Narcolepsy is a primary sleep disorder of the central nervous system characterized by symptoms such as excessive daytime sleepiness, or EDS, and cataplexy--a loss of muscle tone for short periods of time while awake. Patients with narcolepsy also exhibit disordered REM sleep regulation. Research has shown that the cerebral spinal fluid of most patients with narcolepsy lacks a particular type of neurotransmitter called the hypocretins or orexins, indicating that the absence of these molecules causes the disease.
Symptoms of Narcolepsy
Narcolepsy is characterized chiefly by excessive daytime sleepiness, or EDS, following a full night's rest. Patients with narcolepsy experience urges to sleep during they day while doing routine tasks such as reading, watching television, driving or working at the computer. Narcoleptic patients may also experience cataplectic attacks elicited by strong emotions. Cataplectic attacks cause bilateral muscle weakness and loss of muscle tone, which can result in accidental falls. Narcolepsy is associated with hypnagogic illusions and sleep paralysis as well. Hypnagogic illusions are vivid dream-like illusions that occur at sleep onset; this is believed to be caused in part by the disordered regulation of the REM sleep state. Sleep paralysis is the inability to move despite a conscious state of mind during sleep onset or while waking up. Symptoms of narcolepsy typically appear between the ages of 15 and 25.
Testing for Narcolepsy
Testing for narcolepsy typically involves an overnight visit to a sleep-study center followed by a full day of scheduled naps. During the night, the patient is monitored for other sleep disorders such as obstructive sleep apnea or restless legs syndrome. The full day of scheduled naps is known as an MSLT, or multiple sleep latency test. During the MSLT, the patient is asked to lie down for a nap at regularly scheduled intervals while he is monitored to measure the length of time it takes for him to fall asleep. His eye movement and brain activity is also monitored to determine whether he falls directly into a state of REM sleep at sleep onset. Some physicians will also sample the patient's spinal fluid to test for the presence of hypocretin molecules.
The Role Of Dopamine
Dopamine is a monoamine neurotransmitter associated with motivation, appetite, wakefulness, attentiveness and feelings of reward. The activation of neurons that release dopamine relies in part on stimulation from neurons that use hypocretins as neurotransmitters. Narcolepsy is associated with a lack of hypocretin molecules in the cerebral spinal fluid, indicating that the narcoleptic patient's brain is deficient in hypocretin signaling. The absence of stimulation to dopaminergic neurons from hypocretin neurons results in decreased wakefulness.
Treating Narcolepsy
Narcolepsy symptoms are typically treated with drugs that promote wakefulness. These include the amphetamines and related stimulant compounds used to treat attention deficit disorder. Modafinil is a relatively new drug that is used to treat narcolepsy. It works in a way that is pharmacologically different from stimulant drugs and has less risk for dependency. Patients with narcolepsy may also be advised to take regularly scheduled daytime naps to help alleviate EDS. Patients who have narcolepsy with cataplexy may also be treated with tricyclic antidepressants or sodium oxybate to reduce the incidence of cataplectic attacks.
Considerations
Narcolepsy is a relatively rare sleep disorder, affecting roughly .03 to .05 percent of the population in the United States, according to the National Institutes of Health. This statistic, along with the variable constellation of symptoms and their severity, contribute to the fact that most patients suffering from narcolepsy are not properly diagnosed until 10 to 15 years following the first onset of symptoms.
References
- "Sleep and Sleep Disorders: A Neuropsychopharmacological Approach"; Malcolm Lader et al.; 2006
- "Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd Edition)"; Eric Nestler, Seven Hyman and Robert Malenka; 2009
- "Sleep Disorders in Neurology: A Practical Approach"; Sebastiaan Overeem and Paul Reading; 2010
- National Institute of Neurological Disorders and Stroke :Narcolepsy Fact Sheet
