Acoustic Reflection: Review and Clinical Applications for ... - Sleep Group Solutions · 2014. 12. 1. · files aligned at the vocal cords, demonstrating no significant difference

REVIEW ARTICLE

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Sleep and Breathing, volume 6, number 3, 2002. Address for correspondence and reprint requests: John S. Viviano, D.D.S.,SnoreSolutions.com, 4099 Erin Mills Parkway, Mississauga, Ontario, Canada, L5L 3P9. E-mail: [email protected]. 1Privatepractice, SnoreSolutions.com, Mississauga, Ontario, Canada. Copyright © 2002 by Thieme Medical Publishers, Inc., 333 Seventh Avenue,New York, NY 10001, USA. Tel: +1(212) 584-4662. 1520-9512,p;2002,06,03,129,150,ftx,en;sbr00213x.

Acoustic Reflection: Review and Clinical Applications for Sleep-Disordered BreathingJohn S. Viviano, D.D.S.1

ABSTRACT

Sleep-disordered breathing (SDB) affects more than 4% of the adultpopulation with an even higher prevalence within high-risk groups. Nasal con-tinuous positive air pressure, although considered the current gold standard treat-ment for SDB, demonstrates poor patient compliance. Alternative therapies,such as palatal surgeries and airway orthotics, lack validated candidacy selectionprotocols, resulting in varying success rates. Although much has been publishedover the last several years regarding the effect of these therapies on the upper air-way, no publication has presented an accounting of the use of acoustic reflection(AR) to evaluate airway characteristics pre- and post-treatment with these alter-native therapies. This article will review AR and our current knowledge base ofthe pathological airway characteristics that can be assessed through AR. It willinclude the advantages, limitations, and potential clinical usefulness of this diag-nostic modality in the treatment of patients with SDB.

KEYWORDS: Nasal patency, pharyngeal patency, acoustic reflection, palatalsurgery, airway orthotic, obstructive sleep apnea

creased mortality and morbidity. The cost of sleep-related accidents has been estimated to range from$43.15 to $56.02 billon per year.6 Although nasalcontinuous positive airway pressure (nCPAP), thecurrent gold standard of treatment, is highly effec-tive and safe, its long-term compliance rate has beenreported to be 50 to 80%.7 Alternative therapies,

Sleep-disordered breathing (SDB) affectsmore than 4% of the adult population1,2 with cer-tain groups, such as commercial truck drivers,demonstrating a much higher prevalence.3 Moder-ate to severe levels of obstructive sleep apnea (OSA),a component of SDB, are associated with cardio-vascular diseases,1,4 daytime somnolence,5 and in-

130 SLEEP AND BREATHING/VOLUME 6, NUMBER 3 2002

such as palatal surgical procedures and airway or-thotics, successfully increase airway caliber8–12 butreport a variable success rate.13–18 In addition, theylack validated selection protocols that triage pa-tients to the therapeutic approach ensuring thebest outcome.

This article will review acoustic reflection(AR), first described over 25 years ago,19 and howit relates to our knowledge of the airway dynamicsassociated with SDB and their therapies, much ofwhich we have learned in only the last severalyears. Advantages, limitations, and potential clini-cal usefulness of AR will also be discussed.

ACOUSTIC REFLECTION

AR technology provides an objective measurementof the nasal (rhinometry) and pharyngeal (pharyn-gometry) cavities. Many papers have established

its accuracy,19–27 reproducibility,21,25,28–30and appli-cation.31–35 A technical explanation of the modusoperandi of the Eccovision Rhinometer and Pharyn-gometer™ (E. Benson Hood Labs, Pembroke, MA)can be found in various publications.19–21,35 Thesecitations along with the manufacturer’s manual36

adequately review the technology in general and itstechnique of use. Therefore, we will limit our dis-cussion in this regard to current and supplementalfindings (Fig. 1).

Rhinometry

Accuracy Acoustically derived measurements ofnasal anatomy have compared favorably to thosederived by both computerized tomography (CT)23,37

and magnetic resonance imaging (MRI).24,38 Stud-ies evaluating their accuracy and usefulness incomparison to rhinomanometry22,34 have also com-pared favorably and have demonstrated superiorityin that AR has the ability to precisely localize andquantify the most resistive area of the nasal passageand is not dependent on nasal airflow; this allowsfor its use in severely constricted nasal airways.However, acoustic and CT-derived measurementsrelate more favorably in the anterior nasal cav-ity,23,37 indicating that acoustic measurements be-yond the second notch may be of limited clinicalsignificance.37

Reproducibility Lenders and Pirsig34 investigated255 subjects and established the standard of de-viation of repeated measurements of mean cross-sectional area, in the same patient, under constantconditions, to be consistently less than 7%.

Normative Standards Shemen and Hamburg33

demonstrated the utility of AR for evaluating nasalpatency before and after nasal septal surgery, demon-strating a 25% improvement in resistance, a 21%improvement in nasal volume, and a 15% improve-ment in minimal cross-sectional area postsurgery.They also established postoperative norms under

Figure 1 (1) Rhinometer wave tube and nosepiece; (2)Pharyngometer wave tube and mouthpiece. Sound gen-erated by wave tubes is reflected in the airway and thenrecorded by microphones in the wave tubes, thenprocessed by the CPU. The monitor displays both cross-sectional area and distance along the airway.

ACOUSTIC REFLECTION AND SDB/VIVIANO 131

the assumption that patients demonstrating a patentnasal cavity clinically and physically after surgerywould be ideal for determining these values.

In another study, Corey et al30 investigated106 healthy adult subjects and established nasalcaliber norms and a standard rhinometry curveuseful in the evaluation of nasal patency. All mea-surements were made before and after applicationof topical nasal decongestant to eliminate the ef-fects of the nasal cycle. Data analysis consideredrace, sex, height, and weight.

Pharyngometry

Accuracy The accuracy of pharyngometry-derivedmeasurements has been evaluated extensively inthe literature.19,20,25,26 Due to the influences thatlung volume, body posture, and phase of respirationhave on the pharyngeal structures, these early stud-ies evaluated only the trachea in human subjects,demonstrating that acoustic derived measurementscompared favorably to measurements obtainedthrough both anterior/lateral chest radiographsand CT. In a subsequent study, D’Urzo and associ-ates27 demonstrated excellent agreement betweenglottic area measurements of pharyngeal cross-sectional area derived acoustically (1.8 ± 0.8 cm2)as compared to those obtained through CT (1.7 ±0.9cm2). All of these early studies followed a proto-col first described by Fredberg et al,20 which in-volved the use of helium/oxygen gas rather thanroom air in order to reduce the effect of nonidealairway wall characteristics on accuracy outcome.

