VEP were performed by using Data Acquisition and Analysis System, Medicaid Systems, Chandigarh, India in a sample of 58 different drug abusers and 30 controls. All the subjects were males in the age group of 15-45 yrs. The study was done at Guru Gobind Singh Medical College, Punjab during the period from June 2012 to March 2013. The patients were taken from Drug De-addiction Centre of this institute. Prior informed consent was obtained from each individual and the study was approved by the Institutional Ethics Committee. A self designed questionnaire (Proforma) was administered to all the subjects.

The substance abusers which were recruited for this study was alcoholics, opiate addicts e.g. heroin, morphine, tobacco chewers/smokers, pharmaceutical drugs addicts like using cough syrups, pain killers, anti-emetics, etc. The subjects excluded from the study were the persons suffering from any type of post traumatic coma, neurological diseases (multiple sclerosis, optic neuritis, optic neuropathy etc.,), other psychiatric disorders, visual defects, etc. The patient was explained the test to ensure full cooperation. The usual glasses were allowed to put on during the test. The subjects were asked to discontinue any miotic or mydriatic drugs 12 hours before the test. Instructions were given to the subject not to sleep during the procedure and to fix the gaze at the centre of screen.

Equipment set up for VEP study was done as recommended by International Federation of Clinical Neurophysiology (IFCN) committee [7]. Two channels were used:

Channel 1: Oz – Fp_z Channel 2 : O_Z - A₁A₂ (linked ear)

The subjects were allowed to sit comfortably in a fully relaxed state. One eye was tested at a time. The skin at the point of placement of the electrodes was cleared with spirit. Using electrode paste or conducting jelly, the recording electrode was placed at occiput (o_z), the reference electrode at Fp_z or 12cm above the nasion. The ground electrode was placed at the vertex (C_z). The visual stimulus was delivered by LED goggles using red flash of light. To record flash visual evoked potentials, the low cut filter was set at 2 Hz and high cut filter at 200 Hz. Sweep speed was 50ms/div and sensitivity was set at 2μ v/div. About 200 epochs were averaged. The electrode impedance was kept below 5 kohms.

The peak latency and peak to peak amplitudes of different waveforms (N₇₅, P₁₀₀ and N₁₄₅) were measured and the data was compared between different drug abusers and controls and also in between the different groups of drug abusers and analyzed by applying unpaired t-test and one-way Anova test using the SPSS system (version 16.0 SPSS Inc., Chicago, IL, USA). The p-value <0.05 was considered to be statistically significant.

[Table/Fig-1] shows the comparison of the duration of different waveforms of VEP (N₇₅, P₁₀₀, N₁₄₅) of right eye between the drug abusers and controls. It was found that mean ± SD of latency of waveform N₇₅ was 91.56 ± 15.63 among drug abusers while 70.76 ± 9.82 in controls and the difference between the two groups was found to be highly significant statistically (p<0.001). Also, mean ± SD of latency of waveform P₁₀₀ was 129.45 ± 14.54 in drug abusers and 108.37 ± 25.46 among controls and the difference between the two groups was found to be highly significant statistically (p<0.001). Mean ± SD of waveform N₁₄₅ was 165.13 ± 17.22 in drug abusers and 161.56 ± 37.55 in controls and the difference between the two groups was found to be statistically insignificant (p>0.05).

[Table/Fig-2] shows the comparison of the duration of different waveforms of VEP (N₇₅, P₁₀₀, N₁₄₅) of left eye between the drug abusers and controls. It was found that mean ± SD of N₇₅ was 95.13 ± 22.97 in test subjects while 67.43 ± 7.80 in controls and the difference between the two groups was found to be highly significant statistically (p<0.001). Mean ± SD of P₁₀₀ was 125.81 ± 16.95 among tests and 109.20 ± 15.33 among controls and the difference between the two groups was found to be highly significant statistically (p<0.001). Lastly mean ± SD of N₁₄₅ was 160.60 ± 20.58 in abusers while 156.96 ± 43.92 in controls and the difference between the two groups was found to be statistically insignificant (p>0.05).

[Table/Fig-3] shows the comparison of amplitudes of P₁₀₀ of VEP of both the eyes. In right eye, mean ± SD of amplitude was 5.87±3.97 in abusers while 7.72±2.05 among controls and the difference between the two groups was found to be significant statistically (p<0.05). And in left eye, it was 4.86±2.52 in test subjects and 9.68±5.01 in controls and the difference between the two groups was found to be highly significant statistically (p<0.001).

In [Table/Fig-4] the latencies of different waveforms and amplitude were compared between different groups of drug abusers by applying one-way Anova technique and the difference in amplitude and latencies was found to be insignificant statistically (p>0.05) in all the waveforms of VEP of right eye.

