Microsleep versus Sleep Onset Latency during Maintenance Wakefulness Tests: Which One Is the Best Marker of Sleepiness?

Abstract

The interpretation of the Maintenance Wakefulness Test (MWT) relies on sleep onset detection. However, microsleeps (MSs), i.e., brief periods of sleep intrusion during wakefulness, may occur before sleep onset. We assessed the prevalence of MSs during the MWT and their contribution to the diagnosis of residual sleepiness in patients treated for obstructive sleep apnea (OSA) or hypersomnia. The MWT of 98 patients (89 OSA, 82.6% male) were analyzed for MS scoring. Polysomnography parameters and clinical data were collected. The diagnostic value for detecting sleepiness (Epworth Sleepiness Scale > 10) of sleep onset latency (SOL) and of the first MS latency (MSL) was assessed by the area under the receiver operating characteristic (ROC) curve (AUC, 95% CI). At least one MS was observed in 62.2% of patients. MSL was positively correlated with SOL (r = 0.72, p < 0.0001) but not with subjective scales, clinical variables, or polysomnography parameters. The use of SOL or MSL did not influence the diagnostic performance of the MWT for subjective sleepiness assessment (AUC = 0.66 95% CI (0.56, 0.77) versus 0.63 95% CI (0.51, 0.74)). MSs are frequent during MWTs performed in patients treated for sleep disorders, even in the absence of subjective sleepiness, and may represent physiological markers of the wake-to-sleep transition.

Keywords: Maintenance Wakefulness Test; polysomnography; sleep latency; sleepiness; wakefulness.

Figure 1

Examples of microsleeps. The microsleeps (MSs) are highlighted in the red boxes. They mostly present as a disappearance of alpha rhythm (eyes closed) with or without slow theta waves. The duration of the MS is clearly less than half of the epoch, and therefore this epoch is not allowed to be scored as sleep. The black arrows point to slow eye movements typical of the N1 stage and the dotted black arrow points to a vertex wave (EEG channels: bipolar montage (Fp2-C4; T4-T4; T4-O2, Fz-Cz, Cz-Pz); EOG: electro-oculogramm right (R) and left (L); EMG: electromyogram; EKG: electrocardiogram).

Figure 2

Effect of test time on microsleep and sleep episodes. Test 1 is performed at 9:00 a.m., Test 2 at 11:00 a.m., Test 3 at 1:00 p.m., and Test 4 at 3:00 p.m. (A) Mean microsleep latency (MSL) and sleep onset latency (SOL). Latencies are presented as mean with 95 CI. The MSL (black dot) is always significantly shorter than the SOL (grey dot). No significant difference between the 4 tests is observed. (B) Proportion of patients who fell asleep (MS or sleep) for each test. No significant difference between the 4 tests is observed. **** p < 0.05.

Figure 3

Correlation between the microsleep latency (MSL) and the sleep onset latency (SOL). Each point represents a patient defined by his mean MSL and his mean SOL across the 4 tests. (A) All MWT with at least one MS are taken into account (N = 61), even those with no sleep (mean SOL = 40 min). A significant positive correlation is observed between SOL and MSL (r = 0.74, 95% CI (0.63; 0.82), p < 0.0001). (B) The same representation excluding tests with no sleep (N = 35). The positive correlation remains significant (r = 0.63, 95% CI (0.37; 0.80), p < 0.0001).

Figure 4

Diagnostic value of Maintenance Wakefulness Test (MWT) for excessive daytime sleepiness according to whether or not MSs are included in the interpretation.