Manuel Sainz

Using evoked compound action potentials to assess activation of electrodes and predict C-levels in the Tempo+ cochlear implant speech processor.

Objective: In this study, we analyze how electrically evoked compound action potential (ECAP) responses can be used to assess whether electrodes should be activated in the map and to estimate C levels in the Med-El Tempo+ Cochlear Implant Speech Processor. Design: ECAP thresholds were measured using the ECAP Recording System of the Pulsar CI100 implant. Twenty-one postlingually and 28 prelingually deafened patients participated in this study. The relationship between ECAP responses and the activation of electrodes was analyzed. Because an error in the estimation of T levels (behavioral thresholds) has less effect on hearing quality than an error in the estimation of C levels in the Tempo+ cochlear implant speech processor (maximum comfort levels), correlation and regression analyses were performed between ECAP thresholds and C levels. Results: The observation of an evoked potential generally implied that the electrode was activated because only 3.5% of electrodes that yielded measurable evoked responses were deactivated, because of collateral stimulations or an unpleasant hearing sensation. In contrast, the absence of an evoked potential did not imply that an electrode should be deactivated, because 20% of these electrodes provided a useful auditory sensation. ECAP responses did not predict the absolute behavioral comfort levels because of the excessive error between behavioral C levels and those derived from ECAP thresholds (the mean relative error is 43.78%). However, by applying a normalization procedure, ECAP measurements allowed the C-level profile to be predicted with a mean relative error of 6%; that is, they provided useful data to determine the C level of each electrode relative to the average C level of the patient. Conclusions: ECAP is a reliable and an useful objective measurement that can assist in the fitting of the Tempo+ cochlear implant speech processor. From results presented in this work, a protocol is proposed for fitting this cochlear implant system. This protocol facilitates appropriate cochlear implant fitting, particularly for children or uncooperative patients.

Long-Term Evolution of the Electrical Stimulation Levels for Cochlear Implant Patients

Objectives The stimulation levels programmed in cochlear implant systems are affected by an evolution since the first switch-on of the processor. This study was designed to evaluate the changes in stimulation levels over time and the relationship between post-implantation physiological changes and with the hearing experience provided by the continuous use of the cochlear implant. Methods Sixty-two patients, ranging in age from 4 to 68 years at the moment of implantation participated in this study. All subjects were implanted with the 12 channels COMBI 40+ cochlear implant at San Cecilio University Hospital, Granada, Spain. Hearing loss etiology and progression characteristics varied across subjects. Results The analyzed programming maps show that the stimulation levels suffer a fast evolution during the first weeks after the first switch-on of the processor. Then, the evolution becomes slower and the programming parameters tend to be stable at about 6 months after the first switch-on. The evolution of the stimulation levels implies an increment of the electrical dynamic range, which is increased from 15.4 to 20.7 dB and improves the intensity resolution. A significant increment of the sensitivity to acoustic stimuli is also observed. For some patients, we have also observed transitory changes in the electrode impedances associated to secretory otitis media, which cause important changes in the programming maps. Conclusion We have studied the long-term evolution of the stimulation levels in cochlear implant patients. Our results show the importance of systematic measurements of the electrode impedances before the revision of the programming map. This report also highlights that the evolution of the programming maps is an important factor to be considered in order to determine an adequate calendar fitting of the cochlear implant processor.

Generalized alternating stimulation: a novel method to reduce stimulus artifact in electrically evoked compound action potentials.

Stimulus artifact is one of the main limitations when considering electrically evoked compound action potential for clinical applications. Alternating stimulation (average of recordings obtained with anodic-cathodic and cathodic-anodic bipolar stimulation pulses) is an effective method to reduce stimulus artifact when evoked potentials are recorded. In this paper we extend the concept of alternating stimulation by combining anodic-cathodic and cathodic-anodic recordings with a weight in general different to 0.5. We also provide an automatic method to obtain an estimation of the optimal weights. Comparison with conventional alternating, triphasic stimulation and masker-probe paradigm shows that the generalized alternating method improves the quality of electrically evoked compound action potential responses.

