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Neuromuscular Dysfunction and Cortical Impairment in Sleep Apnea Syndrome

MARILLIER, MATHIEU1,2; GRUET, MATHIEU1,2,3; BAILLIEUL, SÉBASTIEN1,2,4; LE ROUX MALLOUF, THIBAULT1,2; WUYAM, BERNARD1,2,4; TAMISIER, RENAUD1,2,4; LEVY, PATRICK1,2,4; PEPIN, JEAN-LOUIS1,2,4; VERGES, SAMUEL1,2

Author Information
Medicine & Science in Sports & Exercise: August 2018 - Volume 50 - Issue 8 - p 1529-1539
doi: 10.1249/MSS.0000000000001625

Abstract

Recognized as a major health concern, obstructive sleep apnea (OSA) syndrome is a chronic disease characterized by repeated partial or complete upper airway obstruction that leads to chronic intermittent hypoxia and sleep fragmentation during the night. These phenomena are associated with increased oxidative stress and neuro-inflammation leading to significant structural and functional brain impairments (see for review [1]). Reduced cerebral blood flow (2) and impaired cerebrovascular reactivity (3,4) have been reported in OSA patients. Several studies suggest that OSA is associated with substantial grey and white matter alterations (5,6). Obstructive sleep apnea is also associated with brain functional impairments as cognitive functions (e.g., memory, attention, learning and executive functions) are widely reported to be negatively impacted in OSA patients (7), resulting in lowered work productivity and daytime functioning (8). Some results also suggest that repeated bouts of nocturnal hypoxemia may lead to cellular and molecular modifications in peripheral muscles with an increase in the proportion and size of type II fibers, protein content and microvascularization (9,10). Peripheral nerve dysfunction likely due to axonal lesions has been reported as another neuromuscular consequence of OSA (11). Hence, OSA syndrome is likely to have repercussions along the whole neuromuscular pathway, from the brain and motor cortex to the peripheral nerves and muscles.

Fatigue is a major complaint in patients with OSA syndrome and is associated with severe daytime repercussion (12). While fatigue is usually characterized from subjective evaluations, no studies to date have provided an objective and comprehensive assessment of neuromuscular fatigue and its functional impact in OSA syndrome. Neuromuscular fatigue can originate from peripheral (i.e., a reduction in force or power output secondary to changes occurring at or distal to the neuromuscular junction) or central (i.e., a reduction in voluntary activation [VA] [13]) mechanisms. Peripheral muscle alterations in strength and endurance have been shown to occur in OSA (14). Whether the cerebral alterations previously reported in OSA may be associated with central activation deficit contributing to these impairments in neuromuscular function remain to be determined.

Previous studies using transcranial magnetic stimulation (TMS) to evaluate peripheral muscles in OSA reported reduced corticospinal excitability and increased intracortical inhibition (15,16) in these patients. However, these assessments were performed at rest or on low level of muscle contraction where TMS is less effective at activating motoneurons due to a reduction in corticospinal excitability (17). Hence, the functional consequences (i.e., muscle strength and endurance) of the abovementioned corticospinal alterations in OSA and whether changes in corticospinal excitability and inhibition during muscle exercise are different in OSA patients compared to healthy individuals remain unknown.

Continuous positive airway pressure (CPAP), the first-line therapy for OSA, is known to improve patients’ symptoms and quality of life (18). Several studies demonstrated that CPAP treatment can reverse, at least in part, cerebral structural alterations and cognitive impairments induced by OSA (5,19). Yet, the effect of CPAP on the central and peripheral components of the neuromuscular function is unknown. Whether the structural and functional brain improvements reported after CPAP treatment could contribute to neuromuscular function normalization in OSA patients remains to be investigated.

The aim of the present study was to investigate the effect of OSA on knee extensor function (strength and endurance) and peripheral and central neuromuscular mechanisms of fatigue during isometric knee extensions, before and after 8 wk of CPAP treatment. We hypothesized that (i) OSA patients would demonstrate reduced knee extensor strength and endurance due to corticospinal impairments and (ii) 8 wk of CPAP treatment would improve exercise muscle function and neuromuscular abnormalities in these patients.

