Applied Radiology

Dr. Donnelly is an Associate Professor and Dr. Strife is a Professor in the Department of Radiology, and Dr. Myer is a Professor in the Division of Otolaryngology at the Children's Hospital Medical Center, Cincinnati, OH.

It is estimated that up to 3% of all children, approximately 2 million in the United States alone, are affected by obstructive sleep apnea syndrome. ^1,2 The most common cause of obstructive sleep apnea is enlarged adenoid and palatine tonsils in otherwise healthy children. Other causes of obstructive sleep apnea include craniofacial anomalies, congenital syndromes (particularly Down's syndrome and achondroplasia), mucopolysaccharidosis, and prior surgery on the airway. ³ Many of these patients are predisposed to airway obstruction at multiple sites. ³ Polysonography is helpful in differentiating between central versus obstructive causes of sleep apnea. ^4,5 However, it provides no accurate information concerning the anatomic level of obstruction in patients with obstructive sleep apnea. In cases of obstructive sleep apnea in which there is a complicated medical history or persistent sleep apnea following a surgical procedure performed to treat sleep apnea, dynamic sleep fluoroscopy has been shown to be a useful adjunct to endoscopic evaluation, affecting management decisions in more than 50% of cases. ³ It is particularly helpful in identifying dynamic abnormalities of the airway, such as functional collapse, as compared with static, fixed obstructions. Despite this, dynamic sleep fluoroscopy in children has received little attention in the imaging literature. In 1979, Felman et al ⁶ described their cinefluoroscopic technique in 9 children using sleep deprivation. We have performed more than 80 dynamic sleep fluoroscopy procedures in children using sedation. The purpose of this review is to describe the indications and technique for dynamic sleep fluoroscopy and the anatomic sites and common causes for airway obstruction.

Indications

When children present with symptoms of airway obstruction, imaging evaluation typically includes frontal and lateral radiographs of the airway and chest and flexible fiberoptic laryngoscopy. If extrinsic tracheal compression is suspected, cross-sectional imaging with computed tomography (CT) or magnetic resonance imaging (MRI) is usually performed. If an intrinsic or dynamic problem of the trachea is suspected, direct laryngoscopy and bronchoscopy under general anesthesia is usually performed.

For obstructive sleep apnea, one of the advantages of dynamic sleep fluoroscopy over flexible fiberoptic laryngoscopy is the ability to evaluate the entire airway simultaneously when the child is sleeping. We use sleep fluoroscopy to evaluate those children with complex medical histories who are at increased risk of multilevel airway obstruction.

Multiple disease processes are associated with an increased risk of multilevel airway obstruction. In Down's syndrome, airway obstruction can occur secondary to macroglossia, lymphoid hyperplasia, congenitally narrow nasopharynx, laryngomalacia, congenital subglottic stenosis, tracheobronchomalacia, or tracheal stenosis. ⁷ Clinical evidence of airway obstruction may be secondary to any, or all, of these potential sites. Children with neuromuscular disorders are also at risk for airway collapse at multiple levels secondary to muscular hypotonia. ⁸ Other children at risk for multilevel airway obstruction include those with congenital craniofacial anomalies, such as Pierre Robin syndrome, or metabolic disorders, such as the mucopolysaccharidoses. ³ In patients who have had previous surgery and have persistent sleep apnea, the problem may be related to residual obstruction secondary to the original cause or secondary to sequelae of the surgical manipulation of the airway. ⁹ In some patients who have had tracheotomies, there may be difficulty with decannulation. These children may have episodes of apnea or respiratory distress during sleep secondary to development of granulation tissue or localized tracheomalacia at the surgical site or recurrence of the primary problem.

At our institution, indications for evaluation with dynamic sleep fluoroscopy include: 1) persistent symptoms of sleep apnea despite normal findings on flexible fiberoptic laryngoscopy; 2) persistent symptoms of sleep apnea after a single site of obstruction has been identified and appropriately treated; 3) potential for obstruction at more than one site within the upper airway because of either previous surgery or an underlying abnormality; and 4) difficulty decannulating a patient following tracheotomy.

Technique

The study is performed and monitored by a pediatric radiologist. Patients are prepared for the procedure according to our departmental sedation program guidelines. ¹⁰ During the procedure, a radiologist, radiology technologist, and radiology nurse are present. Food and drink are withheld for 4 to 8 hours prior to the examination to decrease the risk of aspiration. Patients are sedated with either oral chloral hydrate (70 to 100 mg/kg) or intravenous pentobarbital (3 mg/kg, with repeat dosing up to a total of 7 mg/kg) depending upon patient age. During the entire procedure and sedation recovery, respiratory rate, heart rate and rhythm, and blood oxygen saturation are monitored using transcutaneous pulse oximetry. The child's parents are allowed and encouraged to attend the procedure to reduce the child's anxiety and to verify if the sleep patterns observed are typical of those that occur at home.

The studies are performed in a room equipped with lateral fluoroscopy. The children are imaged in supine position with lateral fluoroscopy. The fluoroscopic portions of the examinations are videotaped with simultaneous audiotaping so that fluoroscopic findings can be correlated with episodes of oxygen desaturation or noisy breathing. Physical observations, which are noted and correlated with fluoroscopic findings, include respiratory effort, thoracic wall motions, and episodes of apnea. Fluoroscopic evaluation is performed for approximately 10 to 20 seconds at areas of anatomic interest when signs of airway occlusion occur. The fluoroscopic evaluation of a child with sleep apnea should be performed at three specific sites: the level of the base of the tongue (oropharynx), the hypopharynx, and the intrathoracic trachea. Rarely, these three areas may be seen simultaneously. In larger children, the intrathoracic trachea may need to be evaluated separately from the hypopharynx. Evaluation of the oropharynx and hypopharynx are performed with the arms positioned at the child's sides. Downward pulling on the arms improves visualization of the neck. Evaluation of the intrathoracic trachea is best performed with the arms extended above the child's head. To limit radiation dose, a maximum of 2 minutes of total fluoroscopic time is used.

