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Standards, Recommendations and Guidelines

VISUAL ELECTRODIAGNOSTICS
A Guide To Procedures
Copyright by ISCEV Publications

CONTENTS

Section 1. Patient Referral

Introduction

This guide has been commissioned by the International Society for Clinical Electrophysiology of Vision (ISCEV), to assist practitioners and administrators. It describes the principles of conducting a practice devoted to the diagnosis and management of patients with various visual disturbances. The need for such a guide has become apparent because the advances in this field have made it the subject of growing and more widespread importance. The guide, it is hoped, will bring to the attention of various medical departments the value of such clinics and the need for their provision as part of an integrated service. In addition,developments in Health Care Economics require that there is a common agreement about which patients may require such investigations, how patients should be treated and the consequent costs. Who are the clients for an electrodiagnostic service?

They will normally be referred from other specialist medical services and their eye condition can be categorised under the following headings:

  1. Symptoms suggestive of known neurological or ophthalmological disease, which require confirmation of the diagnosis.
  2. Unexplained Visual Loss (including medico-legal problems and defects associated with psychiatric disturbances or mental and physical handicap).
  3. Paediatric neurophthalmic practice.
  4. Where opacities in the media of the eye prevent other clinical methods from establishing a diagnosis.
  5. Monitoring health, e. g. , in patients receiving medication which may become retinotoxic or neurotoxic.
  6. Detection of the disease or carrier states of inherited visual disorders.
  7. Providing quantitative assessment of the progress of an eye disease.
  8. Assessment of retinal and optic nerve function following trauma
  9. ResearchThese subheadings are illustrated with numerous examples in Section 5.

Section 2. Tests Available

Electrophysiological:

Complimentary Tests:

Proper interpretation of visual electrodiagnostic procedures frequently requires complementary testing and the most important of these are listed below.

Psychophysical:

Other related services sometimes included in the Visual Electrodiagnostic Clinic repertoire:

Section 3. Patterns of Referral

There are two general groups of patients referred to Electrodiagnostic Departments:

  1. Patients who are referred for specific tests.
  2. Patients who are referred for investigation.

If specific tests are requested, they can be performed by trained technicians,and the results given with the normal values for the clinic. Interpretation of these findings must be a matter for the referring specialist.

In order to be able to deal with patients who are referred for investigation the clinic needs to be suitably equipped and have staff capable of carrying out standard clinical procedures. Reporting on such patients should be done ideally by an Ophthalmologist or a member of staff with a postgraduate degree. The person writing the report should have knowledge of the particular conditions handled by the clinic.

Examples of applications for specific tests are given in Section 6.

Section 4. Standardisation and Costing

Standardisation of tests

The International Society for the Clinical Electrophysiology of Vision has issued, and will continue to issue, standards for performance of the tests listed in Section 2. Condensed versions of the standards so far published are listed in Appendix B. There are several books listed in the bibliography which also describe practical electrodiagnostic methods.

Costing

There are a wide variety of health services and health care provisions. In many cases Electrodiagnostic Departments must charge for each item of service. It is obviously desirable to agree on some principles for charges and this document advances the following tentative outline:

  1. Where patients are referred “for investigation”, it is evident that the charge must include general clinical work up, even if clinical reports are sent with the patient.
  2. Each named test is separately costed.
  3. The costs of the tests include:
    1. The proportionate capital outlay on equipment (cost of equipment/number of tests in its lifetime)
    2. Proportion of staff salary devoted to test.
    3. Clinic overheads. It is suggested that these are similar to a Minor Operating Theatre, without its staffing component.
    4. Ancillary costs: receptionist, secretarial, archives.
    5. Interpretation of results

Many such departments are centres of research and research costs should also be calculated on this basis.

Section 5. Referral Indications

The following is a list of diseases and eye conditions whose detection or assessment may be facilitated by the results of electrophysiological tests. The list is by no means comprehensive and, although an attempt has been made to give examples in each group, the intention is not to limit the large range of possible reasons for referral but rather provide some helpful guidelines.

1. Confirmation of Neurological or Ophthalmological Disease

Where the following diseases are suspected, in many instances electrodiagnostic testing may be helpful:

2. Unexplained Visual Loss

This category includes patients who have visual loss as measured on the Snellen Test type or equivalent visual examination and yet have no other physical abnormalities to account for it. Some examples of this type of referral are as follows:

