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Upper airway imaging in obstructive sleep apnea in adults

Upper airway imaging in obstructive sleep apnea in adults
Author:
Richard J Schwab, MD
Section Editors:
Nancy Collop, MD
Nestor L Muller, MD, PhD
Deputy Editor:
Geraldine Finlay, MD
Literature review current through: Dec 2022. | This topic last updated: Jan 03, 2022.

INTRODUCTION — Upper airway imaging is currently not part of the routine diagnostic evaluation for obstructive sleep apnea (OSA) because it can neither confirm nor exclude the disorder. Despite this, upper airway imaging has several important roles:

Upper airway imaging provides information about soft tissue and craniofacial anatomy, which has provided important insights into the pathogenesis of OSA [1-7]. Upper airway imaging can also identify anatomic risk factors for sleep apnea such as enlargement of upper airway soft tissue structures [7], tongue fat [8], and reduction in the size of craniofacial structures [9]. In addition, upper airway imaging can identify ethnic-specific risk factors for sleep apnea [10].

Upper airway imaging can be clinically useful in patients with OSA who are imaged to identify potential sites of upper airway obstruction prior to surgical intervention, although validation of this approach has not been addressed with well-designed clinical trials.

Upper airway imaging may discover pathologic masses or growths that reduce upper airway size, leading to OSA.

The methods of upper airway imaging and the clinical settings in which upper airway imaging may be helpful are reviewed here. The diagnostic evaluation of OSA is described separately. (See "Clinical presentation and diagnosis of obstructive sleep apnea in adults".)

UPPER AIRWAY ANATOMY — The upper airway is a complicated structure that performs several physiologic functions, including respiration, vocalization, and swallowing. It can be divided anatomically into three regions (image 1A-B):

The nasopharynx (the region between the nasal turbinates and the hard palate)

The oropharynx, which can be subdivided into the retropalatal region (from the hard palate to the caudal margin of the soft palate) and the retroglossal region (from caudal margin of the soft palate to the base of the epiglottis) (image 1A)

The hypopharynx (the region from the base of the tongue to the larynx)

The upper airway is smallest in the oropharynx in both normal subjects and patients with OSA, particularly in the retropalatal region [1-7,11]. Airway closure during sleep occurs in the retropalatal region in the majority of patients with OSA [12,13].

IMAGING IN PATHOGENESIS AND MANAGEMENT — Upper airway imaging has provided important insights into the pathogenesis and treatment of sleep apnea.

Insights into the pathogenesis of sleep apnea with upper airway imaging include the following:

Imaging studies have shown that increased volume of upper airway soft tissue structures (in particular the volume of the tongue, lateral pharyngeal walls, and total soft tissue (figure 1)) is an important risk factor for sleep apnea [7,14].

In addition, studies have shown that patients with obesity who have sleep apnea have significantly more fat in their tongue than obese controls [8]. It has been proposed that tongue fat may explain the relationship between obesity and sleep apnea [8].

MRI has confirmed ethnic differences in the upper airway anatomy related to obstructive sleep apnea. For example, Icelandic patients have larger combined upper airway soft tissue volume, while Chinese patients have larger soft palate volume (in males), as well as smaller retropalatal airway areas and more restricted mandibular and maxillary bone structures [10].

Insights into the management of sleep apnea with upper airway imaging:

Weight loss decreases tongue fat (figure 2) [15] and fat pad size [6]. Data from patients that have lost weight indicate [15] that the reduction in tongue fat volume based on MR Dixon imaging was the primary mediator of the relationship between weight loss and AHI improvement.

CPAP increases the upper airway primarily in the lateral dimension by decreasing the thickness of the lateral walls [16].

Examination of the centroids of the soft palate and tongue has provided important information on the mechanism of action of oral appliances [17].

Upper airway imaging studies have provided new insights into the biomechanical changes associated with upper airway surgery including hypoglossal nerve stimulation [18,19].

MRI studies of the upper airway indicate that the primary risk for OSA in adolescents is enlargement of the tonsils and adenoid, not enlargement of the tongue, lateral walls, or soft palate [20].

