If you are reading this, you've probably had a sleep study or you are considering to do a sleep study and want to know a bit more about the report. A sleep study means that you will spend a night hooked up to devices that generate an encyclopedic amount of information on what's going on in your brain and blood, your lungs and limbs.
Neither you-nor the doctor treating you-will view more than a minuscule fraction of this data. The sleep lab will reduce the 1,000 or so pages of material down to its essence. You will receive a report which is a summary of all the information and it can all be awfully confusing. "People do tend to get lost in the numbers". In order to help you get a better handle on these numbers and a better comprehension of what they quantify and signify, here is a guide to polysomnography.
Quantity of Sleep
The first thing looked at is the total sleep time, or TST. This is an objective measure, based on the readings from electrodes recording brain waves, of the amount of time you actually sleep during your study. Very often, this measurement is at odds with people's subjective perception of how much they’ve slept. They will feel as if they hardly slept a wink, but the report indicates they were out for six hours. The electrodes don't lie.
Sleep Efficiency and Latency
The ratio between the total sleep time and the total recording time, or TRT, is called the sleep efficiency. People who have significant difficulties in either initiating or maintaining sleep have diminished sleep efficiency, which can be related to various conditions and disorders, including depression. The number of minutes between the time the light is turned out and the person falls asleep is the sleep onset latency. Normally, it takes about 15 minutes to fall asleep. A significantly shorter onset latency--the proverbial falling asleep as soon as your head hits the pillow--might seem desirable, but in fact is an indicator of sleep deprivation. This could suggest a disorder, or it could be just a sign to slow down. Regardless of the demands of work and family, and the enticements of television and the web, you need at least seven and a half hours of sleep a night.
Once you drop off, you should progress through four stages of increasingly deep, dreamless sleep and into a fifth stage during which dreaming--characterized by rapid eye movement--occurs. Over the course of the night, you will cycle repeatedly between Non-REM and REM sleep. The structure of these cycles--poetically known as "sleep architecture"--reveals whether you are getting the restorative sleep you need to feel and be well.
Stage 1: The very lightest sleep - should only be 5 percent of the total sleep time, and should only occur at the beginning of the night.
Stage 2: The still fairly light - sleep should represent about 55 to 60 percent of the TST.
Stage 3 and 4: Deep sleep--also known as "slow wave" sleep and should account for about 20 percent of the TST. Your sleep study measures the time spent in each stage, in minutes and as a percentage, and the latencies between the time you fall asleep and the time you enter Stage 4 and REM sleep, respectively.
Arousals and Awakenings
Arousals and interruptions of sleep lasting 3 to 15 seconds can occur spontaneously or as a result of sleep-disordered breathing or other sleep disorders. Each arousal sends you back to a lighter stage of sleep. If the arousal last more than 15 seconds, it becomes an awakening. You are usually not aware of arousals, but may be aware of awakenings. The number of arousals and awakenings is registered in the study, and reported as a total number and as a frequency per hour of sleep, which is referred to as an index. The higher the arousal index, the more tired you are likely to feel, though people vary in their tolerance of sleep disruptions. As few as five arousals per hour can make some people feel chronically sleepy. In the worst cases of SDB, the index can be 100 or more.
The arousals and awakenings that occur when things go awry in your breathing while you're asleep can arise from a glitch in the central nervous system for example the brain "forgets" to breathe, but more commonly have a mechanical cause. The mechanical malfunction occurs when the soft structures in the back of the throat collapse into the airway, reducing the amount of air that makes its way into your lungs and, as a consequence, the amount of oxygen in your bloodstream. In an apnea, airflow is reduced by at least 80 percent; in a hypopnea, the range of the reduction is 50 to 80 percent. The report indicates the number of times each occurs, as a nighttime total and as an index of events per hour. This latter number is referred to as either the apnea-hypopnea index or the respiratory disturbance index. An index of 5 to 14 indicates a mild level of breathing and sleep--disturbance. From 15 to 30 is moderate; greater than 30 is severe. The associated drops in blood oxygen levels, known as desaturations, are also measured and categorized. Normal saturation is around 95 percent. A desaturation to 86 percent is mild, a reduction to 80 to 85 percent is moderate, and a drop to 79 percent or less is severe.
