There is nothing more frustrating for a sleep specialist than a visit from a patient who has been diagnosed with narcolepsy simply based on daytime sleepiness. A more likely scenario is that the patient has sleep apnea. To help decipher between these sleep disorders, it is important to have a good understanding of the differences between obstructive sleep apnea (OSA) and narcolepsy.
Obstructive Sleep Apnea
OSA, the most common type of sleep apnea, is a chronic, potentially lethal condition that causes poor sleep quality and increases the risk for other conditions, including coronary artery disease, congestive heart failure, arrhythmias, refractory hypertension, type 2 diabetes, stroke, and polycystic ovarian disease.1
Approximately 14% of men and 5% of women have OSA, although this can vary depending on the population.1 For example, it is estimated that 70% to 80% of patients being evaluated for bariatric surgery and 60% to 70% of those with a history of stroke or transient ischemic attack have OSA.2,3 Of all the comorbidities, the one most studied is hypertension. Up to 50% of patients with hypertension have OSA.4
The prevalence of OSA is increased in patients between 35 and 60 years of age.5 The prevalence of OSA also is increased in postmenopausal women but remains less than that of men.6 However, the prevalence of OSA in postmenopausal women taking hormone replacement therapy is similar to that of premenopausal women.6
The classic symptoms of OSA include snoring alternating with periods of silence, followed by gasping or snorting. A meta-analysis found that a complaint of choking or gasping during sleep was the most useful clinical indicator of sleep apnea.7
OSA occurs as a result of the collapse of the upper airway during sleep. It often is associated with obesity (body mass index ≥30 kg/m2) but that is not always a reliable indicator, especially in high-risk patients with other comorbid conditions (American Academy of Sleep Medicine [AAPM] OSA screening initiative, “Sleep Apnea Hurts HEARTS [heart failure, elevated blood pressure, atrial fibrillation, resistant hypertension, type 2 diabetes, stroke]).8 Thus, patients at high risk for OSA should undergo a comprehensive sleep evaluation that includes a polysomnogram or home sleep apnea test.1
One of the most common symptoms of OSA is excessive daytime sleepiness (EDS), which is why OSA often is confused with narcolepsy. Daytime sleepiness may be subtle or very pronounced and is a consequence of sleep fragmentation during the night.9 EDS is not present universally in OSA and should not be used as an indicator to determine the severity of apnea.
Narcolepsy is a rare condition — with a prevalence of 0.02% to 0.06% — that affects both sexes equally.10 Most patients with narcolepsy begin to exhibit symptoms near the age of puberty, with a peak age of onset between 15 and 25 years.11 If a patient is older than 30 years of age, clinicians should consider a diagnosis of OSA before narcolepsy.
Narcolepsy is divided into 2 subtypes: narcolepsy type 1 (hypocretin deficiency with cataplexy) and narcolepsy type 2 (hypocretin deficiency without cataplexy).12 Patients with narcolepsy have an 85% to 95% reduction in the amount of hypocretin neurons, which results in instability of wake and sleep states.13 Narcolepsy, which is thought to be an autoimmune disorder of the brain, is closely associated with the human leukocyte antigen (HLA) DQB1*0602 allele.14
The classic triad of symptoms of narcolepsy include EDS with or without cataplexy, sleep paralysis, and hypnagogic hallucinations.15 Unwanted episodes of sleep occur throughout the day, often when a patient is inactive or bored. Patients with narcolepsy dream more frequently and go into REM sleep more quickly compared with patients without narcolepsy, who typically take 90 minutes to enter REM sleep.
Cataplexy occurs, to some degree, in 60% to 70% of patients with narcolepsy. It is an abrupt loss of muscle tone, most frequently occurring during periods of strong emotional reactions such as laughter, anger, or surprise.15 Cataplexy can present with various levels of disability; some patients may fall to the ground, whereas others may experience just weakness in their extremities. A sleep specialist or neurologist can help diagnose milder cases of cataplexy.
Sleep paralysis, which occurs in 25% to 50% of patients with narcolepsy, is extremely frightening for the individual.15 Patients wake unable to move or speak and feel as though they are dying, with associated anxiety. Sleep paralysis can happen in patients without narcolepsy, but any patient who complains of sleep paralysis should be tested for narcolepsy.
Hypnagogic hallucinations occur in 33% to more than 80% of patients with narcolepsy.15 These hallucinations, which occur at sleep onset, can be vivid and unpleasant and present as auditory or visual hallucinations. They can be so real that patients have reported calling the police because they believe an intruder has been in their home, only to realize later that it was part of a dream.
Narcolepsy requires a clinical diagnosis of EDS and a positive Multiple Sleep Latency Test (MSLT). Criteria include a mean sleep-onset latency of <8 minutes on MSLT and ≥2 sleep-onset REM periods during 5 scheduled naps.12 A MSLT should follow an overnight polysomnogram, which evaluates for other sleep disorders such as OSA. Genetic testing also can be used to identify individuals at risk for narcolepsy. The HLA DQB1*0602 allele is the most specific genetic marker for narcolepsy across all ethnic groups and is found in 95% of patients with narcolepsy type 1.12