AUDITORY ELECTROPHYSIOLOGY: EVERYTHING YOU NEED TO KNOW BEFORE STARTING YOUR ASSESSMENTS (PART III) – NUMBER OF SOUND STIMULUS AVERAGES

2MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 The purpose of this bulletin is to help you learn more about the important theoretical and technical factors required to develop effective electrophysiological assessment methods. Before proceeding, we ask you to read Part I (released in March 2025), which addresses a fundamental mathematical concept commonly used in the auditory evoked potential field: standard deviations. We also refer you to a newsletter published in May 2025 that reports on selecting stimulus polarity (rarefaction, condensation, or alternating) in diagnostic auditory evoked response measurement. Continuing this series, the current issue focuses on the quantity of averaged sound stimuli. Fortunately, there are numerous guidelines available today that provide valuable information on the best test parameter settings for conducting auditory electrophysiological investigations. The clinical guidelines and publications that we reviewed in the development of this paper are listed in the references section. Milaine Dominici Sanfins, Piotr Henryk Skarzynski, and James W Hall III AUDITORY ELECTROPHYSIOLOGY: EVERYTHING YOU NEED TO KNOW BEFORE STARTING YOUR ASSESSMENTS (PART III) – NUMBER OF SOUND STIMULUS AVERAGES 3 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 Before auditory electrophysiologic measurement begins, an adequately low inter-electrode impedance (< 5000 ohms) must be verified. As the averaging process begins and the number of stimulus rejections is high (> 10%), then the evaluator must reassess electrode locations and impedances before continuing further. Other possible sources of excessive measurement artifact include electrical interference and myogenic (muscle) artifact associated with patient movement. IMPEDANCE ANALYSIS AND CONTROL 4MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 The extent of measurement artifact control will have an overall impact on the quality of auditory electrophysiologic responses. Small amplitude and high-frequency aberrations, whether from electrical or myogenic sources, make it difficult to accurately analyze response components (waves). Excessive distortion interferes with precise identification of wave peaks, a requirement for correctly calculating latencies. An evaluator needs to consider the number of stimuli that are required for satisfactory measurement (e.g., British Society of Audiology, 2019; British Society of Audiology, 2025). If the patient is calm throughout the test, then averaging 2000 stimuli per wave is usually sufficient to produce minimal measurement noise. Under favorable measurement conditions, it is often possible to record with even fewer stimulus repetitions. The aim is a robust, clear, and reliable response with low residual noise. An optimal measurement scenario includes auditory responses evoked with a high intensity click or a high frequency tone burst from patients with normal hearing sensitivity, ideally sleeping naturally or with the assistance of sedation or anesthesia. Conversely, an evaluator should consider extending the averaging process, i.e., increasing the number of stimulus presentations whenever artifact rejection levels exceed ±10 µV (> 10% rejection). ARTIFACT LEVEL VERSUS NUMBER OF AVERAGED STIMULI 5 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 Table 1: Correlation between artifact rejection level and number of auditory stimuli (Lightfoot & Stevens, 2014). Evaluators should reflect on this important information: the use of a fixed number of elicited stimuli is often not the most appropriate strategy for optimal auditory evoked response measurement. Two undesirable test outcomes are likely if the evaluator consistently presents a fixed number of averaged stimuli without considering the artifact rejection level. Under adverse measurement conditions, with excessive artifact rejection, it is very likely that an inconclusive waveform will be recorded. However, when an auditory electrophysiologic response is recorded with a fixed number under optimal measurement conditions, the test time may be unnecessarily