Near-infrared photonic energy penetration: can infrared phototherapy effectively reach the human brain?

Abstract

Traumatic brain injury (TBI) is a growing health concern effecting civilians and military personnel. Research has yielded a better understanding of the pathophysiology of TBI, but effective treatments have not been forthcoming. Near-infrared light (NIR) has shown promise in animal models of both TBI and stroke. Yet, it remains unclear if sufficient photonic energy can be delivered to the human brain to yield a beneficial effect. This paper reviews the pathophysiology of TBI and elaborates the physiological effects of NIR in the context of this pathophysiology. Pertinent aspects of the physical properties of NIR, particularly in regards to its interactions with tissue, provide the background for understanding this critical issue of light penetration through tissue. Our recent tissue studies demonstrate no penetration of low level NIR energy through 2 mm of skin or 3 cm of skull and brain. However, at 10-15 W, 0.45%-2.90% of 810 nm light penetrated 3 cm of tissue. A 15 W 810 nm device (continuous or non-pulsed) NIR delivered 2.9% of the surface power density. Pulsing at 10 Hz reduced the dose of light delivered to the surface by 50%, but 2.4% of the surface energy reached the depth of 3 cm. Approximately 1.22% of the energy of 980 nm light at 10-15 W penetrated to 3 cm. These data are reviewed in the context of the literature on low-power NIR penetration, wherein less than half of 1% of the surface energy could reach a depth of 1 cm. NIR in the power range of 10-15 W at 810 and 980 nm can provide fluence within the range shown to be biologically beneficial at 3 cm depth. A companion paper reviews the clinical data on the treatment of patients with chronic TBI in the context of the current literature.

Keywords: TBI; class IV laser; depression; infrared; sleep disturbance; traumatic brain injury.

Figures

Figure 1

Hypothesized mechanism of action of near-infrared light (NIR) photobiomodulation. Notes: NIR (600–980 nm) penetrates tissue to variable depth depending on wavelength, coherence, time, and the tissue involved. A portion of the photonic energy reaches the mitochondria and is absorbed by cytochrome c oxidase. In addition to inducing increased adenosine triphosphate (ATP) production, NIR appears to initiate increased production of reactive oxygen species (ROS), reactive nitrogen species (RNS), and possibly (?) nitric oxide (NO). Downstream events include increased early response genes – c-fos, c-jun – and activation of nuclear factor kappa-B (NF-κB), which in turn induces increased transcription of gene products leading to neurogenesis, synaptogenesis, and increased production of growth factors and inflammatory mediators. Abbreviation: ↑, increase.

Figure 2

Ex vivo human skin studies illustrated. Notes: (A) The pad of LEDs is held 2 mm from the surface of the light meter detector. The arrow indicates a row of near-infrared light (NIR) LEDs with a wavelength of 880 nm. The meter reads 0.01 W. (B) Human skin 1.9 mm thick is interposed between the NIR LED and the light meter detector. Thin plastic wrap covers the detector. (C) The NIR LED is covered with thin plastic wrap and placed directly against the sample of human skin. Photonic energy could not be detected passing through 1.9 mm of human skin.

Figure 3

Ex vivo brain tissue studies illustrated. Notes: (A) The photonic energy penetrating a fixed distance (3 cm) of air was determined. (B) A section of ex vivo lamb head was prepared which included skull, tissue, and brain. (C) The section was interposed in the space between the infrared light emitter and the light meter detector, both of which were fixed in place. The amount of infrared light energy penetrating the fixed distance (3 cm) through tissue was determined. (D) The temperature change was determined using a digital thermometer before and immediately after infrared light exposure.