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How do we diagnose narcolepsy? Well, this is really a
clinical diagnosis supported by
polysomnography and other diagnostic testing.
Polysomnography and narcolepsy
demonstrates spontaneous awakening, reduced sleep efficiency
and often increased
light non-REM sleep. REM sleep often begins within 15
minutes so we see early
onset REM sleep in these patients. Patients have a
deficiency of REM sleep and so
they push REM sleep earlier into the night to try and
recover REM sleep that has
been lost but the diagnostic modality of choice for
diagnosing narcolepsy is the
multiple sleep latency test and this is really a test I want
you to remember and
associate with the diagnosis of narcolepsy. To make a
diagnosis of narcolepsy,
we want to look at the findings that we see on the MSLT. The
MSLT is done the
morning after a sleep study or polysomnogram. The patient is
placed in sleep-
inducing environments and allow to fall asleep spontaneously
multiple times. Each
nap session continues for about 15 minutes after sleep onset
to detect when REM
sleep begins after the onset of a nap. Typically, REM sleep
begins many minutes
into a nap and in narcolepsy we see early onset REM. Each
nap each sleep episode
is repeated at 2-hour intervals until the patient has had
4-5 opportunities to nap.
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And patients with narcolepsy take less than 8 minutes to
fall asleep and have early
onset REM during those naps. In terms of laboratory
investigations, we can also
do CSF studies and gene testing to evaluate particularly
narcolepsy type 1. In
the CSF, we can measure orexin levels and that's that
hormone released by the
hypothalamus that is reduced and deficient in patients with
narcolepsy type 1. We
see low levels that are suggestive of narcolepsy type 1 and
this is typically done
when sleep testing cannot be performed. CSF analysis is not
required to establish a
diagnosis of narcolepsy type 1, but when performed and
demonstrating low orexin
levels is highly suggestive of this condition. Genetic
testing for that DQB1 gene
and abnormalities polymorphisms in that gene can also be
performed. Other test
may be supportive of this diagnosis, but again the MSLT is
how we diagnose
narcolepsy. Actigraphy or the use of a movement sensor worn
on the patient's
non-dominant wrist can be helpful to evaluate sleep time and
sleep efficiency and we
see reduced sleep efficiency in narcolepsy. So there are a
number of diagnostic
criteria to establish a diagnosis of narcolepsy type 1. The
first is daily periods of
irrepressible need to sleep, 3 times per week for more than
3 months. So for a
persistent period of time patients have this frequent need
to sleep during the day.
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We also need to establish 1 or both of the following; low
levels of orexin in the
cerebrospinal fluid or cataplexy and an MSLT of less than 8
minutes
demonstrating that early onset of sleep in the MSLT. For
narcolepsy type 2, there
are a number of criteria that are used to establish this
diagnosis, daily periods of
irrepressible need to sleep, a mean sleep latency of less
than 8 minutes. Again, that
MSLT finding. Here are typically cataplexy is absent. There
are normal levels of
orexin in the cerebrospinal fluid. We don't need that to
establish a diagnosis of
narcolepsy type 2, but the presence would be suggestive of
this type of narcolepsy.
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And the findings should not be better explained by other
causes like insufficient
sleep, obstructive sleep apnea which is ruled out on a
polysomnogram, delayed
sleep phase disorder, or effects of medications, illicit
substances or withdrawal
from a medication.