If you have been called for a sleep study, congratulations. For most people with narcolepsy, it can take many years and sometimes decades of battling to be taken seriously. It is only with a referral to a sleep specialist that you may be prescribed a sleep study, a test that remains the most common tool for the diagnosis of narcolepsy.
The sleep study can be usefully separated into two different tests. An overnight polysomnography followed, the next day, by something called a multiple sleep latency test. Let’s look at each in turn.
Polysomnography
The purpose of polysomnography is to find out how you sleep, crucial data that will help diagnose any of the many sleep disorders there are, including narcolepsy. You will usually be asked to have washed your hair, avoid napping during the day preceding the test, eat as normal and stick to any medication regimen you are on
On arrival at the sleep centre you will be shown to the room in which you will spend the night. Once settled, a technician will come to prepare you for the study, attaching between 12 and 22 electrodes to your scalp to monitor the electrical activity across your brain and across your chest to record the electrical activity of your heart. The sleep centre will usually provide dinner and then, towards the end of the evening, the lights will be switched off, you will be fitted with a finger clip that will record the oxygen concentration in your blood and you will be instructed to go to sleep.
During sleep, the brain normally cycles through a predictable sequence of stages.
- Stage 1. From waking, the brain transitions to a light phase of sleep, in which the eyeballs start drifting in their sockets and the frequency of electrical activity across the surface of the brain begins to slow.
- Stage 2. The eyeballs come to a standstill and the brain’s trace is characterised by rapid bursts of activity known as ‘sleep spindles’ and ‘k complexes’.
- Stage 3. The brainwaves slow further still into what is often referred to as deep sleep. The sleep spindles and k complexes are still present but are harder to see.
- Rapid-eye movement (REM) sleep. Around 90 minutes after falling asleep, the brain switches from deep Stage 3 into REM sleep. This period, in which the eyeballs flit wildly behind their lids, typically lasts a matter of minutes and is strongly associated with dreaming. At the end of REM sleep, the cycle starts over. During the course of an eight-hour sleep, the brain usually experiences five or more such sleep cycles.
For a person with narcolepsy, there are several features of the polysomnographic trace that differ from the pattern above, one or more of which may support a diagnosis.
- There are few or no clear sleep cycles
- The brain enters REM sleep far sooner than normal, usually within the first few minutes of falling asleep
- The duration of Stage 3 sleep is reduced
- The frequency of REM sleep is greater than normal, sometimes with 30 or more episodes over the course of a night
Multiple Sleep Latency Test (MSLT)
When you wake, you will be given breakfast and then, at some stage in the morning, the MSLT will begin. Although you have only recently woken from your polysomnography test, you will be instructed to go to sleep again. After 20 minutes, you will be woken and told to get up. This sequence of events will be repeated up to five times, from which the clinician will calculate your sleep latency, the average time it takes you to fall asleep. A mean sleep latency of less than five minutes qualifies as ‘severe’ hypersomnia, five to ten minutes as ‘troublesome’, ten to 15 as ‘manageable’ and 15 to 20 as ‘excellent’, meaning that excessive daytime sleepiness is not an issue at all. Most people with narcolepsy will be asleep within a minute or two, clear evidence of severe hypersomnia.
The MSLT also allows the clinician to look for sleep onset REM periods (SOREMPs). These are defined as bouts of REM sleep that occur within the first 15 minutes of falling asleep. In the general population, SOREMPs are extremely rare. For those with narcolepsy, SOREMPs are extremely common. If you experience a SOREMP on two or more of the MSLT’s five sleep trials, this is considered diagnostic of narcolepsy.
Other sleep disorders
There are many sleep disorders that could account for your symptoms and the clinician will be on the lookout for these. One of the most common is sleep apnea, which is thought to affect around 1 in 20 men and 1 in 50 women. In sleep apnea, breathing stops completely, sometimes for up to a minute at a time and sometimes as many as 30 times an hour. This kind of disruption to the sleep cycle explains the main symptom of sleep apnea – excessive daytime sleepiness. If you have sleep apnea, it does not mean you do not also have narcolepsy, but your clinician is likely to begin by treating your sleep apnea first to see if this improves your sleepiness.