Different kinds of Sleep Studies

There are several different sleep studies that are used when trying to diagnose different sleep disorders. Which study or studies is required depend upon your symptoms and suspected possible diagnoses.

Polysomnogram (PSG)

A polysomnogram is the most common sleep study required to assess for different sleep disorders. This study requires you to sleep in the sleep lab for the entire night. In preparation for the study, you get connected to many different sensors:

Ø You have several electroencephalogram (EEG) leads which record the brainwaves.

Ø You have sensors next to your eyes which measure eye movements.

Ø You have an electromyogram (EMG) lead which records the muscle tone near your chin.

Ø There are airflow or air pressure sensors near your nose and mouth to record movement of air.

Ø There is a microphone which records snoring.

Ø There are bands around the chest and belly which record movement that corresponds with breathing

Ø There is a finger probe which measures the oxygen saturation of the blood

Ø There is an electrocardiogram (EKG) lead to record your heart rate and rhythm.

Ø There are EMG leads on the legs which record leg movements.

Ø In addition, there is often video monitoring that records your activity during sleep.

Ø Some additional measurements are made in certain circumstances.

After all of the leads have been applied, they get connected to a computer which records the data. You are put into a bed and allowed to fall asleep.

After the data has been acquired, the technician scores the study. Scoring consists of determining the stage of sleep, assessing for arousals or awakenings, snoring, respiratory events, a decrease in oxygen levels, abnormal leg movements, or abnormal EKG tracing.

What is done with all of this information? For sleep apnea there are several measurements that are particularly important. The first is the number of sleep apnea events per hour of sleep. There are different definitions used in different sleep labs, but the apnea-hypopnea index (AHI) and the respiratory disturbance index (RDI) are the most common (see definitions). Also important is the effect of sleep apnea on the oxygen level, the sleep efficiency (the percentage of the night spent asleep), the number of arousals and awakenings, and the presence of any additional sleep disorders.

CPAP Titration

A CPAP Titration is a full night study that is very similar to a diagnostic PSG. However, this is only for people who have sleep apnea and need to identify the CPAP pressure that controls the sleep apnea.

During the CPAP titration, all of the same measurements are made as in the PSG. However, during this study, the technician applies CPAP starting at a low pressure and adjusts the pressure until the CPAP appears to control the sleep apnea.

Why does this need to be done? Currently the best way to find the pressure that works for each individual person is by trying multiple pressures. Attempts to predict which pressure would work based upon other factors have not been very accurate.

Split night Polysomnogram

A split night PSG is a combination of a diagnostic PSG and a CPAP titration. This study is only appropriate when the technician can confidently predict that the study will confirm a suspicion of sleep apnea in the first several hours of sleep. After there has been adequate sleep time during the diagnostic phase, the technician applies CPAP just like during a full night CPAP titration.

People who have moderate to severe sleep apnea often meet criteria for a split night study. People with mild sleep apnea, people with difficulty falling asleep, and people who have the majority of their sleep apnea events at the end of the night often do not meet criteria for a split night study, and a full night diagnostic study is then performed. Obviously, people who don’t have sleep apnea cannot get a split night study.

Multiple Sleep Latency Test (MSLT)

The MSLT is a daytime sleep study. It is most often used to assess people who have excessive daytime somnolence despite a nighttime sleep study which did not reveal a cause. In practice this usually means the physician is attempting to diagnose or rule out narcolepsy or a similar disorder. Sometimes these are done to assess for an additional sleep disorder for patients with sleep apnea who are still very sleepy despite good control of the sleep apnea. This study can also be used to assess how sleepy someone is during the daytime related to an already diagnosed disorder such as sleep apnea.

The MSLT consists of similar measurements to the PSG but only records the brainwaves, eye movements, chin tone and EKG. The patient undergoes a series of 5 naps during the day following a night-time PSG and is instructed to not resist falling asleep. The time it takes to fall asleep (up to 20 minutes) is measured and then averaged for the total number of naps. If the patient falls asleep, then the nap is continued for 15 minutes to assess for signs of REM sleep.

Maintenance of Wakefulness Test (MWT)

The MWT is a modified version of the MSLT. The major difference is that the patient is asked to try and stay awake in a quiet setting, instead of allowing themselves to fall asleep. The time to fall asleep is measured and averaged for the 4 naps.