  1. Failure to read test type. Here the results of reading the Snellen Test type are poor and yet there is no other abnormality and visual behaviour is normal.
  2. Post traumatic visual impairment. Routine testing of vision may be inaccurate or difficult to interpret following ocular trauma.
  3. Visual impairment and stress. Unexplained visual loss with a background of stress or domestic upset can require the back-up of the Electrodiagnostic Clinic.
  4. Difficulty seeing in dark. It is important that this surprisingly common symptom is properly investigated by the appropriate psychophysical and electrodiagnostic tests.
  5. Abnormal result of visual field test. Visual field testing may give conflicting results in some patients and electrophysiological tests may be needed especially when the fields of vision appear to be constricted.
  6. Failed driving test for no clear reason. The possible impact of this on the patients career may demand comprehensive tests of retinal function.
  7. Handicapped patients who have difficulty in expressing themselves, foreign patients unfamiliar with the local language or illiterate patients having difficulty with subjective testing. Electrophysiological tests may be useful.
  8. Medico legal claims. The results of routine subjective tests of vision may be biased by the patients' concerns regarding the possible outcome of the case. In such cases the length of the tests may be prolonged by the need to take into account the possibility of skilled dissimulation.

3. Paediatric Neurology

4. Opacities in Media

5. Monitoring Health - Toxicity

6. Detection of the Disease or Carrier States of Inherited Visual Disorders

In the above conditions the Standardised ERG and sometimes the EOG is often useful.

7. Quantitative Assessment of Visual Disease

8. Assessment of Retinal and Optic Nerve Function Following Trauma

The pattern or flash VEP combined with Standardised ERG may be used. In particular medico-legal cases may often be referred with this type of injury.

9. Research

Many Electrodiagnostic Departments have a research commitment and patients with particular eye conditions that are being researched may thus be referred from other units. The tests used would of course depend on the type of research and tests available.

Volunteers are also required to maintain an up to date normal series of results from the various tests employed.

Section 6. Indications for Specific Tests

Psychophysical Tests

Psychophysical tests are usually requested for a specific reason by the referring doctor and in some clinics particular tests have been developed to a high degree for special purposes. It is not therefore practical to lay down general guidelines. The measurement of the visual acuity is generally routine, and for pattern responses the refractive error should be accurately corrected. Some psychophysical tests complement the electrodiagnostic test, for example, dark adaptometry, colour vision testing and measurement of contrast sensitivity.

Electrodiagnostic Tests

Often when electrophysiological tests are needed, the function of the whole visual pathway needs to be assessed. That is to say, for example, a measurement of the VEP alone may give misleading results without an ERG. On the other hand the number of tests performed needs to be minimised on grounds of patient comfort and co-operation. The following are some suggestions as to the appropriate use of ERG, EOG and VEP for some of the more commonly presenting conditions but the choice may be modified in any particular case:

Appendix A. Summary of Potential Indications for Specific Tests

PROVISIONAL DIAGNOSIS

EOG

ERG

BRIGHT FLASH ERG

PATTERN ERG

FLASH VEP

PATTERN VEP

SPECIAL VEP

Inherited retinal dystrophies

+

+

         

Vascular diseases including diabetes

 

+

 

+

 

+

 

Opaque media or trauma

 

+

+

 

+

   

Retrobulbar neuritis*

     

+

 

+

 

Unexplained visual loss

 

+

 

+

 

+

 

Infant with questionable vision

 

+

   

+

 

+

Albinism

 

+**

       

+

Toxic and nutritional eye disease

+

+

 

+

+

   

Glaucoma

     

+

     

Suspected intracranial lesion

     

+

 

+

+

* not helpful during active phase but rather to monitor the recovery

** to exclude other conditions associated with nystagmus

Appendix B. The Standardised Electroretinogram (ERG)

The Original Standards for Clinical Electroretinography were published in 1989 and a 1994. The following is a summary of the requirements:

A standard for five different types of response has been given:

  1. Maximal response in dark adapted eye
  2. Rod response in dark adapted eye
  3. Oscillatory potential
  4. Cone response
  5. Response to flicker

Basic Technology

A full field (Ganzfeld) light stimulus is recommended. The recording electrodes should consist of a corneal contact lens electrode which supports the eyelids and reference electrodes placed centrally on the forehead or near each orbital rim. The ground electrode can be located on the forehead or ear. Skin electrodes should have a resistance of 10KW or less measured at 30 - 200 Hz. The electrodes should be cleaned after use with each patient.

The light stimulus should consist of flashes having a maximum duration of 5ms and a colour temperature near 7000°K. It should be used with domes or diffusers that are visibly white.

The standard stimulus strength is defined as one that produces a stimulus

strength (in luminance time) at the surface of the Ganzfeld bowl of 1.5-3.0 cd·m-2. This is the Standard Flash. The stimulator should also be capable of producing a steady and even background luminance of 17-34 cd·m-2 (5-10 fL). It is also recommended that the stimulus system should be able to modify both stimulus and background intensity, the Standard Flash being attenuated over a range of at least 3 log units. The method of attenuation should not change the wavelength. The International Standardisation Committee recommends that manufacturers of stimulators should provide a suitable photometer as part of the equipment and the stimulus strength on the surface of the bowl should be documented by the user or manufacturer.