METHODS

Subjects

Fifteen severe, newly diagnosed OSA patients were initially recruited in this prospective controlled study. Inclusion criteria were: (1) age ≥18 and <70 yr, (2) body mass index <30 kg·m−2, (3) severe OSA syndrome (apnea–hypopnea index >30 events per hour) and (4) to present a strictly normal neurological evaluation. OSA diagnosis was based on a full-night polysomnography. Two patients presented untreated grade 1 hypertension and 2 patients presented grade 3 hypertension treated by antihypertensive medications (20). Fifteen healthy participants of similar age, body mass index, sex, and self-reported physical activity (by questionnaire, see below) were recruited by advertisement in newspapers and posters in our hospital to be included as control group. They underwent a full-night polysomnography similar to OSA participants to ensure they were free of sleep disorder (defined as an apnea–hypopnea index >5 with the presence of clinical symptoms (e.g., excessive daytime sleepiness) or an AHI > 15 regardless of the presence of clinical symptoms [21]). Subjects refrained from prolonged and intense physical activity on the 2 d before the tests, abstained from drinking caffeinated beverages on test days, and had their last meal at least 3 h before the tests. Participants were asked to maintain their usual physical activity habits throughout the study. This study was performed according to the Declaration of Helsinki and was approved by the local ethics committee (CPP Grenoble Sud Est V, 2012-A00158-35) and registered at clinicaltrials.gov (NCT02854280). All participants gave their written informed consent before their participation in the study and were free to withdraw at any time.

Study Design

OSA syndrome diagnosis was based on a full-night polysomnography performed according to American Academy of Sleep Medicine recommendations (22). Before each neuromuscular assessment test, daytime sleepiness was assessed with the Epworth Sleepiness Scale (23). Self-reported physical activity was assessed with the Baecke Physical Activity Questionnaire (24). Then, OSA patients performed intermittent isometric knee extensions to task failure before and after an 8-wk CPAP treatment. Control participants performed the same evaluations before and after an 8-wk period. Before the first experimental session, participants were familiarized with the different stimulation techniques and with the knee extension ergometer to perform maximal and submaximal voluntary contractions.

Experimental Setup

Subjects sat in an upright position, with hip and knee angles set at 90° of flexion. Their right leg was connected to a strain gauge (Captels, St. Mathieu deTreviers, France) ~3 cm above the tip of the lateral malleolus. Before, during and after the knee extension fatiguing task, neuromuscular evaluations were performed with single- and paired-pulse TMS and femoral nerve electrical stimulation (FNES) to assess maximal VA, corticospinal excitability and inhibition, neuromuscular transmission and muscle contractile properties.

EMG recordings

EMG signals of the right vastus lateralis, rectus femoris, vastus medialis and biceps femoris were continuously recorded using bipolar silver chloride surface electrodes of 20-mm diameter (Contrôle Graphique Medical, Brie-Comte-Robert, France) during voluntary and magnetically/electrically evoked contractions. The recording electrodes were secured lengthwise to the skin over the muscle belly following SENIAM recommendations (25), with an interelectrode distance of 20 mm. The reference electrode was fixed over the patella. Low impedance (Z < 5 kΩ) at the skin–electrode surface was obtained by abrading the skin with fine sand paper and cleaning with alcohol. EMG signals were amplified, band-pass filtered (5–1 kHz; input impedance, 200 MΩ; common mode rejection ratio, 85 dB; gain, 1000) and recorded at a sampling rate of 2 kHz using BioAmp and PowerLab systems (ADInstruments, Bella Vista, Australia) to be stored on a computer for subsequent analysis with LabChart 7 software (ADInstruments).

Femoral nerve electrical stimulation

Single FNES was delivered percutaneously to the femoral nerve via a self-adhesive electrode (20-mm diameter, Ag-AgCl, Controle Graphique Medical) manually pressed by the experimenter into the femoral triangle to minimize stimulus intensity and discomfort. The anode, a 10 cm × 5 cm gel pad electrode (Compex SA, Ecublens, Switzerland), was located in the gluteal fold. Square wave pulses (1-ms duration) were produced via a high-voltage (maximal voltage, 400 V) constant-current stimulator (Digitimer DS7, Hertfordshire, UK). Femoral nerve electrical stimulation intensity (176 ± 28 mA in OSA vs. 161 ± 36 mA in controls, P > 0.05) corresponded to 140% of the optimal intensity, that is, the stimulus intensity at which the maximal amplitude of both twitch force and concomitant quadriceps muscle M-wave were reached.