Dynamic MRI studies during respiration have shown that upper airway caliber is significantly narrower in obese apneics than obese controls in the retropalatal region [21]. Such data provide further evidence that retropalatal airway narrowing plays an important role in the pathogenesis of OSA in patients with obesity.

A systematic review of upper airway imaging studies in patients with sleep apnea found that the most important anatomic finding for the development of OSA was a small minimum cross-sectional area [14].

IMAGING MODALITIES — The ideal upper airway imaging modality would be inexpensive, noninvasive, radiation-free, performed in the supine position during sleep, and dynamic (to visualize apneic events), while providing high-resolution images. Unfortunately, such an ideal modality does not exist and available modalities have both benefits and limitations.

Magnetic resonance imaging (MRI) and nasopharyngoscopy are the best choices among the available options for imaging the upper airway in patients with OSA. Other modalities include cephalometry, computed tomography, acoustic reflection, optical coherence tomography, and ultrasound.

Magnetic resonance imaging — MRI is expensive, but offers numerous advantages:

It is widely available

There is no radiation exposure

Resolution of the soft tissue boundaries of the upper airway is excellent

Upper airway cross-sectional area and volume can be accurately quantified

Sagittal, coronal, and axial images can be obtained (image 1A-B and image 2A-B)

Three-dimensional reconstruction of the airway and soft tissue structures (ie, tongue, soft palate, fat pads, and lateral pharyngeal walls) can be performed (figure 3)

Dynamic imaging can be performed using cine MRI

State-dependent MRI can be done

Dixon imaging can be used to quantify tongue fat [8]

MRI may not be possible if patients have contraindications (eg, class 2 or 3 obesity, claustrophobia, metallic devices, pacemakers). (See "Patient evaluation for metallic or electrical implants, devices, or foreign bodies before magnetic resonance imaging".)

Nasopharyngoscopy and drug-induced sleep endoscopy — Nasopharyngoscopy is widely available and in patients with sleep apnea can evaluate the lumen of the nasal passages, oropharynx, and vocal cords (it cannot visualize the surrounding soft tissue structures). It has been used in numerous studies to evaluate the site of upper airway obstruction, airway luminal changes in patients with OSA, prediction of oral appliance and hypoglossal nerve stimulation outcomes, and the effects of mandibular repositioning devices, weight loss, or uvulopalatopharyngoplasty (UPPP) on airway caliber [22].

Nasopharyngoscopy can be performed with the patient in either the sitting or supine position during wakefulness, spontaneous sleep, or sedative-induced sleep. The latter is typically referred to as drug-induced sleep endoscopy (DISE). Sleep nasopharyngoscopy is thought to be superior to awake nasopharyngoscopy at identifying sites of upper airway closure, but supportive data for this assertion are not robust. Nasopharyngoscopy does not involve radiation exposure, but it is invasive and requires nasal anesthesia.

A Mueller maneuver (ie, inhalation with the nasal passages occluded and the mouth closed) performed during the nasopharyngoscopy may simulate an obstructive apnea and provide insight into the location of upper airway obstruction. Since the Mueller maneuver is effort dependent, both the amount of negative pressure generated during the maneuver and the percentage of airway narrowing during the Mueller maneuver should be quantified in order to compare changes across subjects [23].

In one study that compared DISE with awake nasopharyngoscopy with Mueller maneuver, the degree of obstruction caused by the soft palate and tongue was significantly underestimated by an awake procedure; in addition, 23 percent of patients showed evidence of laryngeal obstruction due to involvement of the epiglottis that was not assessable during wakefulness [24]. In another study, severe retrolingual collapse was identified more often via DISE than the Mueller maneuver (85 versus 36 percent), and the discrepancy was particularly high in individuals with Friedman I and II tongue positions [25]. Studies of the interrater reliability of DISE indicate that the assessment of sites of obstruction is more reliable than the degree of obstruction, and that reliability is better when the procedure is performed by experienced operators [26,27] but large reproducibility studies have not been performed. In addition, state-related changes in the upper airway during DISE should be quantified [28].