Analysis of programming maps and its application for balancing multichannel cochlear implants

Channel balancing is important for correct perception when using multichannel cochlear implants. In this paper we analyse the effect of channel imbalance on hearing perception in cochlear-implanted patients. Based on warble tone audiometry measured for 10 implanted patients, we evaluate how hearing sensitivity is affected by an imbalance. We also study the distribution along the cochlea of the programming parameters for 146 patients. The parameters are compared for different groups of patients (defined taking into account the age at implantation, the stimulation mode, and the presence of severe cochlear damage). The aim of this study is to obtain information that can be useful for balancing the channels for the patients in the different groups, as well as to evaluate the effect of an imbalance on perception. These results provide some complementary information that can be applied to the correct fitting of the cochlear implant processor. Sumario: El balance de los canales cuando se usan implantes cocleares multicanal es importante para lograr una percepción correcta. En este articulo analizamos el efecto de la falta de balance sobre la percepción auditiva en pacientes implantados. Con base en los datos de la audiometria con tonos modulados practicada en 10 pacientes implantados, evaluamos qué tanto se afecta la sensibilidad auditiva por la falta de balance del implante. También estudiamos en 146 pacientes la distribución de los parámetros de programación a lo largo de la cóclea. Los parámetros se com-pararon con distintos grupos de pacientes (definidos según la edad de implantación, el modo de estimulación y la presencia de daño coclear severo). El objetivo de este estudio fue obtener información que pudiera ser útil en el proceso de balance de los canales en los pacientes de los diferentes grupos, así como evaluar el efecto de la falta de balance en la percepción. Estos resultados proporcionan alguna información com-plementaria que puede aplicarse en la correcta adaptación del procesador del implante coclear.

A study of adaptation mechanisms based on ABR recorded at high stimulation rate.

OBJECTIVE This paper analyzes the fast and slow mechanisms of adaptation through a study of latencies and amplitudes on ABR recorded at high stimulation rates using the randomized stimulation and averaging (RSA) technique. METHODS The RSA technique allows a separate processing of auditory responses, and is used, in this study, to categorize responses according to the interstimulus interval (ISI) of their preceding stimulus. The fast and slow mechanisms of adaptation are analyzed by the separated responses methodology, whose underlying principles and mathematical basis are described in detail. RESULTS The morphology of the ABR is influenced by both fast and slow mechanisms of adaptation. These results are consistent with previous animal studies based on spike rate. CONCLUSIONS Both fast and slow mechanisms of adaptation are present in all subjects. In addition, the distribution of the jitter and the sequencing of the stimuli may be critical parameters when obtaining reliable ABRs. SIGNIFICANCE The separated responses methodology enables for the first time the analysis of the fast and slow mechanisms of adaptation in ABR obtained at stimulation rates greater than 100 Hz. The non-invasive nature of this methodology is appropriate for its use in humans.

An improved masker-probe method for stimulus artifact reduction in electrically evoked compound action potentials.

The masker-probe paradigm is a commonly used technique to reduce stimulus artifact in electrically evoked compound action potential registers. This method takes advantage of the refractory properties of the cochlear nerve, combining the responses to different types of stimulation pulses in order to obtain the biological response free of artifact. In this paper we extend the masker-probe paradigm by combining the responses to these stimulation pulses with optimal weights. We also provide an automatic method to obtain an estimation of the optimal weights. A comparison with the conventional masker-probe paradigm shows that the proposed method improves the quality of electrically evoked compound action potential registers.

Auditory brainstem and middle latency responses recorded at fast rates with randomized stimulation

Randomized stimulation and averaging (RSA) allows auditory evoked potentials (AEPs) to be recorded at high stimulation rates. This method does not perform deconvolution and must therefore deal with interference derived from overlapping transient evoked responses. This paper analyzes the effects of this interference on auditory brainstem responses (ABRs) and middle latency responses (MLRs) recorded at rates of up to 300 and 125 Hz, respectively, with randomized stimulation sequences of a jitter both greater and shorter than the dominant period of the ABR/MLR components. Additionally, this paper presents an advanced approach for RSA [iterative-randomized stimulation and averaging (I-RSA)], which includes the removal of the interference associated with overlapping responses through an iterative process in the time domain. Experimental results show that (a) RSA can be efficiently used in the recording of AEPs when the jitter of the stimulation sequence is greater than the dominant period of the AEP components, and (b) I-RSA maintains all the advantages of RSA and is not constrained by the restriction of a minimum jitter. The significance of the results of this study is discussed.

