**3. Results**

Applying the inclusion and exclusion criteria, 54 ears of 27 selected children were studied, with a mean age of 16.7 months (SD = 5.7) and age range between 6 and 24 months, corresponding to 19 males (70.4%) and 8 females (29.6%).

**Table 3** presents the latencies of waves I, III, and V (the most constant) and I-V interlatencies obtained with insertion headphone and free-field stimuli, as well as the differences between them. No significant differences were observed in the interlatency values.

We found statistically significant differences in the latency values of waves I, III, and V, p < 0.001, and Rho values of 0.78, 0.49, and 0.63, respectively. In the assessment of agreement or concordance in the distribution of mean latencies, a significant difference (p < 0.001) was observed in the Wilcoxon test for the three main waves of the ABR.

The V-wave threshold was obtained at 20 dB HL in all ears studied.

**Table 4** represents the results of the thresholds obtained in the ASSR-MF recording with insertion headphone stimulus and in free field.


#### **Table 3.**

*Latencies of waves I, III and V and I-V interlatencies obtained with a stimulus with insertion headphones and free field, at 70 dB HL, as well as the differences between them, p-value and rho value in between them, p-value and rho value in Spearmen's contrast test.*


#### **Table 4.**

*Results of the thresholds obtained in the ASSR-MF recording with insert earphone stimulus and in free field and the differences between the two limits (dB HL).*

In the recordings with insert earphones, the 500 Hz recording was achieved in 50 of the 54 ears studied (7.41%), with responses obtained at all other frequencies. The average response stabilization time was 2.12 minutes.

In the recordings with free-field stimulus, the absence of response at 500 Hz was 22.22%, at 1000 Hz 12.96%, at 2000 Hz 5.55%, and at 4000 Hz 1.85%. The mean response stabilization time was 3.68 minutes which is an increase of 1.56 minutes over the insert phones.

### **4. Discussion**

The children in our study are aged between 6 and 24 months, some of them premature, so latency values may be variable. This is why we decided to apply this age range to minimize variations in latencies due to the immaturity and hypomyelination of the acoustic pathway, which maturity does not end until 12 months [18, 29].

The ABR and ASSR are usually recorded by means of earphones inserted inside the external auditory canal and using surface electrodes placed as described above. The recordings of both tests taken together will give us the hearing thresholds in intensity and frequency, which are necessary for a correct objective diagnosis of hearing loss in children.

The ABR recording is composed of a 5 to 7-wave trace, with the first five waves being the most important, called I, II, III, IV, and V and I, III, and V being the most constant [15, 16] waves that present fundamental characteristics of amplitude and latency [12, 15, 23]. The latencies generate interlatencies, time intervals between waves, the most important being interlatency I-III, III-V, and above all I-V [19, 23].

These waves disappear as the intensity of the stimulus decreases, with the V wave remaining constant, the last recording of which in intensity does not mark the threshold of the response. The average values that we have obtained in the children with normal criteria studied, using acoustic stimuli through insertion headphones, are similar to those observed in the literature [17, 20, 21, 29, 32].

Likewise, the ABR study was carried out in a similar way to that employed by other authors, using the same type of stimulus (click) [33], with a cadence of 44 stimuli/sec, lower than the critical rate of 50 st/sec [34], 2000 stimuli in monaural stimulation and with contralateral masking [35].

After stimulation and stable ASSR response, the software of the device applies the Fast Fourier Transform (FFT) algorithm, which is the recording of the electroencephalographic trace corresponding to the modulated frequency of the presented tone, and calculates the estimated audiometry at frequencies of 500, 1000, 2000, and 4000 Hz, so the normality criteria do not require detailed interpretation [10, 36] and therefore do not require the patient's cooperation or the explorer's intervention [37].

Although it is generally accepted that a stable ASSR response should not occur beyond 8–10 minutes, in our daily practice, and depending on the individual patient, we have come to accept response stabilization times of 12–14 minutes. However, in our study, we have selected cases with response stabilization of not more than 6 minutes.

We agree with the various authors that the most difficult response to record in the ASSR is the frequency of 500 Hz, with all other frequencies being fairly constant and with no differences between the two ears [10, 36, 38].

In the ASSR test, the child is in the same environment, with the same electrodes and their location on the skin as in the ABR test and with stimulation through insert

#### *Precocious Auditory Evoked Potential Recording with Free-Field Stimulus DOI: http://dx.doi.org/10.5772/intechopen.102569*

earphones, the only difference being acoustic stimulation with clicks in the case of ABR and CE-Chirp in the case of ASSR. This difference does not affect the attainment of hearing thresholds, although the CE-Chirp follows a response with higher amplitude and curve quality [39].

