Predicting which patients will benefit from surgery for obstructive sleep apnea: The ENT exam.

Abstract

Airway evaluation is critical for surgical decision making. In patients with obstructive sleep apnea (OSA), a minimal evaluation should include a basic head and neck physical examination to evaluate for overt pathology. An upper airway examination will also provide insight into identifying patients with a higher risk of OSA. For patients who are evaluated for surgery, endoscopy combined with cephalometrics is the most accepted method of identifying patients with retroglossal collapse and obstruction. A new paradigm suggests that most patients have multilevel obstruction, so examination should be directed at assessing risk factors to direct the aggressiveness of surgical intervention.

Introduction

The surgeon's concept of obstructive sleep apnea (OSA) influences how he or she performs and interprets the ENT examination. If surgical options were limited only to tracheotomy, an upper airway examination would not be necessary. In contrast, site-specific surgical procedures require a careful airway evaluation. Despite the examination's importance, sparse data exist to guide the surgeon.

Knowledge of upper airway mechanics provides a map to help guide dogma. What is known about the upper airway suggests that a paradigm shift might be in order. The concept of "an obstructive lesion" inaccurately depicts obstruction during sleep in most individuals. Accumulated data demonstrate that the upper airway in the OSA patient is small, not stenotic. [1] Obstruction is defined not by a specific anatomic site, but by collapse that can involve multiple airway segments. [2] Therefore, the treatment of airflow limitation might involve increasing the stability of unstable segments rather than excising segmental narrowing. The surgery stabilizes the airway proportional to the combined anatomic and physiologic abnormalities. Limited surgery, such as uvulopalato-pharyngoplasty (UPPP), is not likely to be successful when many abnormalities are present. Airway evaluation attempts to identify and quantify abnormalities to direct the aggressiveness of surgery.

The goals of airway evaluation are fourfold. First, the physical examination identifies patients who are at risk for sleep apnea. Second, examination identifies pathology of the upper airway. This can include enlarged lymphoid tissue and other airway masses. Patients who are diagnosed with OSA should have at a minimum an evaluation to identify such pathology. Third, evaluation can attempt to predict surgical outcomes. Considerable data in the surgical literature attempt to support this possibility, but most results continue to disappoint. [3] Last, examination identifies surgically treatable segments that, when corrected, can alleviate OSA. Examination attempts to identify areas where surgery can enlarge, stiffen, or alter the shape of the upper airway. Correction of these characteristics will prevent airflow limitation and decrease the ventilatory effort, obstruction, arousal, and sleep fragmentation that occur with OSA.

There are a number of ways to evaluate the upper airway (table 1). Three have been widely used for clinical examination: the physical examination, fiberoptic endo scopy, and cephalometric radiography. Future research will likely provide new and more accurate methods and techniques.

Population screening

The otolaryngologist examines the upper airway in a large population. Because patients with dysmorphology of the soft and skeletal tissues have a greater risk of OSA and snoring, otolaryngologists can identify large numbers of individuals with OSA. Guilleminault et al have described a morphometric model that predicts OSA with a high level of sensitivity and specificity (table 2). [4] Airway size measured endoscopically can also predict OSA and its severity. [5] Patients who are at an increased risk might warrant further diagnostic testing.

Pathology

In contrast to OSA in children, in whom pathology is common, OSA in adults is infrequently associated with pathologic lesions. Instead, dysmorphology is common. [6] Dysmorphic tissues have also been described as disproportionate anatomy. Disproportionate anatomy is nonpathologic anatomy that consists of redundant, hypertrophic, or normal tissues that contribute to obstruction.

