Armstrong et al. (12) utilized an endoscopic optical technique (optical coherence tomography) that generates quantitative, real-time images of the upper airway that enables accurate determination of shape and size. Optical coherence tomography involves the insertion into the nares of an optical probe (3 mm diameter) that is housed in a catheter. Rotation of this probe within the catheter provides a 360-degree profile of surrounding tissue, and longitudinal movement allows the upper airway to be scanned at multiple sites without irritation of the airway mucosa thereby avoiding waking a potentially sleeping subject. A similar technique has been used to examine microscopic tissue anatomy in the specialties of ophthalmology, dermatology, vascular medicine, gastroenterology, and urology. Simultaneous optical coherence tomography and CT images of the upper airway in five healthy subjects at three different levels of the airway showed comparable airway dimensions within 0.8 mm of each on average. The cross-sectional area derived from optical coherence tomography was on average 14.1 mm2 smaller than CT-derived images. The authors describe their validation of this technique with CT in five healthy subjects (12). Intraobserver and interobserver variability was assessed and the correlation coefficients were stated to be 0.99 and 0.99, respectively. In addition to the capacity to continuously measure changes in the airway dimensions under a variety of conditions, other advantages cited by the group using optical coherence tomography include patient comfort, minimal effect on sleep quality or architecture, and lack of radiation. Early shortcomings appear to be the limited capacity to view the complete circumference of the airway at all sites in all subjects and the inability to track changes in airway caliber when breathing is rapid (12). In addition, this imaging modality examines the airway lumen; it cannot evaluate craniofacial or soft tissue structures that are not adjacent to the airway.
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