Certain maneuvers may be performed during the sleep fluoroscopy to further evaluate the obstruction. When present, tracheotomy tubes may be capped in order to see if the patient develops sleep apnea when the artificial airway is bypassed. The tracheotomy tube may also be removed to evaluate for underlying tracheomalacia, which can be masked when the tracheotomy tube is present and physically prevents the trachea from collapsing. In cases in which a child has a tracheotomy tube that is to be removed intentionally or occluded during the study, an otolaryngologist is present to perform these maneuvers. When an area of airway obstruction is encountered, the effect of treatments, such as positive pressure breathing, on decreasing or eliminating the obstruction can be evaluated fluoroscopically.

Normal findings and commonly encountered abnormalities

One of the advantages of sleep fluoroscopy is the evaluation of dynamic motion abnormalities in addition to static fixed causes of obstruction. In the normal sleeping child, there is little or no motion of the pharynx and trachea. Any dynamic motion of these structures encountered during sleep should be considered abnormal. Commonly encountered dynamic abnormalities include glossoptosis, pharyngeal collapse, laryngomalacia, and tracheomalacia. It must be stressed that such dynamic abnormalities can occur at multiple sites or in conjunction with fixed causes of airway obstruction.

The first line of treatment for many of these dynamic causes of obstructive sleep apnea is the use of positive-pressure airway devices during sleep. ¹¹ Some of these types of obstructive sleep apnea will decrease with increasing age. Therefore, if positive-pressure ventilation can relieve the symptoms, it may be the only necessary therapy until the child outgrows the condition. Knowledge of the specific abnormality is important because the odds of positive-pressure therapy being helpful and the odds of the child outgrowing the condition are different for each specific entity. Therefore, the length of trial for conservative therapy may be influenced. In addition, when conservative management fails, the specific types of surgery that can be performed differ for each of the types of airway obstruction.

Glossoptosis

Glossoptosis is defined as abnormal posterior motion of the tongue during sleep. It is seen most commonly in children with neuromuscular abnormalities, because of an abnormal decrease in muscular tone. ¹² It can also be associated with macroglossia and micrognathia. On fluoroscopy, the tongue "falls" posteriorly during sleep, abutting the velum (soft palate) and posterior wall of the pharynx, obstructing the airway (figure 1). ¹² Glossoptosis can be difficult to detect with endoscopic evaluation. Surgical interventions to either reduce the volume of the tongue or reposition the mandible have been described for those cases refractory to medical management. ^13,14

Pharyngeal collapse

Pharyngeal collapse is another commonly encountered cause of obstruction in this population and, like glossoptosis, can be difficult to detect endoscopically. On fluoroscopy, the anterior wall of the pharynx moves posteriorly and the posterior wall moves anteriorly (figure 2). This differs from glossoptosis in which only the tongue moves posteriorly. With pharyngeal collapse, the posterior pharyngeal wall, velum, and tongue oppose each other, causing nasopharyngeal and oropharyngeal obstruction.

Laryngomalacia and tracheomalacia

Laryngomalacia and tracheomalacia are defined as abnormal collapse of the larynx or trachea during breathing secondary to lack of normal structural integrity of the underlying cartilage. With both, the collapse typically occurs during inspiration. Both the larynx and trachea should be relatively still during sleep, and any motion should be considered abnormal. With laryngomalacia, there is inferior indrawing of the pharynx, and the epiglottis buckles and infolds over the tracheal inlet (figure 3), secondary to a lack of adequate cartilaginous support. Tracheomalacia can be focal or diffuse. With tracheomalacia, the trachea cyclically decreases in caliber (figure 4). Typically, the anterior wall of the trachea bows and collapses posteriorly more prominently than the posterior wall moves anteriorly. Tracheomalacia can occur as a primary weakness of the tracheal cartilage or secondary to extrinsic compression, such as by anomalous vascular structures or masses.

Adenoid size

Enlargement of the adenoid tonsils is one of the more common components of obstructive sleep apnea in children. However, determination of what size constitutes an abnormally enlarged adenoid has been a subject of debate. Several studies have addressed the range of normal sizes of the adenoid tissues during childhood. ^15,16 The expected size of the adenoid tonsils changes with age. In newborns, no adenoid tissue may be appreciated at imaging. ¹⁵ There is a rapid proliferation of the adenoid tissues during infancy with a plateau in size varying from 2 to 14 years of age. ¹⁵ The most typical age of maximal size is 7 to 10 years, at which time the adenoid tissues may range from 10 to 15 mm in diameter on a lateral (or sagittal) image. ¹⁶ Beginning in the second decade, the adenoids begin to decrease in size and continue to do so throughout adulthood. ¹⁶

Conclusion

Dynamic sleep fluoroscopy is a useful adjunct to endoscopy in the evaluation of dynamic abnormalities of the airway. This study aids in identifying the actual site of airway obstruction, particularly when there may be multiple potential causes. AR