Triple semicircular canal occlusion in the guinea pig.

Hearing preservation is possible with translabyrinthine procedures, but the optimal means of sealing the remaining labyrinth has not been determined. The purpose of this study was to compare the effects of mechanical and nonmechanical (CO2 laser-assisted) triple semicircular canal occlusion on hearing in the guinea pig. All three semicircular canals of 19 guinea pigs were treated with one of four techniques: fenestration without packing (control), fenestration with packing, CO2 laser coagulation of the membranous canal without packing, or laser coagulation with packing. Six weeks postoperatively, electrocochleographic thresholds were significantly elevated in one of five ears treated with packing alone, in one of five ears treated with laser and packing, in two of five ears treated with the laser alone, and in all four control ears. Thresholds were significantly elevated in control versus occluded ears (p < 0.05). There were no significant differences between the ears treated with laser or packing. These findings suggest that hearing can be preserved in triple canal occlusion by means of sealing the membranous labyrinth with either CO2 laser coagulation or mechanical packing. Further study on the feasibility of hearing preservation with complete labyrinthectomy is warranted.

Triple semicircular canal occlusion versus labyrinthectomy in the cat.

HYPOTHESIS: We have theorized that surgical occlusion of all three semicircular canals (TCO) may be an effective means to treat vestibular pathology limited to semicircular canal dysfunction while preserving hearing and otolithic function. BACKGROUND: A procedure that would eliminate vertigo while preserving hearing and minimizing postoperative dysequilibrium would be desirable. METHODS: Staged bilateral TCO was performed on four cats and compared to staged bilateral labyrinthectomy in four cats. Balance and gait analysis were performed for 3 weeks after each surgical procedure--a total of 6 weeks of testing. RESULTS: Balance testing, gait analysis, and righting reflex were found to be better among the cats undergoing TCO compared to labyrinthectomy. CONCLUSIONS: Compared to labyrinthectomy in the cat, TCO appears to have advantages for vestibular compensation after unilateral and contralateral surgery.

Carbon dioxide laser occlusion of the guinea pig posterior semicircular canal.

Preservation of hearing is possible with selective ablation of the vestibular system and mechanical occlusion of the semicircular canals. Complete ablation of all three canals would improve exposure of the internal auditory canal fundus (e.g., for acoustic tumor exposure), but mechanical packing of the vestibule would disrupt normal sound transduction. This study was designed to assess the feasibility of preserving hearing with CO2 laser occlusion, without mechanical packing of the posterior semicircular canal membranous labyrinth. Twenty adult Hartley guinea pigs underwent occlusion of the right posterior semicircular canal with one of three techniques: mechanical packing, laser coagulation, or laser coagulation with mechanical packing. Electrocochleographic thresholds to clicks and 1-kHz and 8-kHz tone bursts did not change significantly 6 weeks after posterior semicircular canal occlusion with any of these techniques. Histopathologic examination revealed complete canal occlusion with all methods. These findings suggest that mechanical occlusion and CO2 laser occlusion of the posterior semicircular canal do not significantly affect cochlear function in the guinea pig. CO2 laser occlusion of the membranous labyrinth may prove useful for more extensive selective vestibular ablation by obviating the need for mechanical packing of the labyrinth.

Intraoperative cranial nerve monitoring during posterior skull base surgery.

Intraoperative monitoring of neurophysiologic function is rapidly evolving as an important adjunct during skull base surgery to reduce the incidence of neurologic deficit. Facial nerve monitoring is an excellent model, since electrical and mechanical evoked potentials can be directly presented to the surgeon in real-time through an acoustic loudspeaker display. The lower cranial nerves may also be monitored using similar electromyographic techniques. Auditory system monitoring is more difficult due to the low amplitude response that requires averaging and filtering to extract the evoked potential. In conjunction with auditory monitoring, improved hearing preservation may be further enhanced by concomitant facial nerve monitoring, since the surgeon is alerted to traumatic manipulations that may affect both facial and cochlear nerves. Techniques and interpretative issues are presented to maximize the efficacy and safety of cranial nerve monitoring.

Promontory stimulation following labyrinthectomy: implications for cochlear implantation.

Transtympanic electric promontory stimulation is a psychoacoustic test used to assess residual acoustic neurons in profound sensorineural hearing loss. The test was performed in six patients who had previously undergone transmastoid (N = 5) or transcanal-oval window (N = 1) labyrinthectomy as a means of determining the feasibility of future cochlear implantation. Four patients had unilateral Meniere's disease, one had labyrinthitis, and one had delayed onset vertigo. All patients perceived a definite auditory sensation in the labyrinthectomized ear during stimulation. The results of threshold, dynamic range, and difference limen testing were similar to those obtained during preoperative stimulation of cochlear implant candidates (N = 12) who subsequently became successful users. There was no evidence of response degradations as the time following labyrinthectomy increased. The results of this study suggest the possibility of successful cochlear implantation following labyrinthectomy. Supporting histologic data are reviewed.

