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Sleep Apnea: Introduction and Diagnosis

Sleep apnea is a disorder in which there is a break or pause (apnea) in breathing during sleep. There are two basic types of apnea:

1. Obstructive: An obstructive apnea occurs when the body (especially the chest walls and abdomen) attempts to breathe in, but the air flow is blocked by some obstruction in the upper portion of the airway.

2. Central: A central apnea is also marked by the absence of air flow into the lungs. However, unlike an obstructive apnea, there is no attempt of movement at the chest wall. These apneas are felt to originate in the part of the brain responsible for controlling respiration.

The discussions here deal predominately with obstructive sleep apnea (abbreviated OSA). This site also contains information on snoring and on treatment of obstructive sleep apnea.

Obstructive Sleep Apnea (OSA), A Brief Description

OSA is a disorder in which there is partial or complete obstruction of the upper airway during sleep. While sleeping the muscles of the upper airway relax, and in some individuals this relaxation is so pronounced it produces partial collapse of the throat.

Typical symptoms of obstructive sleep apnea are daytime sleepiness, awakening at night with a choking sensation, morning headaches, restlessness during sleep, and snoring. During sleep, the individual with OSA may block off his or her airway 30 to 100 times an hour. Each blockage causes them to leave the deep stages of sleep and partially awaken until the airway is reopened. As a result, the sleep is severely disrupted and the patient will feel sleepy throughout the day. The daytime sleepiness can be so severe that individuals will fall asleep while driving or while sitting in a chair.

Causes of OSA

The cause of OSA is felt to be a combination of (1) anatomic characteristics of the upper airway and (2) abnormalities in the neuromuscular control of the muscles in the throat. Sleep apnea is known to be more common in individuals with large tonsils, palate, and tongue, and with a short thick neck. This anatomy predisposes the throat to easily collapse. A badly deviated nasal septum or other nasal obstruction can also worsen OSA because it limits the ability to breathe through the nose. Overweight individuals are also at high risk for OSA.

Not all individuals with these anatomic features will have OSA, and OSA occassionally occurs in people with normal-appearing throats. Most researchers therefor believe that some problem must also exist in the nerve supply to the muscles of the throat while sleeping, or in the control mechanism of breathing in the brain. This is still an area of active research.

Sleep apnea is also felt to be worse with certain medical conditions, such as hypothyroidism.

Health Effects of OSA

Diagnosis and treatment of OSA is important because it can cause serious health problems. During each episode of apnea, the oxygen concentration in the bloodstream begins to fall. Under normal circumstances, the oxygen concentration should be 94% or greater. With OSA, oxygen concentration can drop to dangerously low levels. The graph to the left shows oxygen concentration in an individual during OSA. Oxygen drops lower than 50% before the individual is aroused and awakens enough to break the apnea episode. Some individuals will have 50-100 of these apnea events per hour while asleep. As a result, the individual with sleep apnea will be unable to enter the deep stages that are necessary for healthy sleep.

Decrease in oxygen concentration like this can obviously have severe health consequences. Studies have shown that the heart rate typically slows down during apnea and then has a sudden increase during the arousal that ends an apneic period. Cardiac output has been shown to decrease during obstructive apnea. Other heart irregularities such as arrhythmias are also more common during OSA.

Sleep apnea is also associated with increases in blood pressure, both in systemic blood pressure throughout the body's blood vessels and also the large blood vessel going to the lungs (pulmonary artery pressure). In severe cases this can worsen congestive heart failure.

All of these factors contribute to a higher mortality rate in individuals with untreated severe sleep apnea.

Diagnosis of Sleep Apnea

Diagnosis of sleep apnea begins with a careful history and physical examination. The history of daytime sleepiness, awakening at night, and severe snoring all point towards sleep apnea. Sleep apnea must be distinguished from Laryngeal reflux disease, which can also cause shortness of breath at nighttime due to spasm of the vocal folds from acid irritation. Shortness of breath when lying down also can be seen in heart failure, so this must certainly be ruled out in the evaluation of nighttime breathing disorders.

There are certain findings on the physical exam that are classic for OSA. As mentioned earlier, OSA is more common in overweight individuals, and especially in those with thick necks. One often sees large tonsils, a large soft palate (the roof of the mouth), and a large tongue. All of this combines to produce a narrow opening into the upper airway.

During the exam, the physician may assess the upper airway with a fiberoptic laryngoscope. A commonly used test called the Muller's maneuver. This consists of asking the patient to take a deep breath in with the mouth closed and the nostrils pinched, and simultaneously looking with the laryngoscope at the throat. One looks in particular for collapse of certain portions of the airway. We believe that this reproduces what is happening during OSA episodes, but the implication of a positive Muller test is not well known.

The Sleep Study (Polysomnography)

The best way to diagnose sleep apnea is a sophisticated test called a sleep study. This is an overnight test that is done in special sleep centers. Multiple monitors are used to measure a variety of different physiological signals during sleep. Some of the parameters measured include:

1. Respiration: Airflow monitors keep track of how often and how long the apneic events last. An apnea is defined as a cessation in breathing lasting 10 seconds or longer. A hypopnea is usually defined as a decrease in airflow of 50%, associated with a drop on oxygen concentration of four percentage points.

2. Blood oxygen concentration: One of the key findings in OSA is a drop in oxygen concentration.

3. Muscle movement: Movement of the chest wall is monitored to determine if the apnea is central or obstructive. Other types of sleep disorders are associated with leg movements, jaw clenching, and other characteristic movements.

4. Brain wave activity: Sleep is divided into characteristic stages based on brain wave activity. This activity is monitored during the sleep study.

5. Heart activity (ECG): As mentioned earlier, certain heart irregularities are seen during OSA.

6. Body position: Sleep apnea is often more common while lying on one's back. This allows the tongue to more easily fall back and block the airflow.

The sleep study is a very complex test and all of its results must be carefully interpreted in order to properly diagnose the severity of the sleep disorder. There are, however, two measures that are often quoted in order to summarize the results.

First, the number of apneas and hypopneas are summed and averaged over the nighttime to calculate the average number of respiratory disturbances per hour. This is called the Respiratory Distress Index (RDI) or the Apnea Hypopnea Index (AHI). Sleep apnea is generally defined in adults as an RDI of 5 or greater.

The second important parameter is the lowest oxygen concentration observed during sleep apnea. There is no specific number that indicates sleep apnea, but levels below 85% are certainly indications of significant apnea.

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