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) ¹.
• 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 ¹.
• 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 ¹.

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.