Advancing pain assessment in Alzheimer’s disease and related dementias: Functional near-infrared spectroscopy for investigating brain activity

Background Pain assessment in Alzheimer’s disease and related dementias (ADRD) is challenging due to cognitive decline and communication barriers, limiting the reliability of self-report and observational tools. Functional near-infrared spectroscopy (fNIRS) offers a noninvasive measure of cerebral hemodynamic responses and may serve as an objective biomarker for pain. This pilot study evaluated the feasibility of fNIRS for pain assessment in ADRD, using transcranial direct current stimulation (tDCS) solely as a controlled cortical modulation paradigm to test fNIRS sensitivity, rather than as a therapeutic intervention. Methods Forty older adults with mild to moderate ADRD were randomized to active ( n = 20) or sham ( n = 20) tDCS for 5 consecutive days to generate controlled cortical modulation. Pain was assessed using the Numerical Rating Scale (NRS), Mobilization-Observation-Behavior-Intensity-Dementia-2 (MOBID-2), and fNIRS responses to standardized pain stimuli. Hemodynamic changes in prefrontal and somatosensory cortices were analyzed to determine whether fNIRS detected pain-related brain activity. Results NRS and MOBID-2 scores were significantly correlated at baseline ( r = .605, p < .001) and post-intervention ( r = .567, p < .001). In the active tDCS condition, pain stimulation elicited significant cortical hemodynamic changes that correlated with pain scores ( p < .05), supporting fNIRS’s sensitivity for detecting pain-related neural responses. In the sham group, only a few significant correlations were observed post-intervention (e.g., frontal cortex r = .44, p = .049; prefrontal cortex r = .52, p = .017), which were less consistent compared to the active condition. Conclusion fNIRS demonstrated feasibility as an objective pain assessment tool in ADRD. tDCS served only as a probe to induce cortical modulation for evaluating fNIRS performance. In this study, tDCS functioned as a probe to induce cortical modulation for evaluating fNIRS sensitivity, not as a therapeutic intervention. Larger trials are needed to confirm fNIRS validity for clinical application.