Histologic changes after semicircular canal occlusion in guinea pigs.

HYPOTHESIS: Histologic changes occurring after varying degrees of surgical trauma to the inner ear in guinea pigs can reveal the mechanism of hearing preservation/loss. BACKGROUND: Surgical approaches to the inner ear that allow for hearing preservation have gained increasing acceptance in neurotologic surgery. The mechanisms responsible for hearing preservation and hearing loss after partial labyrinthectomy are as yet poorly understood. METHODS: Ten animals underwent semicircular canal occlusion, suctioning of perilymph, ampullectomy, or wide vestibulotomy. Tone-burst auditory brain stem response (ABR) thresholds were performed at weekly intervals after surgery. After 4 weeks, temporal bone specimens were processed to obtain 10-jim sections from plastic-embedded ears. The histologic findings were correlated with the initial and final ABR thresholds. RESULTS: After surgical occlusion of one or more semicircular canals, ABR thresholds were preserved, as the authors reported previously. Suctioning of inner ear fluid led to transient loss of thresholds with recovery. Ampullectomy produced dichotomous results, with some subjects preserving auditory function and others losing auditory function. Wide vestibulotomy resulted in permanent loss of auditory function in most cases. Histologically, there was intraluminal fibrosis and inflammation near the site of surgical entry. Most specimens showed normal cochlear architecture and hair cell counts, irrespective of the degree of hearing loss. Vestibular hair cells were also well preserved, even when they were close to the site of surgical injury. CONCLUSIONS: These findings suggest that electromechanical changes, rather than cell death, are responsible for changes in auditory and vestibular function after partial labyrinthectomy.

Image-guided surgery of the skull base using a novel miniature position sensor.

Image-guided navigational systems have been a useful adjunct for minimally invasive surgery of the skull base. A novel miniature position sensor has been developed that uses a low magnetic field for real-time tracking of surgical instruments. The 1.7-mm-diameter sensor attached to the position and orientation system (Magellan(R), Biosense, Inc., Johnson and Johnson Co., Baldwin Park, CA) was deployed through various surgical instruments or used in a hand-held fashion with a malleable shaft probe. We report on our experience using this electromagnetic system in a series of lesions of the sella and clivus. After patient/image registration, the system was consistently accurate to within 2 mm. We have found this system to be particularly advantageous in endoscopic surgery of cystic lesions of the skull base, where access is limited and anatomy may be distorted. In three patients, this device obviated the need for an extensive external surgical approach. Case histories are presented, which illustrate the specific advantages this miniature system provides during skull base surgery.