The threshold of ASSR responses compared to hearing screening methods, such as tone audiometry or behavioral audiometry, in children has been studied by numerous authors, indicating, with minor adjustments for correction, the similarity of hearing thresholds [37, 38, 40, 41]. In our daily practice, we have found this similarity between threshold levels in older children undergoing tonal audiometry and ABR/ ASSR-MF under sedation [42].

As we have already mentioned, to avoid bias, the ABR/ASSR-MF tests of the selected children, in free field (loudspeaker 70 cm away from the ear to be tested), were carried out in the same clinical act, in the same environment (cabin with acoustic attenuation), by the same explorer as in the tests with insertion earphones, first performing the stimulus with insertion earphones and then, if the child was selected, the stimulus in free field.

The performance of the tests with stimulus in the free field required the modification of the software and hardware of the equipment and its calibration, and we are not aware of any standard or correction coefficient for the performance of these tests in the free field, adjusting ourselves to the calibration performed by the company Audiología, S.L. [43].

The differences in the mean evoked latency of the ABR recording in the tests performed with insertion earphones and those in free field with 70 dB HL stimulation are presented in **Table 3**. We can see that the mean difference in the latency of the main waves (I, III, and V) corresponds to the delay caused by the distance at which the sound source is located (70 cm) in the free-field stimulation. In the conditions in which the test was performed, with an ambient temperature of 22°C, a humidity of 50%, and the cabin being located at sea level (Cartagena, Spain), the speed at which sound is transmitted in the air is 244.4 m/sec [9]. At this distance of 70 cm, the average delay of the arrival of the stimulus at the eardrum (receiver) from the loudspeaker (transmitter) is 2.032 ms, a delay that resembles the average difference of the latencies of the three waves I, III, and V of the ABR response tracing, taking into account possible variations of a few centimeters when placing the loudspeaker in each of the tests or due to the movements of the child's head during the exploration.

Likewise, the interlatencies were similar in both tests, with no significant differences between them, especially in the most important interlatency I-V, interlatencies not affected by the distance of the sound source and which shows the response of the different levels of neural generators at the level of the brainstem.

The ASSR-MF thresholds obtained with a stimulus with insert earphones in our daily practice and the selected cases are similar to those found in the literature [44–47], accepting normal values close to 30 dB HL, although there is a slight decrease in those obtained in free field in relation to those obtained with insert earphones, representing a difference of 10.37 dB HL. In our study, the mean response stabilization time was less than 6 minutes, with the most inconsistent response at 500 Hz, in both different stimulations, and the most constant responses at 1000, 2000, and 4000 Hz.

In the literature, we have found very few studies in which free-field stimulation has been used to obtain ABR and ASSR.

Shemesh et al. carried out a study with 20 patients aged between 24 and 60 years, 10 of whom underwent ASSR recording with insertion headphones, and another 10

patients with hearing aids underwent ASSR recording with free-field stimulus, comparing the thresholds. In this work, there are hardly any indications of the calibration of the equipment, although it uses a booth with acoustic attenuation according to the ISO392-2, 1994 standard. He also recorded the ASSR with and without hearing aids, finding, logically, significant differences in the thresholds with and without hearing aids, but not, on the other hand, between the thresholds with audiometry and ASSR without hearing aids. A control group of 21–24-year-olds with normal hearing recorded audiometric thresholds below 20 dB HL at frequencies between 250 and 8000 Hz and thresholds below 20 dB in ASSR at frequencies of 500, 1000, 2000, and 4000 Hz. They conclude the benefits of ASSR testing on hearing thresholds for objective assessment of the benefit of hearing aids and that it may be determinant in young uncooperative individuals [48].

Arias et al. conducted a study with 14 patients aged 2–14 years with cochlear implants to obtain ASSR-MF thresholds and behavioral audiometry. They used an Audix V, model NDOO1A USB from Neuronic, S, A., calibrated with a sound level meter model 2260 and a microphone type 4144 (Brüel & Kjaer) ensuring that the acoustic energy measured in dB SPL corresponded to its value in dB HL, but no further details are given. They did not record ABR and compared the results of ASSR thresholds with free-field stimuli with behavioral audiometry. However, the study does not give data on distance from the sound source and does not show normality thresholds as these are patients with cochlear implants and therefore profound hearing loss. They conclude that ASSR-MF recording with the free-field stimulus is useful for assessing free-field hearing thresholds in cochlear implant patients [49].

Though clinically useful, the results obtained in these studies are not comparable with those obtained in our studies since none of them examines thresholds of normality in children.

Given that there are no standards or correction coefficients for the ABR and ASSR-MF tests with free-air stimuli and the absence of sufficient literature on studies with free-field stimuli for recording early auditory evoked potentials, we consider our results as a new possibility as a determination of criteria for normality in children in whom stimulation through earphone insertion in the external auditory canal is impossible, such as children with hearing aids or implants and in those who do not cooperate in liminal or behavioral audiometry tests, such as children with down syndrome and autistic spectrum disorders.