More recently, Marshall et al21 acousticallyevaluated the pharyngeal dimensions of 10 normalhuman subjects using room air rather than helium/oxygen gas and compared these measurements tothose obtained through MRI. They superimposedthe acoustic and MRI-derived pharyngeal area pro-files aligned at the vocal cords, demonstrating nosignificant difference in pharyngeal and glottalcross-sectional areas. Due to the “smoothing” ef-fect and resulting loss in anatomical detail associ-

ated with AR, a 35% underestimation of maximalhypopharyngeal area was observed. However, thiswas the only measure that was statistically differentbetween the two modalities. Minimal areas at theoropharyngeal junction and the glottis were similar(within 20%) and estimates of pharyngeal volumealso showed agreement (within 10%). Integratingover the whole airway appeared to minimize theeffect of “smoothing” and increased agreement inmeasurements between these two modalities.

Reproducibility In an early study utilizing helium/oxygen gas, Brooks and colleagues25 determinedthat pharyngometry intrasubject, day-to-day co-efficient of variation (CV) (9 ± 4%) is similar towithin-run measurements performed on the sameday (10 ± 4%) and similar to variability experiencedwith conventional measurements of pulmonaryfunction. In a later study evaluating mean pharyn-geal area, also using helium/gas, Brooks and associ-ates29 demonstrated a within-run CV of 8 ± 4%.They also compared acoustic measurements takenwith a custom-made mouth piece versus a rubberpulmonary function mouthpiece, demonstratingdifferences between the mouthpieces but an excel-lent CV within 10% for the rubber mouthpiece,which is similar to the mouthpiece in use today.Using room air, Marshall et al.21 found equally fa-vorable results: same-day within-run CV for oneindividual and in 10 naïve subjects was 10%, andday-to-day CV in 5 subjects over 21 days was 13 ±3% at the oropharyngeal area minimum and 11 ±3% at the hypopharyngeal maximum and glottalminimum areas. Using the most current version ofthe pharyngometer, which uses room air, Kamal28

recently investigated 350 normal adult subjects andestablished a CV for intrasubject pharyngeal vol-ume of 5 to 7%.

Normative Standards Kamal’s28 investigation of350 normal adult subjects documented sex-specificnorms for airway caliber at various pharyngeal sitesand a standard normal pharyngometry curve.


Limitations in Comparing Measurement

Modalities

There are inherent problems regarding comparingacoustic measurements to other measurement tech-niques. Although AR provides an instantaneousmeasurement of the airway, MRI requires severalminutes, requiring data to be averaged over manybreaths including changes that occur with the res-piratory cycle. MRI also has its inherent drawbacks.Relative to AR, MRI is expensive, time-consuming,and has a level of uncertainty in identifying airwaywalls in cases of poor image quality. Also, technicallimitations in accurately establishing the exact lo-cation of the airway wall on the image at a “pixellevel of contrast” leads to an intrinsic uncertainty inarea estimation, resulting in an irreducible area ofuncertainty21 of ± 19% for an area of 1 cm2 and± 14% for an area of 2 cm2.

Comparison to the CT technique is also notwithout problems. Although CT provides accuratetransverse images free of distortion, inaccuraciescan arise from the airway not being perpendicularto the plane of the scanner, improper selection ofthe window width and level, and inaccuracies indetermining the air-tissue interface.26 Finally, bothCT and MRI are conducted in the supine position,the importance of which will be discussed below.

Limitations of Acoustic Technology

Although studies have demonstrated a strong cor-relation between measurements derived by rhi-nometry, pharyngometry, and other modalities suchas MRI21,24,38 and CT23,26,27,37, unlike these ana-tomic imaging modalities, the acoustic rhinometerand pharyngometer are physiological tools, limitedto providing a measurement of cross-sectional areaaccording to distance along the airway; they do notprovide high resolution of anatomic representationor soft-tissue structures. Therefore, there are limi-tations to each of the studies on AR that relate tospecific anatomical regions. Acoustic derived cross-

sectional measurements are used to establish vol-ume, mean cross-section, and local area maximumsand minimums with pharyngometry, and volume,local area minimums, and a substantially underesti-mated physiological flow resistance with rhinome-try. Notwithstanding this technology’s inability toprovide high-resolution anatomic representation ofthe airway and soft-tissue structures, the favorablecorrelation between measurements derived throughAR, CT, and MRI, and the noninvasiveness, lowcost, and ease of use of AR make it an ideal physio-logical tool useful in contributing to our under-standing of both structural and functional charac-teristics of the airway in health and disease.

Awake versus Sleeping Airway

One of the most challenging aspects of investigat-ing issues pertaining to sleep is that some data arevery difficult to obtain during the sleep state, andin many instances data collected during wakeful-ness are of questionable value. AR performed dur-ing wakefulness has been criticized regarding itsmeaningfulness once the patient falls asleep.

Using the acoustic technique, Rivlin et al39

demonstrated that patients with OSA have asmaller pharyngeal area than controls. They alsofound a significant correlation between the numberof apneas per hour of sleep and the pharyngealcross-sectional area during wakefulness. One pa-tient in this particular study experienced a remark-able decrease in apnea and increase in pharyngealcross-sectional area after a drastic weight loss of 68kg. Many other studies have demonstrated a signif-icant relationship between acoustic pharyngealmeasurements of the awake airway and characteris-tics of that airway during sleep.21,40–45

The existence of a relationship between theawake and sleeping airway has also been demon-strated using other techniques. Suratt et al46 mea-sured nasopharyngeal resistance in the awake airwayand found that it correlated significantly with boththe apnea index and number of desaturation epi-


sodes observed per hour of sleep. They demon-strated that the number of apneic events and arterialoxygen desaturations increased with increasing flowresistance in the upper airway during wakefulness.

Malhotra and associates47 recently publisheda study evaluating two collapsibility measurementtechniques in normal and apneic subjects duringwakefulness and sleep: negative pressure pulses(NPPs) and inspiratory resistive loading. Theirfindings revealed (1) a significant correlation be-tween these two measures of collapsibility, and that(2) collapsibility during wakefulness as measuredby NPPs correlated significantly with collapsibilityduring sleep. Although both controls and apneicsdemonstrated a significant increase in pharyngealcollapsibility during sleep as compared to wakeful-ness, apneics were found to demonstrate signifi-cantly greater pharyngeal collapsibility than controlswhile awake. The authors suggested that “upper-airway collapsibility measured during wakefulness doesprovide useful physiologic information about pharyn-geal mechanics during sleep and demonstrates cleardifferences between individuals with and withoutsleep apnea.”

Exam Technique

An adequate description of the exam techniques forboth rhinometry and pharyngometry can be foundin the manufacturer’s manual36 for the devices. Thefollowing is supplemental information intended tovalidate and further clarify that information.

Pharyngometry Readings Using nasopharyn-goscopy in the sleeping patient, it has been demon-strated that there is a progressive reduction in end-expiratory cross-sectional pharyngeal area prior toobstructive apnea.48 Since end-expiration is the pointat which the pharyngeal airway is most compro-mised and susceptible to suction collapse, pharyn-gometry acoustic readings are taken at that point.Recording acoustic readings consistently in thismanner helps to ensure reproducibility.

