I Department of Ophthalmology, Stanford University School of Medicine; Stanford, ... of Vision and Neuro-ophthalmology, University Eye Hospital of Tiibingen, ...
Documenta Ophthalmologica 91: 291-298, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.
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Guidelines for basic pattern electroretinography R e c o m m e n d a t i o n s by the I n t e r n a t i o n a l S o c i e t y f o r Clinical E l e c t r o p h y s i o l o g y o f Vision
M I C H A E L E M A R M O R 1, G R A H A M E. H O L D E R 2, VITTORIO PORCIATTI 3, GARY L. T R I C K 4 & E B E R H A R T Z R E N N E R 5 for the International Society for Clinical Electrophysiology of Vision IDepartment of Ophthalmology, Stanford University School of Medicine; Stanford, CA, USA; 2Regional Department of Clinical Neurophysiology, Brook General Hospital, London, UK; 3Istituto di Neurofisiologia del Consiglio Nazionale delle Ricerche, Pisa, Italy; 4Department of Ophthahnology, Henry Ford Hospital, Detroit, MI, USA; 5Department of Pathophysiology of Vision and Neuro-ophthalmology, UniversityEye Hospital of Tiibingen, Germany, Dr. Marmor is the Chairman of the ERG Standardization Committee of the International Society for Clinical Electrophysiology of Vision (ISCEV) and Dr. Zrenner is the President of the ISCEV Accepted 4 April 1996
Key words: Clinical electrophysiology,Electroretinogram,Pattern electroretinogram Abstract. The pattern electroretinogram is a retinal response that can be evoked by viewing an alternating grating or checkerboard. It is receiving increasing clinical and research attention because it can provide information about inner retinal cells and the macula. However, clinicians may have trouble choosing between different techniques for recording the Pattern electroretinogram that have been described in the literature. The International Society for Clinical Electrophysiology of Vision has prepared guidelines for a basic pattern electroretinogram recording procedure to aid new users in obtaining reliable responses and to encourage more uniformity among existing users.
Introduction
The pattern electroretinogram (PERG) (Figure 1) is a retinal biopotential that is evoked when a temporally modulated patterned stimulus of constant total luminance (grating or checkerboard) is viewed. The PERG is most often evoked by alternating reversal of a checkerboard pattern. It may be altered in selective dysfunction of the macula or of the inner retina, which do not significantly affect the conventional full-field electroretinogram (ERG). The P E R G is receiving increasing clinical and research attention in both neurologic and ophthalmologic practice. However, the PERG is a very small signal, typically in the region of 0.5-8 #V, depending on stimulus characteristics, and P E R G recording is technically more demanding than the conventional ERG. Recordings published in the literature vary considerably in
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P50
N95
50 ms
Figure 1. IdealizedtransientPERG.
technical quality as well as technique, and new users may find it difficult to choose which technique to use. It is premature to define a 'standard' such as those that exist for conventional (flash) electroretinography [1] and for electro-oculography [2], but the International Society for Clinical Electrophysiology of Vision (ISCEV) believes that there is now a sufficient body of knowledge and clinical experience to propose a set of recommendations for performing a 'basic PERG'. It is hoped that these will act as guidelines to new users and will encourage uniformity among existing users. These guidelines will lead to reliable responses that are interpretable in light of current knowledge, but we acknowledge that there are differing views concerning PERG techniques and evaluation. We recognize that research laboratories will continue to use specialized protocols and techniques tailored to their own requirements. These guidelines are based on equipment and analytic capabilities currently found in most neurophysiologic or ophthalmologic electrodiagnostic clinics. This document addresses recording conditions and technology specific to the PERG, and presumes that readers already have basic understanding and skills in clinical electrophysiology. It should be noted that ISCEV is developing standards for calibration of electrophysiologic equipment, which should be available in 1997. Although much of this document will apply equally to adults and children, these guidelines are not necessarily appropriate to pediatric applications. These guidelines will be reviewed by ISCEV every four years.
Waveform nomenclature and measurement
The waveform of the PERG evoked by pattern-reversal stimuli depends on the temporal frequency of the stimulus. By convention, positivity is displayed upward.