Transcranial magnetic stimulation

A magnetic stimulator (Magstim 200; The Magstim Company, Dyfed, UK) was used to stimulate the motor cortex. Single or paired TMS pulses of 1-ms duration were delivered via a concave double-cone coil (110-mm outside diameter) positioned over the scalp to preferentially activate the left motor cortex (contralateral to the right leg) and elicit the largest motor evoked potential (MEP) in the vastus lateralis, rectus femoris and vastus medialis with only a small MEP in the biceps femoris (amplitude <10% of maximal quadriceps muscle M-wave) during isometric knee extension at 10% maximal voluntary contraction (MVC) with a stimulus intensity of 60% of maximal stimulator power output (part I, Fig. 1). The optimal stimulus site was defined in each session and marked on a cap which was fixed directly to the scalp to ensure reproducibility of the stimulus conditions for each subject throughout the entire session.

F1
FIGURE 1:
Overview of the neuromuscular evaluation (panel A) and the isometric knee extension fatiguing task (panel B). After a standardized warm-up and MVC, neuromuscular assessment consisted in three parts: part I, determination of the optimal site for TMS; part II, determination of the optimal TMS intensity (active motor threshold); part III, assessment of maximal VA, muscle contractile properties, MEP and corticospinal excitability and intracortical inhibition with FNES or TMS. The isometric knee extensors fatiguing task consisted in sets of 17 submaximal contractions interspaced by neuromuscular evaluations, until task failure. MPO, maximal power output of the stimulator.

TMS during brief (3 s) isometric knee extensions at 10% MVC were then performed to determine active motor threshold (AMT) intensity (part II, Fig. 1). Six consecutive contractions were performed at each stimulus intensity, starting at 20% of maximal power stimulator output and increased by 5% until AMT was reached, with 10 s between contractions at the same stimulus intensity and 30 s between sets of 6 contractions. The stimulus intensity that elicited > 3 visually detected MEPs over six contractions in the right vastus lateralis, rectus femoris and vastus medialis was defined as AMT. The intensity that corresponded to 140% AMT (62% ± 10% and 64% ± 13% of maximal stimulator power output in OSA and controls, respectively, P > 0.05) was used throughout the protocol.

Fatigue Protocol and Neuromuscular Evaluations

After a standardized warm-up (3 min) and the initial neuromuscular evaluations, subjects performed a fatiguing task consisting in sets of 17 intermittent submaximal isometric knee extensions (5-s contraction/4-s relaxation, total set duration: 153 s) interspaced by neuromuscular evaluations (duration ~40 s). Target force during the fatiguing task was set at 35% MVC for the two first sets. Intensity was then increased by 5% every two sets of contractions until task failure. Task failure was defined as the inability of the subject to sustain the target force for at least 4 s during two consecutive contractions. Real-time visual feedback of target force levels and soundtrack indicating the contraction-relaxation rhythm were provided to subjects throughout the whole experiment.

Initial neuromuscular evaluations (Fig. 1, part III) consisted of three sets of six voluntary contractions (with 90-s interval between sets) as follows: three submaximal contractions at 35% MVC to assess maximal M-wave, MEP, cortical silent period (CSP), long-interval intracortical inhibition (LICI) and short-interval intracortical inhibition (SICI). Then, one MVC with superimposed single FNES and TMS was performed. Two seconds after the contraction, single FNES was delivered on relaxed muscles. Finally, two submaximal contractions at 75% and 50% MVC (calculated from the MVC of each set) with superimposed single TMS were performed. Immediately after each set of 17 contractions during the fatiguing task, neuromuscular evaluations consisted of three submaximal contractions at 35% MVC and one MVC only. Immediately after task failure, neuromuscular evaluation consisted of part III as before the fatiguing task.

Strong verbal encouragement was given during MVC. The subjects were instructed to return as quickly as possible to the target force level after each TMS pulse elicited during voluntary contractions to allow valid assessment of CSP (26). The SICI and LICI were assessed with paired-pulse stimuli of the motor cortex during contractions at 35% MVC. Short-interval intracortical inhibition consisted of a subthreshold conditioning stimulus at 80% AMT followed 3 ms later by suprathreshold test stimulus at 140% AMT (27). Long-interval intracortical inhibition was measured in a paired-pulse paradigm consisting of suprathreshold (140% AMT) conditioning and test stimuli interspaced by a 100-ms interstimulus interval (28).