Caliber and Compliance In order to facilitate thereading of this document, an explanation of theterms airway caliber and compliance is warranted.Airway caliber, documented at end-expiration dur-ing regular tidal breathing at functional residual ca-pacity (FRC), is representative of cross-sectionalarea of the airway. Airway compliance is docu-mented at end-expiration following exhalationfrom total lung capacity (TLC) to residual volume(RV). The acoustic reading at RV when comparedto FRC documents airway compliance associatedwith lung volume-related changes and is represen-tative of airway collapsibility.

Supine versus Upright Posture Body position isalso an important factor when evaluating the upperairway. For rhinometry, in order to make com-parisons to the norms established in the uprightposition by Corey and colleagues,30 the exam isconducted upright as per the manufacturer’s in-structions. However, in a study using positionalrhinomanometry to evaluate the importance ofnasal resistance in OSA patients, De Vito et al49

demonstrated that 9 of 36 subjects had normalnasal resistance in the seated position and patho-logical resistance in the supine position, suggestingthat it may be useful to evaluate changes in supinenasal patency as compared to norms established inthe upright position.

In a study evaluating the effect of posture onpharyngeal airway caliber, Martin et al50 demon-strated that sleep apnea hypopnea syndrome (SAHS)patients have a smaller upper airway area at theoropharyngeal junction (OPJ) than either snorersor normal subjects in the seated upright positionwhile awake. However, these same patients dem-onstrated no statistical difference in airway cross-sectional areas in the supine or lateral recumbentpositions. Also, these individuals demonstrated asignificantly smaller decrease in OPJ cross-sectionalarea from the seated to either the supine or lateralrecumbent positions. The authors concluded thatthese findings are compatible with SAHS patientsdefending their upper airway more upon lying


down than do snorers or normal subjects whileawake. Another study designed to evaluate the ef-fect of posture on the upper airway dimensions ofnormal subjects provided supporting results; Janand associates31 demonstrated through AR that thepharyngeal areas of normal awake patients aresmaller when supine than while sitting, and thatthis difference persisted in the lateral recumbentposition.

Further support for the conclusion thatSAHS patients defend their airway more than nor-mal subjects in the supine position can be found instudies evaluating neuromuscular function: genio-glossal electromyographic activation is increased inawake apneics while in semi-recumbent and supinepositions in comparison with normal subjects.51,52 Itis generally accepted that airway dynamics differbetween sleep and wakefulness. Although patientssusceptible to airway collapse demonstrate increasedactivation of these muscles while awake, this com-pensatory muscle response is lost with the loss ofmuscle tonus that accompanies onset of sleep, leav-ing the airway susceptible to collapse. Evaluationof the awake airway must consider these state-dependent muscular differences and the extent towhich they manifest themselves in supine versusupright positions during wakefulness.

The literature demonstrates that SAHS pa-tients differentiate themselves from normal sub-jects more distinctly in the seated upright positionwhile awake. However, to date much research hasbeen conducted in the supine state on awake pa-tients with the belief that the supine awake airwaymore closely resembles the sleeping airway.

Pharyngometry Mouthpiece and Baseline Read-ings Over the years, a variety of mouthpiecedesigns have been utilized with the acousticpharyngometer. The Free FlowTM mouthpiece(SensorMedics Corporation, Yorba Linda, CA),currently in use, is designed to aid in patient tongueorientation, allowing the operator to acquire repro-ducible data with minimal patient compliance. This

mouthpiece has a patented restraint bat that gentlypositions the patient’s tongue downward and awayfrom the wave tube opening, virtually eliminatingproximal tongue interference. In a recent review onupper airway imaging, Schwab53 expressed con-cerns regarding the alteration in upper airway anat-omy that accompanies mouth opening associatedwith use of a mouthpiece, thus potentially compro-mising the ability to compare area calculations ob-tained through AR with other imaging modalities.The Free FlowTM mouthpiece has noncompliantbite-pads which in fact open the vertical dimensionup to several mm, potentially altering upper airwayanatomy as Schwab53 describes. However, this ana-tomical alteration is patient-specific due to the factthat some individuals will experience a verticalopening consistent with their habitual rest posture,which can vary greatly between patients.

The temporomandibular joint allows themandible to assume various postures. Habitual orrest posture of the mandible has been described asthat assumed while muscles that elevate or depressit are at rest, while the subject is in the seated up-right or standing position.54 During rest posture,the mandible is held in a muscular sling with theteeth slightly out of contact (creating space be-tween them referred to as freeway space), usually 1to 3 mm in the anterior region, but as much as 10mm or more.54 This is the posture in which acous-tic baseline readings should ideally be performed;when the orofacial musculature is relaxed and inbalance and in a state of minimal tonic contraction;minimally influencing upper airway anatomy.

The bite-pads on the Free FlowTM mouth-piece are easily reduced by scalpel to a wafer-thincompliant pad approximately 1 mm in thickness.With this reduction, the purpose of this pad is nolonger for the teeth to bite down on, but rather away to retain the tongue positioning restraint batdescribed above. This reduced thickness and result-ing increased compliance allow the pads to passthrough the freeway space, minimally interferingwith the assumption of habitual rest posture of the


mandible. It is then possible to obtain an acousticreading at true habitual rest posture and evaluationof the upper airway, uninfluenced by mouth open-ing and muscle contraction not normally present attrue baseline.

Standard Operating Procedure (SOP) The patientis asked to sit comfortably upright in a straight-backed chair, fixing his or her gaze at a pointstraight ahead. Posture is maintained throughoutthe exam during the breathing exercises, particu-larly head, neck, and shoulder posture. For rhinom-etry, have the patient take a relaxed oral breath,exhale half way, then stop breathing or swallowingduring the test. For pharyngometry, take the read-ing at FRC or RV as described above and have thepatient assume a comfortable habitual rest posturewith the mouthpiece in place. This will avoid anyincrease in muscle tonus associated with heavy oc-clusion on the bite-pads.

NASAL AIRWAY PATENCY

Although the relationship between nasal obstruc-tion and SDB is poorly understood, evidence of thisrelationship can be found throughout the literature.

Effect of Nasal Obstruction

Daytime nasal obstruction has been demonstratedto be an independent risk factor for obstructivesleep apnea syndrome (OSAS) .55 Numerous stud-ies involving the creation of an artificial nasal ob-struction have demonstrated the creation of poly-somnography-verified obstructive apneic events innormal subjects.56–59 A population study involvingclose to 5000 subjects found that patients witha symptomatic nasal obstruction were statisticallymore likely to report snoring and chronic daytimesomnolence.60 Taking the opposite approach, Rubin

et al61 demonstrated improvement in daytime som-nolence, sleep quality, and polysomnography-veri-fied apneic events after surgical correction of nasalobstruction. Finally, Loth and Petrusen62 demon-strated a remarkable reduction in snoring and morn-ing tiredness through dilation of the nasal valve re-gion with a nostril dilator in snorers.

