Marina Liebsch

Facial Nerve Monitoring During Cerebellopontine Angle and Skull Base Tumor Surgery: A Systematic Review from Description to Current Success on Function Prediction

BACKGROUND Intraoperative neuromonitoring has been established as one of the methods by which modern neurosurgery can improve surgical results while reducing morbidity. Despite routine use of intraoperative facial nerve (FN) monitoring, FN injury still is a complication of major concern due to severe negative impact on patients quality of life. METHODS Through searches of PubMed, we provided a systematic review of the current literature up to February, 2011, emphasizing all respects of FN monitoring for cerebellopontine angle and skull base tumor surgery from description to current success on function prediction of standard and emerging monitoring techniques. RESULTS Currently, standard monitoring techniques comprise direct electrical stimulation (DES), free-running electromyography (EMG), and facial motor evoked potential (FMEP). We included 62 studies on function prediction by investigating DES (43 studies), free-running EMG (13 studies), and FMEP (6 studies) criteria. DES mostly evaluated postoperative function by using absolute amplitude, stimulation threshold, and proximal-to-distal amplitude ratio, whereas free-running EMG used the train-time criterion. The prognostic significance of FMEP was assessed with the final-to-baseline amplitude ratio, as well as the event-to-baseline amplitude ratio and waveform complexity. CONCLUSIONS Although there is a general agreement on the satisfactory functional prediction of different electrophysiological criteria, the lack of standardization in electrode montage and stimulation parameters precludes a definite conclusion regarding the best method. Moreover, studies emphasizing comparison between criteria or even multimodal monitoring and its impact on FN anatomical and functional preservation are still lacking in the literature.

Transcranial electrocortical stimulation to monitor the facial nerve motor function during cerebellopontine angle surgery.

OBJECTIVE This study was conducted to investigate the success rate of using the facial motor evoked potential (FMEP) of orbicularis oculi and oris muscles for facial nerve function monitoring with use of a stepwise protocol, and its usefulness in predicting facial nerve outcome during cerebellopontine angle (CPA) surgeries. METHODS FMEPs were recorded intraoperatively from 60 patients undergoing CPA surgeries. Transcranial electrocortical stimulation (TES) was performed using corkscrew electrodes positioned at hemispheric montage (C3/C4 and CZ). The contralateral abductor pollicis brevis muscle was used as the control response. Stimulation was always applied contralaterally to the affected side using 1, 3, or 5 rectangular pulses ranging from 200 to 600 V with 50 μs of pulse duration and an interstimulus interval of 2 ms. Facial potentials were recorded from needles placed in the orbicularis oculi and oris muscles. RESULTS FMEP from the orbicularis oris and oculi muscles could be reliably monitored in 86.7% and 85% of the patients, respectively. The immediate postoperative facial function correlated significantly with the FMEP ratio in the orbicularis oculi muscle at 80% amplitude ratio (P = .037) and orbicularis oris muscle at 35% ratio (P = .000). FMEP loss was always related to postoperative facial paresis, although in different degrees. CONCLUSION FMEPs can be obtained reliably by using TES with 3 to 5 train pulses. Stable intraoperative FMEPs can predict a good postoperative outcome of facial function. However, further refinements of this technique are necessary to minimize artifacts and to make this method more reliable.

Safe Resection of Arteriovenous Malformations in Eloquent Motor Areas Aided by Functional Imaging and Intraoperative Monitoring

BACKGROUND: Arteriovenous malformations (AVMs) proximal to motor cortical areas or motor projection systems are challenging to manage because of the risk of severe sensory and motor impairment. Surgical indication in these cases therefore remains controversial. OBJECTIVE: To propose a standardized approach for centrally situated AVMs based on functional imaging and intraoperative electrophysiological evaluation. METHODS: We conducted a retrospective analysis of 15 patients who underwent surgical treatment for AVMs in motor cortical areas or proximal to motor projections. Preoperative assessment included functional magnetic resonance and 3-dimensional tractography. Operations were performed under continuous electrophysiological monitoring aided by direct brain stimulation. We identified critical bloody supply to the motor areas by temporary occluding the feeding vessels under electrophysiological monitoring. Clinical outcome was evaluated with the modified Rankin Scale. RESULTS: Total resection was achieved in 12 cases, whereas electrophysiology limited total extirpation in 3 cases. A significant reduction of motor evoked potentials by up to 15% of the initial values was associated with good recovery of motor function; in contrast, the disappearance of potentials correlated with long-term impairment. The mean follow-up time was 13 months, and clinical assessments revealed overall functional improvement (P < .05). After surgery, 11 patients were asymptomatic or presented with only minor neurological deficits. CONCLUSION: Surgical resection of AVMs in eloquent motor areas can be considered a safe option for selected cases when performed in conjunction with a detailed functional assessment. Possible selection criteria for surgical treatment are discussed in light of the presented clinical data. ABBREVIATIONS: AVM, arteriovenous malformation DSA, digital subtraction angiography DTI, diffusion tensor imaging fMRI, functional magnetic resonance imaging MEP, motor evoked potential mRS, modified Rankin Scale SEP, somatosensory evoked potential

