Introduction
Brainstem auditory evoked potentials or short-latency auditory evoked potentials (SLAEPs) are neurophysiological tests where bioelectrical responses provoked in the auditory system, from the cochlear nerve to the brainstem, are obtained after the presentation of a transient acoustic stimulus (click or CE-chirp)1.
Conventionally, SLAEPs have been incorrectly named “evoked” auditory potentials; however, according to the Royal Spanish Academy, “evoke” means to remember something or someone or to bring them to mind (to call a spirit or the dead), so the correct term is “provoked” auditory potentials, meaning to produce or cause something2.
These have been used in the evaluation of auditory sensitivity in children, in the diagnosis of hearing loss, in intraoperative monitoring, and in the detection of tumors or other retrocochlear disease affecting the conduction of the auditory pathway in the above-mentioned segments3.
The first reported recordings of auditory action potentials were published from Japan by Yoshie, Ohashi, and Suzuki (1967) using an electrode in the external auditory canal; in France by Portmann, Le Bert, and Aran (1967) using a needle electrode inserted through the tympanic membrane onto the cochlear promontory; and in Israel by Sohmer and Feinmesser (1967) who used an electrode on the earlobe. All showed the N1 and N2 waves of the cochlear nerve action potential and the characteristic increase in latency with a decrease in amplitude at decreasing intensity. However, it was Jewett et al. in 1970 who finally clearly described what would become the auditory brainstem response, proposing our current nomenclature for each recorded wave using Roman numerals: I, II, III, IV, V, VI, and VII4. The most important being I and V, which consider structures from the auditory nerve to the lateral lemniscus and inferior colliculus.
During gestational development, the maturation of auditory evoked responses tends to proceed from the peripheral to the central auditory system and in a caudal to rostral direction inside the central nervous system. SLAEPs can be recorded in premature patients from the 25th week of gestation; however, the neurophysiological characteristics of SLAEPs change until 18 months of age, which is when the electrophysiological maturation of the auditory pathway ends, and from this age onwards, the responses are practically identical to those of an adult in all their parameters5.
The interpretation of the results obtained from SLAEP recordings requires values that are representative of a normal population. The fact that there are no universal normal values is due to the many variables that influence the study recordings. The need to standardize SLAEPs is implicitly related to the characteristics of the studied neuroelectrical phenomenon, as it is a signal measured in very small frequency ranges, and several factors can modify these values, such as: environmental conditions under which the study is performed, type of stimulation, polarity, stimulation rate, intensity, population differences in phenotypic and biological characteristics, variability in the manufacturing of recording equipment, accessories to be used, the distance of electrode placement, as well as the patient’s muscle relaxation state6.
Due to the above-mentioned reasons, the use of institutional standards provides a series of important advantages when interpreting recordings. Various studies currently support the recommendation that the equipment of each institution should have its normative values according to the population to be studied7.
This study established SLAEP normality standards for normo-hearing subjects older than 18 months without current or previous neurological disease or current or previous otological disease, which will be representative for adulthood (inter-latency interval values I-III, III-V, total central conduction time with the I-V interval, absolute latencies of components I, III, and V, interaural difference of I-V intervals, amplitude ratio of waves I/V). All these parameters were assessed using different stimulation rates and polarities.
It is necessary to standardize SLAEP values because each device can give slightly different values depending on the characteristics of the population being evaluated and inherent differences in each device8. The eclipse equipment from the Interacoustics brand at the Audiology, Otoneurology, and Phoniatrics service of Hospital General de México (Mexico City, Mexico) has not been standardized in the Mexican population; the equipment has normative values obtained in Denmark in 2012 by evaluating 10 adult subjects representing a different population9,10.
Material and methods
We conducted a cross-sectional and descriptive study. It involved the standardization of a diagnostic test. A total of 34 ears from individuals aged 18 months and older, and 55 years and younger, of both genders, who attended Hospital General de México during the March-June 2024 period and agreed to participate in the study, were evaluated. Tone audiometry was performed to verify normal hearing thresholds, and impedance audiometry was performed to rule out middle ear disease and stapedial reflex arc issues. Instructions were given for hours of wakefulness to attend a 2nd visit for the SLAPP study. This study was conducted with conventional patient preparation, lying on a stretcher in a relaxed state. Cup electrodes were placed and named according to the international 10-20 system for extracranial electrode placement11, with cleaning performed in the scalp regions where they were located; points M1 (left mastoid), M2 (right mastoid), Cz (vertex of the calvarium), and Fpz (forehead), using the eclipse potentials equipment, in a sound-attenuated room through insert earphones (ER-3a), with impedances < 5kΩ, using different rates and polarities with monaural stimuli: CE-chirp and click types, in alternating, rarefaction, and condensation polarities with 2000 averages. The recording window was 20 ms. Initially, for audiological potentials, the auditory threshold of each ear was confirmed down to the identification of the Wave V threshold in decreasing 5dBnHL steps using stimulation rates of 45.1/s and 33.1/s. Subsequently, auditory evoked potentials were recorded in neurological modality at an intensity of 70 dBSL, lowering the stimulation rate to 11.1 stimuli/s with exclusively rarefaction polarity.