Notwithstanding these citations, the exis-tence of a relationship between nasal obstructionand SDB has also been refuted in the literature. Ina study evaluating the relationship between snoringand nasal resistance during sleep, Miljeteig andcolleagues63 failed to demonstrate a relationship be-tween nasal resistance, which varied significantlybetween patients, and both the number of snoresand snore sound intensity.

Nasal Acoustic Evaluation

Acoustic rhinometry has been documented exten-sively in the literature. It has been demonstrated toobjectively compare nasal patency to normative dataestablished by Corey et al,30 to confirm improve-ment in nasal patency after nasal septal/turbinatesurgery,33 and to establish the degree of improve-ment in nasal patency with medical treatment ofnasal allergies.22,34

PHARYNGEAL AIRWAY PATENCY

A great deal of information has been published re-garding the characteristics of both the healthy andpathological pharyngeal airway, both in the awakeand sleeping states. In recent years, much has alsobeen written about how palatal surgery and manip-ulation of the mandible with an airway orthotic af-fects these characteristics. The subject of pharyn-geal airway patency can be subdivided into site ofcollapse/narrowing, caliber/volume, and compliance.These parameters, along with the effect of palatal


surgery and airway orthotics on the pharyngeal air-way, have been evaluated through AR.9,19–21,29,31,44,45

Collapse (Narrowing)

Site analysis for the location of airway collapse canbe technically elusive. Airway closure in OSA hasbeen demonstrated to occur at multiple levels:velopharynx, oropharynx, and/or hypopharynx.64,65

Complicating the issue further, the majority ofOSA patients exhibit more than one site of upperairway obstruction during sleep and the pattern ofthese obstructions varies with sleep stage and bodyposition.66 Studies to date have utilized many tech-niques in both the awake and sleeping states, allwith their own limitations. However, relationshipsbetween the site of obstruction and ultimate thera-peutic success do exist and warrant investigation.67–70

It is generally accepted that dynamic studies duringsleep should be employed to accurately determinethe site of airway obstruction.71 This may be possi-ble in the research setting, but it may not be practi-cal in the clinical setting. Using multiple modalitiesto evaluate the specifics of airway collapse, the lit-erature confirms that patients with OSA demon-strate structural narrowing/collapse of the upperairway which is usually focal and located at thevelopharynx, but extends downward to the oro-pharynx and hypopharynx in approximately 50% ofindividuals.72

Over 15 years ago, Katsantonis and Walsh68

demonstrated through the use of somnofluoroscopya 67% response to uvulopalatopharyngoplasty(UPPP) when maximal narrowing and/or airwaycollapse occurred above the midpoint of the secondcervical vertebra. This compared to a 9% successfulsurgical outcome when it extended below this level.A few years later, Launois et al,69 using fiber opticpharyngoscopy and a pressure catheter to evaluatethe sleeping airway of OSA patients, demonstratedsimilar findings. They reported a successful responseto UPPP in 6 of 7 patients demonstrating upperairway collapse confined to the velopharyngeal seg-

ment, compared to 2 of 11 in whom collapse oc-curred caudal to the velopharynx. In spite of thefact that the existence of a relationship continues tobe demonstrated in the current literature,70 attemptsto date have failed to develop an empirical protocolto select surgical candidates.

A study involving mandibular advancementwith an airway orthotic and propofol-induced sleepreported a remarkable relationship between closureof the airway below the velopharynx and successfultreatment outcome.67 It was discovered that althoughsome patients obtained therapeutic relief when theclosure occurred at the velopharynx exclusively,successful reduction of the apnea-hypopnea indexto below 10 events per hour was less common whenairway closure was confined to this site. In con-trast, all of the patients that experienced closurecaudal to the velopharynx experienced a drop inapnea-hypopnea index to fewer than 6 events perhour. The technique used to determine the site ofairway closure involved a pressure catheter that de-tected the lowest site of closure. As a result, indi-viduals demonstrating closure below the velophar-ynx may have also had undetected closure at thevelopharynx, suggesting that an airway orthoticworks best when dealing with closure caudal to thevelopharynx, regardless of whether there is addi-tional closure at higher levels.

Rubinstein and associates43 used AR to eval-uate upper airway function in awake OSA patientsboth before and after weight loss. They demon-strated a paradoxical inspiratory narrowing of theglottis in some patients and complete resolution ofthis glottic narrowing in 2 of the 3 afflicted patientsafter weight loss, implying that cross-sectional areaat the glottis during wakefulness may be a particu-larly important indicator of airway pathology dur-ing sleep.

Although closure during sleep cannot bedocumented through AR, abnormal sites of upperairway narrowing during wakefulness can be docu-mented and compared to established norms for thenasal cavity, oropharynx, OPJ, hypopharynx, andglottis.19–23,28,30,33


Pharyngeal Caliber (Volume)

In an effort to explain what happens during an ap-neic event, Remmers et al73 proposed that upperairway collapse occurs when the negative pressuregenerated in the airway during inspiration is inade-quately opposed by the pharyngeal dilator muscula-ture. Horner,74 in his review of motor control of thepharyngeal musculature as it relates to OSA, dis-cussed the concept of a pressure-volume relation-ship, whereby decreasing airway volume results in adecrease in the negative pressure required to col-lapse the airway. He concluded that if airway caliberentered a critical region of narrowing (particularlyat end-expiration), it would become susceptible tocollapse with a minimum of inspiratory negativepressure, or even at pressures above atmospheric.

Mandibular advancement through the use ofan airway orthotic has been demonstrated to be ef-fective in enlarging airway caliber.8,10,11 Isono andcolleagues,10 using videoendoscopy, demonstratedwidening of the retropalatal airway as well as thatat the base of the tongue with mandibular advance-ment in 13 OSA patients under general anesthesiawith total muscle paralysis. They also demonstratedthat a more negative pressure was required to causecollapse of the airway when the mandible had beenadvanced. These authors postulated that tensiontransmitted along the palatoglossus muscles to thesoft palate may have been responsible for the air-way stabilization demonstrated in their study.

In contrast, videoendoscopy evaluation ofawake airway caliber with mandibular advancement,while supine, demonstrated an increase in caliber atthe velopharynx and hypopharynx, but no signifi-cant increase at the oropharynx or tongue-baselevel.12 However, one must consider the fact thatthis study was conducted in the awake, supine pa-tient. As discussed above, Martin et al50 demon-strated through AR that the airway caliber of awakeapneics differentiates itself from normal subjectsmore distinctly in the upright position, possibly dueto compensatory muscle tonus in the awake patientwhile supine. This suggests that the above oropha-

ryngeal findings may have been different had thepatient been evaluated in the upright position.

Through the use of AR, Rivlin and associ-ates39 demonstrated that the mean cross-sectionalpharyngeal area is smaller in awake patients withOSA when compared with controls. They also founda significant correlation between the number of ap-neas per hour of sleep and pharyngeal cross-sectionalarea during wakefulness. Katz et al.40 used AR to fur-ther confirm these findings; they concluded that a re-duced cross-sectional area of the awake hypopharynxwas a risk factor for both OSA and snoring.

