Cheung M.C. et al. - Photobiomodulation and Cognitive Functions in Older Adults with Mild Cognitive Impairment

This publication is a series of three case reports, not a clinical trial: three older adults with mild non-amnestic cognitive impairment received 18 sessions of transcranial photobiomodulation over 9 weeks. All three showed a decrease in interference errors on neuropsychological tests, but the absence of a placebo group and randomization precludes attributing these changes to light exposure.

What this study measured

The authors followed three older adults with mild non-amnestic cognitive impairment, a form of the condition that primarily affects executive functions rather than episodic memory. Each participant received 18 sessions of transcranial photobiomodulation spread over 9 weeks.

Assessments were conducted before and after the intervention: neuropsychological tests of memory and executive function, and standardized questionnaires on depressive and anxiety symptoms, overall cognition, and activities of daily living. Intrusion and perseveration errors, recorded during a verbal memory test and a fluency test, served as markers of frontal lobe function.

This is a series of case reports, not a randomized trial or a meta-analysis. The associated level of evidence is the lowest in the hierarchy of clinical studies.

Key Findings

Before the procedure, the errors ranged from borderline normal to severe impairment.

• First participant: errors of intrusion and perseveration shifted from a level below the 1st percentile or at the 2nd percentile—described as moderate to severe impairment—to the 41st–69th percentile, described as a moderate range.
• Second participant: intrusion errors ranged from the 11th percentile to the 83rd percentile.
• Third participant: intrusion errors rose from the 5th percentile to the 56th percentile.
• A participant: depressive and anxiety symptoms went from severe before the intervention to mild afterward.
• Two participants: depressive symptoms progressed from mild to moderate intensity.

The study abstract does not specify the wavelength, irradiance, or duration of the sessions. Therefore, these parameters cannot be reported here.

How Transcranial Photobiomodulation Works

The mechanism remains hypothetical. Red and near-infrared light is thought to be absorbed by mitochondrial cytochrome c oxidase, which would increase ATP production in cortical neurons. Modulation of cerebral blood flow and tissue oxygenation, as well as effects on oxidative stress and local inflammation, are also mentioned in the literature. This case series did not measure any of these parameters.

Limits You Should Know

Three participants, no placebo group, no randomization: the authors themselves note that larger randomized controlled trials are needed. Such a study design does not allow for distinguishing an effect of the light from a placebo effect, a learning effect, or a spontaneous fluctuation.

No negative outcomes have been reported, which is in itself a limitation: the published case series mainly describe favorable outcomes.

The findings pertain to a non-amnestic form of mild cognitive impairment and do not apply to amnestic forms or to Alzheimer's disease. Since lighting parameters are not mentioned in the abstract, no protocol can be inferred from it.

What This Means in Practice

This publication does not change the management of mild cognitive impairment. It documents three individual case reports and serves primarily to justify larger-scale trials. An improvement in frontal lobe function or mood cannot be taken for granted on this basis. In cases of cognitive complaints, a medical evaluation remains the appropriate course of action.

Frequently Asked Questions

Are three cases enough to prove an effect?

No. A series of three cases without a control group is not sufficient to establish a cause-and-effect relationship. The authors acknowledge this and call for larger randomized controlled trials.

What is an intrusion error in a memory test?

An intrusion error occurs when a participant recalls an item that was not included in the learned list. Along with perseveration errors—which involve repeating a response that has already been given—it serves as an indicator of frontal lobe function. It was this type of error that the study tracked before and after the intervention.

What lighting settings were used?

The abstract of the publication does not specify the wavelength, irradiance, or duration of the sessions. Only the number of sessions—18—and the total duration—9 weeks—are specified. The protocol is therefore not reproducible based on the abstract.

Reference

Cheung MC, Lee TL, Sze SL, Chan AS. Photobiomodulation improves frontal lobe cognitive functions and mental health in older adults with non-amnestic mild cognitive impairment: Case studies. Frontiers in Psychology, 2023. PMID 36704674. DOI 10.3389/fpsyg.2022.1095111. https://pubmed.ncbi.nlm.nih.gov/36704674/

This summary is provided for informational purposes only and does not constitute medical advice. Photobiomodulation is not a substitute for prescribed treatment. Consult a healthcare professional.