Diagnosis of Narcolepsy
Diagnosis of narcolepsy is based on the clinical recognition of excessive daytime sleepiness, uncontrollable sleep, observed cataplexy, and the exclusion of other causes of excessive daytime sleepiness. The occurrence of hypnogogic and hypnopompic hallucinations or sleep paralysis suggests narcolepsy, but because they are not exclusive to the condition, they are not essential components of a diagnosis.
It is important to review the entire sleep history. A patient is often asked to keep a two-week sleep diary so that his or her doctor can evaluate the quality of recent sleep and exclude the possible influence of other sleep disorders.
Peripheral Concerns with Narcolepsy
Although the symptoms of narcolepsy seem distinct to the disorder, they warrant differential diagnosis. Symptoms like excessive daytime sleepiness are also indicative of serious physiological diseases and disorders, ranging from brain tumors to heart disease to anemia. Uncontrollable sleep related to these conditions is often more extensive; it lasts longer and does not usually allow the patient to feel refreshed.
Also, affective disorders, including various types of depression, share symptoms with narcolepsy. The extent to which depression is found in cases of narcolepsy has not been thoroughly examined. Complaints of tiredness and disrupted sleep are often the product of depression or stress. Both conditions respond to REM sleep manipulation. Distinguishing the presence of narcolepsy from these conditions may require comprehensive lifestyle assessment.
Diagnostic Tests for Narcolepsy
It usually is difficult to confirm true cataplexy or to rule out other causes of excessive daytime sleepiness, so overnight polysomnography and a multiple sleep latency test (MSLT) are performed. Before polysomnographic testing is performed, a minimum two-week withdrawal period from any drugs with side effects that disrupt sleep is usually required; these include alcohol, antidepressants, narcotics, and other medications.
On rare occasions, blood testing for HLA type may support the clinical theory that a distinct HLA type exists in narcolepsy patients.
Multiple Sleep Latency Test
Once a "normal" night's sleep has been confirmed and other causes of excessive daytime sleepiness, such as obstructive sleep apnea (OSA) and periodic limb movement disorder (PLMD), have been excluded, a multiple sleep latency test (MSLT) is performed, usually the morning after polysomnography. The MSLT is a similar test, but it measures fewer parameters. The MSLT measures EEG, EOG, chin EMG, and usually heart rate. The patient attempts to take four to five 20-minute naps (depending on the protocol) every 2 hours throughout the day. After these naps, the time it takes the patient to fall asleep (sleep latency) is averaged. Sleep latency usually fluctuates in narcolepsy patients, where it may lengthen, but will most often remain shorter than normal latency.
In addition to measuring sleep latency, sleep technicians also assess the patient's REM sleep patterns. Rapid eye movement sleep during the first 15 minutes of sleep is called sleep onset REM (SOREM). The occurrence of SOREM is indicative of severe sleep deprivation or narcolepsy and is almost exclusive to these conditions. A positive MSLT for narcolepsy is generally indicated by two factors, sleep latency of less than 8 minutes and two or more occurrences of SOREM.
Fewer than 10% of patients demonstrate narcoleptic sleep latencies that are longer than 8 minutes. It is fairly common for patients to exhibit SOREM twice during an MSLT; this happens in approximately 16% of cases.
Narcoleptic sleep patterns obtained from the MSLT create a strikingly objective picture that provides an understanding of narcolepsy as an imposing and disturbing condition.
The maintenance of wakefulness test (MWT) and the repeated test of sustained wakefulness (RTSW), also used to measure daytime sleepiness, are similar to the MSLT. However, unlike the MSLT, patients participating in the MWT or RTSW try to stay awake. Pupillometry, the study of pupil diameter, also can be pertinent in narcolepsy cases, because the pupil diameters of narcoleptic patients may vary rhythmically in the dark.