Surprisingly, in this large OSAS patient population, we were unable to correlate the repetitive Sa02 drops, obesity, and to an extent the apneas, with the different degrees of daytime sleepiness measured by our MSLT. Several reasons for these findings, which follow here, can be considered.
Sensitivity of the Test
Perhaps our test was not sensitive enough. Historically, there has been disagreement over the mean MSLT score that would indicate abnormal daytime sleepiness. While in middle-aged adults a mean MSLT score >10 min undoubtedly indicates normalcy,2 a mean MSLT score <10 min also may be normal, depending on the age of the subject. Van den Hoed et al studied 100 patients (excluding those with sleep apnea) referred to a sleep clinic for suspicion of daytime sleepiness.
Clinical symptoms, polysomnographic variables, and the Minnesota Multiphasic Personality Inventory were analyzed. Narcoleptic patients had a mean ±standard deviation (SD) MSLT score of 3.3±3.3 minutes, 83 percent with a score F5 min. Patients diagnosed with central nervous system (CNS) hypersomnia syndrome who took a daily nap had a mean score of 6.5 ±3.9 min, and a new analysis of these data indicated that 81 percent had a score F8 min. In a more recent study, 50 CNS hypersomniacs had a mean MSLT score of 6.3 ±3.5, and 88 percent had a mean score of F8 min. Van den Hoed and his colleagues concluded that, as with any test in medicine, a “grey zone” existed in the MSLT, where mean scores between eight and ten min were recorded. No in-depth study has been done to identify the factors that would better define the limits of this two-minute zone as a function of age, body mass, etc. However, the MSLT has been sensitive enough to discriminate between diverse degrees of daytime somnolence in sleep deprivation or sleep fragmentation protocols, including many pharmacologic investigations. Clinical reports of severe somnolence have always been associated with MSLT scores <5, as in our population. Here, however, we cannot explain MSLT scores by differences in Sa02 drops, obesity, or, to some degree, RDI. One may speculate that the effect of obesity on lung function, as evaluated by lung volume variation or upper airway resistance, may have brought more discriminative power to the analysis, as more quantitative information on respiratory efforts would have been investigated.