Two half days a week I see patients in the sleep center. One of the reasons patients come to see me is for excessive daytime sleepiness (EDS). Patients may voice the above complaint in different ways. Some say [Dr. Sethi my biggest problem is that I feel sleepy during most of the day. I find it hard to concentrate at work or my mind feels "dull"]. Others may say they find it difficult to wake up in the morning in time for work or college. The history may be more worrisome: the patient may have fallen asleep while driving. So how does one differentiate between the various causes of EDS? In this post I shall touch on these very topics.
The various causes of EDS can be broadly listed as follows:
1. Untreated sleep disordered breathing: Obstructive sleep apnea (OSA), Central sleep apnea (CSA), Mixed sleep apnea (MSA).
2. Poor sleep hygiene
3. Circadian rhythm disorder: Delayed sleep phase syndrome (DSPS), Shift work disorder
4. Narcolepsy: primary and secondary
5. Idiopathic hypersomnia.
6. Medical disorders causing poor sleep at night.
Narcolepsy: is characterized by excessive sleepiness. It may or may not be associated with cataplexy. Cataplexy in simple terms refers to loss of muscle tone. Let me explain with the aid of an example. Let us assume a narcoleptic patient hears a joke. He starts laughing and suddenly experiences loss of muscle tone and finds himself on the ground. This sudden loss of muscle tone is referred to as cataplexy. Another example of cataplexy is a runner who collapses when the starter gun goes off. Narcolepsy is usually accompanied with sleep paralysis and vivid hallucinations which are reported either at onset of sleep or when the patient is about to wake up (that is around sleep-wake transitions). A point to note is that if the patient has cataplexy then the diagnosis of narcolepsy is easy to make. It is narcolepsy without cataplexy that is tough to diagnosis. A caveat to remember here is that not “everything” is cataplexy-so weakness in the knees if you have been laughing hard does not constitute cataplexy. Narcolepsy occurs due to the loss of hypocretin neurons in the lateral hypothalamus. So secondary narcolepsy may also occur if these neurons are damaged by a hypothalamic stroke or a hypothalamic tumor or by a infectious/ inflammatory process (limbic encephalitis). The clinical presentation of secondary narcolepsy though is different: example the history is of excessive sleepiness after a stroke or a tumor.
The diagnosis of narcolepsy is made on the basis of an overnight polysomnography (sleep study) which is done to rule out sleep disordered breathing (sleep apnea). One needs to make sure that the patient does not have untreated sleep apnea before the EDS can be attrbuted to narcolepsy. The patient also undergoes a test called multiple sleep latency test (MSLT). During the MSLT the mean sleep latency is calculated. In narcoleptics the mean sleep latency is usually less than 5 minutes and they also have sleep onset REMs (meaning they enter REM sleep as soon as they fal asleep). This is against what is seen in idiopathic hypersomnia which too has a short sleep latency but there is no sleep onset REMs.
The treatment of narcolepsy and idiopathic hypersomnia is virtually identical. The patient is usually prescribed stimulants such as Adderall, modafinil (Provigil) or Nuvigil.
In my next post I shall elaborate on this further.
Nitin Sethi, MD