Postural Strain Affects Upper Airway Muscles in Awake Patients With OSA

In awake patients with obstructive sleep apnea syndrome (OSAS), the genioglossus muscle and accessory muscles assume the inspiratory load that develops with postural strain, regardless of apnea severity, according to the results of a prospective, cross-sectional study (ClinicalTrials.gov identifier: NCT04858399) published in the Journal of Manipulative and Physiological Therapeutics.

During inspiration, the collapsing effect of negative intraluminal is decreased by the tone and the activity of the upper airway dilator muscles. “In awake patients with OSAS, the activity of the upper airway dilator muscles is sufficient to compensate for the inspiratory load,” the researchers explained. “The decrease in pharyngeal dilator muscle activity during sleep disrupts this balance and leads to apnea attacks.” The investigators therefore sought to compare the effects of body posture and severity of apnea on the tone, stiffness, and elasticity of the upper airway muscles of awake patients with OSAS.

The study enrolled a total 80 patients with OSAS from the Sleep Laboratory of the Bitlis State Hospital, located in Bitlis, Turkey, between April 2021 and December 2021. The tone, elasticity, and stiffness of the genioglossus and accessory muscles (ie, sternocleidomastoid and masseter) were measured in all participants while they were awake, sitting upright, and in the supine posture. OSAS was classified as mild, moderate, or severe, based on results from polysomnography and the apnea-hypopnea index (AHI).

A total of 80 individuals with OSAS — 56 men and 24 women — were enrolled in the study. The mean (SD) patient age was 45.0 (11.0) years.  Of the participants, 16 had hypertension, 12 had diabetes comorbidities, and 9 had both. According to their AHI results, 22 patients had mild OSAS, 24 patients had moderate OSAS, and 34 patients had severe OSAS. In all, 32 of the participants smoked and 48 participants did not smoke.

Results of the study showed that among those in the severe OSAS arm, body mass index (4.67 kg/m²), neck circumference (3.44 cm), relative sleep quality index (RSQI; 2.19 points), and Epworth Sleepiness Scale scores demonstrated significant differences from those in the mild OSAS arm (P <.05 for all). Additionally, RSQI and sleep quality index scores were significantly higher in the severe OSAS group than in the moderate OSAS group (P =.007).

Further, sleep stage 1 (N1%) was significantly longer in the mild and severe OSAS arms than in the moderate OSAS arm (P= .032). Mean oxygen saturation (OS) during sleep was 92.91%, 86.29%, and 84.35% in the mild, moderate, and severe OSAS groups, respectively. In contrast, minimum OS and baseline OS both were significantly higher in the mild and moderate OSAS groups than in the severe OSAS group (P <.001).

The tone of the genioglossus, masseter, and sternocleidomastoid muscles was significantly higher in the supine posture than in the sitting posture (P <.05 for all). In addition, the elasticity scores of the sternocleidomastoid and the masseter muscles were significantly higher in the sitting posture (P <.05 for both). A significant interaction effect of postural change and severity of apnea was seen in stiffness scores of the genioglossus muscle (P <.05).

Limitations of the study warrant mention, including: (1) a lack of a control group of healthy participants for making comparisons; and (2) use of myotonometer assessments that were unsupported by electromyography (EMG).

Overall, the investigators found that accessory muscles supported respiration in awake patients with OSAS by increasing their elasticity, while the “Genioglossus muscle responded to positional strain by increasing its stiffness.” Additionally, “Resting tone and elasticity changes of the genioglossus, masseter, and sternocleidomastoid muscles in sitting and supine postures were not affected by the severity of the OSA.” Further studies should be conducted that perform EMG and myotonometer evaluations in a control group of awake patients with OSAS, the researchers noted.