How is obstructive sleep apnea diagnosed and evaluated?
1. History and physical examination
Obstructive sleep apnea can be diagnosed and evaluated by subjective (perceived or biased) and objective (factual, based on empirical data) methods.
2. ESS (Epworth Sleepiness scale)
The Epworth Sleepiness Scale is a self-report test that establishes the severity of sleepiness. A person rates the likelihood of falling asleep during specific activities. Using the scale from 0 to 3 below, the risk of dozing can be ranked from the chart below.
0 = Unlikely to fall asleep
1 = Slight risk of falling asleep
2 = Moderate risk of falling asleep
3 = High likelihood of falling asleep
After ranking each category, the total score is calculated. The range is 0 to 24, with higher scores suggesting greater sleepiness.
0 to 9 = Average daytime sleepiness
10 to 15 = Excessive daytime sleepiness
16 to 24 = Moderate to severe daytime sleepiness
Breaking it down further, excessive daytime sleepiness is greater than 10.
It is important to note that in patients with insufficient sleep (less than 7-7.5 hours for most adults), they can also have daytime somnolence even without obstructive sleep apnea.
Primary snorers usually have a score less than 10, and individuals with moderate to severe sleep apnea usually have a score greater than 16.
Self-reported, subjective measures such as the Epworth Sleepiness Scale usually are combined with a thorough medical history. The history includes questions about:
- work performance,
- daytime sleepiness,
- driving and accident history,
- falling asleep during meetings, and
- Decreased memory.
3. Sleep study:
The primary objective test for obstructive sleep apnea is polysomnography, also referred to as a sleep study. This test measures different physical and physiological parameters while a patient is asleep. During attended polysomnography, a technician observes a person sleeping and monitors recording equipment in the setting of a sleep laboratory.
A typical polysomnography test includes:
- an EEG monitors brain waves,
- measurement of oral and nasal airflow,
- measurement of chest and abdominal movement,
- audio recording of the loudness of snoring,
- blood oxygen levels (Oximetry)
The EEG (electroencephalogram) monitors brain waves and can be used to determine the level of sleep or wakefulness. It is helpful for determining if an event (respiratory or limb movement) disrupts the level of sleep. During REM sleep (dreaming sleep), the eyes typically move from side-to-side. This measurement can help determine the duration of REM sleep. During stage 1-4 sleep there is a baseline muscle tone; however, during REM sleep all muscles relax.
Oral and nasal airflow can be measured by several different methods to help determine the size and frequency of breaths during sleep. Chest and abdominal movements occur with each attempt to breathe and can be used to distinguish between central sleep apnea and obstructive sleep apnea. During central sleep apnea, the signal to take a breath is not given, so the muscles do not attempt to take a breath. During obstructive sleep apnea, the muscles attempt to take a breath, but no air moves. Measurement of the loudness of snoring can be used to quantify snoring. Sometimes a measurement is needed to convince someone that they have a snoring problem. It can also be used to measure changes after treatments for snoring. Oximetry is used to measure the decreases in oxygen in the blood during apneas and hypopneas.
After polysomnography is completed the data are analyzed by a Neurophysiologist. The number of apneas, hypopneas, movements, and desaturations as well as sleep levels are all recorded in a formal report, and a diagnosis is made.
What are the Severity levels in obstructive sleep apnea?
Obstructive sleep apnea can be categorized as mild, moderate, or severe. This stratification assists in determining the direction of treatment. Some treatments that are excellent for mild sleep apnea nearly always will fail for severe sleep apnea. The severity level is measured with polysomnography.
In one grading scale using the apnea-hypopnea index, mild obstructive sleep apnea is 5 to 15 events per hour, moderate obstructive sleep apnea is 15 to 30 events per hour, and severe obstructive sleep apnea is more than 30 events per hour.