DISE is usually performed in the operating room (patients receive propofol), making it difficult to use as an imaging modality in large clinical studies. DISE is also performed in patients undergoing hypoglossal nerve stimulation (HGNS) to determine if they have concentric collapse, which is a contraindication to performing HGNS.

Cephalometry — Cephalometry is a lateral radiograph of the head and neck. It is widely available, easily performed, and inexpensive. However, it requires standardized radiographic equipment and techniques, as well as interpretative skills. In addition, it is performed only while the patient is sitting or standing, and has not been performed when the patient is asleep or supine.

Cephalometry provides two-dimensional evaluation of skeletal and soft tissue structures, providing limited information about anterior-posterior structures and no information about lateral soft tissue structures. It is useful in the evaluation of patients with craniofacial abnormalities, such as retrognathia, and in the evaluation of oral appliances. (See 'Oral appliances' below.)

Computed tomography — Computed tomography (CT) is a widely available but is relatively expensive. Radiation exposure limits its usefulness for performing studies during wakefulness and sleep. Cone beam CT has less radiation than a standard CT scanner but the resolution is also lower.

CT permits accurate determination of upper airway cross-sectional area and volume, with excellent resolution of the airway and bony structures. Spiral (ie, helical) CT provides direct three-dimensional volumetric reconstruction of images, allowing reconstruction of bony structures (cranium, mandible, hyoid) and the airway (image 3). CT is also useful in the evaluation of patients with sleep apnea who are being considered for bony manipulations (dental appliances and maxillomandibular advancement). Dynamic imaging can also be performed with CT scanning.

Acoustic reflection — Acoustic reflection is a technique that allows measurement of airway caliber on the basis of reflected sound waves. It can be performed through the mouth or the nose. The advantages of acoustic reflection are that it is noninvasive, has no associated radiation, and can be readily repeated. However, it is performed in the sitting position and does not provide high-resolution anatomical representation of the airway or soft tissue structures. In addition, if it is performed through a mouthpiece the opening of the mouth will alter upper airway anatomy. Acoustic reflection has been used primarily as a research tool and its clinical utility has not been carefully assessed.

Optical coherence tomography — Optical coherence tomography is a technique for real-time imaging of the upper airway during wakefulness and sleep [29]. It involves placement of a thin, transparent catheter through the nares into the upper airway, through which an optical probe can move freely. As the probe moves along the length of the upper airway, it creates images based upon changes in the characteristics of the light reflected back from the tissues.

Optical coherence tomography avoids exposure to radiation and can be performed without sedation. The major limitation is that tissues can be hidden from the optical probes in patients with a very irregular upper airway. In addition, optical coherence tomography does not directly examine the surrounding deeper soft tissue structures. Evidence suggests that optical coherence tomography correlates well with CT scanning [29].

ULTRASOUND — Ultrasound uses sound reflections to create images. Ultrasound has been used primarily to examine the movement of structures in the upper airway during speech and swallowing [30]. Ultrasound provides a method to quantify the size and motion of the tongue [31-33]. Ultrasound shear wave elastography can also be used to examine biomechanical tissue properties of the tongue [34]. In addition, ultrasound has been shown to be able to measure tongue fat [35].

INDICATIONS — Upper airway imaging is not indicated in the routine evaluation and management of most patients with OSA. It may be helpful in certain circumstances, such as the planning of upper airway surgery or determining why upper airway surgery was ineffective. However, validation of these approaches has not been addressed in well-designed clinical trials.

Uvulopalatopharyngoplasty (UPPP) — UPPP is the most common surgical procedure for patients with OSA [36]. The success rate is related to the site of obstruction, with patients demonstrating retropalatal obstruction having better results than those with retroglossal obstruction [36]. UPPP is discussed in more detail elsewhere. (See "Management of obstructive sleep apnea in adults".)

