Diagnosis of narcolepsy
It is common that people who have some of the symptoms of narcolepsy do not have a definitive diagnosis of the condition until several years after the development of the first symptoms. Misdiagnosis of the condition, and thus inappropriate treatment, is relatively common. Some patients have to wait for over 10 years before they receive a positive diagnosis of narcolepsy. With increasing knowledge of the condition by doctors and other professionals it is hoped that more rapid diagnoses will occur in the future.
Patients who suspect they may have narcolepsy should visit a GP in the first instance. It is useful to take a 'sleep diary' and/or a completed Epworth Sleepiness Scale to the GP. The Epworth scale is an internationally accepted means of measuring of daytime sleepiness, the major symptom of narcolepsy. If your score is 10 or below this indicates a level of daytime sleepiness found in the general population. A score of 18 or more indicates that you have very marked daytime sleepiness and that you should seek medical attention.
If a GP suspects that one of his patients may have narcolepsy he will most likely refer the patient to a specialised Sleep Centre. Once there, a careful history will be taken to determine the types of symptoms the patient is experiencing, the family history, the age when the first symptoms were noticed etc. Unless diagnosis is clear from presenting symptoms such as cataplexy, patients will probably stay overnight and undergo polysomnographic testing. These tests involve a measurement of the electrical activity of the brain, eye and muscle movement and breathing as the patients fall asleep and whilst asleep. How rapidly the subjects fall asleep will be measured - this is called 'sleep latency'. Several such tests are usually carried out - these are called a Multiple Sleep Latency Test (MSLT). Narcoleptic subjects usually fall asleep rapidly and thus have low sleep latency. The type of sleep they enter into from consciousness will be recorded. Evidence that they pass directly into REM sleep (see 'Causes') is regarded as being positive for narcolepsy. Such episodes are called 'Sleep Onset Rapid Eye Movement Periods'. In some centres blood samples will be taken for analysis of tissue type. Increasingly, a sample of cerebrospinal fluid (CSF) will be taken for measurement of orexin concentration. Low CSF orexin values are great value in the diagnostic process. It is hoped that, in the future, orexin levels may be measured from blood (rather than CSF) samples.
According the the 2nd edition of the International Classification of Sleep Disorders there are three types of narcolepsy:
- Narcolepsy with cataplexy
- Narcolepsy without cataplexy
- Narcolepsy due to medical conditions
In the first category, a positive diagnosis of narcolepsy is easy as cataplexy is almost always indicative of narcolepsy. The patient must have had Excessive Daytime Sleepiness daily for at least three months and have definite cataplexy. Ideally, polysomnographic tests should be carried out to confirm the diagnosis or alternatively levels of orexin in the CSF should be measured and found to be low (see 'Causes').
In the second category, a positive diagnosis is more difficult as cataplexy is not present. Again, the patient must have had Excessive Daytime Sleepiness daily for at least three months. Polysomnographic testing must be carried out and should show short sleep latency and Sleep Onset Rapid Eye Movement Periods. In this group of patients CSF orexin levels are typically normal but may be low in some patients.
In the third category, the patient must have had Excessive Daytime Sleepiness daily for at least three months but this must be accompanied by a significant medical or neurological condition. Parkinson's disease, Myotonic dystrophy and some types of brain injury can cause this 'secondary narcolepsy'.
In some centres, testing for the tissue type associated with narcolepsy may be carried out and will provide useful confirmatory information.
There are several sleep centres in the UK and the Republic of Ireland experienced in the diagnosis of narcolepsy. Click here to show details of these specialised centres.