Sensorimotor function of the upper-airway muscles and respiratory sensory processing in untreated obstructive sleep apnea

Mechanisms contributing to the response of upper-airway muscles to changes in airway pressure

This study assessed the effects of inhaled lignocaine to reduce upper airway surface mechanoreceptor activity on 1) basal genioglossus and tensor palatini EMG, 2) genioglossus reflex responses to large pulses (∼10 cmH2O) of negative airway pressure, and 3) upper airway collapsibility in 15 awake individuals. Genioglossus and tensor palatini muscle EMG and airway pressures were recorded during quiet nasal breathing and during brief pulses (250 ms) of negative upper-airway pressure. Lignocaine reduced peak inspiratory (5.6 ± 1.5 vs. 3.8 ± 1.1% maximum; mean ± SE, P < 0.01) and tonic (2.8 ± 0.8 vs. 2.1 ± 0.7% maximum; P < 0.05) genioglossus EMG during quiet breathing but had no effect on tensor palatini EMG (5.0 ± 0.8 vs. 5.0 ± 0.5% maximum; P = 0.97). Genioglossus reflex excitation to negative pressure pulses decreased after anesthesia (60.9 ± 20.7 vs. 23.6 ± 5.2 μV; P < 0.05), but not when expressed as a percentage of the immediate prestimulus baseline. Reflex excitation was closely related to the change in baseline EMG following lignocaine ( r2 = 0.98). A short-latency genioglossus reflex to rapid increases from negative to atmospheric pressure was also observed. The upper airway collapsibility index (%difference) between nadir choanal and epiglottic pressure increased after lignocaine (17.8 ± 3.7 vs. 28.8 ± 7.5%; P < 0.05). These findings indicate that surface receptors modulate genioglossus but not tensor palatini activity during quiet breathing. However, removal of input from surface mechanoreceptors has minimal effect on genioglossus reflex responses to large (∼10 cmH2O), sudden changes in airway pressure. Changes in pressure rather than negative pressure per se can elicit genioglossus reflex responses. These findings challenge previous views and have important implications for upper airway muscle control.

Effects of one-week tongue task training on sleep apnea severity: A pilot study

The aim of the present study was to assess the effects of one-week tongue-task training (TTT) on sleep apnea severity in sleep apnea subjects. Ten patients with sleep apnea (seven men, mean [± SD] age 52 ± 8 years; mean apnea-hypopnea [AHI] index 20.9 ± 5.3 events/h) underwent 1 h TTT in the authors' laboratory on seven consecutive days. A complete or limited recording and tongue maximal protruding force were assessed before and after one-week TTT. One-week TTT was associated with a global AHI decrease (pre-TTT: 20.9 ± 5.3 events/h; post-TTT: 16.1 ± 5.1 events/h; P<0.001) and AHI decrease during rapid eye movement sleep (pre-TTT: 32.2 ± 18.4 events/h; post-TTT: 16.7 ± 6.6 events/h; P=0.03), while protruding force remained unchanged. The authors consider these results to be potentially clinically relevant and worthy of further investigation in a large randomized trial.

Respiratory cortical processing to inspiratory resistances during wakefulness in children with the obstructive sleep apnea syndrome

Children with the obstructive sleep apnea syndrome (OSAS) have impaired respiratory afferent cortical processing during sleep that persists after treatment of OSAS. However, it is unknown whether this impairment is present during wakefulness and, if so, whether it improves after OSAS treatment. We hypothesized that children with OSAS, during wakefulness, have abnormal cortical processing of respiratory stimuli manifested by blunted respiratory-related evoked potentials (RREP) and that this resolves after OSAS treatment. We measured RREP during wakefulness in 26 controls and 21 children with OSAS before and after treatment. Thirteen participants with OSAS repeated testing 3-6 mo after adenotonsillectomy. RREP were elicited by interruption of inspiration by total occlusion and 30 and 20 cmH2O/l per s resistances. Nf at Fz latency elicited by occlusion was longer in children with OSAS at baseline compared with controls (78.8 ± 24.8 vs. 63.9 ± 19.7 ms, P = 0.05). All other peak amplitudes and latencies were similar between the two groups. After OSAS treatment, Nf at Fz latency elicited by 30 cmH2O/l per s decreased significantly (before, 88 ± 26 vs. after, 71 ± 25 ms, P = 0.02), as did that elicited by 20 cmH2O/l per s (85 ± 27 vs. 72 ± 24 ms, P = 0.004). The amplitude of N1 at Cz elicited by occlusion increased from -3.4 ± 5.6 to -7.4 ± 3 μV (P = 0.049) after treatment. We concluded that children with OSAS have partial delay of respiratory afferent cortical processing during wakefulness that improves after treatment.