lengthened. The evaluator must also recognize that the number of averaged stimuli varies according to the type of sound stimulus (e.g., click versus tone burst, highversus low-frequency tone burst, traditional vs. chirp stimuli). The quadratic law can be applied to determine how many stimuli to use and how much artifact rejection is required. A doubling of the artifact rejection level requires triple the number of triggered stimuli (Lightfoot and Stevens 2014). To ensure a successful examination, use the following scheme. Artifact rejection level Number of stimuli elicited (click or chirp) ±5 µV 2000 ±10 µV 4000 ±20 µV 8000 6MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 A good collection strategy must be followed, especially for neurodiagnostic click-evoked auditory brainstem response assessments, which are measures of auditory pathway function. This is because ABR latencies, amplitudes, and interpeak intervals need to be marked accurately. Figure 1 shows waveforms for a neurodiagnostic ABR assessment with click-type stimuli conducted on two females aged 21 (exam A) and 32 years (exam B). We conducted the exams at an intensity of 80 dBnNA, varying the number of stimuli and ensuring the repeatability of the waves in each condition. The six evaluation conditions and the different numbers of averaged stimuli were: It is interesting to note that in exam A there is satisfactory reproducibility of the waves with 1000 to 1500 averaged stimuli. Whereas in exam B, the ABR has good reproducibility of the waves with as few as 500 to 1000 averaged stimuli. These ABR data show that there is variability in responses among different individuals. Evaluators need to be attentive to the recording parameters used and how they can influence each response. According to the BSA recommendations (British Society of Audiology, 2025), no waveform should contain fewer than 1000 averaged stimuli. However, under optimal measurement conditions, a reliable ABR may be recorded with as few as 500 stimuli. Nevertheless, the evaluator must make this decision with deliberate intention. EVALUATIONS WITH DIFFERENT NUMBERS OF STIMULI 1st condition: 120 stimuli. 2nd condition: 250 stimuli. 3rd condition: 500 stimuli. 4th condition: 1000 stimuli. 5th condition: 1500 stimuli. 6th condition: 2000 stimuli. Sanfins, Skarzynski and Hall, 2025 7 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 It is clinically important to mention that the presence of high levels of artifacts with a reduced number of elicited auditory stimuli can lead to the following impairments: • Overlap: High-frequency artifacts can overlap with wave peaks, distorting their shape and making them difficult to distinguish from background noise. • Displacement: The presence of artifacts can lead to errors in identifying wave peaks with compromised accuracy of latency calculations. • Masking: In more severe cases, artifacts can completely mask wave peaks, preventing their identification and analysis. A A B Sanfins, Skarzynski and Hall, 2025 Sanfins, Skarzynski and Hall, 2025 Figure 1: ABR measurement conducted with two female individuals aged 21 (A) and 32 (B) years, respectively. ABRs were recorded using the NeuroAudio equipment and the Neurosoft model. In exam A, all waves are evident and have good reproducibility, starting with 1500 stimulus presentations. In exam B, all waves become evident and have good reproducibility with 500 stimulus presentations. 8MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 Another significant ABR measurement factor that must be considered is the robustness of the V wave. Confident identification of wave V requires an amplitude more than three times the amplitude of residual noise. The following calculation is worth noting: With the signal corresponding to the ABR the noise consisting of any electrical activity that is not ABR (e.g, ambient electrical artifact, myogenic activity, EEG). Signal-to-noise (SNR) = Signal Amplitude x Number of Averages Noise Amplitude Sanfins, Skarzynski and Hall, 2025 9 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 There is a way to confidently record ABRs with a minimal number of stimulus averages. For this, it is essential that the evaluator