Automatic quality assessment and peak identification of auditory brainstem responses with fitted parametric peaks

The recording of the auditory brainstem response (ABR) is used worldwide for hearing screening purposes. In this process, a precise estimation of the most relevant components is essential for an accurate interpretation of these signals. This evaluation is usually carried out subjectively by an audiologist. However, the use of automatic methods for this purpose is being encouraged nowadays in order to reduce human evaluation biases and ensure uniformity among test conditions, patients, and screening personnel. This article describes a new method that performs automatic quality assessment and identification of the peaks, the fitted parametric peaks (FPP). This method is based on the use of synthesized peaks that are adjusted to the ABR response. The FPP is validated, on one hand, by an analysis of amplitudes and latencies measured manually by an audiologist and automatically by the FPP method in ABR signals recorded at different stimulation rates; and on the other hand, contrasting the performance of the FPP method with the automatic evaluation techniques based on the correlation coefficient, FSP, and cross correlation with a predefined template waveform by comparing the automatic evaluations of the quality of these methods with subjective evaluations provided by five experienced evaluators on a set of ABR signals of different quality. The results of this study suggest (a) that the FPP method can be used to provide an accurate parameterization of the peaks in terms of amplitude, latency, and width, and (b) that the FPP remains as the method that best approaches the averaged subjective quality evaluation, as well as provides the best results in terms of sensitivity and specificity in ABR signals validation. The significance of these findings and the clinical value of the FPP method are highlighted on this paper.

A portable, modular, and low cost auditory brainstem response recording system including an algorithm for automatic identification of responses suitable for hearing screening

The recording of auditory brainstem response (ABR) signals is a common measure applied to assess hearing impairments. However, most of the available commercial devices able to record ABR signals can be unaffordable for many medical centers because of their cost and low flexibility. This paper describes a system that allows the recording of high quality ABR. Its low cost, easy handling, high performance, and portability make its use appropriate in low budget institutions. Furthermore, the flexibility and open nature of this system allow its use as a research tool. The ABR recording system includes a new algorithm for automatic evaluation of the quality of responses and the estimation of the latencies and amplitudes of the waves, the fitted parametric peaks (FPP). The performance of this technique is contrasted with a well-established method for quality evaluation based on the correlation coefficient. The encouraging results of this test suggest that the fitted parametric peaks could be used as a method for automatic ABR quality assessment and identification of the peaks.

Analysis of electrical thresholds and maximum comfortable levels in cochlear implant patients

OBJECTIVE It is well known that a proper fitting of the cochlear implant processor is relevant to provide good quality in speech perception. The aim of this study is to extract statistical information to be applied for fitting the processor. METHODS This study is based on the programming maps of 121 patients, aged from 18 months to 68 years at the moment of implantation. All subjects were implanted with the COMBI 40+ cochlear implant at San Cecilio University Hospital, Granada (Spain). The patients were classified into groups based on their age at implantation: younger than 5 years, between 5 and 16 years, and older than 16 years. The patients in each age-based group were divided into two subgroups, considering whether they had recent hearing experience or not. A special group including patients affected by severe damages in the cochlea was also defined. RESULTS Relationships between the programming parameters and factors like the age at implantation, the hearing experience and the presence of severe cochlear damage were found. The THR levels for patients younger than 5 years were significantly lower than those for patients implanted between 5 and 16 years, and this group presented significantly lower THR levels than adults. The MCL levels were not significantly influenced by the age at implantation. A significant increment was observed for both, MCL and THR levels, when patients were affected by severe cochlear damage. A significant increment in the THR levels were observed for patients with no recent hearing experience, while no significant differences were found for MCL levels. This study also analyzes the distribution along the cochlea of the stimulation levels. In the case of patients not affected by severe cochlear damage, the most basal electrodes presented a significant increment in the stimulation levels with respect to the rest of electrodes. CONCLUSION This work provides information of great value for programming the speech processors, particularly when the subjective responses of the patients are not sufficient. The application in our ENT Service has reduced substantially the average time needed to obtain an acceptable fitting of the processor, especially in children. Our study also shows that electrical thresholds are a good indicator of the functionality of the auditory nerve. The analysis of this parameter highlights the importance of an early intervention as well as a deep insertion of the electrode carrier in order to obtain the maximum functionality from the cochlear implant.