The bandpass of the amplifier and preamplifier should include the range of 0.2-300 Hz and be adjustable. The input impedance of the preamplifiers should be at least 1 M ohm. Amplifiers should be AC coupled. Care should be taken to see that the display system represents the full bandpass of the amplification system and current standards for safety of biological recording systems must be observed.

Clinical Protocol

The pupils are maximally dilated and the size of the pupil noted. Dark adaptation for at least 20 minutes is required and the electrodes can be placed under dim red light at the end of this period. The suggested protocol is as follows:

  1. Rod Response
    This is the first signal measured after dark adaptation. A dim white flash 2. 5 log units below the white Standard Flash is used. There should be an interval of at least 2 seconds between flashes.
  2. Maximal Response
    The Standard Flash is used in the dark adapted eye with an interval of at least 10 seconds between flashes.
  3. Oscillatory Potentials
    Using the same Standard Flash, the bandpass filter is reset so that the low frequency is limited to 75-100 Hz and the high frequency to 300 Hz or above. Flashes should be given 15 seconds apart and only the second or subsequent responses retained.
  4. Single Flash Cone Response
    It is recommended that the white Standard Flash is used, with an Interval of at least 0.5 s between flashes, the rods being suppressed by a background with a luminance of 17-34 cd·m-2 (5-10 fL) measured at the surface of the Ganzfeld bowl. Standard ERG recording systems should have an identical intensity ratio between the Standard Flash and the rod suppressing background. Patients should be light adapted for 10 minutes to the background luminance before recording the cone response.
  5. Flicker Response
    Using the same background Standard Flashes are presented at a frequency of 30/second. The first few responses should be discarded.

Reporting

Reports should include two representative waveforms of each of the standard responses to demonstrate the degree of consistency displayed with amplitude and time calibrations and individual responses should be suitably labelled. The reporting form should indicate whether the recordings meet the international standard. Normal values and their variances should be provided with each report. It is recommended that each laboratory establishes its own normal values.

The Standardised Electro-oculogram (EOG)

Basic Technology

The light stimulus should be a Ganzfeld dome, the stimulus intensity being within the range of 50-100 cd·m-2 if the pupils have been dilated and 400-600 cd·m-2with undilated pupils. Skin electrodes are placed as close to each canthus as possible and a ground electrode is attached to the middle of the forehead. Electrodes should be cleaned after use if disposable ones are not used.

Alternating current recording systems are easier to use and a low frequency cut off at 0.1 Hz or lower and a high frequency cut off at no lower than 20 Hz is recommended. Current standards for safety of biological recording systems must be observed.

Clinical Protocol

During the test patients are instructed to make horizontal eye movements every 1 to 2.5 seconds between fixation targets separated by 30 degrees. Responses from a set number of saccades are recorded every minute. Prior to the test patients should be preadapted to ordinary room lighting for at least 15 minutes.

A choice of two methods of recording the light phase is recommended:

The Standardisation Committee recommends ways of eliminating certain artefacts. Each laboratory should establish or confirm normal values for its own equipment.

Reporting

The ISCEV Standardisation Committee recommends that reports or communications of EOG data state clearly whether the ratio of light peak to dark trough (Arden ratio) or light peak to dark baseline method has been used. EOG reports should include the latency of the light peak and the amplitude of the dark trough or dark baseline in microvolts per degree of visual cycle. Published clinical reports should indicate whether the recording technique meets the international standard.

The Standardised Visually Evoked Potential (VEP or VECP)

Basic Technology

Stimulus parameters

Three types of stimulus are commonly used: flash, pattern reversal and pattern onset/offset.

Flash

The stimulus should be a photostimulator which subtends atleast 20 degrees visual angle and has a maximum duration of 5 ms. The stimulator should produce a diffused flash whose luminance approximates 3 candela seconds per metre squared (cd·m-2). Alternatively the flash VEP can be elicited by a standard flash as defined in the ERG standards.

Pattern Reversal

The pattern stimulus consists of black and white checks or black and white gratings which abruptly alternate. There must be no overall change in the luminance of the screen. The stimulus should be defined in terms of the visual angle of each check or the spatial frequency of bars or gratings. At least two pattern element sizes should be 1 degree, and 15 minute checks, or 1. 0 and 4. 0 cycles per degree gratings. The visual field stimulated should exceed 15 degrees.

Pattern onset/offset

For pattern onset/offset, a pattern is abruptly exchanged with an equiluminant diffuse background. Stimuli for pattern onset/offset have similar parameters to those for pattern reversal except that with pattern onset/offset there is a further measure of the luminance of the diffused blank screen interspersed between stimuli. There must be no change in mean luminance as the pattern appears or disappears. We recommend as a convenient standard pattern/blank screen sequence a 200ms pattern separated by at least 400ms diffuse background. The analysis time should include both onset and offset responses.

