Laser Doppler measurements of inner ear blood flow during experimental thrombosis of cochlear blood vessels in the guinea pig

Mild Therapeutic Hypothermia and Putative Mechanisms of Hair Cell Survival in the Cochlea

Significance: Sensorineural hearing loss has significant implications for quality of life and risk for comorbidities such as cognitive decline. Noise and ototoxic drugs represent two common risk factors for acquired hearing loss that are potentially preventable. Recent Advances: Numerous otoprotection strategies have been postulated over the past four decades with primary targets of upstream redox pathways. More recently, the application of mild therapeutic hypothermia (TH) has shown promise for otoprotection for multiple forms of acquired hearing loss. Critical Issues: Systemic antioxidant therapy may have limited application for certain ototoxic drugs with a therapeutic effect on redox pathways and diminished efficacy of the primary drug's therapeutic function (e.g., cisplatin for tumors). Future Directions: Mild TH likely targets multiple mechanisms, contributing to otoprotection, including slowed metabolics, reduced oxidative stress, and involvement of cold shock proteins. Further work is needed to identify the mechanisms of mild TH at play for various forms of acquired hearing loss.

The effect of mannitol upon cochlear dysfunction induced by transient local anoxia

Transient local anoxia of the cochlea was induced by pressing the labyrinthine artery, and compound action potential (CAP) or endocochlear potential (EP) was measured before and after transient local anoxia ranging from 5 to 60 min using 106 albino guinea pigs. The complete interruption of the cochlear blood flow by this procedure and its full restoration after releasing the pressure on the artery was confirmed by a laser-Doppler flowmeter. The anoxia of less than 10 min induced no post-anoxic cochlear dysfunction, whereas the anoxia of a longer duration induced an irreversible dysfunction of the cochlea. It was evident that the post-anoxic recovery of the CAP threshold was worse as the anoxia period was prolonged, and CAP was almost completely abolished after 60-min anoxia. In animals which were administered mannitol intravenously just after the restoration of the cochlear blood circulation, the recovery of the CAP threshold was significantly better than that in the control animals, when the animals were subjected to local anoxia of 15- to 30-min duration. No beneficial effect, however, was observed in the 60-min anoxia group. In conclusion, local anoxia of 10 min or longer caused cochlear dysfunction, which was partially but significantly alleviated by mannitol.

Effects of experimental cochlear thrombosis on oxygenation and auditory function of the inner ear

To elucidate the etiology and pathogenesis of sudden hearing loss, the effect of experimental cochlear thrombosis on oxygenation and the auditory function of the inner ear was investigated in anesthetized guinea pigs. Impairment of cochlear blood flow (CBF) was induced by ferromagnetic obstruction of cochlear blood vessels at lowered body temperature. Perilymphatic oxygen partial pressure (PO2) in the basal scala tympani (about 200 microm below the round window membrane) was measured polarographically using micro-coaxial needle electrodes. Auditory function was examined by recording cochlear microphonic (CM) frequency responses, compound action potentials (CAP) and auditory evoked brainstem responses (ABR). Findings demonstrated a considerable decrease in the mean perilymphatic PO2 of 40%, 2 h after the start of the experiment. Mean CM and N1 CAP amplitudes were reduced by about 25% each and ABR by 18%. No significant changes were observed in the latencies of either CAP or ABR. Mean basal CBF was found to decrease by 35%, as measured by laser Doppler flowmetry in a parallel study. The present findings demonstrate that vascular impairment in the inner ear results in a considerable drop in intracochlear oxygenation, causing a significant loss in the auditory response.