Material and Methods: ASSR-stimulation was performed in 9patients undergoing surgery for vestibular schwannoma or meningi-oma in the cerebello-pontine angle (CPA). ASSR sounds with 5 minduration, 80 dB nHL, 90 Hz modulation and different carrier frequen-cies (500, 1000, 2000 Hz) were used as stimuli. Stimulation was per-formed monaurally on the side of surgery. Evoked responses wererecorded intraoperatively using a �2 channel EEG (right/left mastoidvs. vertex) at the beginning and the end of the surgical procedure.ASSR detections were compared to pre- and postoperative hearingclass.

Results: Sensitivity for hearing class A was 96%, specificity was83% with a positive/negative predictive value (PPV, NPV) of 82%/96%. Sensitivity for serviceable hearing (classes A/B) was 77%, spec-ificity 79%, PPV was 82% and NPV 73%.

Conclusion: ASSR during CPA surgery are viable and show a strongassociation with hearing quality. ASSR may thus represent a valuablealternative or complementary method to BAEP for auditory nervemonitoring.

doi:10.1016/j.clinph.2013.12.021

19. Tailored dura opening in eloquent areas – Is epidural mono-polar stimulation a useful tool?—Andrea Szelényi, Marion Rapp,Lina Nagel, Maria Smuga, Hans-Jakob Steiger, Michael Sabel(Neurosurgical Clinic, University Hospital, Duesseldorf, Germany)

Introduction: Direct cortical stimulation (DCS) is considered thegold standard for intraoperative mapping of motor cortex. As widedura opening for exposure of the motor cortex is not always possible,transdural monopolar stimulation might help for tailored dura open-ing and prevent unnecessary exposure of cortical areas. Thus, thefeasibility of monopolar epidural stimulation (EDS) and its relationto DCS with regard to stimulation intensity and amount of muscleresponses were studied.

Patients and material: 25 patients (57 � 12 years; 11 male) witheloquent area-located lesions and subject to craniotomy exposingthe motor region were studied. After craniotomy, EDS and aftercortex exposure DCS were performed. Localization was videotaped,co-localization was given within a <1 cm range. MEPs of at least 6contralateral muscles were stored on a neuromonitoring device(Inomed Co., Germany). EDS and DCS were performed with an ano-dal monopolar probe (2 mm diameter) and a train-of-5 pulses(0.5 ms individual pulse width, 250 Hz, maximum 30 mA). Motorthreshold was established for each stimulation point.

Results: In 4/25 (40%) patients EDS did not elicit MEPs, but DCSwas successful in 22 (88%) patients. In the 20 patients, where EDSand DCS elicited MEPs, the minimum threshold of EDS was17�6.4 mA compared to 14.1 � 5.3 mA (median 15 mA) in DCS(p = 0.006; paired t-test); thus being true positive in 0.97, true neg-ative in 0.5 cases and resulting in a sensitivity of 0.91 and specificityof 0.67. EDS elicited MEPs in 1.8 � 1.1 muscles compared to 1.6 � 1muscles with DCS.

Discussion: Stimulation intensities to elicit MEPs were expectedlyhigher for EDS, which explains the tendency toward more muscleresponses. The close relation to the hot spot of the DCS allows usingEDS for tailoring dura opening. This might prove a helpful tool espe-cially, in recurrent tumour surgery where dural scarring hampersdura opening.

doi:10.1016/j.clinph.2013.12.022

20. Correlation between blink reflex (br) and corticobulbar motorevoked potentials (comeps) in skull base surgery and role as apredictor of clinical outcome—Isabel Fernández-Conejero a,Alberto Torres a, Andreu Gabarrós a, Vedran Deletis b (a UniversityHospital of Bellvitge, Barcelona, Spain, b St. Luke’s RooseveltHospital, Intraoperative Neurophysiology, NY, United States)

Introduction: In 2009, Deletis et al. were the first to describe amethod for eliciting the R1 component of BR in patients under gen-eral anaesthesia. The R1 includes trigeminal afferents, brainstemconnections between trigeminal and facial nuclei and facial nerve.Therefore this technique might be considered a tool to intraopera-tively monitor the involved structures in the reflex arc. In this workwe compare the neurophysiologic behaviour of R1 component of BRand facial and/or trigeminal CoMEPs in 21 patients.

Material and methods: The study includes 21 patients diagnosed ofskull base tumours that underwent surgery by the retrosigmoidalapproach. Multimodal intraoperative monitoring was performedincluding: BR, lower facial and trigeminal CoMEPs, SEPs, MEPs andcranial nerve mapping. We used the published methodology byDeletis et al. to elicit BR and CoMEPs.

Results: Of the 21 patients, two patients with previous facial pare-sis did not have a BR. Of the 19 who had a BR; nine did not showchanges in BR or CoMEPs during surgery. None of those had facialor trigeminal deficit postoperatively. Eight patients lost BR and pre-sented significant decrement in amplitude of CoMEPs. Of these eight,five recovered all responses after stopping surgery momentarily, theother three didn’t recover responses and presented postoperativefacial paresis. The last two patients lost BR during the surgery whileCoMEPs didn’t show any change. We assumed that these changes inBR were due to changes in depth of anaesthesia.

Conclusion: There is a strong correlation between BR and CoMEPschanges during skull base surgery. BR and CoMEPs help to predictthe clinical outcome of facial and trigeminal nerve function. BRshould be included in the routine methodology for intraoperativemonitoring skull base surgeries since is a reliable tool to monitorthe structures involved in the reflex arc.

doi:10.1016/j.clinph.2013.12.023

21. ION setting for the hearing preservation in acoustic neuromasurgery—Camillo Foresti (Ospedale Papa Giovanni 23, Bergamo,Italy)

Introduction: In the last 15 years, the surgery of the posterior cra-nial fossa, within the methods of neurosurgery and microneurosur-gery, has improved and grown enormously. However, it is still achallenge for the surgeon, because of the importance of functionalneural structures at risk. The recording of auditory pathways, tothe peculiar characteristics of their course and the relationships withthe brainstem structures that undertake this long, gives an effectivesystem of neurophysiological monitoring for the protection of thecentral structures brainstem and for the preservation of hearingitself. The attempt to preserve hearing in the final of the small acous-tic neuroma surgery, it is to be considered almost infeasible withoutthe proper support of a neurophysiological monitoring.

Materials and methods: In the report it will be discussed andexplained how to record Electrocochleography, C-NAP (CompoundAcoustic Nerve Potential), and near field ABR (auditory brainstemresponse), which we record always simultaneously. The report shallalso place the accent on how effective is the monitoring of the differ-ent methods, with an attempt to correlate anatomical and functionalat risk structure.

Society Proceedings / Clinical Neurophysiology 125 (2014) e13–e24 e19