Clinical Protocol

Preparation of the Patient

All stimuli for VEPs should be presented when the pupils of the eyes are unaltered by mydriatic or miotic drugs. Extreme pupil sizes should be noted. For pattern stimulation the patient should be optimally refracted for the viewing distance of the screen. Monocular stimulation should be performed. When a flash stimulus is used with monocular stimulation, care should be taken to ensure that no light can enter the unstimulated eye. Care should be taken in patient preparation to have the patient in a relaxed position to minimise muscle and other artefacts.

Prechiasmal Assessment

To detect pre-chiasmal dysfunction it is essential that monocular stimulation be performed. Transient pattern reversal of checkerboard stimuli is recommended to detect pre-chiasmal dysfunction. Flash stimulation shoulld be employed for difficult to test patients or in cases of media opacities sufficiently dense to prevent a clear image being projected onto the retina, and can in some circumstances provide useful information complementary to that provided by pattern stimulation. Although pre-chiasmal defects can be detected using a single channel with the active electrode placed over Oz, we recommend that three channels should be used with electrodes placed at Oz, O4 and O3 and referred to Fz.

It should be noted that the pattern onset/offset response shows a greater inter-subject variability. Pattern onset/offset is the most effective stimulus for determination of visual acuity or to carry out objective refraction, as it is difficult to deliberately defocus this type of stimulus.

Chiasmal and Post-chiasmal Assessment

To detect chiasmal or post chiasmal defects recordings must be performed over both cerebral hemispheres. The active electrodes should be placed at locations Oz, O4 and O3 and should be referred to a common reference at Fz. Either pattern reversal or pattern onset/offset may be used. Half field stimulation of at least 15 degrees radius should be used to stimulate one hemisphere. Whole field flash stimulation should be used for difficult or unconscious patients, or patients with dense opaciities of the media.

Normal Values

We recommend that each laboratory establish or confirm its own norms using its own stimuli, recording equipment and parameters. The construction of the normal sample for laboratory norms should include the following factors for each of the VEP components: age, gender,and interocular difference. We recommend that laboratory norms make use of descriptive statistics which do not assume a normal distribution but are based on the calculation of the median and percentiles from the observed sample distribution. We recommend the 95% confidence interval as the minimum outer limit of normal (ie the range from 2.5% to 95%)

VEP Measurement and Reporting

Reports or communications of VEP data include two replications of wave forms obtained to each of the standardised conditons. Recording conditions and polarity conventions should be clearly labelled.

For the transient VEP, amplitude measurements are made between peaks and troughs of the deflections. Peak latency measurements ( or implicit time) should be taken from the onset of the stimulus to the peak of the component concerned. The peak should be designated where it would appear on a smooth or idealised waveform.

All reporting (whether for local records, publication, or even for non-standard responses) should include normal values and the limits of normal.

The report should indicate whether the recordings meet this international standard.

Description of the Three Standard Transient Responses

Flash VEP

The visual evoked potential to flash stimulation consists of a complex series of negative and positive waves beginning around 30ms and terminating around 300ms. For the flash VEP evoked by a diffuse stimulus the most common components are the N2 and P2 components around 90ms and 120ms respectively. It should be noted that flash VEP latency is age dependant. In addition, an early positive going wave at around 50ms is seen more frequently in older people. Measurements of amplitude should be taken peak to peak from the preceding peak.

Pattern Reversal VEP

For pattern reversal, the VEP consists of an N75, P100 and N135 component. The nomenclature consists of designating peaks as negative and positive followed by the mean latency.

Pattern onset/offset VEP

The response to pattern onset/offset stimulation consists of 3 main components. The mature components are described as C1 (positive approximately 75ms), C2 (negative approximately 125ms) and C3 (positive approximately 150ms)

Appendix C. Time Taken for Tests

In case of children assessment (under 6-7 years), multiply time by 1. 5

Bibliography

Acknowledgements

Grateful thanks are due to the many members of ISCEV who have taken time and trouble to make modifications to this guide. The Guide was initially conceived by a Working Party as follows; Mr. NR Galloway (Chair), Prof JV Odom (Co-Chair), Dr M Anastasi, Prof GB Arden, Prof W Dawson, Prof GA Fishman, Prof E Zrenner. Since its initial printing helpful suggestions and modifications have been made by Prof Adachi-Usami, Dr Apkarian, Dr Brimlow, Professor Dawson, Prof Harding, Dr Harrison, Dr Kellner, Prof Marmor, Prof Niemeyer, Dr Tremblay, Dr Vaegan, Prof van Lith, Prof Wachtmeister and Prof Zrenner. Dr Marmor recently introduced valuable improvements.

History

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