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 4 to 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.
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