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Transient PERG (see Figure 1) At low temporal frequencies (6 reversals per second or less, equivalent to 3 Hz or less) a transient PERG is obtained. The waveform is characterized by a small initial negative (N) component, at approximately 35 msec, which will be referred to as N35. This is followed by a later and larger positive (P) component (45-60 msec), which is typically denoted as P50. This positive portion of the waveform is followed by a large negative component at about 90-100 msec (N95). For the transient PERG, amplitude measurements are made between peaks and troughs: the P50 amplitude is measured from the trough of N35 to peak of P50, and the N95 amplitude is measured from the peak of P50 to the trough of N95. Latency measurements should be taken from the onset of the stimulus to the peak of the component concerned; inexperienced workers should note that the highest absolute amplitude point on a waveform will not always be appropriate for the definition of the peak if there is contamination from muscle activity or other artifact. The peak should be designated where it would appear on a smoothed or idealized waveform.
Steady-state PERG At higher temporal frequencies (i.e., above 5 Hz), the successive waveforms overlap and a'steady-state' PERG is evoked. The waveform becomes roughly sinusoidal, and Fourier analysis is required to determine the amplitude and phase shift (relative to the stimulus).
Basic technology Electrodes Recording electrodes. Non-contact lens corneal electrodes (thin conductive fibers and foils) are recommended for PERG recording. These types of electrode can usually be positioned without topical anesthesia. Those who perform the test should be aware of possible causes of artifact. Electrode resistance should be checked before insertion, to meet guidelines for each electrode type. Electrodes should be carefully positioned to minimize instability (a major source of artifact or interference). Fiber electrodes should be placed in the lower fomix. Draping the electrode across the lower eyelashes at the medial canthus, or taping it to the cheek, may help to stabilize it. Foil electrodes should be positioned directly under the center of the pupil so that there is minimal or no movement of the electrode when the patient
294 blinks. This is bestachieved by having the foil curve over the lower eyelashes without contacting them, and then tethering the lead to the cheek. The junction of the electrode and lead should form as straight a line as possible and the junction should not touch the skin.
Reference electrodes. Surface reference electrodes should be placed at the ipsilateral outer canthi. Mastoid, ear or forehead locations may result in contamination of the PERG from cortical potentials or the fellow eye. If monocular PERG recording is performed (see recommendations below), the corneal electrode in the occluded eye may be used as a reference. Ground electrodes. A separate surface electrode should be attached and connected to ground; the forehead would be a typical location. Surface electrode characteristics. The impedance between the skin electrodes used for reference and ground, measured on the subject, should be 5 kf~ or less. The skin should be prepared with a suitable cleaning agent, and a suitable conductive paste should be used to ensure good electrical connection. Electrode cleaning and sterilization. electroretinogram [ 1].
See ISCEV standard for full-field flash
Stimulus parameters This guideline presents only a basic protocol for PERG recording. Laboratories may choose to test more conditions or parameters than are described herein.
Field and check size. We recommend using a black-and-white reversing checkerboard with a stimulus field size between 10° and 16°, and a check size of approximately 40 ~. Contrast. The contrast between black and white squares should be maximal (but not less than 80%). Luminance. PERGs are difficult to record with low stimulus luminance, and a photopic luminance level for the white areas of greater than 80 cd/m 2 is recommended. Overall screen luminance should not vary during checkerboard reversals. Transient and steady-state recording. We recommend that laboratories record the transient PERG. It allows separation of the P50 and N95 com-
295 ponents and can be helpful in the evaluation of optic nerve and macular diseases. There are also situations in which the steady-state PERG is useful; some laboratories favor it for glaucoma studies. Since little extra time is required, laboratories should consider recording it as well. Keep in mind, however, that the proper interpretation of steady-state PERGs requires awareness of potential artifacts and measurement of amplitude and phase shift (relative to the stimulus) of the second harmonic by Fourier analysis. We do not recommend steady-state PERG recording without instrumentation for such analysis, and we caution that steady-state stimulation at reversal rates below our recommendation (8 Hz) requires special equipment to modulate contrast sinusoidally.
Reversal rate. For transient PERG, we recommend two to six reversals per second (1-3 Hz); for steady-state PERG, we recommend 16 reversals per second (8 Hz).
Recalibration.
We advise regular stimulus recalibration.
Background illumination.
The luminance of the background beyond the checkerboard field is not critical when the suggested PERG technique is used, as long as dim or ordinary room lighting is used. Care should be taken to keep bright lights out of the subjects' direct view.
Electronic recording equipment Amplification systems.
Alternating-current-coupled amplifiers with a minimum input impedance of 10 M ~ are recommended. Amplification systems should be electrically isolated from the patient according to the current standards for safety of biologic recording systems used clinically. We recommend that the frequency response of bandpass amplifiers should include the range from 1 to 100 n z 1 and that notch filters (that remove the alternating current line frequency) not be used.
