Improvement in sensorineural auditory threshold of the guinea-pig fetus following delivery

The human fetus in utero is hypoxic relative to the neonate due to differences between placental and pulmonary oxygenation. Comparable degrees of hypoxia induced in young and adult animals caused an elevation in hearing threshold of a sensorineural nature. It was hypothesised therefore that the human fetus may also have such an elevation of threshold. This was tested in this study by recording ABR thresholds to bone conducted stimuli in fetal guinea-pigs that were near full term and again in the same animals, after delivery and consequent pulmonary oxygenation. In every animal studied (n = 9), the neonatal threshold was better than that in the fetus. In those fetuses in which a response could be recorded (n = 5), the neonatal threshold was on average 20 dB better than in the fetus. These findings are probably due to a depression of the endocochlear potential induced by the relatively hypoxic state of the fetus. The hypoxic state would lead to a reduced transduction current in the hair cells in response to a stimulus and an elevated hearing threshold. At birth, with the shift to pulmonary oxygenation, the neonate becomes normoxic, the endocochlear potential rapidly reaches near maximal levels and threshold is improved.