There is a large number of features that can individually contribute to the upper airway examination. Although each feature can be described separately, a classification scheme modified from Fujita helps direct pharyngeal surgeries. [7] The three upper airway types parallel the available surgical procedures; they are directed at the palate or upper pharynx, the tongue or lower pharynx, or both. In type I, obstruction occurs in the upper pharynx from tissues related to the uvula, palate, or nasopharynx. In type III, obstruction emanates from the tongue, lingual tonsils, or supraglottis. In type II, obstruction occurs from both. Studies of manometry during sleep demonstrate that 25% of patients with obstructive sleep apnea syndrome have obstructions only at the palate. [8] Only 10 to 20% have obstructions only at the hypopharynx. Most obstructions are of the combined variety. Morrison et al reported that objective endoscopic measures of the upper airway during sleep identified isolated retropalatal obstruction s in only 20% of patients. [9] Combined obstructions occurred in most. In view of the concept that isolated collapse and obstruction to one segment is uncommon, the goal of airway evaluation is to identify the small number of patients who are obstructed at a single segment only.

Prediction

The type of airway and the severity of apnea are variables that correlate with the outcomes of limited pharyngeal surgeries. Sher et al observed that UPPP success rates were 10 times better in patients with only upper pharyngeal obstructions than in those with lower pharyngeal obstructions. [3] An obstruction identified at the tongue base is virtually predictive of UPPP failure (90%). [10] The absence of a lower pharyngeal obstruction, however, is not predictive of success. The literature demonstrates only a 50% success rate in favorable patients without lower pharyngeal obstruction. [3]

There are many reasons for such poor outcomes. Prediction is confounded by subjective evaluation tools and varying definitions of success. Potential quantitative techniques that accurately measure the airway might have a high positive predictive value. [11] Currently, these techniques are not widely practiced or easy to perform.

Surgical evaluation

There is currently no consensus as to which upper airway measures are important. Also, there is no gold standard to validate any characteristic. The surgeon's guide must be an understanding of airway mechanics.

Obstructive sleep apnea syndrome is the result of a structurally small upper airway combined with a loss of muscle tone. [1] In the OSA patient, the upper airway is smaller than normal. [2] The cross-sectional area is smaller in the nasopharynx, oropharynx, and hypopharynx. [12] Increased apnea severity correlates with airway size. [13] The smaller the airway, the more severe the sleep apnea.

The upper airway is often depicted as a two-dimensional structure, with the critical dimension being the posterior airway space. This view is incomplete. Many structural features contribute to airway collapse. For example, airway length is critical. [14] A long pharynx is more unstable than a short one. The lateral wall also plays a prominent role in the collapse of the airway during sleep. [15] It is both more collapsible and thicker in OSA patients than in normal controls.

The upper airway collapses more in OSA patients than in normals. [16] Assessing collapsibility is critical. In fact, airway collapsibility can guide treatment decisions. [17] The critical closing pressure (Pcrit) measures airway collapsibility, and it has been associated with the success or failure of UPPP.

Conceptually, Pcrit is the airway CPAP pressure where complete cessation of airflow occurs. In normal subjects, negative pressure is required to close the airway. In snorers, the airway closure is near ambient (zero) pressure. In sleep apnea, the airway closes at positive pressures. Patients whose closing pressures are near zero have relatively stable airways. Limited procedures such as UPPP might be more successful in these patients. At higher closing pressures, major intervention is required. Collapsible airways occur in patients who are obese, who are older, who have a history of substance abuse, and who have severe OSA.

Physical examination

Nasal examination. Nasal obstruction affects the OSA patient in three ways. First, mouth breathing caused by nasal obstruction results in a loss of nasal reflexes. These reflexes help maintain upper airway muscle tone. [18] Second, mouth breathing can cause the jaw to open during sleep. This can lead to posterior rotation of the mandible and the tongue base, which narrows the airway. Last, the nose also acts as a "starling resistor." [16] Starling resistors describe flow in collapsible tubes. Obstruction in the collapsible segment (i.e., the pharynx) is determined by resistance in the "upstream" noncollapsible segment (i.e., the nose). Nasal obstruction increases the upstream airflow resistance. Upstream resistance increases the collapse of the pharynx. Physical examination remains the primary mode of assessing the nose. Severe nasal obstruction in patients with mild OSA combined with normal cephalometric x-rays can predict a good response to nasal surgery. [19] No other known predictors of nasal surgery exist. Some experts advocate a trial of medical treatment as a physiologic nasal test. Nasal dilators or decongestants such as oxymetazoline can alleviate snoring, but this does not predict the long-term success of surgery.