Tongue fat and its relationship to obstructive sleep apnea

STUDY OBJECTIVES

The objective of this study was to determine whether tongue fat is increased in obese sleep apneics compared to obese subjects without sleep apnea. We hypothesized that excess fat is deposited in the tongue in obese patients with sleep apnea.

DESIGN

Case-control design.

SETTING

Academic medical center.

PATIENTS

We examined tongue fat in 31 obese controls (apnea-hypopnea index, 4.1 ± 2.7 events/h) and 90 obese apneics (apnea-hypopnea index, 43.2 ± 27.3 events/h). Analyses were repeated in a subsample of 18 gender-, race-, age-, and BMI-matched case-control pairs.

INTERVENTIONS

All subjects underwent a MRI with three-point Dixon magnetic resonance imaging. We used sophisticated volumetric reconstruction algorithms to study the size and distribution of upper airway fat deposits in the tongue and masseter muscles within apneics and obese controls.

MEASUREMENTS AND RESULTS

The data supported our a priori hypotheses that after adjustment for age, BMI, gender, and race, the tongue in apneics was significantly larger (P = 0.001) and had an increased amount of fat (P = 0.002) compared to controls. Similar results were seen in our matched sample. Our data also demonstrate that within the apneic and normal tongue, there are regional differences in fat distribution, with larger fat deposits at the base of the tongue.

CONCLUSIONS

There is increased tongue volume and deposition of fat at the base of tongue in apneics compared to controls. Increased tongue fat may begin to explain the relationship between obesity and obstructive sleep apnea.

Metabolic activity of the tongue in obstructive sleep apnea. A novel application of FDG positron emission tomography imaging

RATIONALE

The metabolic activity of the tongue is unknown in patients with obstructive sleep apnea (OSA). Tongue electromyographic (EMG) activity is increased in patients with OSA. This increase in tongue EMG activity is thought to be related to either increased neuromuscular compensation or denervation with subsequent reinnervation of the muscle fibers. Increased glucose uptake in the tongue would support increased neuromuscular compensation, whereas decreased glucose uptake in the tongue would support denervation with subsequent reinnervation of the muscle fibers.

OBJECTIVES

To investigate the metabolic activity of the genioglossus and control upper airway muscles in obese patients with sleep apnea compared with obese control subjects.

METHODS

Obese subjects with and without OSA underwent a standard overnight sleep study to determine an apnea-hypopnea index. Each subject had a positron emission tomography with [(18)F]-2-fluoro-2-deoxy-D-glucose scan in addition to noncontrast computed tomography or magnetic resonance imaging. Glucose uptake was quantified within upper airway tissues with the standardized uptake value.

MEASUREMENTS AND MAIN RESULTS

We recruited 30 obese control subjects (apnea-hypopnea index, 4.7 ± 3.1 events per hour) and 72 obese patients with sleep apnea (apnea-hypopnea index, 43.5 ± 28.0 events per hour). Independent of age, body mass index, sex, and race, patients with OSA had significantly reduced glucose uptake in the genioglossus (P = 0.03) in comparison with obese normal subjects. No differences in standardized uptake value were found in the control muscles (masseter [P = 0.38] and pterygoid [P = 0.70]) and subcutaneous fat deposits (neck [P = 0.44] and submental [P = 0.95]) between patients with OSA and control subjects.

CONCLUSIONS

There was significantly reduced glucose uptake in the genioglossus of patients with sleep apnea in comparison with obese normal subjects with [(18)F]-2-fluoro-2-deoxy-D-glucose positron emission tomography imaging. The reduction in glucose uptake was likely secondary to alterations in tongue muscle fiber-type or secondary to chronic denervation. The reduced glucose uptake argues against the neuromuscular compensation hypothesis explaining the increase in tongue EMG activity in obese patients with OSA.

