|Year : 1998 | Volume
| Issue : 2 | Page : 93-96
A method of scoring automated visual fields to determine field constriction causing blindness
L Dandona, A Nanda
Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India
Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad
Source of Support: None, Conflict of Interest: None
Blindness is usually defined by visual acuity criteria. Patients with markedly constricted visual fields are visually impaired even if they have good visual acuity. To our knowledge, no standardised criteria exist to determine the extent of constriction for fields done with the currently used automated static perimetry. The purpose of this study was to suggest a simple method to do so which would help in determining blindness due to field constriction. We reviewed a number of constricted visual fields obtained with Humphrey automated static perimetry. The central 30° field was divided into six concentric zones. By trial and error, we devised criteria for defining visual field constriction based on absolute loss of sensitivity (≤0 dB) and relative loss of sensitivity (≤5 dB). We suggest that if a zone has at least 75% test points ≤0 dB and no point >10 dB, it be considered to have absolute loss of sensitivity for the purpose of defining visual field blindness. Two exceptions to this are also suggested to prevent this criterion from becoming too rigid. Examples are shown to demonstrate application of these criteria in defining blindness due to visual field constriction to <10° as suggested by the World Health Organization. Standardised determination of visual field constriction with automated perimetry could be useful in more accurate estimation of blindness in surveys, as well as in assessing eligibility for being classified as blind for legal benefits.
Keywords: Visual fields, automated, constriction, criteria, blindness
|How to cite this article:|
Dandona L, Nanda A. A method of scoring automated visual fields to determine field constriction causing blindness. Indian J Ophthalmol 1998;46:93-6
Though the World Health Organization (WHO) has proposed that best corrected visual acuity <3/60 or a visual field <10° around central fixation in the better eye be used as criteria for defining blindness,, only the visual acuity criterion is used commonly. One reason for this is that an easily usable method of defining visual field constriction with the currently used automated static perimetry is not available. Though two recent surveys have reported blindness due to visual field constriction <10° with automated perimetry,, the exact method used to determine field constriction was not defined.
Functional visual impairment is commonly observed in patients with markedly constricted visual fields even if they have good visual acuity. We felt the need for an easily usable standardised method of determining visual field constriction with automated static perimetry for use in a population-based study of visual impairment., In this background, we propose such a method applied to the commonly used central 30-2 and 24-2 threshold Humphrey automated static visual fields.
| Materials and Methods|| |
More than 50 reliable constricted visual fields obtained at our institution with 30-2 or 24-2 threshold strategy of Humphrey automated static perimetry were reviewed. Criteria for visual field constriction were tested for definitions based on absolute sensitivity loss (≤0 dB) and relative sensitivity loss (≤5 dB), which are approximately equivalent to not being able to see V4e and IV4e stimuli in Goldmann kinetic perimetry, respectively., The definition of relative sensitivity loss was considered with the assumption that sensitivity of ≤5 dB may not be enough for practical functioning.
The distance of each stimulus from the centre in the 30-2 and 24-2 Humphrey threshold visual fields was calculated based on the information that the stimulus closest to the central horizontal and vertical meridians are 3° away from the respective meridians, and the separation between two stimuli on the same horizontal or vertical meridian is 6°. Six concentric zones for 30-2 fields, and five for 24-2 fields, were made such that the points similar in distance from the centre were included in the same zone [Figure:1].
In order to arrive at consistent definitions, a trial and error method was used to test various criteria for determining visual field constriction.
| Results|| |
The criteria for visual field constriction that seemed to be a reasonable balance between being neither too strict nor too lenient are shown in [Figure:2]. We suggest that for defining field constriction due to absolute sensitivity loss, each concentric zone has to have at least 75% of the points ≤0 dB, and no point should be >10 dB with the exception that a single point >10 dB could be ignored if it is surrounded by points on all sides ≤0 db or if the zone inner to the one under consideration meets the constriction criteria. For field constriction due to relative sensitivity loss, each concentric zone has to have at least 75% of the points ≤5 dB, and no point should be >10 dB with the exception that a single point >10 dB could be ignored if it is surrounded by points on all sides ≤5 db or if the zone inner to the one under consideration meets the constriction criteria. At those test points where sensitivity is checked twice, the second value (in brackets) is considered.
[Figure:3] shows an example of visual field constriction <5° as defined by absolute sensitivity loss in an eye with visual acuity 6/24. This would meet the WHO blindness criterion of visual field <10°. The field constriction <15°by absolute sensitivity loss in an eye with visual acuity 6/6 is shown in [Figure:4]. This example also illustrates the proposed exception when an isolated point >10 dB (zone I) can be ignored because it is surrounded on all sides by points ≤0 dB.