Oral examination. The oral examination is best performed with a headlight. No single feature predicts the site of obstruction. A lack of abnormal findings in the oropharynx and palate is associated with an obstruction of the tongue base during sleep. [20] The tonsils are graded subjectively on a four-point scale: 0 = absent, 1+ = small, 2+ = easily visualized but not obstructive, 3+ = hypertrophic, and 4+ = apposing in the midline. In OSA, the apparent tonsil size can be misleading because of lateral wall hypertrophy. This medially displaces the tonsils. Obstruction can persist following tonsillectomy.

The webbing of the posterior pillar is highly variable. Webbing can extend all the way to the tip of the uvula. In some patients who have longstanding apnea or snoring, the webbing can extend to the soft tissues of the lateral pharynx. The uvula can be absent, small, medium, or large. Telescoping of the uvular mucosa (observed with or without uvular contraction) and posterior pharyngeal wall folds (rugae) are correlated with snoring and OSA. A routine examination that identifies telescoping, posterior pillar rugae, or significant webbing warrants inquiry into snoring or sleep apnea.

Skeletal anatomy is a major predictor of OSA. [21,22] Facial morphology and dentition reveal skeletal structure. Insight into structure can help direct soft tissue and skeletal surgeries. Skeletal features can also identify patients at risk for OSA. An orthognathic (Angle class 1) relationship refers to normal occlusion and facial proportion. A retrognathic (Angle class 2) designation refers to a small mandible. Angle class 2 is also associated with posterior maxillary constriction. The absence of overjet (overbite) can be misleading in the case of previous orthodontia or retroinclined maxillary incisors. Prognathic (Angle class 3) patients might have a disproportionately large mandible. However, many Angle class 3 individuals are actually maxillary-retrusive. A small maxilla predisposes to OSA.

The tongue dorsum is measured on a three-point scale in relationship to the occlusal plane. A 1+ tongue is at the occlusal plane, a 2+ tongue is easily above the occlusal plane, and a 3+ tongue is massively filling the oral cavity. The Malampatti classification assesses palatal length and tongue size. Ranking is performed by asking the patient to open the mouth and protrude the tongue. Visualization of the margin of the soft palate and a portion of the anterior pillars and uvula is classified as Malampatti I. The free margin of the soft palate that is visible only during phonation is Malampatti II. When no portion of the free margin of the palate is visible on phonation, the patient is classified as Malampatti III. Although intuitively a measure of palatal length, the Malampatti type might be statistically associated with tongue size.

Upper airway endoscopy

Endoscopy might be the best currently available method of evaluating the upper airway and selecting patients for surgery. Combined with control of physiologic variability during sleep, endoscopy correctly predicts UPPP responders and nonresponders. [11] No other method has been shown to be as accurate. Office endoscopy has discriminated populations with and without tongue base obstruction when subsequently evaluated during sleep. [20]

Upper airway endoscopy is performed while the patient is supine and at end expiration to reduce physiologic variability. The airway is smaller in the supine position than in the sitting position in patients with OSA, but not in normals. [23] This is because of the muscle compensation in OSA. Airway muscle tone during wakefulness is augmented to increase its size, especially during inspiration. This "posturing" confounds the assessment of airway size. Dilation can be offset by the negative intraluminal pressures that occur in the partially obstructed upper airway. However, following the collapse that occurs with a change in position, phasic inspiratory dilation might be observed. Endoscopy performed during end expiration reduces the compensatory inspiratory dilator muscle tone. End expiration also reduces the intraluminal airway pressure to near zero. This is a constant benchmark. End expiration also lessens the effects of tracheal tug, which stabilizes and prevents upper airway collapse. [24] All these maneuvers reduce the physiologic factors that increase upper airway size during wakefulness as compared with sleep.