In [Table/Fig-5], the latencies of different waveforms and amplitude were compared between different groups of drug abusers by applying one-way Anova technique and the difference in amplitude and latencies was found to be insignificant statistically (p>0.05) in all the waveforms of VEP of left eye.

Drug abusing is a big upcoming health problem prevalent in all societies. Unfortunately, across the globe and throughout time, drug abuse has manifested itself in one form or another, so it appears that drug-abusing affects both the psychological and physical well being of a person. The negative effects from drug abuse can have immediate and long-term consequences [8].

The recording of potential differences from scalp on giving visual stimuli is known as visual evoked potential and it shows a resultant response of cortical as well as subcortical areas. A normal VEP indicates the intactness of visual system. VEP is primarily a reflection of activity originating in the central visual field which is relayed to the surface of occipital lobe [9]. The P₁₀₀ waveform of VEP is generated in primary visual area and visual association areas (area 18 and 19) due to increased metabolism in these areas on giving stimulation [10].

So it is in this context that visual evoked potential study was done on a group of different drug abusers as VEP is considered to be a very useful technique for assessing the neurological damage done in optic nerve fibers. It is used to check the functional integrity of visual pathway.

This study compared the amplitudes and latencies of different waveforms (N ₇₅, P₁₀₀, N₁₄₅) of VEP in a group of different drug abusers (n=58) with controls (n=30) and also in between the different groups of drug abusers.

In this study, the latencies of waves N₄₅ and P₁₀₀ of both the eyes was found to be prolonged in all the groups of drug abusers as compared to controls. It could be due to delay in conduction velocity associated with demyelination of optic nerve pathways because of chronic intoxication by different drugs. The latency of P₁₀₀ depends upon the surviving fastest conducting fibers [11].

In this study, the amplitude of wave P₁₀₀ of both eyes was also found decreased in different drug abusers. It could be due to axonal loss leading to conduction block of fibers in optic nerve pathway because of chronic and prolonged use of different drugs. But no difference was found in the amplitude and latencies of different waveforms of VEP of both the eyes in between the different groups of drug abusers. So it could be due to that all the drugs are affecting the visual pathway in the same way i.e. by damaging the myelin sheath of the nerve fibres, which is responsible for the fastest conduction in myelinated nerve fibres, leading to the prolongation of the duration of transmission of impulse across the optic nerve pathway. Similar findings were also reported by who found progressive bilateral and symmetric visual loss associated with optic neuropathy in alcoholics [12].

Kriss et al., also found gross VEP abnormalities on central field stimulation in 23 patients with tobacco-alcohol amblyopia [13]. Gross VEP abnormalities is in the form of loss of major positive components and its replacement by a negative wave N₁₀₀ derived from paramacular areas of the field [14]. Chronic alcohol and tobacco smoking intake have been extensively associated to P₁₀₀ amplitude reduction and prolonged latency [6,13,15–17]. Opioids (morphine, fentanyl) stimulates the retino-geniculate-cortex pathway and the thalamus-cortical circuit through the opioid receptors. Hence VEP is considered to be a useful tool for studying the side effects of different drugs on the visual system [18].

Decreased amplitude and increased latency of P₁₀₀ component of averaged evoked potential has also been reported in opiates as compared to normal subjects which can be interpreted as a consequence of brain stem transmission disorders resulting from chronic intoxication [4,5,18,19]. Pharmacological drugs abusers (cough syrups, pain killers, antiemetics) also found increased latency of wave P₁₀₀ [20]. Users showed similar results were shown by researches done on other drugs which includes amphetamine slowed reaction time and reduced early processing. Similarly, inhalant (volatile solvent) and N₂O abusers showed abnormal visual and auditory evoked potentials. N₂O damages the nervous system of chronic abusers. Evoked potentials showed abnormal VEPs with prolonged peak latencies of P₁₀₀ [19–21]. Besides VEP, other evoked potential studies on a group of different drug abusers also showed the similar findings [22].

Therefore, our findings corroborate with the findings of above authors, thereby reiterating the fact that any type of drug abuse could damage the optic nerve pathways leading to long term effects on visual pathway.

The limitations of the study is small sample size and also the effect of duration of exposure to different drugs on VEP can be studied between different drug abusers and be the area of future research.

In this study, the latencies of waves N₄₅ and P₁₀₀ of VEP was found increased and amplitude of waveform P₁₀₀ was found to be decreased in both the eyes among all the groups of drug abusers because of brain stem transmission disorder in visual pathway due to chronic intoxication by different drugs. It could be due to defective myelination of optic nerve fibers and axonal loss leading to conduction block in visual pathway. It can therefore be assumed that VEP testing is a useful tool for evaluating effects of drugs on the visual system so that timely intervention and counseling can be done to prevent the adverse effects on the functional intactness and integrity of the visual pathway and also to motivate the drug abusers to go for de-addiction to prevent the permanent visual loss and to improve the quality of health care.