Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil

Objectives/Hypothesis: The impact of childhood conductive HL (CHL) on devel opment of auditory function has long been debated. The present study was co nducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using a-deoxyglucose (2-DG) uptake as a m easure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central audit ory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA. Study Design: Sham-controlle d study of metabolic effects of CHL during sound stimulation. Methods: Grer bils (aged 21 days or adult), underwent malleus removal, CA, or a sham proc edure. Young animals survived either 48 hours or 3 weeks; adults survived 3 weeks. Each age/survival CHL group contained eight animals; otherwise, eac h group (CA and sham) contained five animals, for a total number of 54. At the appropriate survival time, animals were given an intracardiac injection of C-14-2-DG, and sacrificed under anesthesia after 45 minutes of exposure to normal laboratory sounds. Tissue sections were prepared for exposure to x-ray film for optical density measurements, and alternate sections staine d for identification of nuclei. Measurements from auditory nuclei of experi mental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham grou p, Auditory evoked potential thresholds to both air- and bone-conducted sti muli were obtained in a second group of neonatal and adult animals. Results : Both CHL and CA resulted in a marked decrease in 2-DG; uptake in the majo r ascending projection of the manipulated ear, in both the neonatal and adu lt animals. In young animals, effects of CHL and CA were similar. Effects o f CHL in adult animals were less marked and significantly different from ei ther effects of CHL in young animals or effects of CA in adult animals. HL following malleus removal only was purely conductive and ranged from 38 to 55 dB across frequency. Conclusions: Results suggest that, particularly in young animals, a unilateral CHL may have profound effects on metabolic acti vity in the central auditory system.