The values evaluated from the recording were the latency of intervals I-III, III-V, and the total central conduction time with the I-V interval, the absolute latencies of components I, III, and V, the interaural difference, and the amplitude ratio of waves I/V according to the suggestions of the American Society for Clinical Neurophysiology12.
Results
Electrophysiological measurements were performed on a total of 34 ears at the Audiology, Otoneurology, and Phoniatrics service unit. Four of these were discarded due to various technical reasons that resulted in poor-quality auditory evoked potential recordings. Measurements were taken for absolute latencies of components I, II, III, IV, and V, inter-latency intervals I-III, III-V, and the total central conduction time with the I-V interval, Wave V threshold, amplitude ratio of waves I/V, as well as the interaural difference of V-V intervals using click and CE-chirp stimuli in condensation, rarefaction, and alternating polarity, in patients older than 18 months. A total of 14 of the 30 ears were men’s ears and 15, women’s ears, with an age range from 4 to 32 years. Based on the Gaussian probabilistic model, ± 2 standard deviations (SD) were used in relation to the mean of each of the parameters to be normalized: absolute latencies, inter-latency intervals, total I-V conduction time, Wave V threshold, and the interaural difference. Where using only 1 SD would represent 68.27% of the population, while using 2 SD would represent 95.45%. Therefore, the objective was to find normal representation limits for each of the mentioned parameters (Tables 1–9) in our population and equipment, thereby avoiding erroneous diagnoses and mismanagement of our patients7.
Table 1. Rate 11.1 Stimuli/Sec. Condensation polarity
| Variable | CLICK | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.6 | 1.34 | 1.87 | 1.68 | 1.42 | 1.95 |
| Wave II | 2.59 | 2.24 | 2.93 | 2.79 | 2.52 | 3.07 |
| Wave III | 3.68 | 3.40 | 3.95 | 3.76 | 3.48 | 4.03 |
| Wave IV | 4.85 | 4.53 | 5.17 | – | – | – |
| Wave V | 5.45 | 4.99 | 5.90 | 5.31 | 4.9 | 5.72 |
| Interval I-III | 2.07 | 1.84 | 2.30 | 2.07 | 1.77 | 2.36 |
| Interval III-V | 1.76 | 1.28 | 2.25 | 1.55 | 1.24 | 1.87 |
| Interval I-V | 3.84 | 3.26 | 4.42 | 3.63 | 3.21 | 4.04 |
| Interaural difference V-V | 0.1680 | 0 | 0.39 | 0.17 | 0 | 0.42 |
|
σ: standard deviation. |
||||||
Table 2. Rate 11.1 Stimuli/Sec. Rarefaction polarity
| Variable | Click | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.68 | 1.41 | 1.94 | 1.69 | 1.41 | 1.96 |
| Wave II | 2.63 | 2.21 | 3.06 | 2.78 | 2.52 | 3.04 |
| Wave III | 3.80 | 3.46 | 4.13 | 3.77 | 3.47 | 4.07 |
| Wave IV | 4.90 | 4.51 | 5.29 | – | – | – |
| Wave V | 5.50 | 4.98 | 6.01 | 5.23 | 4.71 | 5.75 |
| Interval I-III | 2.22 | 1.03 | 3.40 | 2.08 | 1.74 | 2.42 |
| Interval III-V | 1.69 | 1.30 | 2.09 | 1.45 | 1.08 | 1.82 |
| Interval I-V | 3.82 | 3.26 | 4.37 | 3.54 | 3.03 | 4.05 |
| Interaural difference V-V | 0.15 | 0 | 0.45 | 0.13 | 0 | 0.35 |
|
σ: standard deviation. |
||||||
Table 3. Rate 11.1 Stimuli/Sec. Alternating polarity
| Variable | Click | CE-Chirp | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.63 | 1.38 | 1.88 | 1.68 | 1.40 | 1.96 |
| Wave II | 2.62 | 2.23 | 3.01 | 2.77 | 2.45 | 3.08 |
| Wave III | 3.72 | 3.38 | 4.06 | 3.74 | 3.44 | 4.05 |
| Wave IV | 4.74 | 4.35 | 5.14 | 4.95 | 4.80 | 5.09 |
| Wave V | 5.46 | 4.99 | 5.93 | 5.23 | 4.79 | 5.68 |
| Interval I-III | 2.09 | 1.81 | 2.36 | 2.06 | 1.79 | 2.34 |
