CONCILIUM OPHTHALMOLOGICUM UNIVERSALE PERIMETRIC STANDARDS and PERIMETRIC GLOSSARY of the International Council of Ophthalmology Dr. W. Junk bv Publishers The Hague/Boston/London 1979 © Dr. W. Junk bv Publishers 1979 Softcover reprint of the hardcover 1st edition 1979 No part of this book may be reproduced and/or published in any form, by print, photoprint, microfIlm or any other means without written permis sion from the publishers. ISBN 978-94-009-9641-0 ISBN 978-94-009-9639-7 (eBook) DOI 10.1007/978-94-009-9639-7 1 PERIMETRIC STANDARDS, 1978 Committee: Jay M. ENOCH (U .S.A., chairman of the IPS I R.G. on Standards, editor), and (alphabetically) Elfriede AULHORN (GFR), Andre DUBOIS POULSEN (France), Frank FANKHAUSER (Switzerland), Allan FRIEDMANN (UK), Erik GREVE (The Netherlands, secretary of the IPS1), H. HARMS (GFR) and Guy VERRIEST (Belgium). Members of this Ad Hoc Drafting Committee to prepare an International Perimetric Standard were selected at a meeting of the R.G.1 on Standards of the IPSI , September 19, 1976, Tiibingen, West Germany. This meeting was held at the Universitats-Augenklinik, Bonn-Venusberg, West Germany, on November 11-12, 1977. The Ad Hoc Committee wishes to express its appreciation to Prof. Best of Bonn for his cooperation and many acts of kindness during the preparation of this draft proposal. The Research Group on Standards would like to thank the many individ uals who have offered thoughtful and constructive criticisms. Many of their proposed changes have been incorporated in this document. 1 R.G., Research Group; IPS, International Perimetric Society 2 CONTENTS 1. Deftnitions of Perimetry and of the Visual Field ............. " 3 2. Need for Speciftcations and Tolerances. . . . . . . . . . . . . . . . . . . .. 3 3. Applicability of this Standard . . . . . . . . . . . . . . . . . . . . . . . . . .. 3 4. Speciftcations of Magnitudes and Units ................... " 3 5. Photometric Speciftcation. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4 6. Background or Adapting Luminance. . . . . . . . . . . . . . . . . . . . . .. 5 A. Speciftcation of luminance . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 B. Preadaptation conditions .. . . . . . . . . . . . . . . . . . . . . . . . . .. 6 C. Diffusely reflecting surface . . . . . . . . . . . . . . . . . . . . . . . . . .. 7 7. Speciftcation of the Location of an Object in the Visual Field . . . . .. 7 8. Target Speciftcation (non -photometric) . . . . . . . . . . . . . . . . . . . .. 8 A. Size, distance and form ............................ " 8 B. Contrast ....................................... 10 C. Duration of presentation ............................. 11 1. Non-moving or static targets ........................ 11 2. Moving or kinetic targets ........................... 12 D. Image sharpness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9. Color Perimetry .................................... 12 10. Other Factors ...................................... 13 A. Attention signal and shutter noise ....................... 13 B. Distractions to be avoided. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C. Relative and absolute scotomas ........................ 13 11. Acceptance and Revision of these Standards. . . . . . . . . . . . . . . ... 14 3 IPS PERIMETRIC STANDARDS, 1978 I. Definitions of Perimetry and of the Visual Field Perimetry is the measurement of visual functions of the eye at topographi cally defined loci in the visual field. The visual field is that portion of the external environment of the observer wherein the steadily fixating eye(s) can detect visual stimuli. 2. Need for Specifications and Tolerances The fundamental purpose of standardization is to provide a common framework for measurement. This allows exchange and comparison of information obtained at different times and in different places. If a com mon measurement scheme can be achieved, then development upon that base can proceed in an orderly manner. In perimetry few standards exist and certain of these are imperfectly specified. This situation needs to be rectified. Specification also implies consideration of tolerances. Tolerances include instrument setting accuracy as well as measurement accuracy. Because tolerances in a clinical office are not comparable with those achievable in a research laboratory, an effort will be made to set standards which define conditions where small errors do not significantly alter results or interpretations of data. 3. Applicability of this Standard This standard is written for all individuals engaged in perimetry and espe cially for clinicians for use in their offices, departments, and clinics. It is directed also towards the manufacturer who provides visual field test equipment. These standards also set minimum criteria for reporting research results. The goal is to set a reasonable minimum set of standards for testing of the visual field. Note that different requirements or strategies may be needed for different tasks. 4. Specifications of Magnitudes and Units This committee makes use of the International System of Units published by the International Bureau of Weights and Measures (Le Systeme Inter- 4 national d'Unites, 1970, OFFILIB, 48 Rue Gay-Lussac, F-75005, Pari~, France, Revised edition 1977). See also: The International System of Units, NBS Special Pubhcation 330,1977, U.S. Department of Commerce, National Bureau of Standards, U.S. Government Printing Office, Washing ton, D.C. 20402 (SD Catalogue No. CI3.l0:330/4). Supplemental use is made of the Vocabulary of the Commission Internationale de l'Eclairage (International Lighting Vocabulary of the Commission Internationale de l'Eclairage, 3rd ed., 1970, Bureau Central de la CIE, 4 Avenue du Recteur Poincare, F-75016, Paris, France). In the Proceedings (Acta) of the XXIInd International Ophthalmolo gical Congress, Paris, 1974, p. 78 and 93, the Concilium Ophthalmologicum Universale published recommendations regarding the use of the Internatio nal System of Units in ophthalmological practice. There is on record an earlier international standard on perimetry which was published by the XIII Concilium Ophthalmologicum 1929, Hollandia, N.V. Boek- en Steendrukkerij, Edward IJdo, Leiden, 9 pages. The present document is meant to supercede this earlier standard. Recently the Committee on Vision, the National Research Council, National Academy of Sciences of the United States of America published the "First Interprofessional Standard for Visual Field Testing," National Academy of Sciences, Washington, D.C., 1975. Certain aspects of the present document are based upon this publication. 