CPAP Treatment

Continuous positive airway pressure treatment was applied with an autotitrating machine (Autoset Spirit, ResMed, UK or Remstar Auto; Philips Respironics, Murrysville, PA) provided by a home care company (Agir à Dom, France). Continuous positive airway pressure compliance and residual events were measured from the machine’s internal microprocessor.

Data Analysis

Before, during and after the fatiguing task, the torque amplitude of potentiated twitch and maximal M-wave (Mmax) peak-to-peak amplitude in relaxed muscles was measured from single-pulse FNES. Voluntary activation was measured with twitch interpolation technique (29) using the superimposed and potentiated twitch amplitudes elicited by single pulse FNES during and after MVC and calculated from the equation: VAFNES = [1 − (superimposed twitch amplitude × potentiated twitch amplitude−1)] × 100 with appropriate correction when the stimulus was not administered exactly at the maximal MVC force. MEP peak-to-peak amplitudes of quadriceps muscles during TMS superimposed on submaximal and maximal contractions were normalized to M-wave peak-to-peak amplitude (Msup) at the same contraction force level (MEP·Msup−1). The duration of the CSP was determined visually and defined as the duration from the stimulus to the return of continuous voluntary EMG (30). For both LICI and SICI, conditioned test stimulus responses (MEP amplitude) of the three initial and final contraction sets were averaged and then expressed as a percentage of the averaged unconditioned single pulse TMS (27,28) at the same force level (35% MVC). A lower conditioned to unconditioned MEP amplitude ratio is indicative of a larger inhibition.

VATMS was quantified by measurement of the force responses to TMS (superimposed twitch) at 100%, 75%, 50% MVC using the method previously described by Todd et al. (17) and validated for the knee extensors (31). When linear regressions were not linear (r < 0.9, for 6% of our measurements), estimated resting twitch was excluded and VATMS was not calculated for the considered set of contractions. Estimated resting twitch was linear for all subjects for at least two sets of contractions both before the fatiguing task and at task failure, permitting VATMS to be determined in all subjects. VATMS was then calculated using the equation: VATMS = [1 – (superimposed twitch at 100% MVC × estimated resting twitch−1)] × 100.

Peak forces measured during stimuli, MEP, CSP, M-waves, and VA before and after the fatiguing task were calculated as the averaged values obtained during the three sets of contractions. Because similar results were found for all quadriceps muscles, the averaged MEP and M-waves amplitudes, LICI, SICI, and CSP values from the vastus lateralis, rectus femoris and vastus medialis were calculated and used for further analysis.

To compare fatiguing tasks of different durations between the two experimental sessions (before and after CPAP treatment or control period) and between subjects, data were compared over four time points: (i) before the fatiguing task, (ii) at 50% and (iii) 100% of the duration of the shortest fatiguing task for a given subject (i.e., to allow comparison of the two sessions at isotime), and (iv) at task failure. If no neuromuscular evaluation corresponded to exactly 50% or 100% of the duration of the shortest fatiguing task for a given subject, the nearest neuromuscular evaluation was considered.

Statistical Analysis

Power assessment for the primary outcome (VATMS) was based on a minimum expected difference of 4% between groups (OSA and controls). Assuming an α level of 5% and power of 90%, 12 subjects were required per group. Data of three OSA patients and four controls were excluded from the analysis because they did not complete the whole protocol due to discomfort during neuromuscular evaluations. Therefore, 12 OSA patients and 11 healthy controls performed both pre- and post-CPAP or control period neuromuscular evaluations and were included in the analysis. Normality of distribution and homogeneity of variances of the main variables were assessed using a Shapiro–Wilk normality test and the Levene test, respectively. Between-group comparisons at baseline for anthropometric characteristics, questionnaires and sleep data were performed using t tests for independent samples or Mann–Whitney U tests when appropriate. Two-way (group–treatment) or three-way ANOVA (group–treatment–time) with repeated measures were performed for each dependent variable collected during the neuromuscular evaluations. Post hoc Tukey tests were applied to determine a difference between two mean values if the ANOVA revealed a significant main effect or interaction effect. For all statistical analyses, a two-tailed alpha level of 0.05 was used as the cutoff for significance. All data are presented as mean ± SD. All statistical procedures were performed on Statistica version 10 (Statsoft, Tulsa, OK).