Quantitative parameters of facial motor evoked potential during vestibular schwannoma surgery predict postoperative facial nerve function

BackgroundFacial motor evoked potential (FMEP) amplitude ratio reduction at the end of the surgery has been identified as a good predictor for postoperative facial nerve outcome. We sought to investigate variations in FMEP amplitude and waveform morphology during vestibular schwannoma (VS) resection and to correlate these measures with postoperative facial function immediately after surgery and at the last follow-up.MethodsIntraoperative orbicularis oculi and oris muscles FMEP data from 35 patients undergoing surgery for VS resection were collected, then analysed by surgical stage: initial, dural opening, tumour dissection (TuDis), tumour resection (TuRes) and final.FindingsImmediately after surgery, postoperative facial function correlated significantly with the FMEP amplitude ratio during TuDis, TuRes and final stages in both the orbicularis oculi (p = 0.003, 0.055 and 0.028, respectively) and oris muscles (p = 0.002, 0.104 and 0.014, respectively). At the last follow-up, however, facial function correlated significantly with the FMEP amplitude ratio only during the TuDis (p = 0.005) and final (p = 0.102) stages for the orbicularis oris muscle. At both time points, postoperative facial paresis correlated significantly with FMEP waveform deterioration in orbicularis oculi during the final stage (immediate, p = 0.023; follow-up, p = 0.116) and in orbicularis oris during the TuDis, TuRes and final stages (immediate, p = 0.071, 0.000 and 0.001, respectively; follow-up, p = 0.015, 0.001 and 0.01, respectively).ConclusionsFMEP amplitude ratio and waveform morphology during VS resection seem to represent independent quantitative parameters that can be used to predict postoperative facial function. Event-to-baseline FMEP monitoring is quite useful to dictate when intraoperative changes in surgical strategy are warranted to reduce the chances of facial nerve injury.

A new concept in the electrophysiological evaluation of syringomyelia

OBJECT The current neurophysiological assessment of syringomyelia is inadequate. Early-stage syringomyelia is anatomically predisposed to affect decussating spinothalamic fibers that convey pain and sensation primarily. Silent periods have been proven to be a sensitive tool for detecting alterations in this pathway. METHODS Thirty-seven patients with syringomyelia were included in this prospective study. Routine electrophysiological measurements were applied including somatosensory evoked potential (SSEP) and motor evoked potential (MEP) recordings for all extremities. The silent periods were recorded from the pollicis brevis muscle, and electrical stimuli were applied to the ipsilateral digiti II. To establish baseline values, the authors had 28 healthy controls undergo monitoring. Sensitivity and specificity values were statistically evaluated according to the main clinical symptoms (paresis, dissociative syndrome, and pain). RESULTS All control individuals had normal silent periods in voluntarily activated muscle. In syringomyelia patients, the affected limb showed pathological silent periods with all symptoms (sensitivity 30-50%). Pain was the most specific symptom (90%), despite SSEP and MEP values that were within the normal range. CONCLUSIONS Silent period testing is a sensitive neurophysiological technique and an invaluable tool for preoperative assessment of syringomyelia. Silent periods are associated with early dysfunction of thin myelinated spinothalamic tract fibers, even when routine electrophysiological measurements still reveal normal values. Conduction abnormalities that selectively abolish the silent periods can distinguish between hydromyelia (a physiologically dilated central canal) and space-occupying syringomyelia.

