Airway evaluation in obstructive sleep apnea

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Chapter 3 Airway evaluation in obstructive sleep apnea

2 CLINICAL EXAMINATION AND CLINICALSCORES

A clinical examination including an endoscopy of the upper airway during wakefulness still constitutes the basis of every airway evaluation in snorers and obstructive sleep apnea (OSA) patients. Anatomic and static clinical findings were the first parameters to be evaluated in order to improve treatment success. The impact of enlarged palatine tonsils became evident in the surgical experiences with children. If performed simultaneously, tonsillectomy was described by most authors as a positive predictive factor for a successful uvulopalatopharyngoplasty (UPPP). All the other anatomic parameters such as the size of the uvula, the existence of longitudinal pharyngeal folds and so forth did not show any relationship to the success rate of UPPP if evaluated separately. In contrast to the significant influence of enlarged tonsils in palatal obstruction, equivalent clinical finding for tongue base obstructions could not be detected. Woodson and Wooten only found hints that the oropharynx was normal in cases with retrolingual obstruction.1

Aware of this dilemma, Friedman et al. developed a clinical four degree staging system incorporating the tonsil size, the position of the soft palate, the tongue size, and the Body Mass Index (BMI).2 This anatomic staging system predicted the success rate better than OSA severity did.3 One may argue that the staging system merely reflects the clinical examination of an experienced sleep physician; nevertheless, such a system may be particularly helpful for less experienced observers.

Whether there are further predictive anatomic para-meters for other surgical strategies has not been evaluated to date. The subjectivity of the assessment and the variability of the nomenclature of the clinical findings are significant limitations in this context.

3 THE MUELLER MANEUVER

Snoring as well as apneas can be simulated by most people and a direct effect of the Mueller Maneuver may be seen during wakefulness. Thus, snoring simulation and the effects of the Mueller Maneuver have been used in upper airway evaluation before surgical intervention in patients to predict surgical outcome and to improve patient selection. Nevertheless, the value of this relatively simple examination has been questioned repeatedly in the past.

3.1 TECHNIQUES OF THE MANEUVER

In order to be able to compare results between different investigators and patients as well as before and after an intervention, the Maneuver should be performed and documented in a standardized fashion. Due to its simplicity the classification according to Sher has been widely used to describe the finding obtained during the Maneuver.3 In this classification, four degrees of airway obstruction at the different levels are defined, ranging from minimal to complete occlusion. Furthermore, any visible obstruction linked to the epiglottis is described. The reproducibility and inter-rater reliability of the results remain problematic. Taking all the available data into account, the reliability of the Mueller Maneuver remains highly questionable and the evaluation of the Maneuver seems highly subjective and hard to reproduce.

3.2 PREDICTING AIRWAY OBSTRUCTION DURING SLEEP AND SURGICAL SUCCESS

There is some evidence that the sites of obstruction detected with the Mueller Maneuver do not reliably reflect the sites of obstruction during sleep. This could be demonstrated through a comparison with videoendoscopy, multi-channel pressure recordings, and dynamic MRI during sleep. Table 3.1 shows the different sites of airway obstruction detected with the different methods of airway evaluation according to selected examples from the literature.

The impact of body position on the significance of the Mueller Maneuver remains unclear. During the Mueller Maneuver, healthy subjects may produce extreme negative pressures of −80 mbar without any signs of pharyngeal collapse.4 This clearly demonstrates the significant differences in upper airway collapsibility during wakefulness and sleep. All the data given do not support the idea that the results obtained by the Mueller Maneuver may be transferred to natural sleep.

Various research groups were not able to better predict the success rates obtained with UPPP when using the Mueller Maneuver. Some authors considered an additional retrolingual collapse during the Mueller Maneuver as an exclusion criterion for a UPPP or performed a partial resection of the epiglottis in UPPP failure patients with laryngeal obstruction during the Mueller Maneuver by partial resection of the epiglottis.