In our practice, we consider performing magnetic resonance imaging (MRI) of the upper airway prior to UPPP since it can identify patients with retroglossal collapse who are not ideal candidates for this type of surgery. Magnetic resonance imaging (MRI) can also provide a baseline anatomic assessment that will allow for a comparison to a second study if the UPPP is ineffective in abolishing the sleep apnea. Whether outcome is improved among those patients selected for UPPP based upon the results of MRI has not been studied. (See 'Magnetic resonance imaging' above.)

Alternatively, it is reasonable to perform drug-induced sleep endoscopy prior to upper airway surgery to try to determine the site of upper airway collapse [28]. (See 'Nasopharyngoscopy and drug-induced sleep endoscopy' above.)

For patients in whom MRI demonstrates primarily retroglossal narrowing or in whom drug-induced sleep endoscopy demonstrates retroglossal collapse, we consider surgery directed at advancing the tongue (eg, geniohyoid advancement, transoral robotic surgery of the base of the tongue or maxillomandibular advancement) in addition to UPPP.

Maxillomandibular advancement — In our practice, we consider performing computed tomography (CT) prior to maxillomandibular advancement, a surgical procedure that moves the top of the jaw (maxilla) and the bottom of the jaw (mandible) forward. CT is the preferred imaging modality because it provides excellent three-dimensional image reconstructions of the airway and craniofacial skeleton (image 3). (See 'Computed tomography' above.)

Cephalometry is an alternative if three-dimensional CT is unavailable [37]. However, it is a second-line modality because it is two-dimensional and provides information only about anterior-posterior structures and not lateral structures. Cephalometry has successfully identified soft tissue and bony abnormalities in patients with OSA [37]. (See 'Cephalometry' above.)

Oral appliances — Cephalometry provides information about the posterior airway space, retrognathia, micrognathia, hyoid position, mandibular position, tongue size, and soft palate size. Cephalometry may be considered for some patients prior to constructing an oral (dental) appliance. However, cephalometry is not necessary for most patients who are going to be treated with an oral appliance. In particularly complicated cases, three-dimensional CT or cone beam CT may better evaluate relationships between the airway and bony skeleton than cephalometry. (See 'Cephalometry' above and 'Computed tomography' above.)

The role of oral appliances in the treatment of OSA is discussed in detail elsewhere. (See "Oral appliances in the treatment of obstructive sleep apnea in adults" and "Management of obstructive sleep apnea in adults".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Sleep-related breathing disorders in adults".)

SUMMARY AND RECOMMENDATIONS

Upper airway imaging is not yet part of the routine diagnostic evaluation for obstructive sleep apnea (OSA) because it can neither confirm nor exclude the disorder. However, we have found imaging to be clinically useful in the planning of upper airway surgery, although validation of this approach has not been addressed with well-performed clinical trials. (See 'Introduction' above and 'Indications' above.)

Upper airway imaging has also provided important insights into the pathogenesis and management of patients with sleep apnea. (See 'Imaging in pathogenesis and management' above.)

Magnetic resonance imaging (MRI) and nasopharyngoscopy (including drug-induced sleep endoscopy) are the best choices among the available options for imaging the upper airway in patients with OSA. Other modalities include cephalometry, computed tomography, acoustic reflection, optical coherence tomography, and ultrasound. (See 'Imaging modalities' above.)

MRI is one of the preferred imaging modalities because upper airway soft tissue resolution is excellent, dynamic imaging can be performed, tongue fat can be measured, and there is no radiation exposure. In addition, it is widely available and both the cross-sectional area and volume of the upper airway can be accurately quantified. (See 'Magnetic resonance imaging' above.)

Nasopharyngoscopy is a widely available and an easy way to evaluate the lumen of the nasal passages, oropharynx, and vocal cords. It can be performed during wakefulness, spontaneous sleep, or sedative-induced sleep, with the patient in either the sitting or supine position. Nasopharyngoscopy does not involve radiation exposure, but it is invasive and requires nasal anesthesia. Drug-induced sleep endoscopy (DISE) should be considered in patients undergoing upper airway surgery in which the site of airway obstruction needs to be determined. DISE is also indicated in patients undergoing hypoglossal nerve stimulation. (See 'Nasopharyngoscopy and drug-induced sleep endoscopy' above.)

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