Influence of BMI and head circumference on variables of auditory evoked potential in young healthy male human participants

Context Age, hearing loss, sex, BMI, and head size are very important parameters that influence auditory evoked potential (AEP) variables. Although the correlation of BMI and head size with AEP parameters has been studied recently, there is no common consensus on it. A few studies show a positive correlation, whereas others report a weaker correlation. Further, normative values of the V/I ratio also show a wide range of normative values from different studies. Aims and objectives This study aimed to evaluate the association of AEP variables with BMI and head size in healthy young male participants and to collect normative data for the V/I amplitude ratio. Materials and methods This is a cross-sectional study in which 30 young healthy male participants with age range 19-25 years and BMI range 15-26 kg/m 2 underwent AEP testing. All were screened for inclusion and exclusion criteria established for the study. Informed consent was obtained and the data obtained were stored in an Excel sheet. A simple correlation regression coefficient was obtained between physical parameters and different AEP variables. Results We observed a strong positive correlation between head size and V wave latency (r = 0.5) and a weaker positive correlation between head size and AEP interpeak latencies (IPLs) I-V and III-V (r = 0.3). No correlation was observed between BMI and AEP variables. The V/I amplitude ratio was 0.98 ± 0.68 and 0.93 ± 0.7 for the left and the right ear, respectively. Conclusion We concluded that BMI had no influence, and head size showed an association with AEP outcome, especially V wave latency and the AEP-IPL difference. The V/I ratio needs to be examined further in studies with larger sample sizes as values were different from those of previous studies.


Introduction
Auditory evoked potentials (AEP) are recorded from the scalp as small voltage potentials after passing auditory stimuli through a headphone [1]. Th ere are mainly fi ve AEP waveforms. Th ese waves represent the neuroelectrical activity that is generated by the neural generators in the auditory pathway between the cochlea and the brainstem. Among the various factors that aff ect AEP, age and hearing loss with defi nitive prolongation of absolute peak AEP latencies are well established [2]. Th e eff ect of sex on brainstem auditory evoked response (BAER) absolute peak latencies (APL) and interpeak latency (IPL) diff erence showed nonsignifi cant diff erences between 3 and 13 years, wheras between 16 and 45 years, sex diff erences were evident. Th ese diff erences can be attributed to hormonal factors, temperature variations, and anthropometric component (head size) [3,4]. In a few studies, the eff ects of BMI and head size were also studied and it was observed that both aff ected the AEP latencies and IPL diff erences independently. Head size is an established variable that independently aff ects the outcome of AEP [5,6]. Although the eff ect of these variables on AEP has been studied extensively outside India, there remains a paucity of data in the Indian context. Further, most studies have not included an important AEP feature, the waveform V/I amplitude ratio, which indicates the involvement of the auditory pathway in hearing loss, whether central or peripheral. In one such study, it was oberved that both IV-V/I and IV-V/III amplitude ratios were independent In uence of BMI and head circumference on variables of auditory evoked potential in young healthy male human participants Balaji W. Ghugare, Sangeeta Jain, Dinesh J. Parmar, Mukesh R. Dinkar, Ravindra Ninama

Context
Age, hearing loss, sex, BMI, and head size are very important parameters that in uence auditory evoked potential ( AEP) variables. Although the correlation of BMI and head size with AEP parameters has been studied recently, there is no common consensus on it. A few studies show a positive correlation, whereas others report a weaker correlation. Further, normative values of the V/I ratio also show a wide range of normative values from different studies.

Aims and objectives
This study aimed to evaluate the association of AEP variables with BMI and head size in healthy young male participants and to collect normative data for the V/I amplitude ratio.

Materials and methods
This is a cross-sectional study in which 30 young healthy male participants with age range 19-25 years and BMI range 15-26 kg/m 2 underwent AEP testing. All were screened for inclusion and exclusion criteria established for the study. Informed consent was obtained and the data obtained were stored in an Excel sheet. A simple correlation regression coef cient was obtained between physical parameters and different AEP variables.

Results
We observed a strong positive correlation between head size and V wave latency (r = 0.5) and a weaker positive correlation between head size and AEP interpeak latencies (IPLs) I-V and III-V (r = 0.3). No correlation was observed between BMI and AEP variables. The V/I amplitude ratio was 0.98 ± 0.68 and 0.93 ± 0.7 for the left and the right ear, respectively.

Conclusion
We concluded that BMI had no in uence, and head size showed an association with AEP outcome, especially V wave latency and the AEP -IPL difference. The V/I ratio needs to be of sex, but the IV-V/I ratio increased signifi cantly with age [7]. Th e eff ect of BMI and head size on the V/I ratio has not been studied so far. Against this background, the present study aims to assess BAEP features including the V/I ratio and to study the eff ects of BMI and head circumference on these features.

Materials and methods
Th e present study was carried out in the Clinical Neurophysiology Laboratory at a medical teaching institute in central Gujarat during May-June 2013. Th e study was approved by the institutional human ethics committee (IHEC). Written informed consent was obtained in vernacular language structured format from all the participants and the study was carried out in accordance with the World Medical Association Declaration of Helsinki.

Selection of participants
Participants were mostly young staff and students from the medical hospital and college. Th irty male participants in the age range 19-25 years were selected after a thorough clinical and otological examination for inclusion and exclusion criteria. Individuals prone to occupational or any other causes of hearing loss, with H/O hypertension, diabetes, hypothyroidism, musculoskeletal disorders, neuromuscular disorders, developmental disorders, facial asymmetry, ototoxic drugs intake, etc. were excluded from the study.