Examining the mechanism of action of a new device using oral pressure therapy for the treatment of obstructive sleep apnea

STUDY OBJECTIVES

The objective of this study was to explore the mechanism of action of the oral pressure therapy (OPT) device, a new treatment for sleep apnea.

DESIGN

Case series.

SETTING

Academic medical center.

PATIENTS

Fifteen subjects with sleep apnea who had been successfully treated (responders) with the OPT device and 4 subjects who were not successfully treated (non-responders) with the OPT device.

INTERVENTIONS

All subjects underwent a MRI (without the device, with the device in place without vacuum and with the device in place with vacuum) to examine the biomechanical changes associated with the OPT device.

MEASUREMENTS AND RESULTS

Oral pressure therapy significantly (P = 0.002) increased the size of the retropalatal airway in both the lateral and anterior-posterior dimensions by moving the soft palate anteriorly and superiorly and the anterior-superior segment of the tongue forward, toward the teeth. The percentage and absolute increase in the cross-sectional area of the retropalatal region, the superior movement of the soft palate, and the anterior displacement of the tongue were significantly greater in the responders than in the non-responders. In responders, there were significant increases in the mean (P = 0.002), maximum (P = 0.0002), and minimum (P = 0.04) cross-sectional areas of the retropalatal region with the OPT device. However, in the retroglossal region, airway caliber decreased with the OPT device.

CONCLUSIONS

In those who responded to oral pressure therapy, it increased airway caliber in the retropalatal region by moving the soft palate anteriorly and superiorly and the anterior-superior segment of the tongue forward.

CITATION

Schwab RJ, Kim C, Siegel L, Keenan BT, Black J, Farid-Moayer M, Podmore J, Vaska M. Examining the mechanism of action of a new device using oral pressure therapy for the treatment of obstructive sleep apnea.

Trazodone increases the respiratory arousal threshold in patients with obstructive sleep apnea and a low arousal threshold

STUDY OBJECTIVES

The effect of common sedatives on upper airway physiology and breathing during sleep in obstructive sleep apnea (OSA) has been minimally studied. Conceptually, certain sedatives may worsen OSA in some patients. However, sleep and breathing could improve with certain sedatives in patients with OSA with a low respiratory arousal threshold. This study aimed to test the hypothesis that trazodone increases the respiratory arousal threshold in patients with OSA and a low arousal threshold. Secondary aims were to examine the effects of trazodone on upper airway dilator muscle activity, upper airway collapsibility, and breathing during sleep.

DESIGN

Patients were studied on 4 separate nights according to a within-subjects cross-over design.

SETTING

Sleep physiology laboratory.

PATIENTS

Seven patients with OSA and a low respiratory arousal threshold.

INTERVENTIONS

In-laboratory polysomnograms were obtained at baseline and after 100 mg of trazodone was administered, followed by detailed overnight physiology experiments under the same conditions. During physiology studies, continuous positive airway pressure was transiently lowered to measure arousal threshold (negative epiglottic pressure prior to arousal), dilator muscle activity (genioglossus and tensor palatini), and upper airway collapsibility (Pcrit).

MEASUREMENTS AND RESULTS

Trazodone increased the respiratory arousal threshold by 32 ± 6% (-11.5 ± 1.4 versus -15.3 ± 2.2 cmH2O, P < 0.01) but did not alter the apnea-hypopnea index (39 ± 12 versus 39 ± 11 events/h sleep, P = 0.94). Dilator muscle activity and Pcrit also did not systematically change with trazodone.

CONCLUSIONS

Trazodone increases the respiratory arousal threshold in patients with obstructive sleep apnea and a low arousal threshold without major impairment in dilator muscle activity or upper airway collapsibility. However, the magnitude of change in arousal threshold was insufficient to overcome the compromised upper airway anatomy in these patients.

