What initial investigations should I consider for a patient presenting with excessive daytime sleepiness and suspected narcolepsy?

Initial investigations for a patient presenting with excessive daytime sleepiness and suspected narcolepsy should include:

• Detailed sleep history and clinical assessment: To identify features suggestive of sleep disorders, including narcolepsy and to exclude other causes such as obstructive sleep apnoea/hypopnoea syndrome (OSAHS) NICE NG202.
• Use of sleepiness assessment scales: Employ the Epworth Sleepiness Scale to preliminarily assess the degree of sleepiness, although it should not be used alone to determine referral NICE NG202.
• Referral for formal sleep studies: Since narcolepsy diagnosis requires objective sleep testing, referral to a sleep service for polysomnography and multiple sleep latency testing (MSLT) is appropriate, although this is not detailed in the provided OSAHS guideline, it is standard practice for narcolepsy diagnosis.
• Exclude other causes of excessive sleepiness: Consider investigations to rule out OSAHS, such as home respiratory polygraphy or oximetry, especially if clinical features overlap, as OSAHS is a common cause of excessive daytime sleepiness NICE NG202.

In summary, initial investigations focus on clinical assessment, use of sleepiness scales, and exclusion of other sleep disorders like OSAHS, followed by referral for specialist sleep studies to confirm narcolepsy.