| Interval III-V | 1.73 | 1.37 | 2.10 | 1.48 | 1.18 | 1.78 |
| Interval I-V | 3.83 | 3.38 | 4.28 | 3.55 | 3.13 | 3.96 |
| Interaural difference V-V | 0.12 | 0 | 0.34 | 0.14 | 0 | 0.36 |
|
σ: standard deviation. |
||||||
Table 4. Rate 33.1 Stimuli/Sec. Condensation polarity
| Variable | Click | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (m) | m − 2σ | m + 2σ | Arithmetic mean (m) | m − 2σ | m + 2σ | |
| Wave I | 1.54 | 1.31 | 1.78 | 1.77 | 1.44 | 2.11 |
| Wave II | 2.53 | 2.16 | 2.90 | 2.82 | 2.40 | 3.24 |
| Wave III | 3.81 | 3.50 | 4.12 | 3.92 | 3.61 | 4.24 |
| Wave IV | 4.91 | 4.46 | 5.35 | 5.17 | 4.90 | 5.44 |
| Wave V | 5.76 | 5.47 | 6.04 | 5.69 | 5.27 | 6.10 |
| Interval I-III | 2.26 | 1.98 | 2.54 | 2.15 | 1.85 | 2.44 |
| Interval III-V | 1.94 | 1.67 | 2.22 | 1.76 | 1.46 | 2.60 |
| Interval I-V | 4.21 | 3.96 | 4.46 | 3.91 | 3.58 | 4.24 |
| Interaural difference V-V | 0.07 | 0 | 0.23 | 0.14 | 0 | 0.45 |
| Threshold | 14 | – | – | 8 | – | – |
|
σ: standard deviation. |
||||||
Table 5. Rate 33.1 Stimuli/Sec. Rarefaction polarity
| Variable | CLICK | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.68 | 1.43 | 1.93 | 1.78 | 1.54 | 2.02 |
| Wave II | 2.69 | 2.14 | 3.24 | 2.84 | 2.53 | 3.14 |
| Wave III | 3.86 | 3.33 | 4.38 | 3.92 | 3.58 | 4.25 |
| Wave IV | 5.03 | 4.95 | 5.11 | 5.03 | 4.93 | 5.13 |
| Wave V | 5.68 | 5.26 | 6.10 | 5.74 | 5.30 | 6.17 |
| Interval I-III | 2.17 | 1.71 | 2.63 | 2.13 | 1.87 | 2.40 |
| Interval III-V | 1.82 | 1.42 | 2.22 | 1.82 | 1.43 | 2.20 |
| Interval I-V | 4.00 | 3.53 | 4.46 | 3.80 | 2.71 | 4.90 |
| Difference V-V | 0.13 | 0 | 0.46 | 0.15 | 0 | 0.38 |
| Threshold | 12.5 | – | – | 8 | – | – |
|
σ: standard deviation. |
||||||
Table 6. Rate 33.1 Stimuli/Sec. Alternating polarity
| Variable | Click | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.66 | 1.29 | 2.04 | 1.77 | 1.49 | 2.06 |
| Wave II | 2.65 | 2.14 | 3.16 | 2.85 | 2.52 | 3.18 |
| Wave III | 3.89 | 3.53 | 4.26 | 3.91 | 3.62 | 4.21 |
| Wave IV | 5.05 | 4.56 | 5.54 | 5.15 | 4.95 | 5.36 |
| Wave V | 5.79 | 5.37 | 6.21 | 5.70 | 5.26 | 6.15 |
| Interval I-III | 2.23 | 1.87 | 2.58 | 2.13 | 1.90 | 2.37 |
| Interval III-V | 1.89 | 1.53 | 2.25 | 1.78 | 1.45 | 2.12 |
| Interval I-V | 4.12 | 3.76 | 4.48 | 3.92 | 3.66 | 4.19 |
| Interaural difference V-V | 0.15 | 0 | 0.48 | 0.07 | 0 | 0.19 |
| Threshold | 12 | – | – | 9.5 | – | – |
|
σ: standard deviation. |
||||||
Table 7. Rate 45.1 Stimuli/Sec. Condensation polarity
| Variable | Click | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.67 | 1.33 | 2.00 | 1.65 | 1.45 | 1.85 |
| Wave II | 2.65 | 2.23 | 3.07 | 2.91 | 2.56 | 3.27 |
| Wave III | 4.00 | 3.48 | 4.52 | 4.05 | 3.70 | 4.39 |
| Wave IV | 4.99 | 4.60 | 5.37 | 5.33 | – | – |
| Wave V | 5.87 | 5.46 | 6.29 | 5.83 | 5.40 | 6.25 |
| Interval I-III | 2.30 | 1.79 | 2.82 | 2.16 | 1.88 | 2.43 |
| Interval III-V | 1.90 | 1.42 | 2.37 | 1.78 | 1.44 | 2.11 |
| Interval I-V | 4.21 | 3.65 | 4.76 | 3.94 | 3.58 | 4.30 |
| Interaural difference V-V | 0.07 | 0 | 0.18 | 0.18 | 0 | 0.54 |
| Threshold | 9 | – | – | 8 | – | – |
|
σ: standard deviation. |
||||||
Table 8. Rate 45.1 Stimuli/Sec. Rarefaction polarity
| Variable | CLICK | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.88 | 1.45 | 2.30 | 1.89 | 1.61 | 2.18 |
| Wave II | 2.73 | 2.26 | 3.19 | 2.96 | 2.55 | 3.37 |
| Wave III | 4.02 | 3.59 | 4.46 | 4.04 | 3.70 | 4.32 |
| Wave IV | 4.96 | 4.73 | 5.18 | 5.25 | 4.90 | 5.57 |
| Wave V | 5.88 | 5.38 | 6.38 | 5.87 | 5.51 | 6.23 |
