Tech Tips: Evoked Potentials, Part I

Pattern-Reversal Visual Evoked Potentials

Introduction 

Pattern-reversal visual evoked potentials (PVEPs) are electrophysiologic responses to patterned visual stimulation.  They remain clinically useful as an objective and reproducible test for optic nerve function.1 An increase in retino-striate conduction time is caused by processes, such as demyelination, while abnormalities in the amplitude and waveform of the PVEPs may be caused by the loss of axons along the visual pathway. 

Setting up the Patient 

The patient’s fixation on the center of the pattern is critical to full field PVEP testing. Defocusing will affect response latency, amplitude and waveform morphology.  Patients should come to the lab awake and alert. Level of consciousness will also affect the response.  If the patient is drowsy you will need to talk to them throughout the testing to ensure that they remain alert and focused on the stimulus. 

Visual acuity should be measured and documented on all subjects.  If the patient has poor visual acuity, larger checkerboard patterns may need to be utilized to obtain well defined reproducible waveforms (normal control values should be collected at varying check sizes for comparison).  A Rosenbaum Pocket Eye Chart is a quick, convenient way to check visual acuity. The eye chart is held at a distance of 14 inches (indicated at the top of the chart) while covering the untested eye. The patient should read the lowest row that they are able to see, then repeat for the other eye.   The best PVEPs are obtained when the central macular area of retina is stimulated. This can be achieved by employing an appropriate and optimal check size of 24-32 minutes of arc (can be achieved with 8mm check size or 32 checks viewed at 1 meter). 2 The visual angle can be approximated by the formula B = 3438*W/D, where W is the width of the check size in millimeters, D is the distance from the screen to the eye in millimeters, and B is the visual angle in minutes of arc.  In most labs the distance from the eye is a fixed 100 cm (1000 mm), and to increase the visual angle, the check size is increased. 

The preferred system for electrode placement is the Queen Square System3.  In the Queen Square System, the electrodes are labeled and positioned as follows: 

  • MO: Midoccipital, in midline 5 cm above inion 
  • LO and RO: Lateral occipital, 5 cm to left and right of MO 
  • MF: Midfrontal, in midline, 12 cm above nasion 
  • A1/A2: At ear (or mastoid), left and right 
  • Ground: At vertex 

One quick way to achieve these measurements is as follows: 

  1. Measure from preauricular point to preauricular point marking halfway between the 2 points. 
  2. Measure from the nasion to inion marking 12 cm back from nasion, halfway between the 2 points, and 5 cm above the nasion. 
  3. Follow the vertical vertex mark up, crossing MF, as well as back, crossing MO 
  4. Placing 5 cm on the mark made for MO, following the circumferential plane, make both marks for LO, at 0 cm on tape measure, and RO, at 10 cm on tape measure. 

Recording 

Routine testing system settings include a system bandpass of 1–100 Hz, an analysis time of 250 ms, and a montage of Channel 1: LO–MF; Channel 2: MO–MF; Channel 3: RO–MF; Channel 4: MF–A1.  

Visual fixation should be at the center of the stimulus screen for full-field stimulation. Visual fixation can be aided by use of a small round sticker in the center of the screen if one is not provided by the manufacturer.  

At least two responses should be recorded.  An adequate number of stimuli should be presented in each trial, values can usually by obtained with 100–200 stimuli per response. 

Records are analyzed to identify N75, P100, and N145 in the occipital regions and N100 in the mid-frontal region.  P100 has a maximal amplitude at the mid-occipital site. 

The following derived measurements are useful:  

  1. Inter-ocular latency difference: the difference in P100 latency measured at the MO site to left and right eye stimulation. 
  2. Interocular amplitude ratio: the ratio of P100 amplitude measured at the MO site to the left and right eye stimulation 
  3. Inter-hemispheric amplitude ratio: the ratio of P100 amplitude measured at the LO and RO sites on stimulation of each eye individually 

Graphic representation of interpretation of amplitude and topographic abnormalities of the PRVEP. Lesion A is at the left optic nerve, lesion B is at the optic chiasm and lesion C is in the left optic radiation. The table shows the amplitude and topography of the P100 with left and right eye full-field stimulation with each of these lesions. V: normal amplitude P100; v: low amplitude P100; —: absent P100; LO/MO/RO, left, middle, right occipital. 

Troubleshooting 

If reproducibility is questionable, try the following: 

  • Make sure that the patient is properly alert and fixated on the center of the screen. 
  • Allow the patient to rest their eye between acquiring responses so eye fatigue does not skew responses. 
  • Increase the check size to see if you can obtain more reliable reproducible waveforms with the increased visual angle. 
  • Hemi-field testing may be indicated if morphology of P100 resembles a double-peaked “W” morphology which cannot be resolved with increasing the visual angle.  

References

[1] Huszar L. Clinical Utility of Evoked Potentials E-medicine [serial online]. 2006 [cited 31st July 2007]. Available from URL: https://emedicine.medscape.com/article/1137451-overview#a1

[2] Kothari R. Influence of visual angle on pattern reversal visual evoked potentials, 2014. Available from URL https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4220397/

[3] American Clinical Neurophysiology Society, Guideline 9B: Guidelines on Visual Evoked Potentials, 2008, https://www.acns.org/.

This article was written by Veronica Cantrell, R. EEG/EP.T, CLTM, R.NCS.T, CNIM , Evokes, LLC