Introduction
sleep apneaSleep apnea encompasses central sleep apnea, obstructive sleep apnea, or a mix of the two. Both disorders interfere with sleep quality. Individuals with obstructive sleep apnea, the most common form of sleep apnea, have repetitive episodes of complete (apnea) or partial (hypopnea) obstructions of their upper airway during sleep. The obstructions are caused by upper airway narrowing and often last between 10 and 30 seconds. In central sleep apnea pauses to breathing from 10 to 30 seconds are seen in some individuals. In other cases of central sleep apnea longer pauses to breathing between 40 seconds up to 90 seconds are typical. Central sleep apnea is caused by a lack of signaling by the brain. Normally brain signals tell the body to breathe. But in central apnea signals to breathe are not properly communicated to the rest of the body. The abnormal breathing of Sleep apnea results in an inability to get enough air into the lungs during sleep. This decreases blood oxygen levels or saturation. Individuals with sleep apnea have periods when they may gasp for air or repeatedly wake up (even though they don’t remember waking) and this exposes their body to large amounts of stress. It results in daytime sleepiness, irritability, mood changes and problems with focus and memory. Other consequences such as a seven-fold increase in motor vehicle accidents as well as increased risk of obesity, diabetes, heart attack, stroke and premature death make sleep apnea something that is really important to recognize and treat.
What is an Apnea?

An apnea is a temporary pause to breathing that occurs during sleep. Apneas are either obstructive or central. Apnea is obstructive when air cannot move in and out due to complete or partial blockage of the upper airway. The block is commonly caused by throat muscles relaxing too much, causing the airway to narrow. When someone is experiencing obstructive sleep apnea, his or her body must work harder to open the airway and bring air into the lungs. Loud snoring or gasping can be usually be heard during obstructive sleep apnea. Though snoring may be present in the absence of apnea.

Central apnea occurs when an individual stops breathing for periods of time during sleep. During central apnea, the brain signal that normally tells the body to breathe is not transmitted to the rest of the body.

Mixed Apnea is a combination of obstructive and central apnea. During mixed apnea both central and obstructive episodes occur in the same sleep cycle. Individuals with mixed apnea usually experience central apnea at the beginning of their sleep cycle. This is followed by episodes of obstructive apnea and then a return to regular breathing.

What are some symptoms?
Sleepiness during the daytime related to dozing off, a general feeling of tiredness and a lack of energy are common daytime symptoms related to decreased quality of sleep. As sleep is needed to maintain alertness and thinking, it is not surprising that problems with memory or concentration are common. While some people become sleepy, other people notice that they have difficulty falling asleep or staying asleep (insomnia) that is related to apnea. Less known symptoms include mood swings, depression, impotence, overactive bladder or frequently waking up in the middle of the night to urinate. Nighttime symptoms include loud snoring, snorting, chocking, waking up gasping for air, waking short of breath and witnessed apneas.
How is apnea diagnosed?

Sleep apnea is confirmed by a diagnostic study, called polysomnography that monitors sleep, breathing and oxygen levels. Apnea is measured by decreased airflow that is either complete (apnea) or partial (hypopnea). Each breath in a sleep study is represented by a wave. This allows doctors to identify the occurrence of apneas. Normal breathing is used as the baseline. An apnea will convey a wave that represents a reduction in a waves height of over 90% lasting for 10 seconds or more. A few apneas can be normal. More than five-per-hour, on average, leads to a diagnosis of sleep apnea.

Sleep studies also measure breathing effort of chest and abdominal muscles. When an obstructive apnea takes place, the brain and body increase effort to breath. This effort is detected during an obstructive apnea. When no effort is detected during an apnea, they are considered to be central apneas resulting from a lack of signaling from the brain telling the body to breathe.

How is Apnea treated?

Continuous positive airway pressure or CPAP is the best treatments for adults with obstructive sleep apnea (OSA). CPAP is worn over the nose or face during sleep. It splints open the airway from blockage or narrowing from OSA allowing the airway to stay open throughout the night. It almost always works well to treat OSA. It may take time for some people to get used to wearing a mask at night. Other treatments include oral appliances for non-severe forms of sleep apnea. Weight loss is also recommended as an adjunct to treatment. In some cases surgery may be considered. For a more detailed analysis see the article on CPAP.

What is Hypopnea and how is it recognized?

Hypopnea is abnormally slow or shallow breathing. It is similar to an apnea but the flow is only partially reduced. Hypopneas are identified in a sleep lab by looking at the height of waves that represent reduced airflow and observing accompanying reductions of blood oxygen levels. There is some controversy about the definition of hypopnea. Generally, wave height reductions of at least 30% resulting in a reduction of blood oxygen level of 3% are considered hypopneas. But several definitions exist. Some require at least a 50% reduction in airflow and a 4% desaturation of blood oxygen levels. A more liberal definition allows for hypopneas with 30% reductions without any desaturation as long as there is evidence of brief disruption of sleep called an arousal.

Combined with apneas, hypopneas make up the apnea-hypopnea index (AHI), which is used to diagnose sleep apnea and determine how severe it is. When someone has at least 5 apneas or hypopneas an hour (AHI of 5) they have sleep apnea. An AHI of 5 – 14 is considered mild, 15-29 is moderate and 30 or greater is considered severe.

What is the difference between Apnea and Hypopnea?

Apneas are complete stopping of breathing (at least 90%), while hypopnea is the reduction of breathing (at least 30% with oxygen level reductions or arousals). Both conditions usually prevent individuals from receiving enough oxygen. Left untreated, this could make other medical conditions worse.

