|Year : 2013 | Volume
| Issue : 2 | Page : 79-81
Visual evoked potential: Head size, sex, and BMI
Jayesh D Solanki, Naisargi H Naisargi, Hemant B Mehta, Chinmay J Shah
Department of Physiology, Government Medical College, Bhavnagar, Gujarat, India
|Date of Web Publication||10-Jan-2014|
Jayesh D Solanki
F1, Shivganga Appartments, Plot no 164, Bhayani Ni Waadi, Opp. Bawaliya Hanuman Temple, Gadhechi Wadlaa Road, Bhavnagar - 364 001, Gujarat
Source of Support: None, Conflict of Interest: None
Background: Visual evoked potential (VEP) is a useful noninvasive neurological diagnostic tool affected by certain physical and physiological parameters, age being the major of all. However, only few baseline studies have been conducted in India and none in Gujarat. Materials and Methods: Monocular pattern reversal VEP (PRVEP) was carried out using a standard protocol in medical students of same age group. Latencies of various waveforms were calculated and effects of gender, body mass index (BMI), and head size were studied. Results: Observations revealed normative VEP latencies in line with other studies. Difference observed for only N70 wave, head size were statistical significant and that for BMI, gender, and between eyes were not. Conclusion: Normative VEP data of age, BMI-matched healthy medical students showed no ethnic variation; disproved gender influence on VEP latencies and slight sex difference observed is due to head size.
Keywords: Body mass index, latencies, normative, visual evoked potential, waveforms
|How to cite this article:|
Solanki JD, Naisargi NH, Mehta HB, Shah CJ. Visual evoked potential: Head size, sex, and BMI. Sudanese J Ophthalmol 2013;5:79-81
| Introduction|| |
Evoked potentials are ancillary neurodiagnostic tools applied to many fields.  They permit conduction velocity assessment of sensory impulses in central  and peripheral nervous system.  It may be useful when EEG is useless  and may diagnose subclinical conditions.  Visual evoked potential (VEP) is affected by certain physical parameters ,,, and physiological parameters. ,,,,,, Very few studies have been done in India and none in Gujarat. The present study tried to evaluate VEP of medical students to find effect of gender and its relationship with BMI, head circumference and inter-eye variation keeping same age, physical factors with normal vision, and uniformity of stimulus.
| Materials and Methods|| |
After taking permission from Institutional Review Board of college and written consent of subjects, a cross-sectional study was carried out at Electrophysiology Lab, Department of Physiology of our college from March 2009 to April 2009. 48 medical students (24 males, 24 females) were selected within age group of 16-18 years, normal pupillary size, field of vision, and fundus. The subjects of the present study coming from various parts of the state formed a fairly representative sample of this region.
Pattern Reversal VEP
Visual stimulus was provided by transient pattern reversal method in the form of a black and white pattern on a video screen with a central red spot. Methodology used was standardized as recommended by the International Federation of Clinical Neurophysiology (IFCN) committee  and International Society for Clinical Electrophysiology of Vision (ISCEV)  using 10-20 International system of EEG placement and Fz-Cz-Oz method. Size of check board (8" × 8"), luminance, Contrast level, and rate of pattern reversal (1.7 Hz) were kept uniformly constant among all subjects using instrument RMS EMG EP MARK II. Latencies of wave N70, P100, and N155 and interocular difference were calculated.
The data was recorded on a predesigned validated proforma and was then transferred onto an Excel spreadsheet. Variables were assessed for approximate normality and then were summarized by mean, standard deviation. Sigmastat 2.0 statistical software was used for data analysis and ANNOVA test was used to check the significance with P-value of less than 0.05 being considered statistically significant.
| Results|| |
The present study tested VEP latencies of age-matched medical students divided into male and female groups.
A comparison of anthropometric data suggested a significant difference in head circumference, height, and weight but not in BMI and age [Table 1].
There is a slight difference between latencies of various waveforms of two groups but statistically insignificant [Table 2].
Table 2: Latencies (ms) of PRVEP waveforms of male (n = 24) and female (n = 24) groups
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Statistically insignificant difference was observed between right and left eye in waveform latencies of both groups as shown in [Table 3].
Table 3: Interocular difference of P100 latencies (ms) of male (n = 24) and female (n = 24) groups
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Gender difference was further evaluated by dividing both groups into two subgroups having low BMI (<18.5) and normal BMI (>18.5) but the effect of gender difference on P100 latency proved statistically insignificant as evident from [Table 4].
Table 4: Effect of BMI on P100 latencies (ms) of male (n = 24) and female (n = 24) groups
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| Discussion|| |
VEP is being used by neurophysiologist, neurologist, and neurosurgeons as visual abnormalities may be a presenting sign for many neurological disorders.  However, standardization of recording parameters is must for its optimum use and validity.  Age being a proven factor ,, affecting VEP, in the present study focus was to highlight the effect of gender, ethnic variation, interocular difference, BMI, and head size.
We found that latencies for N70, P100, and N155 were within normal limits and in line with other studies done elsewhere ,,,, proving no ethnic variation of Gujarat region as shown in table below.
P100 Latencies of VEP in Various Studies
The present study revealed shorter latencies in females for P100 and N155 waves than males which are well supported by other such studies. , This can be attributed to (i) early cerebral maturation in female children as evidenced by increased alpha frequency and greater photic sensitivity in females than males;  (ii) 2-5 ms faster reaction time in females than male;  (iii) short axial eye length in females as compared to males;  (iv)large occipitofrontal circumference in females than males;  (v) involvement of some hormonal factors; , (vi) comparatively smaller brain size in females.  However, N70, in contrast, has longer latency in females than males which was documented only by Allison et al. In our study, the age group was 16-18 years. As proven before, most of the age-related changes in latencies subside at the age of 19 years , and the presence of contrasting result for N70 suggests that this change may be last to occur among all three waves of VEP.
However, this sex difference was not statistically significant. Age and BMI were nearly same hence the smaller difference observed can be due to comparatively smaller head size in males. Any pathway in the brain will vary its length as cube root of brain volume. This gender difference was not significant even when the test groups were divided into low BMI and normal BMI individuals.
A small statistically insignificant interocular difference was observed between two sexes that is due to either lateralization of central nervous system  or due to neuroanatomical asymmetry.  Significant inter eye latency difference rather becomes a proof of some monocular disease. 
The present study disproves sex as one of the possible systemic sources of variance in VEP latencies and head circumference was proven the factor responsible for it. VEP latencies, thus seems to be one of the few parameters that is not much affected by sex. For further support, a study with bigger sample size is required.
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[Table 1], [Table 2], [Table 3], [Table 4]