Very preterm infants have a higher risk of suffering from hearing loss, auditory neuropathy and delayed auditory maturation compared to term born infants. As a normal auditory function is the prerequisite for normal language development, any abnormalities in auditory pathway development should be detected as early as possible to enable prompt intervention. Click-evoked brainstem auditory evoked potentials (BAEP) and brainstem audiometry (BA) recordings are reliable and objective, quantitative methods for evaluation of the auditory function and the pathway from inner ear to brainstem and thus, they are commonly used in very preterm infants. However, laboratory specific reference values are needed for correct interpretation of the BAEP and BA results. Neonatal BAEP and BA methods aid in identifying preterm infants at risk for delayed language development, and in the study of possible risk factors for abnormal auditory pathway development.

The aim of this thesis was to calculate reference values for neonatal BAEP and BA recordings at the Department of Clinical Neurophysiology of Turku University Hospital. The reliability of the new reference limit for BA was evaluated against pure-tone audiometry results at the age of five years. Other aims were to study the association between neonatal BAEP and BA recordings and later language development. The harmful effects of different risk factors in neonatal intensive care unit (NICU) on the hearing of very preterm infants were analysed as well. There is a lack of information and contradictory finding in these areas of research.

The reference values for BAEP and BA recordings were determined to facilitate the interpretation of the results and they are currently in clinical use. The reference limit of 35 decibel normalized hearing level for BA can be considered reliable as it showed good sensitivity in relation to pure-tone audiometry results at the age of five years. We also found that slower auditory conduction in the neonatal BAEP recording of very preterm infants associated with poorer acquisition of receptive language at the corrected age of one year. In addition, this thesis discovered that continuous positive airway pressure (CPAP) treatment during the NICU management may have adversely influenced the developing auditory pathway in very preterm infants.