Abstract
Purpose
Supramaximal facial nerve stimulation is an applied current sufficient to evoke a maximal electromyographic response of facial musculature. It is used during cerebellopontine angle surgery for prognostication of postoperative nerve function. We utilized a rat model to examine safe parameters for intracranial electrical stimulation.
Materials and methods
Intracranial facial nerve stimulation with electromyographic monitoring of 14 rats was performed. Supramaximal current level was determined and 50 additional pulses of supramaximal (4 rats), 3 times supramaximal (4), 10 times supramaximal (3), or zero (3) current were applied. To monitor progression of facial nerve injury, video recordings of vibrissae movements and eye closure were captured at 1, 3 and 28 days after surgery; animals were sacrificed on day 28, when nerve morphometry was performed.
Results
One rat in the supramaximal stimulation group (of 4), and one rat in the 10 times supramaximal stimulation group (of 3) demonstrated persistent impairment of facial nerve function as evidenced by decreased amplitude of vibrissae sweeping and eye closure impairment. The remainder of rats in all experimental groups demonstrated symmetric and normal facial nerve function at all time points.
Conclusions
A novel animal model for supramaximal stimulation of the rat intracranial facial nerve is described. A small proportion of animals demonstrated functional evidence of nerve injury postoperatively. Function was preserved in some animals after stimulation with current order of magnitude higher than supramaximal levels. Further study with this model is necessary to definitively isolate the effects of surgical trauma from those of supramaximal electrical stimulation.
1
Introduction
Use of intraoperative facial nerve (FN) monitoring is well established as the standard of care during microsurgery for vestibular schwannoma (VS) . FN outcomes are being increasingly prioritized by patients, reflected by recent trends towards less aggressive resections and non-operative treatment for VS with the goal of minimizing morbidity . Intraoperative electromyographic (EMG) monitoring of FN activity and the use of electrical stimulation for nerve localization have been clearly associated with improved postoperative FN outcomes .
Evoked EMG techniques can also be used to interrogate the continuity and function of the FN during and after tumor removal . There is no consensus, however, as to how this information can be used to predict postoperative FN function. Several investigators have retrospectively reviewed data collected during surgery in an attempt to correlate immediate and long term FN outcomes with intraoperative threshold stimulation levels, EMG amplitude response, or proximal to distal evoked EMG response ratios . A large variety of stimulation parameters have been used, with varying levels of success in predicting long term FN outcomes. Having a means to accurately predict FN outcome would be useful not only for counseling patients, but also for recommending early facial reanimation procedures for those with little chance for significant recovery.
The experience at our institution with supramaximal stimulation (SMS) for prediction of FN outcomes after VS microsurgery was recently reported . With this technique, the amplitude of maximal evoked EMG compound muscle action potentials (CMAPs) are obtained with stimuli both proximal and distal to the site of tumor resection, and their ratio can be used to calculate a percentage “drop off” of FN function. This requires stimulation of the intracranial FN with current levels higher than used for other techniques, such as proximal minimal stimulation threshold. Data regarding safe levels for electrical FN stimulation are not clearly delineated, and data are particularly sparse regarding stimulation of the intracranial FN, the area most vulnerable to injury due to a lack of protective epineurium . We aim to use an animal model for electrical stimulation of the intracranial FN to define safe stimulation parameters that do not induce nerve injury.
2
Materials and methods
Institutional Animal Care and Use Committee approval was obtained (A47211) and all animal procedures and perioperative care were in accordance with the Guide for the Use and Care of Laboratory Animals as described by the National Institutes of Health .
2.1
Surgical technique
Adult male Sprague–Dawley rats were used for all procedures. Anesthesia was induced with an intraperitoneal injection of Ketamine (100 mg/kg) and Xylazine (10 mg/kg). Maintenance doses were provided in response to paw withdrawal on intermittent toe pinch testing or with any other signs of distress. All procedures were performed by a single surgeon (JTB). The right postauricular area was clipped and prepped. To access the intracranial portion of the FN, we utilized a modified form of the surgical approach described by Burgette et al. and Sharma et al., who recently described their rat model of intracranial FN crush injury . The parafloccular lobe of the cerebellum and petrous portion of the temporal bone were exposed. When other investigators utilized this approach to perform nerve crush injuries, the labyrinthine segment of the FN was decompressed to expose it circumferentially. For purposes of intracranial nerve stimulation, this degree of exposure was not required. As such, the remainder of the procedure was partially modified to simplify the approach and decrease the risk of inadvertent FN injury.
Prior to FN identification, electrodes for nerve stimulation and EMG monitoring were placed. The ground and return electrodes for the stimulation circuit were inserted into the posterior cervical musculature. A bipolar recording needle electrode was inserted into the ipsilateral vibrissal pad ( Fig. 1 A ). EMG monitoring from this electrode was carried out continuously, with its output connected to a loudspeaker, effectively serving as an FN monitor during dissection. Pieces of SURGICEL® Fibrillar™ Absorbable Hemostat (Ethicon Inc., Somerville, NJ) were passed into the craniotomy to retract and protect the cerebellum from trauma during intracranial drilling. Drilling continued ventrally and anteriorly through the petrous temporal bone, sacrificing the cochlea and eventually unroofing the labyrinthine segment of the FN. This was followed medially, and the intracranial segment of the FN was identified ( Fig. 1 B).