1 Some users may encounter severe electromagneticinterferencethat makes it difficult to obtain responses with these filter recommendations.Ideally, such interferenceshould be eliminatedby shieldingor modifyingequipment.If interferencecannotbe reduced,the PERG may be recordableby reducing the upper filter setting to the range of 30-50 Hz. The major PERG components (N35, P50 and N95) will be recognizable with such filters, but rapid response components will be lost and the major waveforrns will be altered in amplitude and latency (dependingon the particular filters used). Laboratories that use such filters must recognize that their responses might not be comparableto those from other laboratories and should note in reports that extra filteringwas necessary.
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Averaging and signal analysis. Because of the small amplitude of the PERG signal, averaging is always necessary. For transient PERGs, the analysis period (sweep time) should be 150 msec or greater. A Fourier analysis program will be needed if steady-state PERGs are to be recorded, and the analysis period should be a multiple of the stimulus interval.
Artifact rejection. Computerized artifact reject is essential, and we recommend that this be set at no higher than 100 #V peak-to-peak, and preferably less. The amplifiers must return to baseline rapidly after artifactual signals to avoid inadvertent storage of nonphysiologic data.
Display systems. Display systems must have adequate resolution to represent accurately the characteristics of this small-amplitude signal. It is necessary, in the absence of a simultaneous display of input signal and average, that the system allow a rapid alternation between input signal display and average display so that the quality of the input signal can adequately be monitored; even with a computerised artifact reject system, it is important that the input signal be monitored for baseline stability and the absence of amplifier blocking.
Clinical protocol
Preparation of the patient Pupils.
The PERG should be recorded without dilatation of the pupils, to preserve accommodation.
Fixation.
A fixation spot in the center of the screen is essential. If there are any doubts about the quality of fixation in an individual patient, the patient should be given a pointer and told to point at the middle of the screen throughout.
Refraction. Because of the nature of the stimulus, PERG examination should be performed with optimal visual acuity at the testing distance. Patients should wear the appropriate optical correction for the test distances.
Monocular and binocular recording. Proper positioning of recording and reference electrodes will permit either monocular or binocular recording of the PERG. Binocular recording of the PERG is strongly recommended for the 'basic PERG' because it is generally more stable, it reduces examination time and it allows fixation by the better eye in cases of asymmetric visual loss.
297 Monocular stimulation is required to record simultaneously the PERG and cortical visual evoked potential. This is not recommended routinely because the tests have different technical requirements, but it may be useful for the evaluation of certain conditions and where binocular fixation is not possible (e.g., strabismus).
Noise trials. Determination of noise level by averaging in the absence of stimulation (i.e., blank trials) should be encouraged, and these values should be reported whenever possible. Recording. It is recommended that averaging be continued until a stable waveform is obtained. A minimum of 150 responses should be included, and more may be needed with a 'noisy' subject. At least two replications of each stimulus condition should be obtained to confirm responses and avoid artifacts. PERG reporting Reporting. It is recommended that all reports contain measurements of P50 and N95 amplitude (see above) and P50 latency (the peak of N95 is often rather broad, precluding accurate latency measurement of this component). If steady-state PERGs are performed, amplitude and phase shift of the second harmonic should be reported. All reports should also contain the normal values for the laboratory concerned. Whenever practical, reporting of PERG results should include representative waveforms with appropriate amplitude and time calibrations. Clinical norms. Each laboratory should establish normal values for its own equipment and patient population. Age norms are essential, since PERGs (both transient and steady-state) vary with age.
Acknowledgment This document was approved by the International Society for Clinical Electrophysiology of Vision on June 23, 1994, in Banff, British Columbia, Canada.
References Marmor MF, ZrennerE, for the IntemationalSociety for ClinicalElectrophysiologyof Vision. Standard for clinicalelectroretinography(1994 update). Doc Ophthalmol 1995; 89: 199-210.
298 2. Marmor MF, Zrenner E, for the International Society for Clinical Electrophysiology of Vision. Standard for clinical electro-oculography. Arch Ophthalmol 1993; 601-4.
Address for correspondence: C. Barber, Secretary-General ISCEV Medical Physics Department, Queen's Medical Centre, Nottingham NG7 2UH, UK Phone 44-115-970-9131; Fax: 44-115-942-2745