RESULTS

Subjects characteristics

Anthropometric characteristics, questionnaires and sleep data of both OSA and control groups are presented in Table 1. No difference was found for age, anthropometric parameters and self-reported physical activity between OSA and controls. Epworth Sleepiness Scale score was higher in OSA compared with controls at baseline (Table 1). Epworth Sleepiness Scale score was reduced in OSA with a decrease from 10.7 ± 5.9 before treatment to 4.8 ± 2.7 after CPAP treatment while no significant change was observed in controls after the 8-wk control period compared to before (group–treatment interaction, F = 6.2, P = 0.017). Apnea–hypopnea index was significantly reduced under CPAP treatment with a decrease from 45.7 ± 13.5 before treatment to 3.7 ± 2.9 events per hour when wearing CPAP (P < 0.001, Table 1).

T1
TABLE 1:
Anthropometric, questionnaires, and sleep data for OSA and CONTROLS groups.

Duration of the knee extensor fatiguing task

Exercise duration to task failure was lower in OSA (before CPAP treatment, 1008 ± 549 s; after CPAP treatment, 975 ± 378 s) compared with controls (before control period, 1476 ± 633 s; after control period, 1274 ± 506 s; ANOVA main effect of group, F = 6.2, P = 0.017). No significant main effect of treatment (F = 0.6, P = 0.45) or group–treatment interaction (F = 0.3, P = 0.59) was observed.

Neuromuscular fatigue

Individual values of the main neuromuscular parameters before the fatiguing task and before CPAP treatment (OSA patients) or control period (controls) are shown in the supplemental figure (see Figure, Supplemental Digital Content 1, Individual data regarding MVC, torque, femoral nerve electrical stimulation, TMS, long-interval cortical inhibition, and cortical silent period, https://links.lww.com/MSS/B255). Maximal voluntary contraction torque before, during, and after the fatiguing task are presented in Figure 2A. MVC torque was lower in OSA compared to controls throughout the fatiguing task (ANOVA main effect of group, F = 8.5, P = 0.004) before and after CPAP treatment. No significant difference in evoked torque of knee extensors was found between OSA and controls (Fig. 2B). Both VAFNES and VATMS were lower in OSA compared with controls throughout the fatiguing task (VAFNES ANOVA main effect of group, F = 5.4, P = 0.022, Fig. 3A; VATMS ANOVA main effect of group, F = 4.0, P = 0.049, Fig. 3B) before and after CPAP treatment. No significant main effect of treatment nor group–treatment interaction were observed for any neuromuscular parameter (all P > 0.05).

F2
FIGURE 2:
MVC (panel A) torque, potentiated twitch torque (Tw, panel B) and average maximal M-wave (Mmax, panel C) amplitude of the knee extensors during the fatiguing task in OSA and control (CONTROLS) groups before (Pre) and after (Post) treatment by CPAP or control period. PRE: before exercise; 50%: 50% of the duration of the shortest test; 100%: 100% of the duration of the shortest test; TF, at task failure. *Significant difference between OSA and CONTROLS (main group effect). Values are mean ± SD.
F3
FIGURE 3:
VA level assessed by femoral nerve electrical stimulation (FNES, panel A) and TMS (panel B) during the fatiguing task in OSA and control (CONTROLS) groups before (Pre) and after (Post) treatment by CPAP or control period. PRE, before exercise; 50%, 50% of the duration of the shortest test; 100%, 100% of the duration of the shortest test; TF, at task failure. *Significant difference between OSA and CONTROLS (main group effect). Values are mean ± SD.

Corticospinal excitability and intracortical inhibition

No significant difference in AMT was found between OSA (before CPAP treatment, 43.6% ± 6.7%; after CPAP treatment, 43.6% ± 9.0%) and controls (before control period, 46.2% ± 8.9%; after control period, 43.5% ± 9.3%, F = 0.3; P = 0.60). A significant main effect of group was observed for Mmax amplitude (F = 13.1, P < 0.001, Fig. 2C) and for MEP amplitudes at both 35% (F = 7.0, P = 0.009) and 100% (F = 5.1, P = 0.025) MVC, with lower values in OSA compared to controls (Table 2). However, no significant difference in MEP·Msup−1 both at 35% (ANOVA main effect of group, F = 0.2, P = 0.90) and 100% MVC (ANOVA main effect of group, F = 0.02, P = 0.99) was found between OSA and controls (Table 2). A significant main effect of group was observed for CSP duration at 35% (F = 16.4, P < 0.001) and 100% (F = 9.7, P = 0.002) MVC with higher values in OSA compared with controls (Table 2). A significant main effect of group was observed for LICI (F = 6.4, P = 0.012) with lower conditioned to unconditioned MEP amplitude ratio in OSA compared with controls (Fig. 4A). However, no significant difference in SICI (F = 1.0, P = 0.33) was found between OSA and controls (Fig. 4B). No significant main effect of treatment nor group–treatment interaction were observed for any corticospinal excitability and intracortical inhibition parameter (all P > 0.05).

