Abstract
Brain injuries substantially change the entire landscape of oscillatory dynamics and render detection of typical sleep patterns difficult. Yet, sleep is characterized not only by specific EEG waveforms, but also by its circadian organization. In the present study we investigated whether brain dynamics of patients with disorders of consciousness systematically change between day and night. We recorded ~24 h EEG at the bedside of 18 patients diagnosed to be vigilant but unaware (Unresponsive Wakefulness Syndrome) and 17 patients revealing signs of fluctuating consciousness (Minimally Conscious State). The day-to-night changes in (i) spectral power, (ii) sleep-specific oscillatory patterns and (iii) signal complexity were analyzed and compared to 26 healthy control subjects. Surprisingly, the prevalence of sleep spindles and slow waves did not systematically vary between day and night in patients, whereas day-night changes in EEG power spectra and signal complexity were revealed in minimally conscious but not unaware patients.
Highlights
When it comes to classifying awareness it is deceiving to rely on pure observation as an individuum can be well aware of his environment with closed eyes or completely unaware even though his eyes are open, as believed in some post-comatose states
The recordings were divided into periods of lightsomeness, which corresponds to circadian day, and periods of darkness, which corresponds to circadian night
In the present study we systematically focused on day-night variations of oscillatory EEG activity and sleep patterns in a large sample of Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS) patients which were recorded over 24 hrs at bedside
Summary
When it comes to classifying awareness it is deceiving to rely on pure observation as an individuum can be well aware of his environment with closed eyes or completely unaware even though his eyes are open, as believed in some post-comatose states. Survivors, who awake from coma and show periods of eye opening and closing, seem to exhibit fluctuations of arousal that resemble circadian sleep-wake cycling of healthy individuals This putative manifestation of vigilance is a hallmark for the clinical diagnosis of the Unresponsive Wakefulness Syndrome[1] (UWS; cf Fig. 1). Patients with capability of covert command following in the fMRI were found to have preserved electroencephalographic (EEG) organization across sleep and wakefulness including sleep spindling[10] This intricate relationship between consciousness, circadian rhythmicity, neural connectivity and brain oscillations constitutes an intriguing and compelling puzzle which may help to shed some light on the biggest mystery in humans, their fluctuating conscious awareness of themselves and their surrounding world. Discoveries in this field should have further relevance for diagnosis, prognosis[11] and the more general understanding of the neural correlates of consciousness[12]
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