Adult obstructive sleep apnoea

Obstructive sleep apnoea is an increasingly common disorder of repeated upper airway collapse during sleep, leading to oxygen desaturation and disrupted sleep. Features include snoring, witnessed apnoeas, and sleepiness. Pathogenesis varies; predisposing factors include small upper airway lumen, unstable respiratory control, low arousal threshold, small lung volume, and dysfunctional upper airway dilator muscles. Risk factors include obesity, male sex, age, menopause, fluid retention, adenotonsillar hypertrophy, and smoking. Obstructive sleep apnoea causes sleepiness, road traffic accidents, and probably systemic hypertension. It has also been linked to myocardial infarction, congestive heart failure, stroke, and diabetes mellitus though not definitively. Continuous positive airway pressure is the treatment of choice, with adherence of 60-70%. Bi-level positive airway pressure or adaptive servo-ventilation can be used for patients who are intolerant to continuous positive airway pressure. Other treatments include dental devices, surgery, and weight loss.

Effects of inhaled fluticasone on upper airway during sleep and wakefulness in asthma: a pilot study

STUDY OBJECTIVE

Obstructive sleep apnea is prevalent among people with asthma, but underlying mechanisms remain unknown. Inhaled corticosteroids may contribute. We tested the effects of orally inhaled fluticasone propionate (FP) on upper airway (UAW) during sleep and wakefulness.

STUDY DESIGN

16-week single-arm study.

PARTICIPANTS

18 (14 females, mean [ ± SD] age 26 ± 6 years) corticosteroid-naïve subjects with mild asthma (FEV1 89 ± 8% predicted).

INTERVENTIONS

High dose (1,760 mcg/day) inhaled FP.

MEASUREMENTS

(1) UAW collapsibility (passive critical closing pressure [Pcrit]); (2) tongue strength (maximum isometric pressure-Pmax, in KPa) and endurance-time (in seconds) able to maintain 50% Pmax across 3 trials (Ttot)-at anterior and posterior locations; (3) fat fraction and volume around UAW, measured by magnetic resonance imaging in three subjects.

RESULTS

Pcrit overall improved (became more negative) (mean ± SE) (-8.2 ± 1.1 vs. -12.2 ± 2.2 cm H2O, p = 0.04); the response was dependent upon baseline characteristics, with older, male gender, and worse asthma control predicting Pcrit deterioration (less negative). Overall, Pmax increased (anterior p = 0.02; posterior p = 0.002), but Ttot generally subsided (anterior p = 0.0007; posterior p = 0.06), unrelated to Pcrit response. In subjects studied with MRI, fat fraction and volume increased by 20.6% and 15.4%, respectively, without Pcrit changes, while asthma control appeared improved.

CONCLUSIONS

In this study of young, predominantly female, otherwise healthy subjects with well-controlled asthma and stiff upper airways, 16-week high dose FP treatment elicited Pcrit changes which may be dependent upon baseline characteristics, and determined by synchronous and reciprocally counteracting local and lower airway effects. The long-term implications of these changes on sleep disordered breathing severity remain to be determined.

Effect of rostral fluid shift on pharyngeal resistance in men with and without obstructive sleep apnea

BACKGROUND

Obstructive sleep apnea (OSA) relates to overnight rostral fluid shift, possibly because fluid accumulation around the pharynx increases pharyngeal resistance (Rph). We hypothesised that Rph will increase more in men with than without OSA in response to rostral fluid redistribution.

METHODS

Seventeen men with, and 12 without OSA were randomized to lower body positive pressure (LBPP) for 15min or control, then crossed over. Leg fluid volume (LFV) and Rph were measured before and after each period.

RESULTS

LBPP displaced similar amounts of fluid from the legs in both groups. However, compared to the non-OSA group, Rph increased significantly more during LBPP in the OSA group (-0.38±2.87 vs. 2.52±2.94cmH2O/l/s, p=0.016). Change in Rph during LBPP correlated directly with baseline Rph in the OSA group, but inversely in the non-OSA group.

CONCLUSION

OSA patients have increased susceptibility to pharyngeal obstruction in response to rostral fluid redistribution, which could predispose to pharyngeal collapse during sleep.

Respiratory and auditory cortical processing in children with obstructive sleep apnea syndrome

RATIONALE

Children with obstructive sleep apnea syndrome (OSAS) have impaired cortical processing of respiratory afferent stimuli, manifested by blunted sleep respiratory-related evoked potentials (RREP). However, whether this impairment is limited to respiratory stimuli, or reversible after successful treatment, is unknown. We hypothesized that, during sleep, children with OSAS have (1) abnormal RREP, (2) normal cortical processing of nonrespiratory stimuli, and (3) persistence of abnormal RREP after treatment.