Videoendoscopy,10,12 MRI,11 and AR44 haveall been used successfully to evaluate and documentthe effect an orthotic has on the caliber of a patho-logically small airway. AR, along with the airwaycaliber norms established by Kamal,28 may be use-ful to evaluate the ability of an orthotic to nor-malize the caliber of a pathological airway duringwakefulness.

Pharyngeal Compliance (Collapsibility)

Stienhart et al75 analyzed pharyngeal collapsibilityat the palatal and tongue-base levels during propo-fol-induced sleep using endoscopy. They foundthat patients with OSA syndrome demonstratedsignificantly stronger collapsibility compared withsnorers and that it was most evident at the tongue-base level. Collapsibility at the tongue base was alsofound to have a strong correlation to higher valuesof RDI (respiratory disturbance index) as recordedby standard polysomnography.

Airway compliance during wakefulness hasbeen demonstrated to distinguish apneics from non-apneics. Hoffstein and colleagues42 found that theawake airway of obese apneics was smaller in cal-iber and collapsed to a greater degree during lungvolume-related changes in comparison to nonap-neic obese controls. This increase in airway collapsefrom 30 to 54% implies increased compliance forthe apneic airway of obese patients. Increased pha-ryngeal compliance of the awake airway of obese


apneics was also demonstrated by Brown and asso-ciates45 and then further confirmed in a study eval-uating the relationship between pharyngeal areaand weight loss, which demonstrated through ARthat the post-weight-loss airway became statisti-cally less compliant.43 Of particular note, althoughthese patients demonstrated the anticipated in-crease in FRC after weight loss, they did not expe-rience a significant change in pharyngeal caliber ateither FRC or RV; this suggests that the improve-ment in OSA after weight loss was associated withairway function rather than solely with airwaystructure.

The existence of obesity seems to play a rolein the presentation of the apneic airway as mea-sured through AR. Bradley et al,41 in their acousticstudy of awake, nonobese normal subjects, snorers,and apneics, also demonstrated a reduction of air-way caliber in the pathological airway, but foundvariable compliance in differentiating these pa-tients. These authors demonstrated that patientswith OSA and simple snoring have abnormalitiesof anatomical and mechanical features of the phar-ynx that distinguish them from each other and fromnormal subjects. They found that the nonapneicsnorer, when compared to the apneic snorer, hadsimilar airway caliber at FRC but smaller lungvolume-related changes at RV, implying a less com-pliant airway and allowing for maintenance of air-way patency during sleep. They also demonstratedthat although snorers had a smaller airway caliberthan controls at FRC, they experienced less com-pliance with lung volume-related changes at RV,resulting in no difference in caliber at RV thancontrols. This differentiated those airways as beingsusceptible to snoring due to narrow airway caliber,but not statistically narrower enough than controls’airways at RV to compromise airway patency. Fi-nally, in contrast to the findings of Hoffstein et al42

in their study of obese apneics, although the non-obese apneics in this study also demonstrated asmaller airway than controls at both FRC and RV,they experienced a similar level of compliance withlung volume-related changes. This suggests that

airway structure may be more significant thanfunction when evaluating nonobese apneics.

The ability to repeat AR readings at 0.2-second intervals allows for the dynamic study ofairway characteristics and measurements associatedwith lung volume-related changes, thus facilitatingthe evaluation of airway compliance.

Site of Collapse/Narrowing versus

Caliber/Volume versus Compliance

The relative importance of these variables is diffi-cult to ascertain. Regarding site of collapse/narrow-ing, a study utilizing AR to evaluate improvementin upper airway function after weight loss demon-strated a post-weight-loss improvement in apneaaccompanied by normalization of abnormal nar-rowing at the glottis.43 Site of collapse/narrowinghas been confirmed by many other studies to play asignificant role in the pathogenesis of SDB.67–70

However, the existence of multiple sites of obstruc-tion64,65 and variability based on sleep position andstage of sleep66 make it difficult to study.

Regarding caliber and compliance, their rel-ative importance appears to be quite elusive in theliterature. It has been demonstrated that the criti-cal pressure at which airway collapse occurs can bealtered by either an increase in airway caliber or adecrease in airway wall compliance.74 However,using AR to evaluate the airways of nonobese ap-neics as compared with nonobese normal subjects,the apneics differentiated themselves as having asmaller airway caliber, but not through a differencein compliance.41 On the other hand, another acous-tic study involving obese apneics and studies usingother modalities, in both the sleeping and awakeairway, have demonstrated that apneic airways aremore compliant than normal ones.42,47,75 The incon-sistent interstudy variable of obese versus nonobeseparticipants may play a role in the differences ob-served in the significance of compliance.

Although the importance of complianceseems to vary in the literature, caliber at end-


Figure 2 Relative airway caliber and compliance for nonobese normal subjects, snorers, and apneics. (Reprinted withpermission of Bradley TD, Brown IG, Grossman RF, et al.41)

expiration is consistently found to be important. Astudy by Issa and Sullivan76 demonstrated that thesmaller caliber of the pathological airway at end-expiration makes it more susceptible to suction col-lapse upon inspiration. This disposition to collapsebecomes more pronounced during REM sleep withfurther loss of muscle tonus. This corresponds withthe pressure-volume relationship discussed above,whereby a decrease in airway volume results in adecrease in the negative pressure required for air-way collapse. Acoustic studies have demonstratedthat apneics demonstrate a reduction in airway cal-iber at both FRC and RV; however, the role ofcompliance appears to be associated with the exis-tence of obesity (see Figs. 2 and 3).

The literature demonstrates that site of abnor-mal airway narrowing, airway caliber, volume, andcompliance all have the potential to be critical para-meters in the evaluation of airway patency. Airwaypatency seems to be dependent on both the ana-

tomical factors of site of narrowing, caliber, andvolume, and the physiological factor of compliance.

AIRWAY DYNAMICS AND CRITICALPARAMETERS

Critical Parameters and the Asleep Airway

There have been many attempts to validate para-meters that must be maintained in order to ensureairway patency. One example is “critical pressure.”Airway patency has been found to be dependent onmaintaining a differential of 4 cm H2O betweenatmospheric pressure and the “critical pressure” re-quired to collapse the airway.77 This parameter op-erates on a continuum, with a differential of 8 cmH2O required to eliminate snoring. Individuals witha differential of less than 4 cm H2O can increase it

Figure 3 Relative airway caliber and compliance for obese controls and apneics. (Reprinted with permission of Hoff-stein V, Zamel N, Phillipson EA.42)


in one of two ways: increasing atmospheric pres-sure as is accomplished with nCPAP, or decreasingthe critical pressure required to collapse the airway.It has been established that any therapy intendedto maintain airway patency must maintain this dif-ferential.77–79

In a study designed to evaluate the relation-ship between critical pressure and the response toUPPP, Swartz and associates78 found a 100% cor-relation between apneics who responded favorablyto UPPP and reduction in critical pressure follow-ing surgery. Those individuals not experiencing adecrease in critical pressure to at least �4 cm H2Opostsurgery were found to be nonresponders.

