What are Visual Evoked Potentials (VEPs)?

Visual Evoked Potentials (VEPs): Clinical Applications and Interpretation

Visual evoked potentials (VEPs) are electrophysiological responses recorded from the scalp overlying the visual cortex that measure the functional integrity of the visual pathways from the retina to the occipital cortex. These responses are extracted from the electroencephalogram through signal averaging techniques 1.

Types of VEPs

There are three basic types of VEP recordings according to the International Society for Clinical Electrophysiology of Vision (ISCEV) standard:

1. Pattern-reversal VEPs - Most commonly used clinically

   • Elicited by checkerboard stimuli with large (1°) and small (0.25°) check widths
   • Most reliable and reproducible type of VEP
   • Produces a prominent positive peak (P100) at approximately 100 ms

2. Pattern onset/offset VEPs

   • Elicited by checkerboard stimuli that appear and disappear
   • Useful when nystagmus or poor fixation affects pattern-reversal VEPs

3. Flash VEPs

   • Elicited by a flash stimulus covering at least 20° of visual field
   • Used when pattern stimulation is not possible (infants, uncooperative patients)
   • Less sensitive but useful in specific clinical scenarios 2

Recording Technique

• Standard recording uses a midline occipital active electrode
• Reference electrode typically placed on the forehead or earlobe
• Multi-channel recordings may be used for evaluation of chiasmal and post-chiasmal lesions
• Recording is performed monocularly to detect asymmetries between eyes 1, 2

Clinical Applications

1. Multiple Sclerosis Diagnosis

• VEPs are highly valuable for detecting demyelinating lesions in the optic pathways
• Delayed P100 latency is a sensitive indicator of demyelination
• Can detect subclinical optic nerve involvement even when routine ophthalmological examinations are normal
• Included in MS diagnostic criteria as an abnormal VEP can support dissemination in space when combined with specific MRI findings 1, 3, 4
• VEPs are more sensitive than somatosensory or brainstem auditory evoked potentials for detecting silent lesions in MS 4

2. Optic Nerve and Visual Pathway Assessment

• Quantifies functional integrity of the visual pathways
• Detects subclinical lesions affecting the optic nerves
• Helps differentiate between organic visual impairment and non-organic visual disorders
• Useful in monitoring diseases that can affect the visual pathways:
  • Optic neuritis
  • Compressive lesions (e.g., pituitary tumors)
  • Toxic and nutritional optic neuropathies
  • Hereditary optic neuropathies 5

3. Pediatric Applications

VEPs are particularly valuable in pediatric patients who cannot communicate visual symptoms or cooperate for standard vision assessment 6:

• Detecting lesions causing visual pathway dysfunction
• Differentiating visual impairment from visual inattention in infants
• Quantifying visual impairment to facilitate early intervention
• Monitoring patients at risk for visual complications from:
  • Hydrocephalus
  • Neurofibromatosis
  • Neurosurgical procedures
  • Chemotherapy
• Establishing prognosis in conditions like perinatal asphyxia and cortical blindness 6

4. Pituitary Adenomas and Sellar Lesions

• VEPs can detect compression of the optic pathways by pituitary macroadenomas
• Used in the assessment of visual function when standard visual assessment is difficult
• May complement other visual function tests in patients with pituitary lesions
• Not recommended as the sole method for long-term visual surveillance 1

Interpretation of Results

The main parameters evaluated in VEP interpretation include:

1. Latency - Time from stimulus to response peak (most important parameter)

   • Delayed latency suggests demyelination or conduction defects
   • P100 latency is the most clinically relevant measurement

2. Amplitude - Size of the response

   • Reduced amplitude may indicate axonal loss or conduction block
   • More variable than latency and affected by technical factors

3. Waveform morphology - Shape of the response

   • Abnormal waveforms may indicate specific pathologies
   • Absence of response suggests severe dysfunction

Advantages and Limitations

Advantages:

• Non-invasive
• Objective assessment of visual pathway function
• Can detect subclinical lesions
• Useful in uncooperative patients or those who cannot communicate

Limitations:

• Generally nonspecific to exact location or type of lesion
• Requires specialized equipment and expertise
• May be affected by technical factors and patient cooperation
• False positives and negatives can occur, particularly in bitemporal visual field defects 4

Clinical Recommendations

1. For suspected multiple sclerosis: VEPs should be considered when MRI findings are insufficient to demonstrate dissemination in space, particularly when there is clinical suspicion of optic pathway involvement 1, 3.

2. For pituitary macroadenomas: Visual assessment should include visual acuity, visual field testing, and assessment of optic nerve integrity. VEPs may be used as a complementary test but should not replace standard visual assessment 1.

3. For pediatric patients: VEPs are recommended when standard visual assessment is not possible due to age or cooperation issues, particularly in monitoring conditions that put the visual pathways at risk 6.

4. For post-chiasmal lesions: Multi-channel VEP recordings with hemi-field stimulation are more effective than standard full-field VEPs 7.

VEPs remain an important tool in clinical neurophysiology, providing valuable information about the functional integrity of the visual pathways when appropriately applied and interpreted in the context of the clinical presentation.