Specialized Procedures

Pediatric VEP recording

In principle, the stimulation and recording methods recommended in the ISCEV standard can be applied to all populations. However, in infants, young children or people with disabilities, modifications to VEP recording methods and testing strategies may be required to optimize the quality and pertinence of the result to diagnosis and visual assessment to the clinical question.

All VEPs in children should be compared with appropriate age related normal values. When recording the VEP in infants the sweep duration should be at least 500 ms post-stimulus to record the full VEP waveform in young infants. By six months of age, the peak time of the main positive peak of the pattern reversal VEP for large checks (1°) is usually within 10% of adult values.

Paediatric VEPs should be recorded when the infant or child is alert and attentive. Direct interaction with the child can help maintain attention and fixation, and two testers are beneficial; one to work with the child and the other to control data acquisition. Data quality and reliability will be improved if a recording trial can be paused or interrupted when fixation wanders and then resumed as the child resumes adequate fixation. To facilitate compliance, an infant may view the stimulus while held on a lap or over the shoulder. The order of stimulus presentation should be flexible and selected to ensure that responses most critical to the diagnostic question are obtained within an individual child's attention span. Binocular pattern stimulation, which facilitates attention and fixation, may be useful to evaluate overall visual function. Monocular testing to at least one stimulus is desirable to assess the function of each eye. It is particularly important to replicate VEPs in children to assure that the response measured is a reliable signal and not an artifact. Reports should note the degree of cooperation and arousal of the child. As for adults, additional channels of recording may be important for diagnosis of chiasmal and postchiasmal dysfunction. When pattern VEPs cannot be reliably recorded, flash testing, which is less dependent upon fixation, can usually be achieved.

Multi-channel recording for assessment of the posterior visual pathways

Multi-channel VEP recording is not required by the standard. However, intracranial visual pathway dysfunction may require multi-channel recording for accurate diagnosis. With dysfunction at, or posterior to, the optic chiasm or in the presence of chiasmal misrouting (as seen in ocular albinism), there is an asymmetrical distribution of the VEP over the posterior scalp. Chiasmal dysfunction gives a "crossed" asymmetry whereby the lateral asymmetry obtained on stimulation of one eye is reversed when the other eye is stimulated. Retrochiasmal dysfunction gives an "uncrossed" asymmetry such that the VEPs obtained on stimulation of each eye show a similar asymmetrical distribution across the hemispheres. We suggest that pattern stimuli for multi channel investigations of visual pathway dysfunction should be presented with a field of 30 degrees (double the minimum size required by this standard). A minimum of two channels is needed for detection of lateral asymmetries. We suggest a minimum of three active electrodes, two lateral electrodes placed at O1 and O2, and a third midline active electrode at Oz. All three active electrodes should be referenced to Fz. Additional electrodes placed at PO7 and PO8, also referred to Fz, may increase sensitivity to lateral asymmetries. The position of the lateral electrodes is illustrated in Figure 1B. For all stimulus conditions, normative data should include amplitude and peak time comparisons between homologous left and right occipital channels. Particular caution is needed when interpreting multi-channel pattern-reversal VEPs because of paradoxical lateralization. This phenomenon, in which the response recorded at a lateral scalp location is generated by activity in the contralateral hemisphere of the brain, occurs with a large field, large check reversal stimulus and common reference recording to Fz.

REFERENCES

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1. Odom JV, Bach M, Barber C, Brigell M, Holder G, Marmor MF, Tormene AP, Vaegan (2004) Visual evoked potentials standard. Doc Ophthalmol 108:115-123.
2. Brown M, Marmor MF, Vaegan, Zrenner E, Brigell M, Bach M. ISCEV Standard for Clinical Electro-oculography (EOG) 2006 . Doc Ophthalmol. 2006; 113:205-212 .
3. Marmor MF, Fulton AB, Holder GE, Miyake Y, Brigell M, Bach M. Standard for clinical electroretinography (2008 update). Doc Ophthalmol. 2009; 118:69-77
4. Holder GE, Brigell MG, Hawlina M, Meigen T, Vaegan, Bach M. ISCEV standard for clinical pattern electroretinography - 2007 update. Doc Ophthalmol 2007, 114: 111-116.
5. Brigell M, Bach M, Barber C, Moskowitz A, Robson J. Guidelines for calibration of stimulus and recording parameters used in clinical electrophysiology of vision. Doc Ophthalmol 2003;107:185-193
6. Marmor, M.F., Hood, D.C., Keating, D., Kondo, M., Seeliger, M.W. & Miyake, Y. Guidelines for basic multifocal electroretinography (mfERG). Documenta Ophthalmologica 2003;106:105-115.
7. American Clinical Neurophysiology Society. Guideline 5: Guidelines for standard electrode position nomenclature. J Clin Neurophysiol. 2006;23:107-10. Available at https://www.acns.org/

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