In a second study involving weight loss,79 adirect relationship to critical pressure was also dem-onstrated, resulting in the conclusion that weightloss affects OSA by reducing the collapsibility ofthe pharyngeal airway in proportion to the amountof weight lost. The authors also proposed an expla-nation for why post-weight-loss individuals havevariable success in the reduction of their RDI. Al-though a direct relationship was demonstrated be-tween weight loss and critical pressure; they foundthat regardless of the amount of weight loss and re-sulting reduction in critical pressure, if the end crit-ical pressure was above �4 cm H2O, the patients’RDI would not be eliminated. Therefore, an indi-vidual experiencing a substantial weight loss wouldstill experience airway collapse during sleep if hisor her critical pressure remained above �4 cm H2O.

Critical Parameters and the Awake Airway

Many studies that validated relationships regardingboth the “awake” and “sleeping” airway have beendiscussed. Upper airway collapsibility measuredduring wakefulness provides useful physiologic in-formation about pharyngeal mechanics duringsleep.47 Weight loss results in a reduction in RDI,80

in critical pressure,79 in compliance of the sleepingairway,79 and in compliance of the awake airway.43

Elimination of RDI with weight loss occurs only

when the critical pressure resulting from the weightloss is at or below �4 cm H2O.79 The critical pres-sure at which airway collapse occurs can be loweredby either an increase in airway caliber or a decreasein airway wall compliance.74 Finally, airway com-pliance in the “awake” airway has been found to berelated to airway collapse during sleep in apneics asverified by polysomnography.42

In their presentation of pharyngeal criticalpressure, Gold and Schwartz77 discuss a correlationbetween increasing levels of critical pressure and in-creasing levels of sleep-related airflow obstruction.They propose that different levels of critical pressureon a continuum results in a range of symptoms com-mencing with normal subjects, to snorers, to apneics.Horner74 reports the existence of a pressure-volumerelationship whereby once a reduction in airway vol-ume to a critical level takes place, the airway will col-lapse during sleep. In a study using AR to evaluatethe pharyngeal size of awake normal subjects, snor-ers, and apneics, Bradley and colleagues41 describe acontinuum regarding pharyngeal size that includes acritical level of pharyngeal narrowing during wake-fulness, below which one will tend to snore, and an-other more severe level, below which airway occlu-sion occurs during sleep. See Figure 4.

The above suggests that both critical pressure77

and critical volume74 during sleep are parametersthat must be maintained to ensure airway patencyduring sleep. The relationship between these parame-ters and airway compliance and/or caliber/volumeduring wakefulness suggests that there may be airwayparameters during wakefulness that are also criticalto the maintenance of patency during sleep.

Critical Parameters and Acoustic Reflection

As discussed above, the literature supports the con-cept of “critical pressure” as a parameter necessaryto maintain airway patency. We also discussed therelationship between “critical pressure” in the sleep-ing airway and parameters that can be evaluatedthrough AR in the awake airway. Hoffstein et al42


Figure 4 Critical parameters during wakefulness and sleep.

demonstrated that the apneic airway of obese pa-tients is more compliant than controls’. Bradley etal41 found that the apneic airway of nonobese pa-tients does not demonstrate more compliance thancontrols but rather a reduced caliber. Rubinsteinand associates43 found that apneic airways of obesepatients that do not demonstrate increased compli-ance demonstrate abnormal narrowing at the glot-tis which resolves with weight loss and reduction inapnea. All of these studies involved assessment ofthe awake airway, measured in the sitting uprightposition using AR.

Site of pharyngeal narrowing, airway cali-ber, volume, and compliance can all be accuratelyevaluated, documented, and compared with normsthrough AR.19–21,28,39–43,45 However, the relative im-

portance of each of these variables is difficult toascertain; it is not known if their effects are cumu-lative or independent. The fact that a decrease inpharyngeal compliance has been observed in theabsence of any meaningful change in pharyngealcaliber indicates that airway caliber alone may notprovide all the information required to predictairway stability.43 Perhaps it is not airway caliberat FRC that is critical, but the caliber establishedafter maximum compliance has occurred. In otherwords, airway narrowing to a critical caliber mayin fact be critical to maintaining airway patency.Perhaps, as long as this caliber remains larger thanthe critical caliber required for maintenance ofpatency, the airway does not collapse, regardlessof the degree of compliance.


Factors Influencing Acoustic Critical

Parameters

Gender, age, and obesity have been demonstratedto influence airway structure and function. Theseinfluences must be understood and considered whenevaluating the airway through AR.

In his evaluation of the normal pharynxthrough AR, Kamal28 demonstrated a smaller air-way caliber in normal females versus males. Thisgender difference is supported by other investiga-tions of normal subjects81,82 and recently in a studyevaluating obese apneic males and females.83 Greatercompliance has also been demonstrated in malesversus females.81,82,84 However, these differences instructure and function were all demonstrated inupright posture through AR and become less dis-tinguishable when investigated in supine subjects.82

One possible explanation for these genderdifferences may involve the relationship betweenfemale hormones (progesterone) and increased ge-nioglossus muscle activity.85 Further support forthis explanation lies in the fact that although fe-males experience less severe OSA during nonREMsleep, both genders demonstrate a similar severityof OSA during REM sleep, suggesting that theloss in muscle tonus that accompanies REM sleepleaves females as susceptible as males to airway ob-struction.86 In contrast, although a recent study87

evaluating airway response to resistive loadingduring nonREM sleep found that males exhibit

markedly greater pharyngeal collapsibility in re-sponse to externally applied load, no significantdifference in either tensor palatini or genioglossusmuscle activity was noted for either gender. Theseauthors suggested that gender differences may bebetter explained by a fundamental difference inupper airway anatomy and/or tissue characteristics.

The literature demonstrates conflicting dataregarding the affect of age on the airway. AlthoughMartin et al84 demonstrated that airway caliber de-creases with increasing age for both genders, Huangand colleagues82 found no change for either genderwith age. However, this group did find that malesdemonstrate an increase in compliance with age,suggesting that airway function may play a role inthe increased incidence of OSA in older males.

As discussed above, obesity seems to affectthe relevance of caliber and compliance in OSApatients. See Figures 2 and 3.

The literature suggests that acoustic evalu-ation of the upper airway must consider gender,age, and obesity when developing normativestandards and evaluating both pathology andnormalization of the airway. The fact that genderdifferences disappear when the airway is investi-gated in the supine subject provides further sup-port for evaluating the awake airway in the up-right position.