5. Photometric Specification For proper control of visual stimuli in perimetry, provision of one or more light sources is necessary. That is, the background, test stimuli, and all supplementary targets or display fields need to be specified photometrically. The specification of visual stimuli is complex. Properly, radiant energy determinations should be made, followed by suitable luminous conversions for different field areas and stimulus conditions. For most practical clinical situations the IPS recommends that the visual stimulus in perimetry be specified in luminance units measured at the center of the entrance pupil of the eye. The visual stimulus is essentially dermed by this luminance, the direction of the stimulus in the field of view and the area of the entrance pupil of the eye. Since we often cannot control pupil size in the clinical environment, the least we can achieve is to specify luminance at the center of the entrance pupil of the eye, and to request the examiner to record this luminance and the entrance pupil size at the time of the measurement. 5 In certain conditions special additional calibration requirements exist, e.g., for short duration or colored stimuli. We ask the manufacturer to specify the operating conditions of his instrument. Ideally this would include the complete specifications of lamps, fIlters (including transmission curves), and desired operating condi tions. Luminance at the center of the entrance pupil of the eye should be specified for defmed operating conditions of properly centered light sources and associated optics. Similarly, the spectral distribution at the entrance pupil of the eye should be defined. In addition, definition of desired operating color temperature and CIE coordinates is highly desir able. A simple scheme for assuring that the instrument is functioning within reasonable tolerances of these specified values should be provided. Included would be some test of luminance and/or indication for replace ment of light sources. The international unit of luminance is the candela per meter squared, cd/m2 or cd.m ~. Other units are now regarded as obsolete. Although strictly speaking not the same units2, conversion to apostilb and milli lambert values can be made using the following relationships: IO/1T candela/m2 = I millilambert = 10 apostilbs, where IO/1T = 3.183 (approximate). While this group would prefer luminance measurement of perimetric devices by objective small field test instruments, an acceptable alternative would be to provide a measure convertible into luminance at the center of the entrance pupil of the eye. 6. Background or Adapting Luminance A. Specification of luminance For routine perimetric instruments used in clinical offices it is recommended that a value of background luminance be chosen such that it is photopic and it falls within that range of background luminances over which the Weber fraction remains constant, i.e., boL/LB = constant.3 boL is defined as 2 See discussion in the recently published U.S. Standard relative to this point. 3 For example, see E. Aulhorn, H. Harms, and M. Raabe, Documenta Ophtha!. 20, 538-556, 1966; and J. Enoch, Physiology (Olapter 3, pp 202-289) in A. Sorsby, Modern Ophthalmology, Vol I, First Ed., 1963; and the recent U.S.A. Standard refer enced above. 6 the just detectable luminance difference between test target and back ground,4 and LB is background luminance (also see section on contrast). TIle proposed background level is generally higher than that found in peri meters in use today. This setting criterion is recommended because (a) it requires less sensitive calibration equipment, (b) it is less sensitive to modest fluctuations (or changes) in light source output, (c) the result is less depen dent upon modest variations in eye pupil size, (d) visual functions are tested at clearly defined photopic levels, and (e) fixation control is easier than at low luminance adaptation levels. If this background luminance cannot be achieved, it is recommended that for routine office purposes no less than 10 candelas/m2 be used. A background luminance of 10 candelas/m 2 is below, but near the level where t.L/LB = constant over an extended range of values. Other light or adaptation levels offer advantages. Lower levels may provide an extended range of contrast values for testing, cataract patients may be better evaluated, and rod anomalies may be more effectively studied, etc. Thus, where adequate calibration capability exists and careful studies are conducted to rule out loss of confidence due to increased measurement variance, lower photopic or mesopic background luminance levels can serve a useful purpose. Similarly, higher background luminance values can be useful in tests of the visual fatigue factor or for the develop ment of color perimetric tests. Thus the IPS recommends that instruments be constructed to be capable of calibration over a range of values. The IPS suggests that the standard be a minimum test condition rather than a limiting condition. We encourage careful research on this rather complex and crucial set of questions. We recognize that stability of determinations in special disease conditions, e.g., glaucoma, cataract, chorioretinal degenerations, optic neuropathies, etc., may require use of special or specific background luminance levels and spacial purpose instruments suitable for advanced diagnostic labora tories. B. Preadaptation conditions It is highly desirable that the patient be adapted to the luminance of the background field before commencement of the perimetric test. A longer 4 The specification of ilL is somewhat arbitrary, because the probability of detecting the test target varies between 0 and lover a small range of luminances. ilL is com monly specified as the luminance increment or difference corresponding to a detec tion probability of 0.5 (50% frequencY-Qf-5eeing). 