncomfortable. If the diagnosis is not made on physical examination, a single lateral radiograph of the neck should be obtained, usually with the patient erect or in whatever position that allows the patient to breathe comfortably. Children with epiglottitis should never be made to lie supine against their will to obtain a radiograph because it can result in acute airway obstruction and, potentially, death.
With epiglottitis, on the lateral radiograph, there is marked enlargement of the epiglottis. A normal epiglottis typically has a thin appearance with the superior aspect being sharply pointed. The swollen epiglottis has been likened to the appearance of a thumb. With epiglottitis, there is also thickening of the aryepiglottic folds ( Figs. 2-4 and 2-5 ). The aryepiglottic folds are the soft tissues that extend from the epiglottis anterosuperiorly to the arytenoid cartilage posteroinferiorly and normally are convex downward. When the aryepiglottic folds become abnormally thickened, they appear convex superiorly. Symmetric subglottic narrowing, similar to croup, may be seen on frontal radiography (if obtained); do not let that be confusing.
An obliquely imaged, or so-called omega-shaped, epiglottis may artifactually appear wide because both the left and right sides of the epiglottis are being imaged adjacent to each other. This should not be confused with a truly enlarged epiglottis. The absence of thickening of the aryepiglottic folds can be helpful in making this differentiation. With an omega-shaped epiglottis (normal variant), often both the left and right walls of the epiglottis are visible.
In current times, related to both the uncommon occurrence of epiglottitis and the frequent reliance on computed tomography (CT) to evaluate more common inflammatory neck conditions (such as retropharyngeal abscess), it is increasingly more common to see and diagnose epiglottitis on CT rather than on radiography. Although not classically advocated as a diagnostic tool for epiglottitis (given the risks of laying such patients supine and giving them intravenous [IV] contrast), the findings of epiglottitis are easily identified on CT (see Fig. 2-5 ). Findings include swelling and low-attenuation edema of the epiglottis and aryepiglottic folds associated with inflammatory stranding in adjacent fat.
Exudative tracheitis (also known as bacterial tracheitis, membranous croup, or membranous laryngotracheobronchitis) is another uncommon but potentially life-threatening cause of acute upper airway obstruction. The disorder is characterized by a purulent infection of the trachea in which exudative plaques form along the tracheal walls (much like those seen in diphtheria). Affected children are usually older and more ill than those with standard croup; typically their ages range from 6 to 10 years. Although initial reports described most cases to be secondary to infection by Staphylococcus aureus , other reports have noted multimicrobial infections. It is unclear whether the disease is a primary bacterial infection or a secondary bacterial infection that occurs following damage to the respiratory mucosa by a viral infection. A linear soft tissue filling defect (a membrane) seen within the airway on radiography is the most characteristic finding. A plaquelike irregularity of the tracheal wall is also highly suspicious ( Fig. 2-6 ). Nonadherent mucus may mimic a membrane radiographically. In cooperative patients, having them cough and then repeating the film may help to differentiate mucus from a membrane. Other findings include symmetric or asymmetric subglottic narrowing in a child too old typically to have croup and irregularity or loss of definition of the tracheal wall. Membranes and tracheal wall irregularities may be seen on frontal or lateral radiographs and often seen on one but not the other; therefore it is important to get both views.
If one of these exudative “membranes” is sloughed into the lumen, it can lead to airway occlusion and respiratory arrest. Therefore children who are suspected to have exudative tracheitis are often evaluated endoscopically, the exudative membranes are stripped, and elective endotracheal intubation is performed.
A number of controversies regarding exudative tracheitis exist. First, it is seen with great frequency at some institutions and not at all at others. Second, although it is considered a life-threatening condition, to my knowledge, no patient has ever died at home of this disease—which seems odd. Both of these points raise the question of the validity of this diagnosis. My take is that there are definitive cases of this disease, but it is probably overdiagnosed and overtreated at some institutions.
Retropharyngeal Cellulitis and Abscess
Retropharyngeal cellulitis is a pyogenic infection of the retropharyngeal space that usually follows a recent pharyngitis or upper respiratory tract infection. Children present with sudden onset of fever, stiff neck, dysphagia, and occasionally stridor. Most affected children are young, with more than half of the cases occurring between 6 and 12 months of age. On lateral radiography, there is thickening of the retropharyngeal soft tissues ( Fig. 2-7 ). In a normal infant or young child, the soft tissues between the posterior aspect of the aerated pharynx and anterior aspect of the vertebral column should not exceed the anterior-to-posterior diameter of the cervical vertebral bodies. If these soft tissues are thicker, an abnormality should be suspected. Apex anterior convexity of the retropharyngeal soft tissues provides supportive evidence that there is true widening of the retropharyngeal soft tissues (see Fig. 2-7 ).
However, in infants, who have short necks, it is common to see pseudothickening of the retropharyngeal soft tissues when the lateral radiograph is obtained without the neck being well extended ( Fig. 2-8 ). If it is unclear on the initial lateral radiograph whether the soft tissues are truly rather than artifactually widened, it is best to repeat the lateral radiograph with the neck placed in full extension (see Fig. 2-8 ). Fluoroscopy can also be used to evaluate whether the pseudothickening is persistent. The only radiographic feature that can differentiate abscess from cellulitis is the identification of gas within the retropharyngeal soft tissues.
CT is commonly performed to define the extent of disease and to help to predict cases in which a drainable fluid collection is present (see Fig. 2-7 ). On CT, a low-attenuation, well-defined area with an enhancing rim is suspicious for a drainable fluid collection (see Fig. 2-7 ). Cellulitis without abscess is actually more common than the presence of a drainable abscess.
▪ Lower Airway Obstruction
The most common cause of wheezing in children is small airway inflammation, such as is caused by asthma and viral illness (bronchiolitis). When the wheezing persists, presents at an atypical age for asthma, or is refractory to treatment, other reasons for lower airway obstruction are entertained. Other causes of lower airway obstruction can be divided into those that are intrinsic to the airway (such as bronchial foreign body, tracheomalacia, or intrinsic masses) and those that cause extrinsic compression of the trachea (such as vascular rings). The initial radiologic screening procedure for wheezing is frontal and lateral radiography of the airway and chest. Radiographs are used to exclude acute causes of upper airway obstruction, evaluate for other processes that can cause wheezing (such as cardiac disease), and help to categorize the abnormality as being more likely to be an intrinsic or an extrinsic airway process. Important findings to look for on the radiographs include evidence of tracheal narrowing, position of the aortic arch, asymmetric lung aeration, radiopaque foreign body, and lung consolidation. When tracheal compression is present on radiography, it is important to note both the superior to inferior level of the compression and whether the compression comes from the anterior or posterior aspect of the trachea because various vascular rings present with different patterns of tracheal compression ( Fig. 2-9 ).