OBJECTIVES

To measure sleep RREP and auditory evoked potentials in normal control subjects and children with OSAS before and after treatment.

METHODS

Twenty-four children with OSAS and 24 control subjects were tested during N3 sleep. Thirteen children with OSAS repeated testing 4-6 months after adenotonsillectomy.

MEASUREMENTS AND MAIN RESULTS

RREP were blunted in OSAS compared with control subjects (N350 at Cz -27 ± 15.5 vs. -47.4 ± 28.5 μV; P = 0.019), and did not improve after OSAS treatment (N350 at Cz pretreatment -25.1 ± 7.4 vs. -29.8 ± 8.1 post-treatment). Auditory evoked potentials were similar in OSAS and control subjects at baseline (N350 at Cz -58 ± 33.1 vs. -66 ± 31.1 μV), and did not change after treatment (N350 at Cz -67.5 ± 36.8 vs. -65.5 ± 20.3).

CONCLUSIONS

Children with OSAS have persistent primary or irreversible respiratory afferent cortical processing deficits during sleep that could put them at risk of OSAS recurrence. OSAS does not seem to affect the cortical processing of nonrespiratory (auditory) afferent stimuli during sleep.

Defining phenotypic causes of obstructive sleep apnea. Identification of novel therapeutic targets

RATIONALE

The pathophysiologic causes of obstructive sleep apnea (OSA) likely vary among patients but have not been well characterized.

OBJECTIVES

To define carefully the proportion of key anatomic and nonanatomic contributions in a relatively large cohort of patients with OSA and control subjects to identify pathophysiologic targets for future novel therapies for OSA.

METHODS

Seventy-five men and women with and without OSA aged 20-65 years were studied on three separate nights. Initially, the apnea-hypopnea index was determined by polysomnography followed by determination of anatomic (passive critical closing pressure of the upper airway [Pcrit]) and nonanatomic (genioglossus muscle responsiveness, arousal threshold, and respiratory control stability; loop gain) contributions to OSA.

MEASUREMENTS AND MAIN RESULTS

Pathophysiologic traits varied substantially among participants. A total of 36% of patients with OSA had minimal genioglossus muscle responsiveness during sleep, 37% had a low arousal threshold, and 36% had high loop gain. A total of 28% had multiple nonanatomic features. Although overall the upper airway was more collapsible in patients with OSA (Pcrit, 0.3 [-1.5 to 1.9] vs. -6.2 [-12.4 to -3.6] cm H2O; P <0.01), 19% had a relatively noncollapsible upper airway similar to many of the control subjects (Pcrit, -2 to -5 cm H2O). In these patients, loop gain was almost twice as high as patients with a Pcrit greater than -2 cm H2O (-5.9 [-8.8 to -4.5] vs. -3.2 [-4.8 to -2.4] dimensionless; P = 0.01). A three-point scale for weighting the relative contribution of the traits is proposed. It suggests that nonanatomic features play an important role in 56% of patients with OSA.

CONCLUSIONS

This study confirms that OSA is a heterogeneous disorder. Although Pcrit-anatomy is an important determinant, abnormalities in nonanatomic traits are also present in most patients with OSA.

Localized cortical thinning in patients with obstructive sleep apnea syndrome

STUDY OBJECTIVES

To investigate differences in cortical thickness in patients with obstructive sleep apnea (OSA) syndrome and healthy controls.

DESIGN

Cortical thickness was measured using a three-dimensional surface-based method that enabled more accurate measurement in deep sulci and localized regional mapping.

SETTING

University hospital.

PATIENTS

Thirty-eight male patients with severe OSA (mean apnea-hypopnea index > 30/h) and 36 age-matched male healthy controls were enrolled.

INTERVENTIONS

Cortical thickness was obtained at 81,924 vertices across the entire brain by reconstructing inner and outer cortical surfaces using an automated anatomical pipeline.

MEASUREMENTS

Group difference in cortical thickness and correlation between patients' data and thickness were analyzed by a general linear model.

