Measurement of P300 and sleep characteristics in patients with hypersomnia: do P300 latencies, P300 amplitudes, and multiple sleep latency and maintenance of wakefulness tests measure different factors?

French consensus. Hypersomnolence: Evaluation and diagnosis

Sleepiness is one of the most frequently reported complaints in adults and children during specialised sleep consultations. It is responsible for an alteration that can be severe in quality of life, a lowering of academic or professional performance, and domestic or work accidents. Hypersomnolence is the first cause of road accidents on the highway, responsible for a third of fatal accidents. Furthermore its presence is associated with an increased risk of morbi-mortality related to cardiovascular and neurodegenerative pathologies. Hence, its represents a real public health issue. Recent revisions in international classifications have clarified confusing terminology, and the complaint of hypersomnia has now been replaced by the terms hypersomnolence or excessive sleepiness. It is clinically defined as an excessive quantity of sleep over 24hours, and/or by an alteration in the quality of arousal defined as incapacity to maintain a satisfactory level of vigilance during the day or in the morning on awakening (defined as sleep inertia). The evaluation of sleepiness requires a rigorous clinical approach, completed by subjective and objective measurements. The Epworth Sleep Scale, Multiple Sleep Latency Tests and the Maintenance of Wakefulness Test are the most studied and used in clinical practice. However, to date, no gold standard measurement of excessive sleepiness exists, and there are no quantifiable biological markers. It is therefore important to optimise our evaluation tools, improve our pathophysiological understanding of sleepiness, and define genetic and environmental risk factors.

Auditory evoked potentials remain abnormal after CPAP treatment in patients with severe obstructive sleep apnoea

OBJECTIVE

To assess the effects of 3 months of optimal CPAP treatment on auditory event related potentials (AERP) in patients with severe obstructive sleep apnoea (OSA) compared with healthy controls.

METHODS

Auditory odd-ball related N1, P2, N2 and P3 AERP components were assessed in 9 severe OSA subjects and 9 healthy controls at baseline evaluation and at ∼3 months follow-up in both groups, with OSA subjects treated with continuous positive air-way pressure (CPAP) during this period.

RESULTS

Severe OSA subjects showed significantly delayed, P2, N2 and P3 latencies, and significantly different P2 and P3 amplitudes compared to controls at baseline (group effect, all p<0.05). At follow-up evaluation P3 latency shortened in treated OSA patients but remained prolonged compared to controls (group by treatment interaction, p<0.05) despite high CPAP compliance (6h/night). The earlier AERP (P2 and N2) components did not change in either controls or OSA patients at follow-up and remained different in patients versus controls.

CONCLUSIONS

This study demonstrates that in severe OSA patients AERP responses show minimal or no improvement and remain abnormal following 3 months of optimal CPAP treatment.

SIGNIFICANCE

Persistent cortical sensory processing abnormalities despite treatment in severe OSA may have implications for daytime neurobehavioral performance and safety in OSA patients. AERP responses may help identify residual performance deficits and risks.

The use of evoked potentials in sleep research

Averaged event-related potentials (ERPs) represent sensory and cognitive processing of stimuli during wakefulness independent of behavioral responses, and reflect the underlying state of the CNS (central nervous system) during sleep. Components measured during wakefulness which are reflective of arousal state or the automatic switching of attention are sensitive to prior sleep disruption. Components reflecting active attentional influences during the waking state appear to be preserved in a rudimentary form during REM sleep, but in a way that highlights the differences in the neurochemical environment between wakefulness and REM sleep. Certain ERP components only appear within sleep. These begin to emerge at NREM sleep onset and may reflect inhibition of information processing and thus have utility as markers of the functional status of sleep preparatory mechanisms. These large amplitude NREM components represent synchronized burst firing of large number of cortical cells and are a reflection of the nervous system's capacity to generate delta frequency EEG activity. As such they are useful in assessing the overall integrity of the nervous system in populations not showing substantial amounts of SWS as measured using traditional criteria. While requiring care in their interpretation, ERPs nonetheless provide a rich tool to investigators interested in probing the nervous system to evaluate daytime functioning in the face of sleep disruption, the ability of the sleeping nervous system to monitor the external environment, and the ability of the nervous system to respond to stimuli in a manner consistent with the initiation or maintenance of sleep.

