Obstructive Sleep Apnea and Arousal Threshold in Patients With Post-traumatic Stress Disorder

Obstructive sleep apnea (OSA) is a clinically relevant disease that is associated with cardiovascular, metabolic, and neurocognitive consequences. OSA is a very common disease; the Wisconsin Sleep Cohort Study found that OSA affects 2% of women and 4% of men aged 30-60. However, these data predate the obesity epidemic and use of more modern diagnostic equipment, such that more recent studies have reported prevalences of moderate to severe OSA as high as 24% in women and 49% in men.

The prevalence of OSA has been found to be particularly high in patients suffering from post-traumatic stress disorder (PTSD). One study of veterans with combat related PTSD found that 67.3% of this population was diagnosed with OSA after undergoing polysomnography. This finding is consistent with those of Mysliwiec et al who found rates of OSA in 63% of post-deployment soldiers and 51% of active duty soldiers. Despite increased recognition of this association, there has been little progress in establishing a pathophysiological link between the two diseases, and no study to our knowledge examining the possibility that PTSD might drive the development of OSA. Furthermore, individuals with PTSD have been found to have significantly lower compliance with continuous positive airway pressure (CPAP) therapy than the general population, with claustrophobia, mask discomfort, and air hunger as the most commonly cited reasons for non-adherence. Despite this, studies have shown both benefit in sleep quality and PTSD related nightmares with OSA treatment. Alternative OSA therapies are therefore particularly important in this group, but will rely on determination of targets other than those treated by CPAP.

While a propensity towards upper airway collapse, as seen in obesity, has classically been considered the principle determinate of OSA pathogenesis, more recent work has shown that non-anatomical variables including the ventilatory arousal threshold (ArTH) are also important. During wakefulness pharyngeal dilator muscles remain active to maintain airway patency however during sleep these muscles lose activity in all individuals but in those with OSA airway collapse occurs and results hypoxia and hypercapnia. Both accumulation of carbon dioxide and increased negative pressure can cause recruitment of the upper airway dilator muscles resulting in pharyngeal patency if sleep is maintained. Cortical arousal during apnea for most individuals represents a protective mechanism as the airway is restored with resolution of hypoxia and hypercarbia. However individuals with low ArTH (individuals who wake easily) do not have sufficient time for accumulation of respiratory stimuli, recruitment of pharyngeal muscles is then unable to occur which results in disruption of sleep. ArTH is variable amongst individuals and is dependent on the magnitude of negative intrathoracic pressure tolerated before awakening occurs and is independent from other factors.

A central feature of PTSD is a state of persistent hyperarousal that chronically persists following a traumatic event. Individuals with PTSD have been found to have important physiologic changes of the hypothalamic-pituitary axis (HPA) and sympathoadrenal system including increased levels of sympathetic neurotransmitters such as norepinephrine and increased activity of α2-adrenergic and glucocortocoid receptors. Both the HPA and sympathetic nervous system are associated with attention and arousal. Prolonged activation of these systems has been shown to induce important biochemical changes associated with disordered sleep. Indeed, studies in norepinephrine deficient knockout mice have demonstrated that increased stimulus is required to induce cortical arousal in the norepinephrine deficient mice and results in higher acoustic ArTH when compared to controls. Our hypothesis is that patients suffering from PTSD will have a lower ArTH when compared to subjects without PTSD. This work is significant because if lower ArTH is associated with PTSD these patients may benefit from therapies that raise the ArTH.

Prior studies have indicated that trazodone, a serotonin reuptake inhibitor commonly used for insomnia and occasionally for depression, may raise the arousal threshold without suppressing upper airway muscle activity. This medication therefore presents a potentially attractive alternative treatment to CPAP therapy for patients with OSA related to a low arousal threshold.