Journal Club: Shining infrared light (1871 nm) into the cochlea - one day infrared light-based medical technology?

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Today's journal article

Okamoto A, Uenaka M, Ito Y, Kuroki Y, Miyasaka T, Toda K, Hiryu S, Kobayasi KI, Tamai Y. Safety evaluations for transtympanic laser stimulation of the cochlea in Mongolian gerbils (Meriones unguiculatus). 

Why I picked this article

This is an interesting study to use light, more specifically, infrared light, to stimulate our auditory nerve to recover some perception of hearing. Potential future MedTech! 

When the damage occurs to our peripheral organ of hearing, the cochlea, we develop sensorineural hearing loss. Currently, the large majority of sensorineural hearing loss cannot be treated with pharmacological treatment, and management of sensorineural hearing loss is by prosthetic devices such as cochlear implants and hearing aids. Cochlear implants work by using electrical stimulation to directly stimulate the auditory nerve to restore some perception of hearing. 

This study builds on the concept from earlier research that suggests light can be used instead of electrical stimulation to activate the auditory nerve. How light does this is unclear, but there have been suggestions from research that thermal reaction from light can activate nerves, or that some protein like TRPV4 can activate nerves. The advantage of using light may be that it could be less invasive or potentially safer. This study used Mongolian gerbils to test the safety and efficacy of the concept of using infrared light to stimulate the auditory nerve. 

Some of the research findings

Animal models:

  • Mongolian gerbils (3-18 months old)
  • Microscopy and histology were used to evaluate the cochlea. 
  • Animals were trained to have auditory-evoked behaviour, which was monitored, as the sign of hearing ability. 

Light stimulation:

  • A diode laser system (BWFOEM-1850; B&W TEK, Delaware, USA) was used.
  • Infrared light of a wavelength of 1871 nm was used.
  • Individual pulse was 100micro-seconds
  • Optical fibre (diameter: 100 μm; NA, 0.22) was inserted through the ear canal so that the tip was 0.7 mm before the tympanic membrane (=ear drum)
  • The lateral second turn of the cochlea was stimulated.
Approach to shine the light to the cochlea (arrowheads) through the ear canal. *This is adopted not from Journal club article, but from a part of Fig 2A of Tamai Y et al. (2020) PLoS ONE 15(10): e0240227. Which this study (Okamoto et al (2025) builds on and references. 


Finding: 
  • The temperature profile by laser was compared for different strengths of the laser light. 
  • There is a clear heating with higher strength 26.4, 52.8 and 105W/cm2 (high), and with the highest, auditory neurons were clearly damaged and lost. 
  • The behaviour of animals did not change with weaker infrared light, but with a higher dose, it decreased, indicating less auditory response. 
  • According to the authors, the thermal profile suggests that ~half of the infrared light hitting the "lateral" part of the cochlea has reached the "medial" side, where auditory neurons are.
Taken together, infrared use of 6.6 W/cm2 or weaker appeared safe, but 26.4 W/cm2 or above risked spiral ganglion neuron damage in this animal model. 

Haruna's takeaway

A continued study like this could one day lead to a different type of cochlear implant or nerve stimulation medical device as a treatment option. This was an interesting publication to read. While this publication was about safety checks and did not show effective stimulation of the nerve, I found it an interesting read, relevant to our work as we work with light and various med-tech related projects. Thermal profiling and some ways of describing the impact of the treatment (like behavioral monitoring) may be applicable for our future studies. 

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This is Haruna's 17/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.