Cochlear microphonics in oxygen at high pressure

Hyperbaric oxygen upregulates cochlear constitutive nitric oxide synthase

Background

Hyperbaric oxygen therapy (HBOT) is a known adjuvant for treating ischemia-related inner ear diseases. Controversies still exist in the role of HBOT in cochlear diseases. Few studies to date have investigated the cellular changes that occur in inner ears after HBOT. Nitric oxide, which is synthesized by nitric oxide synthase (NOS), is an important signaling molecule in cochlear physiology and pathology. Here we investigated the effects of hyperbaric oxygen on eardrum morphology, cochlear function and expression of NOS isoforms in cochlear substructures after repetitive HBOT in guinea pigs.

Results

Minor changes in the eardrum were observed after repetitive HBOT, which did not result in a significant hearing threshold shift by tone burst auditory brainstem responses. A differential effect of HBOT on the expression of NOS isoforms was identified. Upregulation of constitutive NOS (nNOS and eNOS) was found in the substructures of the cochlea after HBOT, but inducible NOS was not found in normal or HBOT animals, as shown by immunohistochemistry. There was no obvious DNA fragmentation present in this HBOT animal model.

Conclusions

The present evidence indicates that the customary HBOT protocol may increase constitutive NOS expression but such upregulation did not cause cell death in the treated cochlea. The cochlear morphology and auditory function are consequently not changed through the protocol.

The effects of hyperbaric oxygen on experimental noise damage to the ears

The effect of hyperbaric oxygen on cochlear microphonics, action potential of the auditory nerve, and brain stem response damaged by short exposure to noise is varied. Of 26 guinea pigs 14 showed a positive influence as measured by amplitudes. In 12 animals oxygen had little or no effect. The reason for this could lie in the individual reaction to the anesthetic of the section of the auditory nerve tested. The reaction of postmortem cochlear microphonics after hyperbaric oxygen treatment indicates oxygen diffusion through the round window.

Hyperbaric oxygen and stellate ganglion blocks for idiopathic sudden hearing loss

Ninety-one patients suffering from idiopathic sudden hearing loss are presented. Twenty-two patients were given medical treatment (vasodilators, steroid hormones and vitamins) alone (group 1). Forty-nine patients were treated with stellate ganglion block (SGB) plus oxygen hyperbaric therapy (OHP) (group 2) and 20 patients were treated with SGB plus OHP along with medical treatment (group 3). The SGB plus OHP treated patients were given bupivacaine, which induced Moor's anterior approach of SGB and then exposed to oxygen at a pressure of 2.4 ATA for 90 min. In group 1, 69% of the patients treated within one week after onset exhibited over 10 dB pure tone average improvement, with only 33% patients treated one to two weeks after onset experiencing over 10 dB. However, 74% of the patients in group 2 and 100% of the patients in group 3 who were treated within two weeks after onset exhibited over 10 dB improvement. More significantly, of the patients which experienced complete loss of hearing, 83% in group 2 and 100% in group 3, exhibited over 10 dB improvement, compared to only 33% in group 1. Moreover, 8 (40%) patients in group 3 recovered to within 20 dB of their normal hearing levels. In group 2, 17 patients were treated two to six weeks after onset and 12 (71%) patients had over 10 dB improvement. SGB plus OHP therapy was shown effective in the treatment of sudden idiopathic hearing loss even when patients were treated more than two weeks after onset.

Preliminary report on the effects of hyperoxia on cochlear potential in chinchilla

Anesthestized chinchilla with round window electrodes were placed in a Plexiglas chamber. When they inhaled 100% oxygen at 1 atm, two types of neural effects were observed: 1. fast and reversible AP depression in response to brief exposure to 100% O2; and 2. a slowly developed chronic depression of AP and latency shift due to multiple hyperoxic exposures. The severity of the latter is a function of the dosage of the O2 inhaled, i.e., the concentration of oxygen and the length of time of exposure. For the type-1 effect, carbon dioxide deprivation is felt to be the immediate cause while the type-2 effect is mainly due to O2 toxicity developed at the intracellular level by high O2 concentration. The possible role of superoxide in the auditory O2 toxicity was discussed.

[On the effect of hyperbaric oxygen (OHP) at the guinea pig's cochlea insulted by hypoxia (author's transl)]

The effect of OHP (oxygen under high pressure) by examining 17 CO-intoxicated guinea pigs. The following effects have been observed: The microphonics recovered significantly faster after hypoxia caused by CO than under air. Hyperbaric oxygen has a protecting effect on the inner ear. If OHP is applicated before CO, the microphonics decrease less than under normal breathing. The postmortal microphonics increased to a level above the known postmortal slope, suggesting a diffusion of oxygen through the round window.