Inter- and intra-individual variability in performance near the circadian nadir during sleep deprivation

The study purpose was to assess inter- and intra-individual variability in neurobehavioral function near the circadian nadir during sleep deprivation and conduct exploratory factor analyses to assess relationships among alertness and performance measures during sleep deprivation. Twenty-five healthy individuals (16 females) aged 18-25 years participated. Participants were sleep deprived for two nights under controlled laboratory conditions using a modified constant routine procedure. A comprehensive battery of neurobehavioral performance tests, subjective sleepiness (SSS), and objective alertness (MWT) were assessed. Seventeen of the 22 neurobehavioral measures were impaired by sleep deprivation (all P < 0.01). The use of multiple neurobehavioral performance measures revealed impairments for all individuals during sleep deprivation. However, sleep deprivation effects were task dependent within and between individuals. Gender contributed minimally to inter-individual variability in performance. Exploratory factor analysis reduced the 22 measures to seven independent factors. Our findings indicate that no individual was especially vulnerable or resistant to the performance impairing effects of sleep deprivation. Instead, inter- and intra-individual variability in performance during sleep deprivation was task dependent. The finding that subjective sleepiness and objective alertness were not related to any performance measure during sleep deprivation suggests that these measures may assess independent brain functions.

Pupillometry in clinically sleepy patients

OBJECTIVES

Investigators have suggested using pupillometry to assess alertness in hypersomnolent patients. In this study we assessed hypersomnolent patients and normal volunteers by using pupillometry and examined the usefulness of this technique for the diagnosis of pathologic sleepiness in individual patients.

METHODS

Forty-nine patients were examined by pupillometry and their sleepiness was assessed by using the multiple sleep latency test (MSLT). Thirty-three normal well-rested volunteers were also examined by pupillometry. The patients were classified as having 'mild', 'moderate', or 'severe' sleepiness, based on their mean MSLT sleep latency. Several dynamic variables of pupil diameter were calculated from the pupillograms and correlated with the mean MSLT sleep latency, and were compared between severity groups of patients and the well-rested normal subjects.

RESULTS

All but two pupillometric variables were significantly correlated with sleep latency. All except the same two pupillometric variables of the sleepiest group were significantly different from those of normal subjects. However, only 51% of patients with mean sleep latencies less than 10 min and 35% of patients with mean sleep latencies of less than 5 min could be correctly identified by pupillometry.

CONCLUSIONS

Pupillometry is clearly associated with differences in alertness between groups of patients. However, pupillometric assessment cannot substitute for the MSLT in most cases.

Sleep and respiratory stimulus specific dampening of cortical responsiveness in OSAS

The application of inspiratory occlusion stimuli produces cortical responses called respiratory-related evoked potentials (RREPs). During wakefulness the RREP waveform consists of early P1 and Nf components, an N1 and a P300. During non-REM sleep the predominant component is an N550, best seen in the averages of elicited K-complexes. Obstructive sleep apnea syndrome (OSAS) patients have been previously shown to have a normal wake RREP but to have a reduced amplitude N550 and a smaller proportion of elicited K-complexes than controls. The present study tested the hypothesis that this reflects a sleep-specific dampening peculiar to inspiratory effort-related stimuli, by assessing both respiratory and auditory evoked potentials (AEPs) during wakefulness and non-REM sleep in OSAS patients and controls. Auditory tones were presented in an oddball sequence during wakefulness and as a monotonous series during stage 2 sleep. Inspiratory occlusions, delivered for 500 msec via an nCPAP mask were also presented during wakefulness and stage 2 sleep, every three to five breaths. Data were collected from ten OSAS patients and ten controls. There were no significant differences in the amplitudes of the auditory N1 and P3 or the respiratory P1, Nf, N1 or P3 components during wakefulness. The amplitude of the auditory N550 and the proportion of elicited K-complexes did not differ between groups for auditory stimuli presented during stage 2 sleep. The respiratory N550 and K-complex elicitation rate were both significantly reduced in the OSAS group, despite there being no differences in the mask occlusion pressure response to the occlusion. The results confirm a blunted cortical response to inspiratory occlusions that is specific to sleep. The absence of significant group differences in the responses to auditory stimuli highlight that the sleep-related differences seen in OSAS patients are specific to the processing of inspiratory effort related stimuli.

