Effects of cerebellar retraction on brainstem auditory evoked potentials in an experimental animal model of cerebellopontine angle tumor.

The effects of cerebellar retraction on brainstem auditory evoked potentials (BAEPs) were studied in 15 New Zealand rabbits. In the first series, a Fogarty catheter was placed in the cerebellopontine angle of 5 rabbits. When a balloon volume of 0.2 mL was produced, only the latencies of waves III-V of the ipsilateral side increased. With a volume of 0.4 mL the ipsilateral BAEPs were irreversibly lost and the contralateral one reversibly changed. A volume of 0.6 mL caused loss of the BAEP on both sides and death of the animal. In another series, the cerebellum of 10 rabbits was retracted by a self-retaining retractor laterally to medially, so an approach to the cerebellopontine angle was simulated. With a retraction of up to 36 power units (p) the ipsilateral BAEPs were reversibly changed. A retraction of 54 p caused irreversible loss of wave V on both sides. With 84 p ipsilateral and contralateral waves III-V were immediately lost. The animals died within 7 minutes. Our experimental investigation shows that not only a defined volume in the cerebellopontine angle can cause irreversible impairment of brainstem function, but also uncontrolled retraction of the cerebellum.