Endoscopy can be either dynamic or passive. The dynamic examination involves performing Muller's maneuver. Technically, the endoscope is positioned directly above the segment to be evaluated, and the patient inspires at end expiration against occluded nostrils. The degree of collapse is then scored. The examination is subjective, variable, and affected by patient effort. In some patients, the airway dilates, which is not accounted for in the classification. Muller's maneuver can predict UPPP success when criteria are stringent (i.e., hypopharynx collapse [less than or equal to]25%). [25] Looser criteria demonstrate only a 50% predictive success rate. [26] The observation of tongue obstruction during Muller's maneuver is associated with only an 11% UPPP success. [11] Muller's maneuver, however, does predict failures.

Passive endoscopy assesses airway size at end expiration without an inspiratory maneuver (figure). The key points of this examination are a relaxed patient, examination in the supine position, and evaluation at end expiration. When the patient is supine, findings correlate with the severity of apnea and with the site of obstruction during sleep.

During endoscopy, re-creation of the palatal snore might identify the sites of vibration: the uvula and distal soft palate alone, the entire soft palate, or (rarely) only the lateral pharyngeal wall or supraglottic tissues. Having the patient protrude the tongue and jaw and observing the tongue base movement can indicate the effects of limited genioglossus advancement. A submucous cleft palate, notching of the uvula, large palatal blood vessels, and poor lateral wall movement can be identified. The presence of any of these findings can contraindicate UPPP.

Endoscopy and physical examination are not the only predictors of outcomes. The severity of disease, the patient's age, and obesity are important risk factors to consider when assessing surgical patients. Patients with mild disease do much better than those with severe disease. Identical anatomy in two patients who have different degrees of apnea severity can require very different treatment approaches.

Cephalometry 

Cephalometric x-rays measure facial skeletal landmarks. Analysis has been extended to include soft tissue landmarks. The advantage of cephalometric x-rays is that they are objective methods of evaluation. Results depend on technique. Films must be taken in a standard head position, with gaze parallel to the horizon, with the teeth in light apposition, and on end expiration. Cephalometric analysis identifies lingual obstruction as well as nonspecific predictors of surgical outcome. Retrognathia, mandible plane to hyoid distance, and posterior airway length have been reported. Normal cephalometric values are important predictors, based on values outside of two standard deviations from the mean. Treatment planning with x-rays can be useful in predicting outcomes as well as in planning skeletal advancement procedures.

Characteristics that can be useful in discriminating normal subjects from OSA patients might not be the same characteristics that are useful in selecting patients for particular procedures. Few landmarks have demonstrated correlation to surgical outcomes, and there have been few consistent associations with outcomes. Measurements correlated to outcomes include a longer distance between the hyoid bone and the mandibular plane, the size of the posterior airway space, the distance from the tip of the tongue to the base of the valleculae, and the length of the posterior airway. [27-30] Differences in skeletal subtype, gender, and race can confound these measurements, so controlling for these variables can improve outcomes.

In conclusion, airway evaluation for OSA is an evolving science. Each clinical tool--the physical examination, endoscopy, and cephalometric x-ray--can contribute to the surgeon's assessment. A better understanding of the mechanics and physiology of airway obstruction will ultimately improve patient outcomes.

From the Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, Milwaukee.

Reprint requests: B. Tucker Woodson, MD, Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, 7200 W. Wisconsin Ave., Milwaukee, WI 53226. Phone:[phone]; fax [phone]; e-mail:[e-mail]

References

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Methods of evaluating the upper airway

Acoustic reflection

Cephalometric x-rays [*]

Computed tomography

Endoscopy [*]

Fluoroscopy 

Magnetic resonance imaging 

Pharyngeal manometry

Physical examination [*]

(*.) Office methods.

Formula for morphometric airway analysis

Morphometric index = morphometric component + obesity component

Index = P + ([Mx - Mn] + 3 x OJ) + (BMI - 25) x (NC/BMI)

A value [greater than]70 indicates a risk of obstructive sleep apnea syndrome.

Key:

P Height of the palate from the incisors (mm)

Mx Maxillary molar width (mm)

Mn Mandibular molar width (mm)

OJ Incisor overjet (mm)

BMI Body mass index (kg/[m.sup.2])

NC Neck circumference (cm)