Relation Between Sleep Apnea and Narcolepsy

Narcolepsy may be associated with other comorbid sleep disorders such as RBD, OSA, PLMS, sleepwalking, and nightmares. It is often associated with an increased

BMI, which predisposes to the development of OSA. On average, narcoleptics have a BMI 10% to 20% higher than the normal population (70,71). A reduced metabolic rate, decreased motor activity or abnormal eating behavior have been suggested as possible explanations. SDB is found in 10% to 20% of patients (72). Chokroverty (73) documented repeated apneic episodes in 11 out of 16 narcoleptic subjects. Sleep apnea was predominantly central but obstructive and mixed apneas were also noted. In 1972, Guilleminault et al. (74) reported central sleep apnea in two patients with narcolepsy and later extended this observation and found central and OSA in a large number of patients with narcolepsy (75). Laffont et al. (76) noted sleep apnea (both central and obstructive) in five patients with narcolepsy.

In cases with possibilities for other coexistent sleep disorders liable to produce EDS, narcolepsy may be considered after appropriate and adequate treatment of associated comorbid sleep disorders, and can subsequently be confirmed by an overnight multiple sleep latency test (MSLT) (77). While the combination of EDS and cataplexy is highly sensitive and specific for narcolepsy, the diagnosis based on EDS requires further evaluation. Narcolepsy without cataplexy represents 10% to 50% of the narcoleptic population. A PSG will provide supportive evidence in terms of disturbed nocturnal sleep with frequent awakenings, decreased sleep latency, short nocturnal REM sleep latency, and fragmented night sleep. These are nonspecific and can be seen in other conditions, for example, prior sleep deprivation. A sleep-onset REM period (SOREMP) of less than eight minutes during a PSG is observed in 25% to 50% narcolepsy with cataplexy and is highly specific. A PSG will also help evaluate the presence of other or coexistent sleep disorders, for example, OSA or PLMS. The MSLT demonstrates a mean sleep latency of less than eight minutes, typically less than five minutes and two or more SOREMPs (78). A mean sleep latency of less than eight minutes can be found in up to 30% of the normal population but two or more SOREMPs are considered highly suggestive of narcolepsy in an appropriate clinical context. However, 15% of patients with narcolepsy with cata-plexy especially older than 36 years of age may have a normal or more frequently borderline MSLT result (sleep latency of eight minutes or longer or only one SOREMP). Also population-based studies show that approximately 1% to 3% of adults may have multiple SOREMPs during random MSLTs. Up to 30% of patients presenting with EDS and both a mean sleep latency < 5 minutes + 2 SOREMPs have a condition other than narcolepsy (79). Bishop et al. (80) screened 139 healthy subjects and documented two or more SOREMPs in 17% of the subjects. These individuals were more likely to be male, younger, and sleepier than those with one or zero SOREMPs. In another study, Chervin and Aldrich (81) showed that 4.7% of 1145 patients suspected or confirmed to have OSA had two SOREMPs on the MSLT. Multiple SOREMPs can thus be seen in association with other sleep disorders, for example, SDB or behaviorally induced insufficient sleep syndrome. Measuring hypocretin-1 levels in CSF may be useful when the MSLT is difficult to interpret, in subjects already treated with psychoactive drugs, or in patients with other sleep disorders, for example, SDB, RBD, PLMD or insufficient nocturnal sleep (82,83).

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