4 X-RAY CEPHALOMETRY

Over the years, lateral x-ray cephalometry has become one of the standard diagnostic tools in patients with SDB, especially with regard to the evaluation of the skeletal craniofacial morphology. Not specifically developed for the fields of SDB, imaging techniques and standards for data analysis have been incorporated from the field of maxillofacial surgery, where it has already been used for decades.

4.1 PROVIDING INSIGHTS INTO THEPATHOPHYSIOLOGY OF SDB

Extensive literature is available comparing upper airway anatomy and dentofacial structures using x-ray cephalo-metry between OSA patients and healthy controls. In siblings a significantly longer distance from the hyoid bone to the mandibular plane has been documented in those affected by SDB.5 Further differences were described by different working gropus. The concrete results are often difficult to compare, as the authors not only use different landmarks and parameters but also sometimes rather complex calculated indices and ratios to describe the differences they found. Therefore, the following findings in OSA patients can only be a selection: longer soft palates, reduced minimum palatal airway widths, increased thickness of the soft palate, differences in calculated craniofacial scores, increased pharyngeal lengths, retroposition of the mandible or the maxilla, micrognathia, increased mid-facial heights, and differences in hyoid bone position. In general, the differences are more pronounced in non-obese patients, suggesting that craniofacial changes play a dominant role in this subgroup. Furthermore, substantial differences in maxillofacial appearance of different ethnic groups need to be taken into account.

Various authors could demonstrate that the aberrations in craniofacial morphology they found in OSA patients were more pronounced in patients with severe OSA. Dempsey et al. demonstrated that in non-obese patients and in patients with narrow upper airway dimensions, four cephalometric dimensions were the dominant predictors of Apnea/Hypopnea Index (AHI) level, accounting for 50% of the variance.6 Rose et al. questioned the diagnostic relevance of x-ray cephalometry for OSA, as they found no direct correlation between skeletal cephalometric findings and OSA severity; nevertheless, they also reported a correlation with hyoid bone position.7

5 CT SCANNING

Compared to lateral x-ray cephalometry, CT scanning significantly improves soft tissue contrast and allows precise measurements of cross-sectional areas at different levels as well as three-dimensional reconstruction and volumetric assessment. Fast scanning times and relatively quiet scanning conditions even allow a dynamic assessment of the airway during a respiratory cycle as well as measurements during natural sleep. Nevertheless, ionizing radiation remains problematic.

Providing insights into thepathophysiology of SDB

The majority of published data points to potential differences in upper airway structures and dimension between OSA patients and healthy controls or snorers. In general, the upper airway is described as smaller in apneic patients compared to controls, especially with regard to the retropalatal region. Cross-sectional areas were found to be significantly narrower in affected patients. Inversely, retropalatal tissue was described as being greater in OSA patients compared to controls and larger tongue and soft palates dimensions and volumes were found. Schwab et al. have pointed out the differences in upper airway configuration with an anterior–posterior configuration – a result that is in line with data obtained from magnetic resonance imaging.10

Different authors have described anatomic conditions that reflect the severity of the disease and have correlated their measurements with polysomnographic data. A high apnea index seems to be associated with large tongue and soft palate volumes, and a significant correlation of the retropalatal space and its lateral diameter with the Respiratory Disturbance Index was documented. A combination of the smallest cross-sectional area, the upper airway resistance, and the Body Mass Index was used to predict the severity of OSA and a narrower cross-sectional area and a thicker soft palate were found in severely affected patients compared to patients with only mild to moderate OSA.

With the help of dynamic and ultrafast CT further insights into airway obstruction were gained. In addition to the fact that the naso- and oropharyngeal airways were smaller in OSA patients compared to weight-matched controls, an increased collapsibility in affected patients was found. During a respiratory cycle, substantial changes in cross-sectional areas were seen in patients with SDB, the velopharyngeal segment being the narrowest and most collapsible region.11 These results were essentially confirmed later, showing that patients with severe OSA have significantly narrower cross-sectional areas at the velopharyngeal level12 (see Figure 3.1).

5.4 CT SCANNING IN PATIENT MANAGEMENT

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