Imaging the obstructed Airway:

Introduction: The structure and the neural control of the upper airway have evolved to serve four important physiological functions: 1) respiration 2) deglutition 3) speech and 4) local immunity. The upper airway is collapsible in order to accommodate these functions. During wakefulness, upper airway collapse can be prevented by an increase in pharyngeal neuromuscular tone (1). However, this mechanism is decreased during sleep, predisposing the upper airway to obstruction (2). The obstructive sleep apnea syndrome (OSAS) refers to a breathing disorder characterized by recurrent, partial or complete episodes of upper airway obstruction, commonly associated with intermittent hypoxemia and sleep fragmentation (3). OSAS affects individuals of all ages, from neonates to the elderly. However, it is still not known whether OSAS represents a continuum of a disorder that places pediatric patients at risk for the disease as adults (4), or whether OSAS during different stages of life comprises distinct clinical entities (5-8). The anatomic factors predisposing to OSAS differ over the lifespan. However, a smaller upper airway is noted in patients with OSAS in all age groups, and probably predisposes to airway collapse during sleep. Despite the known anatomic factors, such as craniofacial anomalies, obesity, and adenotonsillar hypertrophy, that contribute to OSAS throughout life, a clear anatomic factor cannot always be identified. This suggests that alterations in upper airway neuromotor tone also play an important role in the etiology of OSAS. The present chapter will focus on the known anatomic risk factors leading to OSAS during child development, with emphasis on studies using magnetic resonance imaging (MRI) that provide the most quantitative and reproducible data. In preschool children, the incidence of OSAS is estimated to be 2% (9, 10), whereas primary snoring is more common and is estimated as 6-9% in school-aged children (11). Although the exact mechanism for OSAS in children is not fully understood, important anatomic risk factors have been identified, and are linked to the anatomical structures surrounding the airway that affect the size and shape of the airway. The Waldeyer's ring, which is the lymphoid immunocompetent tissue within the upper airway, is comprised of the pharyngeal tonsil or adenoid, the paired palatine tonsils, and the lingual tonsil. These tend to enlarge during childhood in response to somatic growth (12-14) and are a potential focus for infection and inflamation (15). Therefore, in this age group, in the absence of obesity and when no apparent craniofacial anomalies or neurological disorders exist, adenotonsillar hypertrophy is considered the most significant anatomic risk factor for OSAS.