Table 1 summarizes our current knowledgeregarding assessment of the pharyngeal airwaythrough AR. The literature supports a distinct con-

Table 1 Acoustic Verified Relationships39,40–43,81,82,84

Airway Airway Glottic

Caliber Compliance Dysfunction

Nonobese Snorers versus Controls Smaller Smaller Not InvestigatedNonobese Apneics versus Controls Smaller Equal Not InvestigatedNonobese Apneics versus Equal Greater Not Investigated

Nonobese SnorersObese Apneics versus Controls Smaller Greater Not InvestigatedObese Apneics Pre- versus Equal Greater Greater*

Post-Weight LossMales versus Females Larger Greater Not InvestigatedOlder Males versus Younger Males Smaller Greater Not Investigated

*Glottic dysfunction: witnessed in obese apneics who demonstrate normal compliance.


tinuum of airway characteristics from apnea to snor-ing to controls, whereby both caliber and compli-ance alternate in their role to distinguish patientsat each level. In keeping with the multifactorial na-ture of the pathogenesis of SDB, the variables ofgender, age, and obesity appear to influence airwaydynamics in the awake airway as documentedthrough AR.

AIRWAY ASSESSMENT BY ACOUSTIC REFLECTION

Advantages

Although lateral cephalographs are limited to pro-viding a two-dimensional view, they have beenused extensively to assess the upper airway. Schwabet al,11 using MRI on awake snorers, demonstrateda specific pattern of airway dilation with mandibu-lar advancement; increase in caliber was greater inthe lateral than the anterior-posterior (AP) dimen-sion. Another study evaluating the effect of CPAPon soft-tissue structures demonstrated a similarairway dilation pattern.88 Evaluation of the lateralpharyngeal walls of normal and apneic airwaysdemonstrated that narrowing in the apneic airwaywas predominantly in the lateral dimension.89 It isof significant importance that no difference in theAP dimension of normal and apneic subjects wasdemonstrated. Unlike cephalographs, AR accuratelyevaluates the airway in three dimensions, allowing fordocumentation of caliber and volume over a givenlength of airway. 21

Demonstrated to be noninvasive, accurate,and reproducible, AR provides an objective mea-surement of the upper airway from the nasal valveto the glottis in three dimensions.19–30 The exam isquick and inexpensive to perform and as a result,easily repeatable. Although it requires patient co-operation, it results in a high patient acceptance.Because the exam makes it possible to take read-ings at 0.2-second intervals, it allows for dynamicassessment of the airway.

Limitations

For both rhinometry and pharyngometry, AR pro-vides a measurement of cross-sectional area ac-cording to distance along the airway. However, itdoes not provide high resolution anatomic repre-sentation of the airway or soft-tissue structures.

Acoustic rhinometry accurately documentsthe nasopharynx.22–24,34,37,38 However, values may beinfluenced by a significant constriction,90 move-ment of the soft palate,91 perforations in the nasalseptum causing an acoustic leak,24 or asymmetricbranching of the nasal cavity at the epipharynx.24

Regarding acoustic pharyngometry, althoughupper airway collapse occurs most often at the levelof the velopharynx, extending caudally as low as thehypopharynx approximately 50% of the time,64,65,72

conventional pharyngometry provides a calculatedapproximation of the velopharynx and an accurateassessment of the remaining upper airway com-mencing at the oropharynx moving caudal to theglottis.19–21,25–27

The conventional AR technique cannot beused during sleep. However, Huang and associ-ates92 recently published a new AR techniquewhich involves introduction of the acoustic wavesinto the nostrils during spontaneous nasal breath-ing while asleep. The authors claim accuracy inmeasuring not only the oro- and hypopharynx, butalso the nasopharynx, which is not possible usingthe conventional technique. They also suggest thatthis new technique can be used as a monitor ofupper airway dimensions during standard noctur-nal polysomnography.

Potential Clinical Usefulness

The literature establishes a clear relationship be-tween acoustic derived measurements of the awakeairway and pathological airway behavior duringsleep.39–43,45 This sleep-wake relationship of airwaydynamic characteristics has also been demonstratedthrough other modalities.47 Shown to be accu-rate19–27,34,37,38 and reproducible21,25,28–30,34 in the


assessment of the upper airway, the potential forclinical usefulness of AR will be discussed based onour current understanding of upper airway dynam-ics and the unique characteristics afforded us bythis imaging modality.

Patient Screening As a result of insufficient pub-lic and professional awareness,93 only a small por-tion of individuals afflicted with SDB have beendiagnosed and treated. With increase in awareness,the development of a validated protocol designedto facilitate the initial screening process would helpto control the resulting increase in patient load atfacilities already working to full capacity. Acousticassessment of nasal patency and pharyngeal char-acteristics of individuals in the general population,or perhaps in high-risk groups, could isolate thosethat warrant further evaluation through polysom-nography, ultimately reducing the burden on cur-rent medical facilities all the while increasing thenumber of individuals isolated for treatment.

Positional Therapy Evaluation AR has been usedto evaluate airway response to positional therapy.Hsu et al.94 used acoustic technology to assess theeffect head extension through cervical repositioninghas on airway caliber. They demonstrated a signifi-cantly increased caliber with cervical repositioningthrough the use of a patented special pillow.

Surgery Candidacy Relationships between thesite of obstruction and therapeutic success withUPPP have been established.68–70 The need for apreoperative investigation of the upper airway inorder to establish candidacy for UPPP prior to sur-gery has been emphasized in the literature.70 Al-though site of airway closure is impossible todemonstrate in the awake patient using AR, theacoustic pharyngometer accurately documents thecross-sectional area and abnormal upper airwaynarrowing caudal to the velopharynx.21 Research byKamal28 documenting acoustic derived pharyngealcross-sectional minimum area norms provides nor-mative standards for evaluation of the airway re-garding caliber and sites of abnormal narrowing.

Postsurgery Evaluation AR has been used toevaluate changes in pharyngeal structure and func-tion after UPPP surgery. Wright et al9 demonstrateda postsurgical increase in pharyngeal area as well asa reduction in lung volume-related changes. It hasalso been demonstrated to be useful in the evalua-tion of postsurgical nasal patency33 and postmed-ical treatment of nasal allergies.22,34

Orthotic Candidacy It is difficult to determinewhether an airway orthotic stabilizes the pharyn-geal airway by increasing caliber or by decreasingcompliance. A small airway cannot tolerate asmuch compliance without collapsing as a large air-way, and a large airway may be less likely to bechallenged to comply due to the fact it would expe-rience a lower critical pressure.74,76 Horner74 dis-cussed the concept of a critical airway volume belowwhich airway collapse occurs. However, althoughcaliber has differentiated apneics from nonapneicswhen investigating nonobese patients,41 it was com-pliance and not caliber that improved after weightloss in a study involving obese apneics.43 Even so,in the absence of abnormal compliance in obeseapneics, abnormal narrowing at the glottis has beendocumented.43 The literature suggests that airwaypathology manifests itself through a variety of vari-ables when assessed through AR, possibly due tothe multifactorial etiology of SDB. See Table 1.

