Sleep apnea is divided into two types: obstructive and central, with obstructive being much more common (UMMC 2013a; NSF 2014). In obstructive sleep apnea, the airways collapse and block airflow episodically during sleep. In central sleep apnea, the respiratory control centers in the brain do not provide sufficient stimulation to maintain regular breathing during sleep. However, it appears that elements of both obstructive and central mechanisms occur simultaneously in some people with sleep apnea (Ramirez 2013; Khan 2014; ASAA 2014; Hoffman 2012; Lehman 2007).
Obstructive Sleep Apnea
Swelling, narrowing, or anatomical abnormalities of structures of the air passageway can obstruct airflow, causing a pause in respiration that must be overcome by strong signals from the brain’s respiratory center to trigger inhalation (Jordan 2014; Schwab 2011; ATS 2014). Posture often plays a crucial role in obstructive sleep apnea; the gravitational pull associated with a back-sleeping position can cause an increased tendency for the airway to collapse and block airflow (Bilston 2014).
Central Sleep Apnea
The involuntary breathing reflex is regulated by the brain in response to blood carbon dioxide levels. Normally, as blood carbon dioxide levels rise, the respiratory center of the brain triggers increased breathing (Nattie 1999; Guyton 1990). In central sleep apnea, the brain does not adequately regulate breathing. Central sleep apnea is often caused by another health condition. For instance, central sleep apnea is common in congestive heart failure and with chronic opioid drug use (Javaheri 2013; Floras 2014). Central sleep apnea can also occur without a clear cause (Ramirez 2013; ASAA 2014).
Mixed or Complex Sleep Apnea
Mixed sleep apnea, sometimes referred to as complex sleep apnea, involves both obstructive and central components. Up to18% of people with sleep-disordered breathing—the broad category that includes apneas as well as less severe hypopneas (shallow and/or slow breathing)—may have mixed sleep apnea. In a significant percentage of cases, central apnea is revealed during treatment of the obstructive aspect of sleep apnea with CPAP (Khan 2014; Hoffman 2012; Lehman 2007).
Association with Other Diseases
Sleep apnea is associated with a number of serious conditions (Thompson 2012; Martinez Ceron 2014; Yu 2011; Canales 2008):
Cardiovascular disease. Intermittent hypoxia (periods of low oxygen levels) caused by sleep apnea can contribute to chronic high blood pressure and endothelial damage (Ziegler 2011). People with obstructive sleep apnea are more likely to suffer from high blood pressure, coronary artery disease, and abnormal heart rhythms, and they have a higher risk of heart failure, heart attack, and stroke (Jean-Louis 2008; Gottlieb 2013; Pepin 2014; Ali 2014; Hohl 2014; Oldenburg 2014).
Type 2 diabetes and obesity. Up to 40% of people with obstructive sleep apnea have type 2 diabetes, and as many as 53% of overweight or obese people with type 2 diabetes have obstructive sleep apnea (Bonsignore 2013; Nannapaneni 2013). Sleep apnea is also independently associated with insulin resistance (Martinez Ceron 2015). Obesity in general, and especially abdominal or central obesity, is a major risk factor for obstructive sleep apnea (Hoffstein 1992; Schwartz 2008; Thompson 2012; Stadler 2009; Hayes 2014).This relationship between obesity and obstructive sleep apnea appears to be bidirectional. While obesity likely plays a causal role in obstructive sleep apnea, sleep apnea may contribute to weight gain (Romero-Corral 2010; Pillar 2008; Yu 2011).
Cancer. In a study that followed 386 adults over 20 years, moderate-to-severe obstructive sleep apnea was associated with a 2.5 times higher risk of new cancer diagnosis and a 3.4 times higher risk of cancer death (Marshall 2014). In another study, individuals with mild sleep disordered breathing (SDB) had 1.1 times the risk of dying of cancer, those with moderate SDB had 2 times the risk, and those with severe SDB had 4.8 times the risk, compared with healthy subjects (Nieto 2012).
Gout and hyperuricemia. Gout is a painful arthritic condition caused by an excess of uric acid in the blood (hyperuricemia), which accumulates as crystals in the joints (NLM 2014). People with gout appear to be at increased risk of sleep apnea (Roddy 2013). Furthermore, individuals with high uric acid levels have been shown to be more likely to snore more than five nights per week and to experience daytime sleepiness (Wiener 2012). Uric acid level is known to rise in concert with increasing frequency of apneic episodes and has been suggested to be a marker of sleep apnea severity (Cantalejo Moreira 2013; Hirotsu 2013; Kanbay 2014; Wiener 2012).
Cognitive and neurological problems. Studies suggest chronic sleep deprivation and interruption may increase the risk of chronic cognitive problems and neurological disease, including Alzheimer’s disease (Lucey 2014; Buratti 2014).
Pregnancy complications. Obstructive sleep apnea is common in pregnancy, and symptoms associated with sleep apnea such as snoring generally worsen as pregnancy progresses (Sarberg 2014; Calaora-Tournadre 2006; Facco 2012; Sahota 2003; Lefcourt 1996). A rigorous scientific literature analysis concluded that, in pregnant women, sleep disordered breathing—a designation for a group of related conditions that includes sleep apnea—is associated with a 2.34-fold increased risk of gestational high blood pressure and a 1.86-fold increased risk of gestational diabetes compared with those without sleep apnea (NIDDK 2013; NLM 2015b).
Erectile dysfunction. It has been estimated that half of men with sleep apnea also have erectile dysfunction, and vice versa. This association could be explained by dysfunction of the cells lining the insides of blood vessels (endothelial dysfunction), a condition common to both erectile dysfunction and obstructive sleep apnea (Hoyos, Melahan, Phillips 2014).
Low testosterone levels. Several studies have confirmed that serum testosterone levels are lower in men with obstructive sleep apnea. A study that compared 36 men with stable mild-to-severe obstructive sleep apnea to healthy controls found that lower serum testosterone levels were associated with greater sleep apnea severity and body mass index (Canguven 2010). One study found that men with obstructive sleep apnea had 17% lower serum testosterone levels than controls (Bercea 2014); another found that obese men with severe obstructive sleep apnea had significantly lower testosterone levels than matched, healthy controls, and that lower testosterone was significantly correlated with symptoms of depression in the obstructive sleep apnea group (Bercea 2013).
Systemic conditions. Obstructive sleep apnea and accompanying intermittent hypoxia are associated with increased oxidative stress, systemic inflammation, and endothelial dysfunction (Alzoghaibi 2012; Badran 2014; Nadeem 2013; Chen 2013; Hoyos, Melehan, Liu 2014; Drager 2013; Ziegler 2011).
Other conditions. In a five-year study following 1411 people with obstructive sleep apnea and 7055 without, obstructive sleep apnea was associated with 1.9 times the risk of developing an autoimmune disease (Kang 2012). Obstructive sleep apnea has also been associated with depression, glaucoma, interstitial cystitis, and chronic kidney disease (Chung 2014; Lin 2011; Perez-Rico 2014; Cross 2008; Dantzer 2008; Adeseun 2010; Chou 2011).