Recording of BERA
All participants were subjected to BERA testing according to standard techniques on an RMS Portable Aleron EP-Electromyograph machine manufactured by RMS Recorders and Medicare System (Chandigarh, India). Recording of BERA was carried out in a quiet and dimly lit room with the participant in the supine position. Participants were briefl y informed about the procedure. Restless, irritable, and apprehensive participants were allowed to relax for 5-10 min before testing. Surface electrodes were placed at the vertex (CZ), both ear lobes (Ai and Ac), and the forehead (ground). Monaural auditory stimulus consisting of rarefaction clicks of 100 s 2 pulse were delivered through an electrically shielded earphone at a rate of 11.1/s. Th e contralateral ear was masked with pure white noise 30 dB below that of the BERA stimulus. A band pass of 10-3000 Hz was used to fi lter out undesirable frequencies in the surrounding. Responses to 2000 click presentations were live averaged to obtain a single BERA waveform pattern. Waveforms were obtained at 25, 40, 60, and 90 dB in each ear. Data of waveforms obtained at 90 dB were used for analysis.
APL of waves I, III, V, IPL of I-III, III-V, I-V wave forms, and V/I ratio were considered for assessment. Figure 1 shows the BERA waveforms with diff erent parameters recorded in a normal participant.

Statistical analysis
Th e Graph Pad Prism statistical tool was used for analysis (GraphPad Software, Inc., California, USA). Range, reference limit, and mean ± SD for all BERA waveforms latencies and latency diff erences (ms) and V/I ratio (%) were obtained. Simple correlation regression (r) was performed to determine the eff ect of BMI and head circumference with diff erent AEP features.

Results
In the present cross-sectional study, 30 male participants age range 19-25 years, BMI range 15-26 kg/m 2 , and head circumference range 50-58 cm underwent BAEP electodiagnostic study. Th eir mean and SD values are shown in Table 1. Table 2 shows diff erent variables of BAEP, its range, reference limits, and mean values with SD. No sideto-side statistically signifi cant diff erence was observed among any of the variables under study (P > 0.05). Figure 2 shows the correlation of Vth wave latency with head circumference. Table 3 shows the correlation  coeffi cient values of diff erent parameters with BMI and head circumference. A signifi cant positive correlation was observed between Vth wave latency and head circumference. No correlation was observed between diff erent variables and BMI.

Discussion
Th e present study observed the eff ect of head circumference and BMI on diff erent parameters of AEP in young healthy male participants. As we had already excluded sex and age factors from the study, it clearly accounts for the eff ect of only these two variables on AEP. We observed that of seven AEP parameters under study , APL-V waveform, IPL diff erence I−V, and III−V showed a positive correlation with head circumference. V waveform latency showed a very strong correlation. Th e other IPL diff erence showed a weaker correlation with head size. Th ere was no correlation between any of the AEP parameters and BMI.
A positive correlation between head size and AEP latencies, IPL diff erences reinforces the fact that head circumference refl ects brain size, which aff ects the conduction time of the neural pathway, and hence should be considered an independent variable while interpreting AEP results [8,9]. It is well established that adul t AEP IPL latency has a strong correlation with brain-stem size and a weaker correlation with head size [10]. In a study with a large clinical sample, head size and IPL I-V were found to be very poorly correlated and it was concluded that even IPL has no clinical relevance [11].
We also explored the V/A amplitude ratio, which is an essential measure for delineation of central and peripheral auditory pathway involvement. Normative values of the ratio obtained from the present cohort (0.98 ± 0.68 Lt ear and 0.93 ± 0.7 Rt ear) were diff erent from those of previous studies by Gathe et al. [12] (2.03 ± 0.76 and 2.15 ± 0.81) and Th akur et al. [13] (3.3 ± 5.83 and 1.87 ± 0.95). Interstudy diff erences observed in the values may be attributed to variable cohort and sample sizes in diff erent study populations from diff erent geographical areas. Further, the present study found a lower and upper limit of normal of 0.3, 0.23 (Lt, Rt) and 1.66, 1.63 (Lt, Rt), respectively. Th e literature suggests that if the amplitude ratio is below 0.5, it may be consistent with central impairment and if it is above 3.0, it may be consistent with peripheral auditory pathway impairment [14]. Th us, with wide diff erences in the normative values obtained in each study and its importance in delineating the central versus the peripheral auditory pathway, further research in this domain with a large sample size is essential. Correlation between BERA V wave latency and head circumfere nce.

Figure 2
BMI-related observations in AEP are in accordance with a previous study by Solanki et al. [6]. In a comparison study of AEP parameters between obese participants (>30 BMI) and controls (<30 BMI) among healthy young adults, signifi cant diff erences were observed [15]. Th ese fi ndings were not corroborative with observations in present study. Both studies have limitations of smaller sample size and hence results can not be extrapolated to generalized population. Hence further studies with larger sample size are required to explore association of BMI with AEP parameters. Although, association of BMI with peripheral neuropathies have been well established but it's association with cranial neuropathies remains unanswered.

Conclusion
Th e small sample size and the small BMI range remain the major limitations of this study. It can be concluded that head size remains an independent variable aff ecting AEP parameters. Although the V/I amplitude ratios were within the normal range, a study with a larger sample size should be carried out for better interpretation.

Financial support and sponsorship
Nil.

Con icts of interest
Th ere are no confl icts of interest.