Sleep-Disordered Breathing in Neurological Diseases

Episodes of hypoxia and hypercapnia occurring during apneas significantly dilate blood vessels in the brain (both hypercapnia and hypoxia are potent stimuli of cerebral blood vessels dilation Guyton, 2005). This results, together with a concomitant increase of mean arterial pressure, in average blood flow increase in the cerebral vessels. Studies in healthy volunteers (Przybylowski, 2003) have shown that episodes of breath apneas cause an increase of cerebral blood flow compared with resting conditions (43% on average). Following an episode of apnea, hyperventilation (with normoxia and hypocapnia) significantly decreases flow in the middle cerebral artery as compared to quiet breathing by 20% (Przybylowski, 2003). In normal subjects sleep reduces the vasodilatation response to a hypoxia (Meadows, 2004). A number of different mechanisms triggered during sleep apneas can influence the blood flow in the brain. The increase in intracranial pressure, together with a negative pressure in the chest, may reduce the perfusion of the brain (Jennum, 1989). More frequent significant carotid artery stenosis (Silvestrini et al, 2002; Nachtmann et al, 2003) and flow disturbances in the intracranial arteries (Behrens et al, 2002; Nachtmann et al, 2003) were found in patients with sleep-disordered breathing as compared with a population of healthy controls. There was also found that: cerebrovascular autoregulation reserve and hypercapnia triggering cerebral blood vessels dilatation are reduced in patients with obstructive sleep apneas as compared with the control group (Balfors, 1994). Similarly, studies of cerebral blood flow autoregulation in patients with sleep disordered breathing show impaired and delayed expansion of cerebral blood vessels in response to hypoxia (Urbano, 2008). Short-term mechanisms, associated with airway obstruction and hypoxia during sleep, are of paramount importance in the pathophysiology of cerebral circulation disorders and ischemic stroke. During obstructive apnea there is a temporary increase in blood flow through the brain vessels due to hypoxia and hypercapnia, but this increase is smaller than that of healthy people. Hyperventilation which follows the apnea causes hypocapnia and normoxia with significant reduction of blood flow through the brain vessels. Obstructive sleep apnea promotes a substantial fall in cerebral blood flow (Culebras et al, 2004; Netzer et al, 1998). It seems that short-term mechanisms, associated with apnea during sleep, underlie the observed periodicity of brain ischemic stroke occurrence during the day and more frequent prevalence of ischemic stroke in the early morning hours (Mohsenin, 2003; Yaggi, 2003).