have access to objective measurement tools that meet the recommended criteria for 97% or more confidence in the presence of responses. With ABR equipment that includes objective analysis features, an evaluator can confidently conclude the assessment before reaching the previously calculated number of stimulus presentations. One example of a clinical objective measurement approach is the Fsp or Fmp (F statistics for single points or multiple points) which statistically differentiates the ABR versus residual noise (Elberling and Don, 1984). The Fsp statistical index quantifies the relationship between the variance of a sample of the averaged electrophysiological data which includes the magnitude of the ABR, the residual background noise, and the variance of the background noise itself. In essence, the Fsp acts as an indicator of the true ABR in relation to noise. High Fsp values reflect an ABR with greater magnitude compared to background noise, providing greater confidence in identifying the response. F STATISTICAL INDEX (SIGNAL-TO-NOISE RATIO) Don and Elberling (1994) proposed an evolution of the Fsp, called Fmp, which is distinguished by the analysis of multiple points along the waveform instead of a single point. This multi-point approach allows for a more comprehensive and accurate assessment of the signal-tonoise ratio. 16 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 COMMENTS We strongly recommend that evaluators review published auditory evoked response guidelines to deepen their knowledge and stay up-to-date on current best practices. We invite you to follow us in our new publications for more detailed information. 17 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 18 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 REFERENCES: Sanfins MD, Medeiros B, Santillo MEA, Skarzynski PH. Electrophysiology of hearing: everything you need to know before starting youe evaluations (part I) - choosing the standard deviation.MEDINCUS - DOI: 10.5281/ ZENODO.14362257 - VOL.24, MARCH/2025. Sanfins MD, Santillo MEA, Skarzynski PH. Electrophysiology of hearing: everything you need to know before starting youe evaluations (part II) - polarity of stimuli. MEDINCUS - DOI: 10.5281/ZENODO.14718558 VOL.26, MAY/2025. Ogleznev V, Zaretsky A, Shesterikov A. Small amplitude high frequency artifact, whether electrical or myogenic in origin, interferes with precise estimation of the wave peaks and, therefore, influences accuracy of latency calculations. Electroencephalography and clinical neurophysiology. 1983; 56(1), 105-108. Stuart A, Cobb KM. Effect of stimulis and number of sweps on the neonate auditory brainstem response. Ear & Hearing. 2014; 35, 5, 585-588. Elberling C, Don M. A direct approach for the design of chirp stimuli used for the recording of auditory brainstem responses. J Acoust Soc Am. 2010; 128, 2955–2964. Elberling C, Kristensen SG, Don M. Auditory brainstem responses to chirps delivered by different insert earphones. J Acoust Soc Am. 2012; 131, 2091–2100. Fobel O, Dau T. Searching for the optimal stimulus eliciting auditory brainstem responses in humans. J Acoust Soc Am. 2004; 116(4 Pt 1), 2213–2222. Lightfoot G, Stevens J. Effects of Artefact Rejection and Bayesian weighted Averaging on the Efficiency of Recording the Newborn ABR. Ear and Hearing. 2014, 35(2), pp.213–20. NHSP Recommended stimulus reference levels for ABR systems. 2012 . Retrieved: Dec 1, 2012 from http://hearing.screening.nhs.uk/ audiologyprotocols#fi leid16502 BRITISH SOCIETY OF AUDIOLOGY (2025) Recommend Auditory Brainstem Response (ABR) Testing in Babies, [Online]. Available from: https://www.thebsa.org.uk/guidanceand-resources/current-guidance/?subject=ele ctrophysiology BRITISH SOCIETY OF AUDIOLOGY (2019a) Recommended Procedure Auditory Brainstem Response (ABR) Testing in Babies, [Online]. Available from: https://www. thebsa.org.uk/wp-content/uploads/2019/04/ RecommendedProcedure-for-ABR-Testingin-Babies-FINAL-Feb-2019.pdf. Accessed 17/04/2019 Stevens, J. et al., 2013. Predictive Value of Hearing Assessment by the Auditory Brainstem Response Following Universal Newborn Hearing Screening. International Journal of Audiology, 52(7), pp.500–6. Picton TW, Ouellette J, Hamel G, Smith AD. Brainstem evoked potentials to tonepips innotched noise. Journal of Otolaryngology. 