DEVELOPMENT OF VESTIBULAR AND AUDITORY FUNCTION - EFFECTS OF HYPOTHYROIDISM AND THYROXINE REPLACEMENT THERAPY ON NYSTAGMUS AND AUDITORY-EVOKED POTENTIALS IN THE PIGMENTED RAT

The functional development of semicircular canals and some brainstem s tructures of the auditory system was followed in parallel with time in control and propylthiouracyl-induced hypothyroid pigmented rats by re spective recording of postrotatory nystagmus response and auditory evo ked brainstem potentials, with the aim of discovering the timing of pe rmanent alterations of these responses in congenital hypothyroidism. A group of hypothyroid rats which underwent thyroxine-replacement thera py from postnatal day 12 onward was also included in our studies to co rroborate the involvement of thyroid hormones in these effects. Postro tatory nystagmus and auditory evoked responses were absent in congenit al hypothyroid rats. In the thyroxine-replaced group postrotatory nyst agmus values showed no differences from the control group from postnat al day 28 onward. Auditory evoked potentials in thyroxine-replaced ani mals could not be elicited at 30 dB, but by increasing the intensity o f stimulus to 70 dB, values of latencies of the four waves composing t he response were indistinguishable from controls from postnatal day 39 and thereafter. These results show that hypothyroidism affects both s emicircular canal and auditory function, the latter more severely than the former, but that these effects can be prevented when thyroxine re placement treatment is started in early stages of postnatal developmen t. Copyright (C) 1996 ISDN.