fNIRS for Disorders of Consciousness

Functional near-infrared spectroscopy (fNIRS) is a non-invasive neuroimaging method that measures changes in oxygenated (HbO) and deoxygenated hemoglobin (HbR) in cortical tissue. Through neurovascular coupling, neural activity in specific brain regions is associated with increased metabolic demand and localized changes in cerebral blood flow, resulting in measurable shifts in HbO and HbR concentrations. These changes form the basis of the blood oxygen level-dependent (BOLD) signal measured in functional magnetic resonance imaging (fMRI) and the optical signals detected by fNIRS.

Assessing level of consciousness in patients with severe brain injury remains a clinical challenge. Standard bedside assessments rely primarily on observable behaviors such as eye movements, motor responses, or verbal output. When these behaviors are impaired by brain injury, observable behavior may not accurately represent preserved cognition. Functional neuroimaging studies have demonstrated that some individuals diagnosed with disorders of consciousness (DoC) can generate brain responses during cognitive tasks even when behavioral responses are absent. However, many neuroimaging methods, particularly fMRI, are difficult to implement in critically ill patients because they require specialized facilities and patient transport outside the intensive care unit. fNIRS provides a portable, bedside method for measuring hemodynamic responses associated with neural activity, making it potentially suitable for use in the neuro-intensive care unit.

This study is a non-therapeutic observational pilot investigation designed to evaluate fNIRS signal responses and the feasibility of implementing a structured cognitive paradigm during bedside monitoring. The study is conducted in two stages.

In the first stage, the protocol is implemented in healthy adult participants to characterize the magnitude and consistency of fNIRS responses during a series of sensory and cognitive paradigms. These data establish baseline response patterns and help identify task conditions that produce reliable hemodynamic signals.

In the second stage, the refined protocol is implemented in patients with DoC in the neuro-intensive care unit to evaluate feasibility and obtain preliminary observations of cortical hemodynamic responses in this population.

During fNIRS monitoring, participants are exposed to a series of paradigms designed to engage different levels of sensory and cognitive processing. These include somatosensory stimulation, auditory processing tasks, semantic language processing, spatial navigation imagery, and motor imagery. Tasks are presented in a block design with alternating task and rest periods to allow detection of hemodynamic changes associated with neural activation.

The goal of this pilot study is to determine whether bedside fNIRS monitoring can produce reliable hemodynamic signals during these paradigms and whether the protocol can be feasibly implemented in a Neuro-ICU environment. Findings from this study will provide preliminary data to inform future research on objective methods for assessing brain activity in patients with DoC.

Inclusion criteria

Healthy Control Participants:

• Age 18 years or older
• Proficient in English language
• Intact cognition
• Montreal Cognitive Assessment (MoCA) score > 26

DoC Patients:

• Age 18 years or older
• Proficient in English language
• Surrogate decision maker available
• Admission to the intensive care unit within 28 days
• Documented Glasgow Coma Scale (GCS) score
• Auditory Function subscale score ≥1 on the Coma Recovery Scale-Revised (CRS-R)

Exclusion criteria

Healthy Control Participants:

• Known baseline hearing impairment without available hearing aids
• Neurological or psychiatric history

DoC Patients:

• Known baseline hearing impairment without available hearing aids
• Inability to obtain informed consent
• Extensive bilateral frontal injury on available imaging (CT/MRI)
• Patients who have undergone craniectomy surgery
• IV sedation in the past 24 hours
• Absent auditory startle response (<1 from Auditory Function subscale score of CRS-R)