T2
TABLE 2:
MEP amplitude, MEP to superimposed maximal M-wave ratio (MEP·Msup−1) and CSP duration in OSA and CONTROLS groups before and after treatment by CPAP or control period.
F4
FIGURE 4:
LICI (panel A) and SICI (panel B) interval cortical inhibition assessed by TMS during the fatiguing task in OSA and control (CONTROLS) groups before (Pre) and after (Post) treatment by CPAP or control period. PRE: before exercise; 50%: 50% of the duration of the shortest test; 100%: 100% of the duration of the shortest test; TF, at task failure. *Significant difference between OSA and CONTROLS (main group effect). Values are mean ± SD.

DISCUSSION

This is the first study to assess the effect of OSA on cortical motor function and its repercussion during a limb muscle fatiguing exercise before and after CPAP treatment. We found that (i) OSA patients have reduced knee extensor strength and endurance, (ii) OSA patients display central VA deficit, part of which originating from supraspinal mechanisms, (iii) intracortical inhibition is increased and peripheral neuromuscular transmission is impaired in OSA, and (iv) 8 wk of CPAP treatment, despite suppressing abnormal respiratory events and improving subjective daytime sleepiness, is ineffective to reverse the abovementioned neuromuscular and functional impairments.

Peripheral muscle function in OSA patients

To the best of our knowledge, only one study has previously highlighted peripheral muscle dysfunction in OSA patients (14). These authors reported lowered maximal strength and endurance during isokinetic exercise of the knee extensors in OSA patients compared to controls. By using isometric knee extensions, our results extend these previous findings because this contraction regimen is known to induce lower cardiorespiratory stimulation and therefore enable us to focus specifically on the neuromuscular mechanisms of limb muscle dysfunction in OSA.

The OSA patients reported a level of physical activity which was not significantly different compared with controls (Table 1). Although objective measures, such as actigraphy, would have been necessary to support this result, the similar level of physical activity in both groups suggests that other factors than spontaneous physical activity may underlie the differences in knee extensor function between groups. Sauleda et al. (9) showed an increase in the size of type II fibers, in muscle protein content and larger metabolic activity (upregulation of cytochrome oxidase and phosphofructokinase activities) in the quadriceps of OSA patients. Wahlin Larsson et al. (10) also reported changes in lower limb muscle properties (increased microvascularization and proportion of fast type muscle fibers) in OSA patients together with reduced exercise capacity. Intermittent hypoxemia and subsequent oxidative stress is one potential mechanism contributing to limb muscle structural and metabolic changes leading to reduced functional capacity as previously shown in respiratory muscles (32). Reduced fitness level (e.g., maximal oxygen consumption during cardiopulmonary exercise test) has been reported in OSA patients (33). Although the underlying mechanisms to explain this reduction remain unclear (e.g., impaired cardiovascular and metabolic adaptations to exercise [34–36]), it may have also contributed to the impairment in isolated muscle function as observed in the present study.

VA in OSA patients

Previous results have suggested that VA may be reduced in OSA patients at the start of exercise (14) and that these patients were not able to increase motor unit recruitment at maximal exercise (37). Our results objectively demonstrate a central VA deficit (reduced VAFNES) with a supraspinal component (reduced VATMS) in OSA patients compared with controls before as well as during exercise. O’Leary et al. (38) suggested that the ability to maintain high knee extensor VA levels contributes to enhanced exercise endurance performance. Hence, the reduced VA levels measured at the start of exercise until exhaustion in OSA patients likely contributed to their reduced knee extensors endurance.