Loube44 used AR to evaluate hypopharyn-geal changes produced by mandibular advancementin the awake patient. He found that no change involume was 95% predictive of failure and that anincrease in volume was 60% predictive of a success-ful treatment outcome. Airway area was less accu-rate than volume as an outcome predictor. AlthoughLoube’s study indicates a very strong associationbetween airway volume and treatment outcome,the 60% predictive value for success associated withairway dilation indicates that variables other thanvolume may play a role in determining success.

The theory postulated by Isono and col-leagues,10 that mandibular advancement increasesairway muscle tonus, may explain those instancesin which a decrease in compliance in the absence of


Figure 5 Mandibular condyle is displaced vertically as itis advanced with an airway orthotic due to guidance ofanterior wall of mandibular fossa.

airway dilation is observed with mandibular ad-vancement. Since Loube’s investigation was lim-ited to changes in airway caliber/volume, improve-ment or the absence of improvement in compliancewent unnoticed. Notwithstanding this, the factthat nonresponse to mandibular advancement is95% predictive of orthotic failure is useful informa-tion when considering candidacy for airway or-thotic therapy. However, as discussed above, wheninvestigated through AR, the apneic airway differ-entiates itself from normal airways more readily inthe upright position; that these patients were in-vestigated in the supine position could have poten-tially altered this studies outcome.

That an airway orthotic works best whendealing with closure caudal to the velopharynx, re-gardless of whether there is additional closure athigher levels,67 could be a useful consideration whenevaluating airway narrowing through AR. Com-parison to established norms28 could aid in assess-ing the beneficial effects of an airway orthotic onthe caliber of a pathological airway.

The use of AR to predict successful orthotictherapy may be enhanced by further investigationof the relationship between airway patency duringsleep and the effect on the airway of an orthoticduring wakefulness. The literature suggests that thefollowing variables warrant evaluation: critical mini-mum caliber, minimum volume, maximum compli-ance, and/or site specific area norms. Much in thesame way that violation of “critical pressure” dis-cussed above results in airway collapse, violation ofone or more of these parameters as measuredthrough AR may also result in collapse. Consider-ing the correlation between nasal patency and SDB,it would also be of value to investigate the effectuntreated nasal blockage has on the success of or-thotic therapy.

Orthotic Construction A degree of mandibularvertical displacement is unavoidable with mandib-ular protrusion. Anterior and downward movementof the mandibular condyle occurs due to the anat-omy of the anterior wall of the mandibular fossa,which dictates condyle movement as it glides for-

ward during protrusion (Fig. 5). Also, the mandib-ular anterior teeth are guided downward by the lin-gual surface of the maxillary anterior teeth as themandible is translated forward (Fig. 6).

The effect on the airway of this vertical dis-placement of the mandibular apparatus and associ-ated musculature is not clear. Consequently, thedegree of benefit derived from an airway orthoticthat results from anterior repositioning as opposedto this accompanying vertical displacement is un-known. To date, it has been popular to minimizevertical opening when using an airway orthotic.However, an anecdotal finding in my own clinicalpractice, which I have verified through personalcommunication with other practitioners and casestudies,95 is that some patients benefit from thevarying of vertical posture of the mandible beyondthat associated with mandibular protrusion.

The vertical and protrusive settings that beststabilize the airway with an oral bite-jig may play arole in the construction of an airway orthotic.Through the use of a chair-side constructed bite-jig, the mandible could be manipulated in both theAP and vertical dimensions, providing an opportu-nity to evaluate through AR the various critical air-way parameters discussed thus far; aiding the clini-cian in determining ideal construction parameters.

Orthotic Titration The current protocol involvesadvancing the mandible until subjective relief ofsymptoms is experienced and then verified objec-tively through standard polysomnography. It is not


Figure 6 Mandibular teeth are displaced vertically asmandible is advanced with an airway orthotic due to guid-ance of the lingual surface of the maxillary anterior teeth.

uncommon for an orthotic to be advanced furtherthan therapeutically necessary in order to ensure asuccessful outcome with the polysomnogram. How-ever, from a neuromuscular point of view, it is im-portant that an airway orthotic not advance themandible so much as to result in hyperextension ofthe masticatory and cervical muscles. Unnecessarymandibular advancement may lead to increased pa-tient discomfort and ultimately lower complianceto therapy.

Mandibular advancement past the point oftherapeutic effectiveness to ineffectiveness has beenreported. L’Estrange and associates,96 demonstratedthat although the airways of most patients increasein caliber with mandibular advancement, reductionin caliber occurs in some patients with advancementpast 75% of full protrusive position, possibly due toskeletal differences in the subjects.

Once the orthotic has been constructed, itcould be optimized further through acoustic evalua-tion of the airway’s response to mandibular manipula-tion with the orthotic in place, thus ensuring orthotictitration that results in the most ideal management ofthe airway. This would help to minimize the possibil-ity of inadvertent advancement past the ideal point ofeffectiveness into a position that would unnecessarilystrain the masticatory and cervical muscles and/or re-duce the effectiveness of the orthotic.

Orthotic Maintenance The importance of regularfollow-up is generally regarded as mandatory when-ever ongoing therapy is prescribed; its value regard-

ing orthotic therapy was highlighted in a recentpublication97 demonstrating that patients returningregularly for adjustments and follow-up visits expe-rienced a better long-term effect than patients con-tinuing to use their original orthotic. Orthotic effec-tiveness may be affected by factors such as weightgain, development of allergies, medication use, andso on. A yearly examination of the orthotic and den-tition along with subjective evaluation through pa-tient consultation is considered standard protocol;an acoustic exam at these visits could objectivelysubstantiate that the orthotic is still ideally titratedto maintain airway patency during sleep.

SUMMARY AND CONCLUSIONS

The potential clinical usefulness of AR in thetreatment of patients with SDB involves all stagesof treatment: initial screening of patients, estab-lishing patient candidacy, evaluating nasal patency,determining mandibular posture that optimizes air-way patency, determining orthotic titration settings,and verifying continued efficacy of orthotic set-tings at follow-up. The use of AR could facilitatefront-line efforts in isolating afflicted individuals;ensure a higher level of success for surgical, posi-tional, and airway orthotic therapies; eliminate thepossibility that an undiagnosed nasal obstructioncould interfere with successful treatment; establishorthotic construction parameters; and objectivelyverify the orthotics’ continued effectiveness. Al-though much evidence is currently found in theliterature, continued research to evaluate criticalparameters of the awake airway as documentedthrough acoustic reflection would further validatethe use of this diagnostic modality in the treatmentof patients with SDB.

ACKNOWLEDGMENTI wish to thank Randy Clare for his AR technicaladvice, critical reading of the manuscript, and themany useful discussions.


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Acoustic Reflection: Review and Clinical Applications for ... - Sleep Group Solutions · 2014. 12. 1. · files aligned at the vocal cords, demonstrating no significant difference