Visual field constriction <10° by relative sensitivity loss in an eye with visual acuity 6/9, and <20° in an eye with visual acuity 6/15, are shown in [Figure:5] and [Figure:6], respectively. [Figure:5] illustrates the other proposed exception when an isolated point >10 dB (zone I) can be ignored because the inner zone (II) meets the constriction criteria.
| Discussion|| |
An easily usable standardised method of determining visual field constriction with automated perimetry to assess visual field blindness would be helpful in obtaining more complete data on prevalence of blindness from population-based surveys. It would also be helpful in assessing the eligibility of patients with good visual acuity but constricted fields for legal benefits available to the blind.
The method proposed by us for defining visual field blindness with the commonly used 30-2 or 24-2 threshold Humphrey visual fields can be used easily by superimposing a transparency with the concentric zones [Figure:1] on the visual field printout.
The WHO criteria of blindness are best corrected visual acuity <3/60 or central visual field constricted to <10° in the better eye. [1, 2] In India the commonly used definition of blindness has been presenting visual acuity <6/60. The corresponding visual field constriction that could qualify as blindness may be considered as <20°. Though this is arbitrary, a justification for this could be that the visual angle doubles from 6/60 to 3/60, and if visual field constriction <10° is considered equivalent to visual acuity <3/60 for defining blindness in the WHO criteria, field constriction <20° could be considered equivalent to visual acuity <6/60.
Whether the absolute or relative sensitivity loss criteria proposed by us should be used in defining field constriction is debatable. Sensitivity of ≤5 dB with Humphrey threshold perimetry is approximately equivalent to not being able to see the IV4e stimulus of Goldmann perimetry. It is possible that sensitivity between 1-5 dB in the central field may not be enough for practical visual functioning, though we are not aware of any definite proof of this. Therefore, a study correlating the magnitude of sensitivity loss that leads to difficulty for the patient would help answer whether the stricter absolute sensitivity loss (≤0 dB) criteria are necessary or the relative sensitivity loss (≤5 dB) criteria would suffice in defining blindness based on visual field constriction. Until such data are available, we suggest that the proposed absolute sensitivity criteria (≤0 dB) be used to define visual field blindness.
That our proposed method of visual field scoring deals only with the central 30° visual field may be considered a limitation. However, the objective of our visual field scoring was to be able to readily determine if the central visual field is constricted to <10° or <20°. This would help in deciding if blindness due to visual field constriction is present. We have proposed criteria for the central 30-2 and 24-2 threshold visual fields because these are the most commonly used fields in clinical practice. These criteria can also be modified for use with automated perimeters other than Humphrey by taking into account the threshold equivalensce between the different perimeters.
An important question regarding definition of blindness due to visual field loss is whether types of field loss other than constriction should also be considered. The answer to this can be obtained by investigating what extent and types of visual field loss lead to functional disability. Until reliable information regarding this is available, the field constriction criteria may have to be used.
A method of scoring visual fields based on relative weights given to different points in the field has previously been proposed., Though this is a sophisticated method of scoring visual fields, it does not readily provide an answer to the question when to score the central visual field as constricted to <10° in the better eye to meet the WHO criterion of blindness. The method proposed by us, though less elaborate as compared with this previous method, does provide this answer. This application of the proposed method may find use in surveys that attempt to detect blindness due to constriction of visual fields done with automated perimetry in addition to that due to decreased visual acuity.
| References|| |
|1.|The Prevention of Blindness. WHO Technical Report Series, No 518
. Geneva: World Health Organization; 1973. p 10-11.
|2.|International Statistical Classification of Diseases and Related Health Problems. Tenth Revision
. Geneva: World Health Organization; 1992. Vol 1. p 456-57.
Foster PJ, Baasanhu J, Alsbirk PH, Munkhbayar D, Uranchimeg D, Johnson GJ. Glaucoma in Mongolia: a population-based survey in Hovsgol Province, Northern Mongolia. Arch Ophthalmol
Murdoch IE, Jones BR, Cousens S, Liman I, Babalola OE, Dauda J, et al. Visual field constriction as a cause of blindness or visual impairment. Bull World Health Organ
Dandona R, Dandona L, Naduvilath TJ, Nanda A, McCarty CA. Design of a population-based study of visual impairment in India: the Andhra Pradesh Eye Disease Study. Indian J Ophthalmol
Dandona L, Dandona R, Naduvilath TJ, McCarty CA, Nanda A, Srinivas M, et al. Is current eye-care-policy focus almost exclusively on cataract adequate to deal with blindness in India? Lancet
Anderson DR, Feuer WJ, Alward WLM, Skuta GL. Threshold equivalence between perimeters. Am J Ophthalmol
Lynn JR, Fellman RL, Starita RJ. Principles of perimetry. In: Ritch R, Shields MB, Krupin T, editors. The Glaucomas
. St. Louis: Mosby-Year Book, Inc; 1996. Vol 1. p 491-521.
Anderson DR. Perimetry With and Without Automation
. St. Louis: The C.V. Mosby Company; 1987. p 50.
Esterman B. Grid for scoring visual fields. II. Perimeter. Arch Ophthalmol
Esterman B. Functional scoring of the binocular field. Ophthalmology