7 time period of preadaptation to this field is necessary for lower background luminance levels. It should also be longer if the patient enters the examina tion chamber from an intensely luminous environment. It is desirable that the manufacturer and examiner determine the light adaptation period which provides relatively stable response for the instrument and conditions used. Preadaptation conditions can also be important when testing individ uals manifesting certain types of pathology. c. Diffusely reflecting surface It is desirable that the background field be a diffusely reflecting surface, i.e., a non-glossy surface which at least approximates Lambert's Law. 7. Specification of the location of an Object in the Visual Field A polar coordinate system should be used when derming (a) the half meridian and (b) the eccentricity of the center of the test target, both expressed in degrees. The zero degree half-meridian is defined to the right of the patient (as seen by the patient). The specified half-meridian then proceeds counterclockwise through 3600 about the fixation target (as seen by the patient). The fixation point is dermed at having zero degree eccentricity. This assumes the patient has normal fixation. This system does not allow fine specification of the area of a scotoma or of an isopter because of non-linearity of representation,S except in the case of a hemisphere where proportional solid angles are present. This requires the center of the entrance pupil of the eye to lie at the center of curvature of the hemisphere. In this special case, two equal solid angles located at different loci in the visual field subtend equal areas on the sur face of the hemisphere. This condition does not exist if a flat test surface is used for examination of the visual field, e.g., when a tangent screen is employed. The same statement may be applied to the cartographic deforma tion of the field as expressed on a flat sheet of paper. There exist carto graphic projections which attempt to represent areas proportionately. It is highly desirable to keep the tolerances for location, registration, and replication of test object position within narrow limits. If this cannot be achieved, the reliability of subsequent determinations is limited, espec ially for static (target not moving) perimetry. Accuracy is limited by the size and nature of the fixation target, the stability of patient fixation, 5 Distinguish between ability to specify a location and an area. 8 and the mechanical capabilities of the perimetric device.6 In tum, these factors influence the selection of the smallest useful target size. Optimal fIXation targets have not yet been defmed. This is an important question which needs clarification through research. Obviously, it is desirable to monitor patient fixation directly. When central vision is impaired, special fixation targets or displays are often needed. It is desirable that measured test points should be indicated on the test record in an obviou~ manner. Oearly the more points tested, the better the characterization of the visual field. The more repetitions of evaluations made at a single point, the greater the reliability of the determination. It is desirable that one or more points be evaluated more than once in order to defme the approximate reliability of the test. It is desirable that interpolation or analysis techniques employed be clearly defmed. 8. Target Specification (non-photometric) A. Size, distance, and form Ideally target dimensions should be specified in terms of the solid angle subtended at the center of the entrance pupil of the eye and measured in steradians. Practically, this is not done, nor do we recommend such desig nation as essential at this time. A conceptually simpler scheme is the specification of the diameter of the target in terms of visual angle subtended at the center of the entrance pupil of the eye. This assumes that a round target is located at the point of fixation. If the target is not round, the diameter of the equivalent round target subtending the same area at the point of fIXation may be used. Target diameters should be expressed in degrees, minutes, and seconds of arc. It is highly desirable to specify test target distance from the eye, because lumi nance is dependent on test distance for perimetric test targets of small dimension. (The same is not true for extended background fields.) Thus, for proper specification, it is highly desirable that both angular subtense and target distance be specified. Other factors, such as image blur resulting from several causes, also make specification of target distance desirable (see below, Image sharpness). As an example of proper specification, a 6 Lens factors also influence accuracy of location and re.,Iocation of a target in the visual field. Apparent location of a target is influenced by power and centration of the lens correction, vertex distance, base curve and lens thickness. It is desirable that the lens(es) used and the vertex distance be noted. There is an advantage in keeping vertex distance small.