Airway Evaluation in Obstructive Sleep Apnea

3.1

Techniques of the Maneuver

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. Because of its simplicity, according to Sher et al., the classification has been widely used to describe the finding obtained during the maneuver. 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, multichannel 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 280 mbar without any signs of pharyngeal collapse. 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.

3.3

Significance of the Mueller Maneuver

The Mueller maneuver is a safe and simple examination that does not exert relevant strain on the patient. The reliability of the maneuver is insufficient, and the results cannot be transferred to natural sleep. A hypopharyngeal collapse may indicate the exclusion of patients from UPPP. Altogether, the Mueller maneuver does not facilitate patient selection for the varying surgical interventions used in OSA patients.

4.1

Providing Insights Into the Pathophysiology of SDB

Extensive literature is available comparing upper airway anatomy and dentofacial structures using x-ray cephalometry 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. Further differences were described by different working groups. 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 midfacial heights, and differences in hyoid bone position. In general, the differences are more pronounced in nonobese 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 nonobese 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. 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.

4.2

X-Ray Cephalometry and Therapeutic Interventions

One of the dominant indications for performing x-ray cephalometry has been treatment with oral appliances. Especially with regard to the evaluation of potential predictive parameters for treatment success and dental side effects, x-ray cephalometry has been the standard diagnostic tool. As early as 1995, Mayer and Meier-Ewert, two of the fathers of treatment with oral appliances in Europe, looked for cephalometric predictors of treatment success and reported that specific cephalometric variables were indeed predictive for the therapeutic effect. Other authors have confirmed the existence of predictive cephalometric parameters, especially in relation to hyoid bone position and oropharyngeal airway dimension. Nevertheless, the problems related to different nomenclature and selection of airway parameters described earlier remain.

X-ray cephalometry has also been evaluated with regard to potential predictive parameters for postoperative results of UPPP alone or in combination with other approaches. To date, there is no convincing evidence that skeletal measurements obtained with x-ray cephalometry could predict the outcome of UPPP. Nevertheless, lateral x-ray cephalometry is the standard tool in the preoperative evaluation of the craniofacial skeletal anatomy before maxillomandibular advancement surgery. Its value is not questioned.

4.3

X-Ray Cephalometry in Patient Management

X-ray cephalometry has provided substantial insights into the pathophysiology of OSA, demonstrating significant craniofacial characteristics associated with this disease. Although the results are not easy to compare, specific cephalometric characteristics have been repeatedly mentioned as a risk factor for OSA and correlate with the severity of the disease. Selected cephalometric parameters indicate favorable results of mandibular advancement by oral appliances. Nevertheless, no cephalometric parameter exists that would reliably rule out treatment success with an oral appliance, and surgical outcome cannot be predicted. This may explain why x-ray cephalometry has not become a routine procedure in the diagnostic workup of OSA as long as maxillomandibular surgery is not planned.

5.1

Techniques and Standards

Despite its widespread use in airway assessment in patients with SDB, no standardized scanning protocol exists for this indication, and the nomenclature of the soft tissue structures is not uniform. In addition to a two-dimensional assessment of the upper airway, three-dimensional techniques were used to assess volumes of soft tissue structures and airway spaces. As early as 1987, ultrafast or dynamic CT was inaugurated in this field to evaluate dynamic changes of the upper airway dimensions during respiratory cycles. Whereas the vast majority used CT imaging during wakefulness, several authors also used scanning protocols under hypnotic relaxation, sleep, and sleep during apneas, and also used direct comparisons between wakefulness and sleep.

5.2

Providing Insights Into the Pathophysiology of SDB

The majority of published data points to potential differences in upper airway structures and dimensions between OSA patients and healthy controls or snorers. In general, the upper airway is described as smaller in apneic patients compared with 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 with controls, and larger tongue and soft palate 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 MRI.

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 BMI 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 with 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 nasopharyngeal and oropharyngeal airways were smaller in OSA patients compared with 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, with the velopharyngeal segment being the narrowest and most collapsible region. These results were essentially confirmed later, showing that patients with severe OSA have significantly narrower cross-sectional areas at the velopharyngeal level ( Fig. 3.1 ).

5.3