| Interval I-III | 2.14 | 1.73 | 2.55 | 2.15 | 1.77 | 2.52 |
| Interval III-V | 1.87 | 1.34 | 2.40 | 1.82 | 1.57 | 2.07 |
| Interval I-V | 4.00 | 3.55 | 4.45 | 3.97 | 3.57 | 4.38 |
| Interaural difference V-V | 0.14 | 0 | 0.33 | 0.19 | 0 | 0.52 |
| Threshold | 10 | – | – | 8.5 | – | – |
|
σ: standard deviation. |
||||||
Table 9. Rate 45.1 Stimuli/Sec. Alternating polarity
| Variable | Click | CE-CHIRP | ||||
|---|---|---|---|---|---|---|
| Arithmetic mean (μ) | μ − 2σ | μ + 2σ | Arithmetic mean (μ) | μ − 2σ | μ + 2σ | |
| Wave I | 1.80 | 1.37 | 2.23 | 1.89 | 1.57 | 2.20 |
| Wave II | 2.68 | 2.19 | 3.18 | 2.90 | 2.53 | 3.27 |
| Wave III | 3.96 | 3.55 | 4.36 | 4.03 | 3.69 | 4.37 |
| Wave IV | 5.03 | 4.36 | 5.69 | 5.33 | 5.15 | 5.51 |
| Wave V | 5.92 | 5.57 | 6.27 | 5.81 | 5.42 | 6.21 |
| Interval I-III | 2.15 | 1.73 | 2.57 | 2.13 | 1.71 | 2.56 |
| Interval III-V | 1.96 | 1.61 | 2.31 | 1.78 | 1.41 | 2.15 |
| Interval I-V | 4.12 | 3.71 | 4.53 | 3.92 | 3.50 | 4.34 |
| Interaural difference V-V | 0.16 | 0 | 0.38 | 0.14 | 0 | 0.50 |
| Threshold | 10.5 | – | – | 7.5 | – | – |
|
σ: standard deviation. |
||||||
Conclusion
We suggest that the values found should be taken as a reference for normality when performing SLAPP studies, using stimulation rates of 45.1, 33.1, and 11.1 stimuli per second with click and CE-chirp stimuli in condensation, rarefaction, and alternating polarity, with the Interacoustics Eclipse equipment. This is because the ranges represent 95.45% of our sample population with normal hearing. These values can be compared when conducting studies with patients who have different audiological conditions that may fall outside these ranges, considering these values as “not normal.” A better separation of waves IV and V was also observed when using the click stimulus and a greater Wave V amplitude in recordings with CE-chirp; therefore, we recommend using the click stimulus for performing SLAPP in their neurological modality and the CE-chirp stimulus for performing SLAPP for auditory threshold search (Wave V).
It is suggested to complement the results with further studies, including a larger sample size with a population in different age ranges < 3 years, and conduct additional evaluations with the same parameters of our study in patients with different auditory pathologies to compare the results obtained from this study.
Of note, unlike the click stimulus, which produced inter-latency intervals with very similar values regardless of the stimulation rate and stimulus polarity, the CE-chirp showed great variability. Therefore, we do not suggest the use of the latter stimulus for neurological evaluation, only for the determination of electrophysiological auditory thresholds.
Acknowledgments
The authors would like to thank Dr. L. Reyes-Contreras for her support in conducting this study.
Funding
The authors declare that they have not received funding.
Conflicts of interest
The authors declare no conflicts of interest.
Ethical considerations
Protection of humans and animals. The authors declare that no experiments involving humans or animals were conducted for this research.
Confidentiality, informed consent, and ethical approval. The study does not involve patient personal data nor requires ethical approval. The SAGER guidelines do not apply.
Declaration on the use of artificial intelligence. The authors declare that no generative artificial intelligence was used in the writing of this manuscript.