Evidence of a sleep-specific blunted cortical response to inspiratory occlusions in mild obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) patients have elevated non-rapid eye movement (REM) sleep arousal thresholds to inspiratory loading. To test the hypothesis that this is due to sleep-specific dampening of cortical responses to inspiratory effort, respiratory-related evoked potentials (RREPs) were evaluated in six mild OSAS patients and six age- and body mass index-matched controls during wakefulness and Stage 2 non-REM sleep. Electroencephalogram was recorded from six scalp sites (Fz, FCz, Cz, CPz, Pz, and O(2)). Electrooculogram, electromyogram, and mask pressure signals were also recorded. During sleep, pharyngeal pressure was recorded using a Millar pressure catheter placed 2 cm below the glottis. The RREP waveform was broadly similar in the two groups during wakefulness, but was markedly different during Stage 2 non-REM sleep. During wakefulness, only the N1 component showed reduced amplitude in the OSAS group. During sleep, the occlusion stimulus elicited fewer K-complexes in the OSAS patients. In addition, the N550 component in the average of K-complex responses was smaller in amplitude in the OSAS group. The data suggest that patients with mild OSAS have a "blunted" response to the respiratory occlusion stimulus. This appears not to be related to compromised mechanoreceptor function, as the RREP was normal in the patients when they were awake.

Rethinking the assessment of sleepiness

The consequences of dozing off when intending to stay awake, e.g. while driving or at work, are potentially catastrophic. The accurate assessment of this tendency is important, but is currently difficult. Several different methods give disparate results. A way out of this dilemma is suggested that involves modification of existing concepts of sleep and wakefulness to conclude the powerful influence of behaviour on sleep propensity. This propensity at a particular time depends, hypothetically, on a mutually inhibitory interaction between a sleep and a wake drive, not on the magnitude of either drive alone. Measurements of sleep propensity are partly situation-specific, whether measured objectively by laboratory tests or subjectively by a questionnaire such as the Epworth Sleepiness Scale. The latter is believed to measure a general characteristic, the average sleep propensity across a range of specified situations in daily life. Any one situational sleep propensity is not always an accurate predictor of another, even in the same subject. The Multiple Sleep Latency test should not be a gold standard for such measurements. Wider discussion and more research into "sleepiness" is needed.

A normative study of the maintenance of wakefulness test (MWT)

The maintenance of wakefulness test (MWT) is a daytime polysomnographic procedure which quantifies wake tendency by measuring the ability to remain awake during soporific circumstances. We present normative data based on 64 healthy subjects (27 males and 37 females) who adhered to uniform MWT procedural conditions including polysomnographic montage, illuminance level, seating position, room temperature, meal timing, and subject instructions. When allowed a maximum trial duration of 40 min, subjects' mean sleep latency to the first epoch of sustained sleep was 35.2 +/- 7.9 min. The lower normal limit, defined as two standard deviations below the mean, was 19.4 min. Calculation of data on the basis of a maximum trial duration of 20 min and sleep latency to the first appearance of brief sleep (a microsleep episode or one epoch of any stage of sleep) yielded a mean sleep latency of 18.1 +/- 3.6 min and a lower normal limit of 10.9 min. Sleep latency scores were significantly higher than those previously reported in patients with disorders of excessive somnolence. Therefore, the MWT appears to be a useful procedure in differentiating groups with normal daytime wake tendency from those with impaired wake tendency and in identifying individuals with pathologic inability to remain awake under soporific circumstances.