What are Visual Evoked Potentials (VEPs)?

Visual Evoked Potentials (VEPs)

Visual evoked potentials (VEPs) are electrical potentials recorded from the scalp overlying the visual cortex that have been extracted from the electroencephalogram by signal averaging, used to measure the functional integrity of the visual pathways from the retina to the occipital cortex. 1, 2

Definition and Basic Concepts

VEPs represent the brain's electrical response to visual stimuli and provide objective measurements of visual function at the level of the occipital cortex. Key characteristics include:

• Recorded using electrodes placed on the scalp, typically at the midline of the occipital region 2
• Extracted from the electroencephalogram through signal averaging techniques 1
• Used to quantify the functional integrity of the optic nerves and visual pathways to the brain 2

Types of VEP Recordings

According to the International Society for Clinical Electrophysiology of Vision (ISCEV), there are three standard VEP protocols 3:

1. Pattern-reversal VEPs:

   • Elicited by checkerboard stimuli with large (1°) and small (0.25°) check widths
   • Most commonly used and clinically reliable type
   • Particularly sensitive for detecting demyelinating conditions

2. Pattern onset/offset VEPs:

   • Also uses checkerboard stimuli with large (1°) and small (0.25°) check widths
   • Useful when pattern-reversal responses are degraded or when testing patients with nystagmus

3. Flash VEPs:

   • Elicited by a flash stimulus that subtends at least 20° of the visual field
   • Particularly useful for testing infants, sedated patients, or those unable to fixate

Clinical Applications

VEPs have several important clinical applications:

1. Multiple Sclerosis (MS) Diagnosis and Monitoring:

   • Highly valuable for detecting demyelinating lesions in the optic pathways 1
   • Delayed P100 latency is a sensitive indicator of demyelination
   • Can detect subclinical optic nerve involvement even when routine ophthalmological examinations are normal 1, 4

2. Optic Neuritis Assessment:

   • Useful in predicting the extent of recovery after optic neuritis 4
   • Can capture disabling effects of clinical and subclinical demyelination events

3. Other Neurological Conditions:

   • Helpful in diagnosing compressive lesions affecting the visual pathways 1
   • Valuable in assessing toxic and nutritional optic neuropathies
   • Used in evaluating hereditary optic neuropathies 1

4. Additional Clinical Uses:

   • Objective assessment of visual acuity and refractive error 5
   • Evaluation of amblyopia and binocularity
   • Assessment of psychogenic visual disorders
   • Pre-surgical prediction of post-surgical visual function 5

Recording Techniques and Considerations

• Standard recording uses a single channel with a midline occipital active electrode 3, 6
• For patients over 3 years of age, VEPs are usually recorded using a video monitor presenting patterned stimuli 2
• In sedated patients and infants, flashes of light from a strobe or LED array are used 2
• Multi-channel VEPs are required for evaluation of chiasmal and post-chiasmal lesions 3, 6

Interpretation and Clinical Significance

• Abnormalities in VEP waveforms can indicate dysfunction anywhere along the visual pathways
• Common abnormalities include:
  • Increased latency (delayed response) - typical of demyelinating conditions like MS
  • Reduced amplitude - may indicate axonal loss or conduction block
  • Abnormal waveform morphology - can suggest various pathologies

Limitations and Considerations

• VEPs assess the function of the entire visual pathway, so abnormalities must be correlated with clinical findings and other tests
• Results can be affected by poor fixation, inattention, or technical factors
• Interpretation requires knowledge of age-related normal values and technical parameters
• Temperature of the patient should be considered in interpretation 7

VEPs remain a valuable, non-invasive tool for objectively assessing visual pathway function, particularly useful when combined with other diagnostic modalities like MRI in conditions such as multiple sclerosis.