Effects of temperature and elevated intracranial pressure on peripheral and brain stem auditory responses in dogs

Far-field recordings of central (P2 through P4) and peripheral (cochlear microphonic; and compound action potential of the eighth nerve) auditory responses were used to assess changes in auditory function resulting from elevated intracranial pressure. Normative data for eight dogs were obtained. The relationship between response latency and core temperature was examined. A mean slope of -0.17 ms/°C resulted for the temperature range of 35.0 to 40.0°C. Systemic arterial pressure was measured in order to identify the cerebral ischemic response. Responses were not altered significantly unless the intracranial pressure approached within 15 to 30 mm Hg of mean systemic arterial pressure. Changes in the response consisted of both enhancement and deterioration during intracranial pressure elevation and were accompanied by increases in systemic arterial pressure during that elevation. Supernormal amplitudes of the action potential also occurred during recovery periods. Results suggest that: (i) during elevated intracranial pressure, changes in both central and peripheral auditory function result from ischemia rather than pressure-induced distortion of the cochlea or central neural assemblies. (ii) Far-field auditory responses may include an O₂-dependent cochlear microphonic. (iii) An unknown process causing enhancement of central and peripheral neural responses exists and operates in connection with intracranial hypertension. Possible mechanisms underlying enhancement of response components are discussed.