Pupil dilation response elicited by violations of auditory regularities is a promising but challenging approach to probe consciousness at the bedside

Pupil size as a biomarker of cognitive (dys)functions: Toward a physiologically informed screening of mental states

The objective assessment of cognitive processes is of critical importance to understanding the mechanisms underlying various mental functions and dysfunctions. In recent years, the technological innovations related to the eye-tracking industry made the time right to organically integrate pupillometry in the assessment of cognitive profiles. Here, we review evidence showing that pupillometry offers a uniquely sensitive biomarker of the functioning of several distinct networks, cognitive functions, emotional states, and individual differences in their instantiation. We outline why and how pupillometry can be effectively exploited to enrich current research and behavioral paradigms, including those designed for clinical testing. By making the cases of anxiety disorders (both generalized and specific - e.g., generalized anxiety vs. math anxiety) and substance use disorders, we then exemplify how pupillometry can be leveraged to obtain clinically meaningful variables. We finally conclude by arguing that measuring pupil size has the potential to complement more traditional, but coarse assessment methods, to return a more graded, objective, and physiologically informed picture of cognitive functioning.

Covert Consciousness in Acute Brain Injury Revealed by Automated Pupillometry and Cognitive Paradigms

BACKGROUND

Identifying covert consciousness in intensive care unit (ICU) patients with coma and other disorders of consciousness (DoC) is crucial for treatment decisions, but sensitive low-cost bedside markers are missing. We investigated whether automated pupillometry combined with passive and active cognitive paradigms can detect residual consciousness in ICU patients with DoC.

METHODS

We prospectively enrolled clinically low-response or unresponsive patients with traumatic or nontraumatic DoC from ICUs of a tertiary referral center. Age-matched and sex-matched healthy volunteers served as controls. Patients were categorized into clinically unresponsive (coma or unresponsive wakefulness syndrome) or clinically low-responsive (minimally conscious state or better). Using automated pupillometry, we recorded pupillary dilation to passive (visual and auditory stimuli) and active (mental arithmetic) cognitive paradigms, with task-specific success criteria (e.g., ≥ 3 of 5 pupillary dilations on five consecutive mental arithmetic tasks).

RESULTS

We obtained 699 pupillometry recordings at 178 time points from 91 ICU patients with brain injury (mean age 60 ± 13.8 years, 31% women, and 49.5% nontraumatic brain injuries). Recordings were also obtained from 26 matched controls (59 ± 14.8 years, 38% women). Passive paradigms yielded limited distinctions between patients and controls. However, active paradigms enabled discrimination between different states of consciousness. With mental arithmetic of moderate complexity, ≥ 3 pupillary dilations were seen in 17.8% of clinically unresponsive patients and 50.0% of clinically low-responsive patients (odds ratio 4.56, 95% confidence interval 2.09-10.10; p < 0.001). In comparison, 76.9% healthy controls responded with ≥ 3 pupillary dilations (p = 0.028). Results remained consistent across sensitivity analyses using different thresholds for success. Spearman's rank analysis underscored the robust association between pupillary dilations during mental arithmetic and consciousness levels (rho = 1, p = 0.017). Notably, one behaviorally unresponsive patient demonstrated persistent command-following behavior 2 weeks before overt signs of awareness, suggesting prolonged cognitive motor dissociation.

CONCLUSIONS

Automated pupillometry combined with mental arithmetic can identify cognitive efforts, and hence covert consciousness, in ICU patients with acute DoC.