RESULTS

Localized cortical thinning in patients was found in the orbitorectal gyri, dorsolateral/ventromedial prefrontal regions, pericentral gyri, anterior cingulate, insula, inferior parietal lobule, uncus, and basolateral temporal regions at corrected P < 0.05. Patients with OSA showed impaired attention and learning difficulty in memory tests compared to healthy controls. Higher number of respiratory arousals was related to cortical thinning of the anterior cingulate and inferior parietal lobule. A significant correlation was observed between the longer apnea maximum duration and the cortical thinning of the dorsolateral prefrontal regions, pericentral gyri, and insula. Retention scores in visual memory tests were associated with cortical thickness of parahippocampal gyrus and uncus.

CONCLUSIONS

Brain regions with cortical thinning may provide elucidations for prefrontal cognitive dysfunction, upper airway sensorimotor dysregulation, and cardiovascular disturbances in OSA patients, that experience sleep disruption including sleep fragmentation and oxygen desaturation.

Respiratory Movement of Upper Airway Tissue in Obstructive Sleep Apnea

STUDY OBJECTIVES

To measure real-time movement of the tongue and lateral upper airway tissues in obstructive sleep apnea (OSA) subjects during wakefulness using tagged magnetic resonance imaging.

DESIGN

Comparison of the dynamic imaging of three groups of increasing severity OSA and a control group approximately matched for age and body mass index (BMI).

SETTING

Not-for-profit research institute.

PARTICIPANTS

24 subjects (apnea hypopnea index [AHI] range 2-84 events/h, 6 with AHI < 5 events/h).

METHODS

The upper airway was imaged awake in two planes using SPAtial Modulation of Magnetization (SPAMM). Tissue displacements were quantified with harmonic phase analysis.

MEASUREMENTS AND RESULTS

All subjects had dynamic airway opening in the sagittal plane associated with inspiration. In the nasopharynx, the increase in airway cross-sectional area during inspiration correlated with minimal cross-sectional area of the airway (R = 0.900, P < 0.001). AHI correlated negatively with movement of the nasopharyngeal lateral walls (R = - 0.542, P = 0.006). Four movement patterns were observed during inspiration: "en bloc" anterior movement of the whole posterior tongue; movement of only the oropharyngeal posterior tongue; bidirectional movement; or minimal movement. Some subjects showed different inspiratory movement patterns with different breaths. A low AHI (< 5) was associated with en bloc movement (P = 0.002).

CONCLUSIONS

Inspiratory movement of the tongue varied between and within subjects, likely as a result of local and neural factors. However, in severe OSA inspiratory movement was minimal.

CITATION

Brown EC; Cheng S; McKenzie DK; Butler JE; Gandevia SC; Bilston LE. Respiratory movement of upper airway tissue in obstructive sleep apnea. SLEEP 2013;36(7):1069-1076.

A simplified method for determining phenotypic traits in patients with obstructive sleep apnea

We previously published a method for measuring several physiological traits causing obstructive sleep apnea (OSA). The method, however, had a relatively low success rate (76%) and required mathematical modeling, potentially limiting its application. This paper presents a substantial revision of that technique. To make the measurements, continuous positive airway pressure (CPAP) was manipulated during sleep to quantify 1) eupneic ventilatory demand, 2) the level of ventilation at which arousals begin to occur, 3) ventilation off CPAP (nasal pressure = 0 cmH(2)O) when the pharyngeal muscles are activated during sleep, and 4) ventilation off CPAP when the pharyngeal muscles are relatively passive. These traits could be determined in all 13 participants (100% success rate). There was substantial intersubject variability in the reduction in ventilation that individuals could tolerate before having arousals (difference between ventilations #1 and #2 ranged from 0.7 to 2.9 liters/min) and in the amount of ventilatory compensation that individuals could generate (difference between ventilations #3 and #4 ranged from -0.5 to 5.5 liters/min). Importantly, the measurements accurately reflected clinical metrics; the difference between ventilations #2 and #3, a measure of the gap that must be overcome to achieve stable breathing during sleep, correlated with the apnea-hypopnea index (r = 0.9, P < 0.001). An additional procedure was added to the technique to measure loop gain (sensitivity of the ventilatory control system), which allowed arousal threshold and upper airway gain (response of the upper airway to increasing ventilatory drive) to be quantified as well. Of note, the traits were generally repeatable when measured on a second night in 5 individuals. This technique is a relatively simple way of defining mechanisms underlying OSA and could potentially be used in a clinical setting to individualize therapy.

