context: Obstructive sleep apnea is a common and serious disorder in which breathing repeatedly stops for 10 seconds or more during sleep. The disorder results in decreased oxygen in the blood and can briefly awaken sleepers throughout the night. Sleep apnea has many different possible causes. In adults, the most common cause of obstructive sleep apnea is excess weight and obesity, which is associated with soft tissue of the mouth and throat. During sleep, when throat and tongue muscles are more relaxed, this soft tissue can cause the airway to become blocked. But many other factors also are associated with the condition in adults. In children, causes of obstructive sleep apnea often include enlarged tonsils or adenoids and dental conditions such as a large overbite. Less common causes include a tumor or growth in the airway, and birth defects such as Down syndrome and Pierre- Robin syndrome. Down Syndrome causes enlargement of the tongue, adenoids and tonsils and there is decreased muscle tone in the upper airway. Pierre- Robin syndrome actually has a small lower jaw and the tongue tends to ball up and fall to the back of the throat. Although childhood obesity may cause obstructive sleep apnea, it's much less commonly associated with the condition than adult obesity. Regardless of age, untreated obstructive sleep apnea can lead to serious complications, including cardiovascular disease, accidents, and premature death. So it's important that anyone with signs and symptoms of obstructive sleep apnea -- especially loud snoring and repeated nighttime awakenings followed by excessive daytime sleepiness -- receive appropriate medical evaluation. In addition to obesity, other anatomical features associated with obstructive sleep apnea -- many of them hereditary -- include a narrow throat, thick neck, and round head. Contributing factors may include hypothyroidism, excessive and abnormal growth due to excessive production of growth hormone (acromegaly), and allergies and other medical conditions such as a deviated septum that cause congestion in the upper airways. In adults, smoking, excessive alcohol use, and/or the use of sedatives is often associated with obstructive sleep apnea. More than half of people with obstructive sleep apnea are either overweight or obese, which is defined as a body mass index ( BMI) of 25-29.9 or 30.0 or above, respectively. In adults, excess weight is the strongest risk factor associated with obstructive sleep apnea. Each unit increase in BMI is associated with a 14% increased risk of developing sleep apnea, and a 10% weight gain increases the odds of developing moderate or severe obstructive sleep apnea by six times. Compared to normal-weight adults, those who are obese have a sevenfold increased risk of developing obstructive sleep apnea. But the impact of BMI on obstructive sleep apnea becomes less significant after age 60. BMI isn't the sole marker of obesity that's important. Men with a neck circumference above 17 inches (43 centimeters) and women with a neck circumference above 15 inches (38 centimeters) also have a significantly increased risk of developing obstructive sleep apnea. In addition, extreme obesity (defined as a BMI above 40) is associated with obesity-hypoventilation syndrome ( Pickwickian syndrome), which can occur alone or in combination with obstructive sleep apnea. In this syndrome, which affects up to 25% of the extremely obese, excess body fat not only interferes with chest movement but also compresses the lungs to cause shallow, inefficient breathing throughout the day and night. Although modest weight loss improves obstructive sleep apnea, it can be difficult for fatigued and sleepy patients to lose weight. In extremely obese patients, bariatric surgery is associated with an 85% success rate in improving the symptoms of obstructive sleep apnea. In middle-aged adults, the prevalence of obstructive sleep apnea is estimated to be 4%-9%, although the condition is often undiagnosed and untreated. Among people over age 65, it's estimated that at least 10% have the condition. Aging affects the brain's ability to keep upper airway throat muscles stiff during sleep, increasing the likelihood that the airway will narrow or collapse. Obstructive sleep apnea is up to four times as common in men as in women, but women are more likely to develop sleep apnea during pregnancy and after menopause. In older adults, the gender gap narrows after women reach menopause. Postmenopausal women who receive hormone replacement therapy are significantly less likely than those who don't to develop obstructive sleep apnea, suggesting that progesterone and/or estrogen may be protective. But hormone replacement therapy is not considered to be an appropriate therapy for the condition, because it can affect health in other ways. Other factors associated with obstructive sleep apnea include: Family history. About 25%-40% of people with obstructive sleep apnea have family members with the condition, which may reflect an inherited tendency toward anatomical abnormalities. Ethnicity. Sleep apnea also is more common in African- Americans, Hispanics, and Pacific Islanders than in whites. Increasing evidence suggests that obstructive sleep apnea is strongly associated with conditions such as high blood pressure (hypertension), stroke, heart attack, diabetes, gastroesophageal reflux disease, nocturnal angina, heart failure, hypothyroidism, and an abnormal heart rhythm. About half of sleep-apnea patients have hypertension, and untreated obstructive sleep apnea increases the risk of heart-related illness and death. In addition, obstructive sleep apnea is associated with excessive daytime sleepiness, which increases the risk for motor vehicle accidents and depression. Some complications may be related to the release of stress hormones, which may be triggered by frequent decreases in blood oxygen levels and reduced sleep quality. Stress hormones can increase heart rate and also can lead to the development or worsening of heart failure. Medical treatment -- which includes control of risk factors, use of continuous positive airway pressure ( CPAP) or oral appliances, and surgery -- may improve signs and symptoms of obstructive sleep apnea and its complications.