1979; 8, 289-314. Hayes D, Jerger J. Auditory Brainstem Response (ABR) to tone-pips: results innormal and hearing impaired subjects. Scandinavian Audiology. 1982; 11, 133-142. Stapells D. Threshold estimation by the tone-evoked ABR: a literature meta-analysis. Journal of Speech-Language Pathology and Audiology. 2000; 24, 74-83. Elberling C, Don M. Quality estimation of averaged auditory brainstem responses. Scand Audiol. 1984; 13 , 187 – 197. 01. 02. 03. 04. 05. 06. 07. 08. 09. 10. 11. 12. 13. 14. 15. 16. 19 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 Authors - Professor of the Audiology da Universidade Federal de São Paulo (UNIFESP); - Research group member, Institute of Physiology and Pathology of Hearing, Kajetany, Poland. - Professor of the post-graduate program in Clinical Audiology at the Albert Einstein Israelite Institute of research and teaching; - Postdoc at the World Hearing Center, Warsaw, Poland; - Sandwich Doctorate by School of Medical Sciences, Universidade Estadual de Campinas (FCM-UNICAMP) and by Università degli Studi di Ferrara/Italy; - Specialist in Audiology by Federal Council of Speech Therapy and Audiology; PROF. DR. MILAINE DOMINICI SANFINS - Speech Therapist and Audiologist, Master by Medical School of University of São Paulo (FMUSP); - Member of the Teaching and Research Commission of the Brazilian Academy of Audiology (2024-2026); - Rapporteur of the Research Ethics Committee of the Federal University of São Paulo; - Reviewer of scientific articles in the area of Neuroaudiology, Neuroscience, Electrophysiology and Audiology; - Instagram @misanfins / email: [email protected]om.br and [email protected] - Professor, ENT, Master and Doctorate by Medical University of Warsaw; - Research, didactic, clinical, and organizational work in World Hearing Center of Institute of Physiology and Pathology of Hearing, Institute of Sensory Organs and Medical University of Warsaw; - Specialist in ENT, pediatric ENT, audiology and phoniatrics, and public health. Participated in the 3rd Stakeholders Consultation meeting during which the World Hearing Forum of WHO was announced; - Member of the Roster of Experts on Digital Health of WHO, Vice-President and Institutional Representative of ISfTeH; - President-elect of International Advisory Board of AAOHNS, member of Congress and Meeting Department of EAONO, Regional Representative of Europe of ISA, VicePresident of HearRing Group, Auditor of EFAS, PROF. DR. PIOTR HENRYK SKARZYNSKI member of the Facial Nerve Stimulation Steering Committee; - Board Secretary of the Polish Society of Otorhinolaryngologists, Phoniatrists and Audiologists. Member of Hearing Committee (2018–19); - Goodwill Ambassador representing Poland at the AAOHNSF 2021 Annual Meeting & OTO Experience, and since 2021 a member of Implantable Hearing Devices Committee and Otology & Neurotology Education Committee of AAOHNS; - Consultant Committee of International Experts of CPAM-VBMS (by special invitation), honorary member of ORL Danube Society, and honorary member of Société Française d’Oto-Rhino-Laryngologie; - Member of the Council of National Science Center; - Expert and member of numerous national organizations. 20 MEDINCUS - DOI: 10.5281/ZENODO.15213743 - VOL.27, JUNE/2025 - Professor of Audiology at Salus University and the University of Hawaii, Extraordinary Professor at the University of Pretoria in South Africa, and adjunct professor at numerous universities in the USA and internationally; - Internationally recognized audiologist with over 40 years of clinical, teaching, research, and administrative experience; - Founder of the American Academy of Audiology; - Has held leadership roles within the American Academy of Audiology; PROF. DR. JAMES HALL III - Author of over 200 peer-reviewed publications, invited articles, and book chapters; - Author of 12 textbooks in the field of audiology; - Holds a bachelor’s degree in Biology from American International College, a master’s degree in Speech Pathology from Northwestern University, and a Ph.D. in Audiology from Baylor College of Medicine under the direction of James Jerger.