What is Obstructive Sleep Apnea Syndrome (OSAS or OSAHS) and how is it Diagnosed?

With the addition of daytime sleepiness, OSA is referred to as a syndrome. Since Obstructive Sleep Apnea is commonly defined by abnormal numbers of apneas as well as hypopneas, when it is accompanied by daytime sleepiness it is occasionally referred to as Obstructive Sleep Apnea Hypopnea Syndrome (OSAHS). The shorter term obstructive sleep apnea syndrome and its acronym OSAS is used more often. Since apneas and hypopneas disturb sleep quality, sleepiness or dozing off during the daytime is a common symptom. The more severe the sleep apnea, the more likely it is that OSAS is present.

What is Respiratory Effort–Related Arousal (RERA)?

Respiratory effort-related arousal (RERA) is reduced airflow that causes one to “arouse” from sleep. Normal sleep usually calmly cycles between lighter and deeper levels that can be viewed on the sleep study. Arousals abruptly disrupt these levels of sleep. They may lead to waking in the middle of the night, or one may sleep through them. People are usually unaware of an arousal unless they wake up. However, arousals are not benign. They reduce sleep quality. They are associated with physiological changes such as increased heart rate, blood pressure and sympathetic activity. They may also lead to feelings of daytime sleepiness. To identify a RERA, a wave height reduction that doesn’t meet criteria for an Apnea or Hypopnea, lasting for at least 10 seconds and resulting in an arousal must be identified. These arousals may be noted by examining the brain waves that are monitored in the sleep study to identify sleep levels or stages. RERAs contribute to the Respiratory Disturbance Index (RDI) scale that can also be used to diagnose sleep apnea. The RDI represents the number of apnea, hypopnea and RERAs that take place in one hour. Severity of apnea using RDI is determined in the same way as AHI—5 – 15 is mild, 15 – 29 is moderate and 30 or greater is severe.

What is Upper Airway Resistance Syndrome (UARS)?

The term Upper Airway Resistance Syndrome (UARS) originated from a study conducted on a small group of individuals who had symptoms related to Obstructive Sleep Apnea Hypopnea Syndrome, but who did not experience apneas. In the most recent version of the International Classification of Sleep Disorders (third version), UARS falls under the category of sleep apnea as individuals have similar dysfunction.

People with UARS experience limited airflow, have arousals, but do not experience significant apneas (temporary pauses of breathing). Traditionally, sleep apnea is identified by using a measure of complete (apnea) and partial (hypopnea) obstructions to breathing called the Apnea Hypopnea Index or the AHI. But another measure called the respiratory disturbance index (RDI)* includes respiratory events called RERAs. RERAs are Respiratory Effort Related Arousals. The RDI includes Apneas, Hypopneas and RERAs in its index. Patients with UARS do not meet the criteria of sleep apnea using the criteria of AHI, which usually requires at least 5 apneas or hypopneas per hour, to be diagnosed. But these individuals still experience symptoms of sleep apnea and have respiratory events events that disturb their sleep. Usually their RDI is greater than 5 per hour, though their AHI is less than 5. People with UARS experience fragmented sleep. As a result, daytime sleepiness or insomnia is described as manifestations of the condition.

The most current classifications of sleep disorders considers UARS and OSA to be one and the same, though confusion still surrounds the disorder. Controversy has developed over the vague and ill-defined definitions that have been associated with this condition.

What is Sleep-Disordered Breathing (SDB)?
Sleep disordered breathing is an umbrella term that is used to describe abnormal respiratory events during sleep. Disturbances such as snoring, obstructive apneas, central apneas, RERAs, and full-blown sleep apnea fall into the category of events that make up SDB. There is a distinction between the sleep disordered breathing events and the frequency of events that are necessary to define a disorder such as AHI or RDI.
What is the Apnea-Hypopnea Index (AHI)?

Sleep apnea is confirmed by a diagnostic study, often using sensors of sleep, breathing and oxygen levels. It is measured by decreased airflow that is either complete (apneas) or partial (hypopneas). The number of apnea and hypopnea events per hour make up the Apnea-Hypopnea Index (AHI).

The Apnea-Hypopnea Index is the most frequently used benchmark to diagnose obstructive sleep apnea-(OSA). The AHI also determines the severity of the condition in a patient. In a single night study, AHI is defined as the total number of apneas plus hypopneas divided by the total number of hours of sleep. An AHI of less than 5 is considered normal, 5 – 14 is mild, 15-29 is moderate sleep apnea and 30 or more is severe.

The AHI is considered a substandard way of measuring OSA because it may not measure arousals from sleep , improper beating of the heart (cardiac arrhythmias), and the amount and duration of oxygen desaturation.

What is the Respiratory Disturbance Index (RDI)?

The respiratory disturbance index (RDI) is a measure of how often abnormal respiratory events occur in every hour of sleep. Unlike the AHI, more sleep disorders and conditions are grouped under the RDI, such as RERAs. Because more events are captured by the RDI, the use of the RDI could result in a more sleep disordered breathing diagnoses than the AHI.

References:
1. Principles and practice of sleep medicine. 5th ed. St. Louis:Elsevier/Saunders; 2010.
2. International Classification of Sleep Disorders, 3rd ed, American Academy of Sleep Medicine, Darien, IL 2014.
About the editor: Dr. Michael Morgenstern is a board certified Neurologist, expert in Sleep Medicine and  founder of the American Sleep Apnea Society. He is Director of the Cedarhurst Sleep Center in Long Island, New York and also sees patients with neurologic conditions.

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