The role of intraoperative neuromonitoring in adults with Chiari I malformation

OBJECT Suboccipital decompression and duraplasty is considered the treatment of choice for Chiari-I-malformation. Several studies have shown improvement of neurophysiological parameters during decompressive surgery in pediatric patients. However, there is no evidence of the beneficial role of intraoperative neuromonitoring in adults. METHODS A total of 39 consecutive patients (25 female, age 41.1±14.5 years [mean±SD]) underwent suboccipital decompression for the treatment of symptomatic Chiari-I-malformation senior neurosurgeon and his team. Continuous intraoperative monitoring of somatosensory-evoked potentials of the median nerve (m-SSEPs) and the posterior tibial nerve (t-SSEPs) and motor-evoked potentials to the hand (APB-MEPs) and foot (TA-MEPs) were applied to all patients, alone or in combination. The m-SSEP-N20, t-SSEP-P40, APB-MEP and TA-MEP amplitudes and latencies were recorded at the beginning of the surgery (baseline) and after dura closure. Neurological assessments of the patient were performed prior to surgery, after surgery and during each follow-up visit (3, 6 and 12 months). RESULTS There was no significant change in m-SSEP-N20, t-SSEP-P40 or APB-MEP and TA-MEP amplitudes or latencies between the baseline and final measurements (p>0.05, Students t-test). The average time between both recordings was 125±48min (mean±SD). Postoperatively, none of the patients presented new neurological deficits. During the mean follow-up period of 22.4±20.3 months, 92.6% of the symptoms improved or remained stable. Three patients (8.1%) exhibited a relapse of symptoms after 25.7±7.6 months, and only one patient (2.5%) needed secondary decompression after 24 months. CONCLUSION Intraoperative neuromonitoring (INM) during the primary treatment of Chiari-I-malformation shows only subtle non-significant changes in SSEPs/MEPs without clinical correlation during suboccipital decompression. INM is not considered a prerequisite for a safe suboccipital decompression when operated by an experienced surgical team.

Predictors of Preoperative Tinnitus in Unilateral Sporadic Vestibular Schwannoma

Objective Nearly two-thirds of patients with vestibular schwannoma (VS) are reporting a significantly impaired quality of life due to tinnitus. VS-associated tinnitus is attributed to an anatomical and physiological damage of the hearing nerve by displacing growth of the tumor. In contrast, the current pathophysiological concept of non-VS tinnitus hypothesizes a maladaptive neuroplasticity of the central nervous system to a (hidden) hearing impairment resulting in a subjective misperception. However, it is unclear whether this concept fits to VS-associated tinnitus. This study aims to determine the clinical predictors of VS-associated tinnitus to ascertain the compatibility of both pathophysiological concepts. Methods This retrospective study includes a group of 478 neurosurgical patients with unilateral sporadic VS evaluated preoperatively regarding the occurrence of ipsilateral tinnitus depending on different clinical factors, i.e., age, gender, tumor side, tumor size (T1–T4 according to the Hannover classification), and hearing impairment (Gardner–Robertson classification, GR1–5), using a binary logistic regression. Results 61.8% of patients complain about a preoperative tinnitus. The binary logistic regression analysis identified male gender [OR 1.90 (1.25–2.75); p = 0.002] and hearing impairment GR3 [OR 1.90 (1.08–3.35); p = 0.026] and GR4 [OR 8.21 (2.29–29.50); p = 0.001] as positive predictors. In contrast, patients with large T4 tumors [OR 0.33 (0.13–0.86); p = 0.024] and complete hearing loss GR5 [OR 0.36 (0.15–0.84); p = 0.017] were less likely to develop a tinnitus. Yet, 60% of the patients with good clinical hearing (GR1) and 25% of patients with complete hearing loss (GR5) suffered from tinnitus. Conclusion These data are good accordance with literature about non-VS tinnitus indicating hearing impairment as main risk factor. In contrast, complete hearing loss appears a negative predictor for tinnitus. For the first time, these findings indicate a non-linear relationship between hearing impairment and tinnitus in unilateral sporadic VS. Our results suggest a similar pathophysiology in VS-associated and non-VS tinnitus.

Closer to the Edge—The Value of Intraoperative Brain Mapping

he article, “Intraoperative subcortical fiber mapping with subcortical-cortical evoked potentials,” is a highly valued T contribution to the literature evaluating subcorticalcortical mapping in glioma surgery. It extends our knowledge in identifying motor cortical fiber tract integrity after resection of intrinsic brain tumors in eloquent areas. At present there is increasing amount of clinical and experimental data demonstrating various fiber tract identification either by imaging or by functional means.

Surgery of Metastases in the Sensorimotor Cortex Performance and Clinical Results

Tumor resection in the rolandic region, also known as sensorimotor cortex, is a challenge. This study aims at reviewing a series of patients undergoing resection of metastases in the sensorimotor cortex using a multimodal concept including neuronavigation, sonography, and intraoperative electrophysiological monitoring. Eleven patients suffering from metastases located in precentral (8) and postcentral gyrus (3) were analyzed concerning their functional motor outcome. Improvement of motor function could be seen in 5 patients 1 week after surgery, 5 patients remained unchanged, and only 1 deteriorated. Median survival time averaged 15 months. A multimodal approach, including preoperative and intraoperative neuronavigation, intraoperative sonography, and intraoperative electrophysiological monitoring can lead on to excellent functional outcome in surgery of metastases in the sensorimotor cortex.