Correlation of daytime sleepiness with urine metabolites in patients with obstructive sleep apnea

The apnea-hypopnea index (AHI) is closely related with the severity of daytime sleepiness, but excessive daytime sleepiness (EDS) is not presented on all patients with obstructive sleep apnea (OSA). It is unclear why daytime sleepiness is not always present in OSA patients even if their sleep is disrupted from the perspective of polysomnographic findings. This study aimed to analyze the correlation between sleepiness and urine metabolites of neurotransmitters involved in the arousal system.

On the basis of AHI in polysomnography, 49 consecutive OSA patients were included. According to their Epworth sleepiness scale (ESS), 23 non-sleepy patients (ESS <11) and 26 sleepy patients (ESS ≥11) were included. Urine samples were collected before and after polysomnography and analyzed by gas chromatography-mass spectrometry with selective ion monitoring. Six metabolites of dopamine, norepinephrine and serotonin were analyzed.

The dopamine metabolites, homovanillic acid (r = 0.366, P = 0.017) and 3,4-dihydroxyphenylacetic acid (DOPAC; r = 0.584, P < 0.0001), were significantly correlated with ESS after adjusting for age, sex, body mass index, AHI, sleep efficiency and total sleep time. A two-by-two table analysis revealed that the overnight increase in the urine DOPAC was more frequent in sleepy patients while its decrease was more frequent in non-sleepy patients (odds ratio = 3.54, P = 0.032).

Urine dopamine metabolites may identify sleepy patients with OSA. In particular, the overnight change of urine DOPAC may indicate OSA patients with EDS.