Prolonged Exposure of CBA/Ca Mice to Moderately Loud Noise Can Cause Cochlear Synaptopathy but Not Tinnitus or Hyperacusis as Assessed With the Acoustic Startle Reflex.

This was a mouse study, not a human clinical trial, so it cannot show what happens in patients. Only 21 mice were tested after acclimation, and the main acoustic startle results were presented in detail for 9 mice exposed to 70 dB SPL. For mice exposed to 75 dB SPL, the acoustic startle comparison used a different protocol than the main 70 dB SPL experiment reported in detail. The paper does not describe random allocation or blinding for the animal groups. Noise exposure was continuous 8 to 16 kHz octave-band noise, which the authors note is not representative of real-world noise. Histology was not performed, so synaptopathy was inferred from ABR Wave 1 changes rather than direct synapse counts. The 75 dB SPL ABR Wave 1 reductions were significant only as main effects across stimulus levels, not at individual levels after correction. ABR and DPOAE testing was done 2 weeks after exposure to allow recovery from temporary threshold shifts, but temporary threshold shifts were not confirmed. Tone PPI may be insensitive to plasticity at the thalamocortical level, where moderate noise effects may occur.

Do not read the negative tinnitus and hyperacusis tests as proof that 75 dB SPL noise is harmless. In these mice, 75 dB SPL exposure still reduced ABR Wave 1 input-output functions at 8, 11, and 16 kHz, consistent with mild cochlear synaptopathy.