Functional role of neural injury in obstructive sleep apnea

The causes of obstructive sleep apnea (OSA) are multifactorial. Neural injury affecting the upper airway muscles due to repetitive exposure to intermittent hypoxia and/or mechanical strain resulting from snoring and recurrent upper airway closure have been proposed to contribute to OSA disease progression. Multiple studies have demonstrated altered sensory and motor function in patients with OSA using a variety of neurophysiological and histological approaches. However, the extent to which the alterations contribute to impairments in upper airway muscle function, and thus OSA disease progression, remains uncertain. This brief review, primarily focused on data in humans, summarizes: (1) the evidence for upper airway sensorimotor injury in OSA and (2) current understanding of how these changes affect upper airway function and their potential to change OSA progression. Some unresolved questions including possible treatment targets are noted.

Genioglossus fatigue in obstructive sleep apnea

Obstructive sleep apnea (OSA) is a prevalent disorder that may cause cardiovascular disease and fatal traffic accidents but the pathophysiology remains incompletely understood. Increased fatigability of the genioglossus (the principal upper airway dilator muscle) might be important in OSA pathophysiology but the existing literature is uncertain. We hypothesized that the genioglossus in OSA subjects would fatigue more than in controls. In 9 OSA subjects and 9 controls during wakefulness we measured maximum voluntary tongue protrusion force (Tpmax). Using surface electromyography arrays we measured the rate of decline in muscle fiber conduction velocity (MFCV) during an isometric fatiguing contraction at 30% Tpmax. The rate of decline in MFCV provides an objective means of quantifying localized muscle fatigue. Linear regression analysis of individual subject data demonstrated a significantly greater decrease in MFCV in OSA subjects compared to control subjects (29.2 ± 20.8% [mean ± SD] versus 11.2 ± 20.8%; p=0.04). These data support increased fatigability of the genioglossus muscle in OSA subjects which may be important in the pathophysiology of OSA.

Neurogenic changes in the upper airway of patients with obstructive sleep apnea

RATIONALE

Controversy persists regarding the presence and importance of hypoglossal nerve dysfunction in obstructive sleep apnea (OSA).

OBJECTIVES

We assessed quantitative parameters related to motor unit potential (MUP) morphology derived from electromyographic (EMG) signals in patients with OSA versus control subjects and hypothesized that signs of neurogenic remodeling would be present in the patients with OSA.

METHODS

Participants underwent diagnostic sleep studies to obtain apnea-hypopnea indices. Muscle activity was detected with 50-mm concentric needle electrodes. The concentric needle was positioned at more than 10 independent sites per subject, after the local anatomy of the upper airway musculature was examined by ultrasonography. All activity was quantified with subjects awake, during supine eupneic breathing while wearing a nasal mask connected to a pneumotachograph. Genioglossus EMG signals were analyzed offline by automated software (DQEMG), which extracted motor unit potential trains (MUPTs) contributed by individual motor units from the composite EMG signals. Quantitative measurements of MUP templates, including duration, peak-to-peak amplitude, area, area-to-amplitude ratio, and size index, were compared between the untreated patients with OSA and healthy control subjects.

MEASUREMENTS AND MAIN RESULTS

A total of 1,655 MUPTs from patients with OSA (n = 17; AHI, 55 ± 6/h) and control subjects (n = 14; AHI, 4 ± 1/h) were extracted from the genioglossus muscle EMG signals. MUP peak-to-peak amplitudes in the patients with OSA were not different compared with the control subjects (397.5 ± 9.0 vs. 382.5 ± 10.0 μV). However, the MUPs of the patients with OSA were longer in duration (11.5 ± 0.1 vs. 10.3 ± 0.1 ms; P < 0.001) and had a larger size index (4.09 ± 0.02 vs. 3.92 ± 0.02; P < 0.001) compared with control subjects.

CONCLUSIONS

These results confirm and quantify the extent and existence of structural neural remodeling in OSA.