Reduced VA has been reported in chronic obstructive pulmonary disease (39), another condition characterized by chronic hypoxic exposure. Furthermore, nocturnal hypoxemia has been associated with brain damage in these patients (39), suggesting a close relationship between hypoxia during sleep, brain structural alterations and reduced muscle VA. The extent of the impairment in limb muscle function (strength and endurance) and neuromuscular parameters (VA level and intracortical inhibition) observed in OSA patients in the present study appears, however, to be smaller compared with previous observations in patients with chronic obstructive pulmonary diseases (40–43). Alterations in cerebral blood flow (2) and impaired diurnal (44) and nocturnal (45) cerebral hemodynamics may participate to brain functional alterations in OSA patients. Reduced gray matter volume in specific brain areas has been reported in OSA and was associated with neurocognitive dysfunction (5). Reduction in white matter has also been reported in numerous brain regions implicated in motor planning, contraction or control (6). Hence, it is likely that these hemodynamic and structural alterations have functional repercussions, such as the inability to properly activate the muscle as observed in the present study.

Corticospinal excitability and inhibition in OSA patients

When accurately normalized to compound muscle action potential (M-wave), MEPs have been reported to be similar between OSA patients and controls (16). Although absolute MEPs amplitude (mV) was reduced in OSA patients compared to controls in the present study, we also measured reduced M-wave in OSA patients, leading to similar normalized MEPs (MEP·Msup−1) in OSA and controls. This clearly suggests that corticospinal excitability, based on MEPs amplitude accurately normalized to M-wave, is not impaired in OSA patients. Yet, it also brings to light that peripheral neuromuscular transmission and sarcolemmal excitability are impaired in OSA patients. These results reinforce previous findings concerning peripheral nerve dysfunction in OSA patients reported on relaxed muscle by our group (11). Motor threshold assessed during muscle contraction (i.e., active motor threshold) did not significantly differ between OSA patients and controls in accordance with Opie et al. (46). Conversely, other studies have shown increased resting motor thresholds (i.e., measured on relaxed muscles) in OSA patients (15,46). Corticospinal excitability measured during muscle contraction is however more likely to reproduce usual physical activity. Overall, corticospinal excitability seems to be preserved in OSA patients both before and during exercise when MEPs are accurately normalized to M-wave and measured during muscle contraction.

Increased intracortical inhibition based on the lengthening of CSP is the most robust finding evidenced by TMS in OSA patients (15,16). Although previous studies have investigated CSP independently of any exercise-induced fatigue, the present study is the first to document increased CSP duration in OSA before and during a fatiguing exercise. Cortical silent period is thought to be mediated by the activation of long-lasting GABAB receptors (47). Hence, a lengthening in CSP suggests an increase in GABAergic activity in OSA patients. Since LICI is also mediated by the activation of GABAB receptors (47), an increase in this parameter can be expected in these patients, although CSP rather reflects the duration of the inhibition whereas LICI reflects its magnitude (47). Only one study (46) had assessed LICI in OSA patients and reported no difference compared to controls, but this assessment was performed on relaxed muscle. The increased LICI (i.e., lower conditioned to unconditioned MEP amplitude ratio) in OSA patients measured during muscle contractions in the present study is likely to be more representative of motor cortex behavior during exercise. During knee extensors exercise, development of central and supraspinal fatigue occurred together with an increase in intracortical inhibition (48). Hence, the increased intracortical inhibition illustrated by the larger CSP duration and LICI from the start until the end of exercise may have contributed to the reduced VA (and exercise duration) observed in OSA patients.

The present work is the first to show no difference in SICI during exercise between OSA patients and controls, reflecting similar intracortical inhibitory activity of GABAA (49). Previous findings have suggested unmodified SICI on relaxed muscles in OSA (15,46). Thus, the differences between groups in CSP and LICI but not SICI suggest that a specific alteration of GABAB receptors may underlie the enhanced intracortical inhibition in OSA patients.

Several mechanisms may underlie increased intracortical inhibition in OSA. Previous results suggest that sleep fragmentation, a main feature of OSA, is unlikely to explain the differences in corticospinal excitability and inhibition between OSA patients and controls (50). Grippo et al. (16) reported a significant positive correlation between PaCO2 and CSP duration in the morning, suggesting a key role of hypercapnia in the enhanced intracortical inhibition of OSA patients. In an animal model of chronic intermittent hypoxia and hypercapnia, Dergacheva (51) showed that GABAergic pathway is enhanced in the brain of exposed rats. Hence, intermittent hypoxia and hypercapnia during sleep in OSA patients may contribute to an increase in GABA activity leading to the increased intracortical inhibition reported in the present study.

