The clinical diagnosis of consciousness is mainly based on bedside observation of the patient's responses using standardized neurobehavioral scales. By definition, it is common to observe vigilance fluctuation in patients in minimally conscious state (MCS) who would show reproducible but fluctuating signs of consciousness [1]. As the probability to detect voluntary responses depends on the patient's level of vigilance at the time of assessment, multiple assessments are needed in order to detect signs of consciousness and avoid misdiagnosis [2]. If this fluctuation is known in disorders of consciousness (DOC), it remains poorly understood and characterized. In this study, we investigated the relationship between pupil diameter (suggested as an objective physiological measure of alertness level in healthy subjects [3-6]) and behavioral responsiveness in DOC patients. To this end, five patients with chronic DOC (1 unresponsive wakefulness syndrome [UWS; ie, reflexive responses], 2 MCS- [ie, signs of consciousness but no signs of language preservation, 2 MCS+ [ie, signs of language preservation]; 3 males; age=47±15.16 (median ± SD), median TSI=284 days) were enrolled. For each patient, four behavioral assessments were performed in a single day using the Coma Recovery Scale-Revised. Before each assessment, pupil response was recorded for 10 minuttients (MCS-) was excluded from the analysis due to eye closure during whole recording period. Pupil diameter was recorded using Phasya Drowsimeter R100 glasses (eye images acquired at 120 Hz with a high-speed camera integrated into the glasses). Eye closure periods were marked manually. Several parameters were investigated: eye opening percentage (EOP), as well as median, variance, entropy, and Lempel-Ziv complexity of the pupil diameter. We here provide preliminary descriptive results for this small sample.
We observed lower EOP and median pupil diameter when the patients were unresponsive (i.e., diagnosis of UWS) vs. when they were responsive at bedside (i.e., MCS; median EOP=74.78% vs 99.6%, median pupil diameter=21 vs 28). Variance did not show any specific pattern; however, complexity measures of entropy and Lempel-Ziv were also lower in the UWS (median entropy=9.83 vs 10.58 and median Lempel-Ziv complexity=121 vs 328). Median pupil diameter also seemed to be more sensitive to behavioural changes across different assessments. These preliminary data suggest that higher responsiveness is related to higher median and complexity of the pupillometry signal and eye opening percentage at rest, supporting that pupillometry markers could be used as potential predictor of behavioral responsiveness in DOC patients.

References: 

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Pupil diameter and Behavioral Responsiveness in Disorders of Consciousness