Clinical electrophysiologists should be aware that international standards or guidelines exist for the major clinical electrophysiologic responses.^1,5–7,9 These standards, established and sanctioned by the International Society for Clinical Electrophysiology of Vision (ISCEV), describe a set of basic stimuli that give rise to standard responses that should be recorded whenever electrophysiologic tests are performed clinically. Other responses or protocols may be added freely at the discretion of the examiner, but the standard responses should always be included. This ensures that electrophysiologic testing will always produce a core of data that is recognizable and comparable everywhere, whether for clinical or research purposes. This program of standardization has been highly successful. Most of the publications on clinical electrophysiology in recent years display or refer to the standard responses, and the major manufacturers of clinical electroretinographic equipment have incorporated the standards into their stimulus protocols. The standards that were current at the time of writing are reprinted in this book, but these are living documents that are revised every few years. Therefore, readers are urged to check the ISCEV Web site (www.iscev.org) to see the latest versions. There is also a useful set of ISCEV-sanctioned calibration guidelines,² which give advice about the evaluation and calibration of equipment for electrophysiologic testing.

Clinical electrophysiological testing became practical in the middle of the twentieth century, but a wide variety of procedures were used by different laboratories to elicit responses. Some of this variability related to changing technology, as amplifiers and recording equipment improved with the advent of transistors and later computers. By the 1970s and 1980s, the major electrophysiological tests (ERG, EOG, VEP) were established procedures, but reading the clinical electrophysiological literature was still a major challenge because the tests were performed under many different conditions. The ERG, for example, was recorded in different units after different periods of dark adaptation, with different types of lenses, with different electrical filtration settings, with different intensities and durations of light flashes, and with different colors of light. To compare data among research or clinical reports was very difficult, and this impeded both clinical care and the interpretation of clinical research.

ISCEV had recognized the need for standardization at its founding in 1961,^3,9 but serious discussions of standardization did not take place until 1987, when the National Retinitis Pigmentosa Foundation (now called Foundation Fighting Blindness) joined with ISCEV to form an international committee that collated views and practices from around the world and wrote the first standard, for clinical electrophysiology (ERG), in 1989.⁴

Once published, the ERG standard had an immediate impact. Within just a few years, most clinical electrophysiology papers incorporated the standard ERG tests or described any variations in technique relative to the ISCEV standards. Data from different patients and from procedures around the world became immediately comprehensible and comparable. This success prompted ISCEV to develop standards for the EOG and VEP and then to write guidelines for newer procedures, the pattern ERG (PERG) and multifocal ERG (mfERG). Because all of the standards rely on accurate calibration of the light stimuli and of the recording equipment, ISCEV formulated a set of calibration guidelines to summarize the best available techniques.

It is important to recognize both the power and the limitation of these ISCEV standards. The standards provide a basic core of tests that can be performed with readily available instrumentation and that are relevant to the most common clinical indications for each test. However, the standards cannot cover all possible clinical situations, all variants of testing procedure, or all of the options within the technology of testing. The introduction to each of the standards or guidelines acknowledges that there are many other test, or more specialized test, that could be performed—and indeed that should be performed in certain clinical situations. The standards responses should be obtained as a part of virtually every electrophysiological testing session but not to the exclusion of other procedures that individual clinicians or laboratories may choose to do, either routinely or as an add-on for specific patients. Furthermore, the existence of standards should not stifle innovation with respect to the development of new procedures that may eventually become standards.

The ERG standard⁶ was the first to be written, and it defines five basic waveforms: dark-adapted rod response, dark-adapted maximal response, oscillatory response, light-adapted cone response, and flicker cone response. These serve to separate rod and cone function, to document inner versus outer retinal function, and to allow evaluation of waveform timing. The ERG standard uses only white-light stimuli, although some laboratories use colored stimuli as well. Additional ERG procedures, such as following responses during dark adaptation or recording responses to high-intensity flashes to show photoreceptor activity more accurately, are not included at present and remain at the discretion of individual laboratories. In clinical practice, the five standard responses will suffice to answer most diagnostic questions, but they will be amended as research shows that additional or modified responses are clinically important.

The EOG standard⁷ shows the basic technique for recording the standing potential across the RPE indirectly, as the voltage changes between lid canthus electrodes when the eye looks alternately left and right. The EOG response, or light response, represents the rise in this voltage in light relative to darkness. Because there have been two established ways of setting a dark baseline, the standard allows either for the recording of a dark trough (after room illumination) or for dark-adapting over a longer time to a steady baseline level.

Establishing an effective VEP standard⁸ has been a challenge because of the wide variety of clinical recording conditions that are used by different laboratories for different clinical questions. Because it is impractical for a standard to have too many variations, ISCEV settled on one basic configuration of electrodes on the scalp (central inion) and three common stimulus configurations (flash, alternating pattern, on-and-off pattern). Depending on the clinical indications, at least one of these standard responses should be included as a part of every VEP examination, even if other electrode locations or other stimuli are used as well.

As the PERG was developed and its usage increased, ISCEV elected to publish a set of guidelines that represented good practice by established laboratories. After several more years of experience, clinical expectations were codified sufficiently that standards¹ could be written. In this same spirit, guidelines have now been prepared for the mfERG,⁵ which is an even newer procedure that is gaining acceptance rapidly and is proving to have many applications in clinical ophthalmology. These guidelines provide information on how to record a basic good quality mfERG and avoid artifacts. They reflect procedures that are followed by many of the major laboratories around the world, even if it is still too early to establish binding standards.

The calibration guidelines² that ISCEV has prepared are intended to aid electrophysiologists in the practicalities of standardizing stimulus and recording equipment. These include recommendations on how to measure stimulus luminance, check electrodes, calibrate amplifiers, and so forth. These procedures are necessary for all types of electrophysiological recordings, since a display of standard responses has little meaning unless the stimulus and recording conditions were truly what they were intended to be.