Muscle function and neuromuscular impairments after CPAP treatment

Eight weeks of effective CPAP treatment did not result in the hypothesized improvement in limb muscle function nor in the investigated neuromuscular parameters. Hence, despite CPAP therapy effectively removed nocturnal respiratory events, neuromuscular impairments remained after treatment. This suggests that these impairments may not be the direct consequences of nocturnal exposure to intermittent hypoxia and hypercapnia. Several studies assessed the effect of CPAP treatment on upper airway and respiratory muscles function as well as on brain structure/function and provided controversial results. Barreiro et al. (32) reported that 6 months of CPAP treatment was ineffective to reverse muscle dysfunction and the associated oxidative stress in the external intercostal muscles. Hence, one may suggest that only 8 wk of CPAP treatment did not reverse the potential oxidative stress and associated muscle impairments in the knee extensors of our patients, as suggested by the persistence of muscle dysfunction observed in the present study. Peripheral nerve transmission also remained altered after CPAP treatment which is supported by previous findings from our group (11). Changes in cerebrovascular reactivity have also been assessed after CPAP treatment. Although some results showed a normalization of cerebrovascular reactivity after CPAP treatment (3,52), others showed limited improvements despite effective CPAP treatment (4). Thus, the present results as well as previous studies suggest that neuromuscular impairments may persist in patients with OSA after CPAP treatment despite the removal of nocturnal exposure to intermittent hypoxia and hypercapnia. This suggests either that nocturnal respiratory events and impaired blood gases are not the main factors responsible for neuromuscular alterations or that neuromuscular damage induced by OSA are, at least in part, not reversible.

Castronovo et al. (19) reported that 3 months of CPAP treatment only enabled limited changes in white matter integrity despite substantial improvement in cognitive performance. Longer treatment duration (i.e., 12 months) was needed to witness an almost complete reversal of white matter abnormalities. Altogether, these results suggest that the duration of the present CPAP treatment, which was effective to reduce subjective daytime sleepiness, may not be sufficient to improve potential brain structural alterations, to normalize intracortical inhibition and subsequently to reverse muscle VA deficit.

The relatively small sample size reflects the difficulty to recruit OSA patients for demanding neurophysiological and exercise evaluations. It was however sufficient to demonstrate a significant between-group difference in our primary outcome. In the present study, severe OSA patients appear relatively heterogeneous regarding the degree of neuromuscular impairments, suggesting different phenotypes as frequently observed in chronic respiratory diseases (40,53). Although we found no correlation between OSA severity and neuromuscular impairments (results not shown), further studies are required to better characterized neuromuscular dysfunction in patients with OSA of different severities. Although our study design did not include a sham-CPAP group, the present results demonstrate that 8 wk of CPAP are ineffective in improving neuromuscular dysfunctions. While the present study demonstrates cortical impairments during one-leg isometric knee extensions, future studies should evaluate whether similar mechanisms affect whole-body exercise response such as walking or cycling in OSA which may better reflect daily life activity.

In conclusion, severe OSA patients demonstrated reduced central VA together with increased intracortical inhibition compared with controls. These impairments in neuromuscular function are likely to contribute to the reduced knee extensors strength and endurance observed in OSA patients. Eight weeks of effective CPAP treatment did not reverse the abovementioned cortical impairments nor the associated muscle dysfunction. The role of these cortical alterations regarding OSA patient functional capacity (e.g., gait, whole body exercise tolerance) and the effect of longer CPAP treatment duration remain to be further investigated.

This work has been funded by the Fond de dotation AGIR pour les maladies chroniques and by the Fonds de dotation Recherche en Santé Respiratoire.

M. M., M. G., S. B., T. L. R. M., B. W., R. T., P. L., and S. V. have nothing to disclose. J.-L. P. reports grants and personal fees from Philips, RESMED, Fisher & Paykel, grants from Fondation de la Recherche Médicale, Direction de la Recherche Clinique du CHU de Grenoble, Fonds de dotation “Agir pour les Maladies Chroniques,” personal fees from Astra Zeneka, SEFAM, Agiradom, outside the submitted work. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

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Keywords:

EXERCISE; MUSCLE DYSFUNCTION; NEUROMUSCULAR FATIGUE; OBSTRUCTIVE SLEEP APNEA SYNDROME; TRANSCRANIAL MAGNETIC STIMULATION

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