Magnetic stimulation of the facial nerve.

Electrical testing of the facial nerve has evolved into an important means of assessing neural injury. However, the inability to stimulate the intratemporal facial nerve electrically results in a delay in diagnosis, because axonal degeneration must progress distal to the stylomastoid foramen before testing can be meaningful. To circumvent this problem, we began an investigation of magnetic stimulation of the facial nerve, because pulsed magnetic fields can pass unattenuated through all body structures, including bone. Normal volunteers and one patient with acute facial paralysis were studied with both magnetic and electric stimulation of the facial nerve. The results indicate that (1) magnetic stimulation was more comfortable because high current levels were not required at the skin surface to assure indepth stimulation, (2) magnetic and electric stimulation of the extratemporal facial nerve resulted in nearly identical compound muscle action potentials, indicating that the sites and mechanisms of neural depolarization are similar, and (3) transtemporal magnetic stimulation appears to allow depolarization of the proximal intratemporal nerve. These preliminary results are encouraging and indicate that magnetic stimulation of the facial nerve warrants further investigation.

Brain stem auditory evoked response correlates in patients with spinocerebellar lesions.

Brain stem auditory evoked responses were obtained in three patients with spinocerebellar dysfunction for assessment of brain stem involvement and lateralization. Computed tomography scan in the three cases showed negative findings for the brain stem. The evoked potentials demonstrated involvement of the pontine tegmentum. Correlation was found for lateralization to the site of greater neurologic deficit. Brain stem auditory evoked response abnormalities reflect disturbance of neural function rather than the underlying anatomic change of that disturbance. It therefore appears to be a more sensitive type of testing than the computed tomography scan in detecting lesions that alter electrophysiology but do not produce alterations of morphology and radiodensity.

Pathology of deafness and disequilibrium in head injury:a human temporal bone study.

A Caucasian male suffered a skull fracture at age twenty-two. There was bilateral deafness with partial recovery of hearing in the left ear. He was able successfully to wear a hearing aid in this ear for only thirteen years. His balance remained impaired until death at age fifty of unrelated causes. Temporal bone histologic studies revealed bilateral transverse fractures extending through the ampullated end of the three semicircular canals and vestibula. The cochleae were not involved. In the right ear, the organ of Corti was missing in the basal 14 mm and shrunken in the remainder of the cochlea. In the left ear, the organ of Corti was missing in the basal 14 mm and present with partial hair cell loss in the remainder of the cochlea. Spiral ganglion neuron loss correlated in severity with the organ of Corti lesions bilaterally. The membranous vestibular labyrinth showed distension and ruptures with total loss of sensory epithelium of the cristae and maculae bilaterally. Progressive deterioration of left ear residual hearing cannot be explained on the basis of sensorineural deficit; it probably was the result of biochemical alterations. Total loss of vestibular sensory epithelium bilaterally explained the persistent disequilibrium.

Ultrasonic irradiation through the round window. Functional and morphological findings in sound-conditioned cats.

Ultrasound was presented though the round window in 9 sound-conditioned cats at levels approximating clinical usage. Threshold shifts were mild to moderate and confined to 4 kHz and higher. There was total loss of function at 16 kHz in 2 animals. Threshold shifts correlated with cochlear histological findings as shown by reconstruction. There were three main types of lesions: abnormalities of supporting structures, hair cell loss, and lesion of Reissner's membrane. Supporting structure damage was the most frequent. The cristae of the semicircular canals did not show any lesions, though some saccular damage was noted. These changes were not observed in 4 control animals. Some conductive damage was noted as a result of probe placement. High frequency loss can be expected with the round window approach at irradiation levels of 52 mW for 20 min, or stronger.

Air calorics: a technique and results.

A major problem with air calorics appears to be one of technique. A pilot study led to the design of irrigating tips which allow consistant air presentation and simultaneous measurements of irrigating temperature at the delivery orifice (adding a second sensor). Preset temperatures of 24 C and 50 C in our system yielded air equilibration mean temperatures of 27.4 C and 45 C at the delivery orifice during testing. A normal study was carried out at these temperatures with an air flow of six liters per minute for 60 seconds. The range of caloric responses, mean maximum speed, and standard deviation are comparable to values previously reported with water stimulations. Statistical testing indicated no difference between ears for either warm or cold. Also, there was no significant difference for warm versus cold responses. We have performed over 2000 clinical examinations that incorporate this technique with satisfactory results and remarkable acceptance by the patients. The normal or "standard" probe size has been found to be adequate for the majority of the clinic population.