VEP Test in Ranchi — Visual Pathway Assessment
Advanced electrophysiological testing of the optic nerve and visual brain pathways at Neurovision Clinic, Ranchi.
What is VEP — Visual Evoked Potential?
A Visual Evoked Potential (VEP) test is a non-invasive electrophysiological procedure that measures the electrical activity generated in the visual cortex of the brain in response to visual stimuli. During the test, the patient sits in front of a screen displaying a checkerboard pattern that alternates at a fixed frequency, while small recording electrodes placed on the scalp over the occipital region detect the brain's response. The time it takes for the visual signal to travel from the retina, through the optic nerve, optic chiasm, and optic tract, to reach the visual cortex is recorded as the P100 latency — the key measurement in standard pattern-reversal VEP.
A delayed P100 latency indicates demyelination or conduction block somewhere along the visual pathway, most classically in optic neuritis. Though the test involves visual stimulation, it is fundamentally a neurophysiological investigation of brain function rather than an eye examination. At Neurovision Clinic in Ranchi, Dr.
Yuvraj Lahre (DM Neurology, AIIMS, Gold Medalist) interprets all VEP tracings, correlating the electrophysiological findings with the patient's clinical presentation and neuroimaging results. The test takes approximately 30 to 45 minutes, is entirely painless, and does not require any injections or radiation exposure.
Why is VEP — Visual Evoked Potential Done?
- •To confirm the diagnosis of optic neuritis when a patient presents with acute or subacute painful vision loss, especially in young adults where multiple sclerosis is a concern.
- •To detect subclinical lesions in the optic pathway in patients with known or suspected multiple sclerosis, as VEP can reveal demyelination even before symptoms or MRI changes appear in the optic nerves.
- •To evaluate unexplained visual loss when ophthalmological examination and retinal imaging (OCT, fundoscopy) are normal, helping distinguish functional visual loss from organic pathology.
- •To monitor disease progression or treatment response in neuromyelitis optica spectrum disorder (NMOSD) and MOG antibody-associated disease, where serial VEPs track optic nerve function over time.
- •To assess the integrity of the visual pathway in compressive lesions such as pituitary adenomas, craniopharyngiomas, or optic pathway gliomas.
- •To aid in the diagnosis of paediatric visual pathway disorders, including amblyopia, cortical visual impairment, and optic nerve hypoplasia, where reliable visual acuity testing may be difficult in young children.
- •To provide objective evidence of optic nerve dysfunction in medico-legal and occupational health assessments where visual function is disputed.
How VEP — Visual Evoked Potential is Performed
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Scalp Preparation and Electrode Placement
The skin over the occipital scalp (back of the head) is gently cleaned with a mildly abrasive gel to reduce electrical impedance. Three to five small recording electrodes are attached with conductive paste at standardised positions according to the International 10-20 system — typically at Oz (midline occipital), with reference and ground electrodes at the vertex and forehead.
- 2
Visual Stimulus Setup and Refraction
You are seated one metre in front of a monitor that displays a black-and-white checkerboard pattern. The pattern reverses (black squares become white and vice versa) at a rate of approximately two reversals per second. If you wear corrective lenses, you keep them on. Each eye is tested independently, with the non-tested eye covered with an opaque patch.
- 3
Signal Acquisition and Averaging
For each eye, the reversing checkerboard is presented for 200 or more trials. The EEG machine records the brain's electrical response after each reversal and averages the signals to extract the VEP waveform from background EEG noise. This averaging process yields a clean trace where the P100 wave — a positive deflection occurring approximately 100 milliseconds after stimulation — is clearly identifiable.
- 4
Latency and Amplitude Analysis
The technician measures the P100 latency (in milliseconds) and the N75-P100 amplitude (in microvolts) for each eye. These values are compared against laboratory normative data adjusted for age, sex, and visual acuity. A delayed P100 latency exceeding the normal range or a significant interocular latency difference suggests demyelinating pathology.
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Neurologist Interpretation and Discussion
Dr. Yuvraj Lahre reviews the raw waveforms, latency values, and amplitude data. He integrates the VEP findings with your clinical history, neurological examination, and any available MRI scans of the brain and orbits. He then explains the results to you in plain language, outlines the diagnostic implications, and discusses the next steps — which may include further testing, referral to an ophthalmologist when combined eye disease is suspected, or initiation of treatment.
How to Prepare
- •Wash your hair the night before or the morning of the test and avoid applying hair oil, gel, spray, conditioner, or any styling product. Clean, product-free hair ensures good electrode contact and reliable signal quality.
- •Bring your prescription glasses or contact lenses if you use them. The test requires optimal visual acuity, and corrective lenses should be worn during the procedure.
- •Get a good night's sleep before the test. Fatigue and drowsiness can increase EEG background noise and make VEP waveforms harder to interpret.
- •Inform the clinic if you have dense cataracts, significant corneal opacities, or nystagmus, as these can affect the reliability of pattern VEP and may require the use of flash VEP as an alternative.
- •Eat your usual meals and take your regular medications. There is no need to fast, and no sedation is required. The test is completely painless with no after-effects.
- •If the patient is a young child, light sleep or natural drowsiness may actually help reduce muscle artifact, so scheduling the test around nap time can be beneficial. Let the clinic know in advance so appropriate accommodations can be made.
Related Conditions
VEP — Visual Evoked Potential helps diagnose and monitor these conditions. Explore our condition pages for more detailed information about each.
Frequently Asked Questions
What is the purpose of a VEP test, and who should get one?
A Visual Evoked Potential test measures the functional integrity of the entire visual pathway from the retina to the occipital cortex. It is recommended for patients with unexplained vision loss, suspected optic neuritis, or possible multiple sclerosis. Dr. Yuvraj Lahre at Neurovision Clinic in Ranchi uses VEP to detect conduction delays in the optic nerve that may not be visible on routine eye examinations or even on MRI, providing crucial objective evidence that can accelerate diagnosis and treatment.
Is the VEP test painful or invasive?
No, the VEP test is completely non-invasive and painless. Small electrodes are placed on the surface of your scalp with a conductive gel — there are no needles, injections, or radiation involved. You simply sit in a comfortable chair and look at a checkerboard pattern on a screen. Dr. Yuvraj Lahre and his technician guide you through each step, and the only requirement is that you remain relaxed and maintain focus on the screen. The entire experience is comparable to watching a flickering screen for half an hour.
What does an abnormal VEP result mean?
An abnormal VEP typically shows a delayed P100 latency, which indicates that the visual signal is taking too long to reach the brain — most commonly due to demyelination (loss of the myelin insulation around nerve fibres) in the optic nerve. This pattern is strongly associated with optic neuritis and multiple sclerosis. Dr. Yuvraj Lahre will interpret your VEP results in the context of your full clinical picture. A delayed VEP does not automatically mean you have multiple sclerosis; it is one piece of a larger diagnostic puzzle that includes your symptoms, neurological examination, and imaging findings. In some cases, an abnormal VEP may also point toward compressive lesions, ischaemic optic neuropathy, or toxic-metabolic optic nerve damage.
How much does a VEP test cost in Ranchi, and when do I get the results?
At Neurovision Clinic, Ranchi, the VEP test is priced transparently based on whether one or both eyes are tested and whether additional evoked potential modalities (such as brainstem auditory evoked response) are added on the same visit. Dr. Yuvraj Lahre provides a detailed cost estimate during your initial consultation so there are no surprises. Results are available the same day — the waveforms are analysed and interpreted by Dr. Lahre personally, and you will receive both a printed report and a digital copy during your post-test discussion, allowing you to move forward with treatment planning immediately.