Measuring Salivary Cortisol to Assess the Effect of Natural Environments on Stress Level in Acute Patients With Severe Brain Injuries: An Exploratory Study

Background: Salivary cortisol is a safe and non-invasive measure of hypothalamic-pituitary-adrenal axis function and is used as a biomarker of the human stress response. Natural environments are recognized to contribute to help reduce the effect of stress. Objective: To determine the feasibility of a salivary cortisol collection protocol for acute severely brain-injured patients, and to explore the influence of exposure to natural settings on salivary cortisol concentration as an index of stress level. Methods: An exploratory study on 17 acute patients with severe brain injury was performed. We collected salivary samples in a closed hospital ward and a therapeutic garden at the start of the session and after 30 minutes of rest time. Physiological parameters, level of communication, and subjective well-being were also assessed. Results: The primary objectives regarding the feasibility of the protocol were met overall. We found no significant differences in cortisol values when including the whole population. However, cortisol values were significantly higher in the indoor environment in patients with communication attempts. Conclusions: A salivary collection protocol with brain-injured patients in the acute phase is feasible and safe, and this type of measurement could pave the way for future research supporting the benefits of nature as an additional resource in their neurorehabilitation.