The effect of sitting position on consciousness levels and pupillary light reflex

Critically ill patients are currently mobilized whilst still intubated in the intensive care unit (ICU) because of its preventive effect against muscle weakness ¹ and delirium. ² Only a few studies have reported on the effect of early mobilization on consciousness levels. ³

Some studies have recently demonstrated that the measurement of quantitative pupillary light reflex (PLR) is useful for predicting neurological outcome in comatose patients. ⁴ Other studies have shown that some parameters associated with PLR reflect mental and cognitive activity. ⁵ The primary purpose of this study was to investigate the effects of sitting position on parameters of PLR.

Between May and December 2018, 20 critically ill patients aged 20 or above, undergoing mechanical ventilation in the ICU at The Jikei University Kashiwa Hospital, were selected as participants. Patients who had vision abnormalities before admission were excluded. Patients who had a history of drug abuse that affected their autonomic nervous system were also excluded from the analysis. Glasgow coma scale (GCS) scores were evaluated and quantitative PLR was measured by automated pupillometry (NPi-200) in participants by physiotherapists. Participants had been supine for more than 1 h before measurements. Therefore, measurements were conducted first in supine and subsequently within 1 min after sitting. Otherwise, it was so difficult to unify the time to keep sitting, because the time to be able to keep sitting varies from patient to patient. Evaluators were blinded to the purpose of this study and clinical information about the participants in order to reduce bias. The evaluators were not involved in analysing and recording the data.

To reveal whether the neurophysiological effect of sitting position is specific to ICU patients, we measured quantitative PLR in 20 healthy persons with no previous medical history at Kimura Hospital. The measurement in healthy subjects was conducted in light and dark conditions. The study was approved by the ethics committees of The Jikei University School of Medicine and Kimura Hospital. The patients' informed consent to participate was obtained through an opt-out methodology. Written informed consent was obtained from healthy subjects.

The mean age of the patients was 65.5 ± 15.6 years; 14 patients were male and 6 patients were female. Admission diagnoses were medical in 12, surgical in 4, and neurosurgical in 4 patients. The period between ICU admission and evaluation was 12.0 ± 14.6 days. The mean age of the healthy subjects was 35.0 ± 8.7 years; 11 were male and 9 were female. The total GCS score was significantly higher for the sitting position than the supine position (10.5 (9.00–11.0), 9.0 (7.25–11.0), p = 0.002). The pupillary constriction rate mean was significantly higher for the sitting position than for the supine position in patients (22.4 ± 9.1, 19.4 ± 9.3, p = 0.010) and healthy persons (under light condition; 27.6 ± 5.0, 24.8 ± 4.2, p = 0.001, under dark condition; 40.0 ± 4.0, 38.5 ± 4.8, p = 0.027). There was a significant correlation between the total score of GCS score and the mean of pupillary constriction rate, both in the supine (rs = 0.693, p = 0.001) and sitting positions (rs = 0.564, p = 0.010).

Our findings suggest that pupillary constriction rate in critically ill patients could be increased if they are seated. Furthermore, measuring the constriction rate of a patient's pupils might be useful for monitoring their consciousness levels, especially when GCS cannot detect change of consciousness levels, for example, when GCS score is 3.