Pediatric Color Vision Testing
Jay M. Cohen
This chapter, written for the clinician, is intended to be a practical and realistic clinical guide for the color vision testing of the pediatric patient. Discussion is limited mostly to color vision tests that are widely used and accepted, and are currently commercially available. The physiology and development of color vision and color deficiency is covered elsewhere in the text (see Chapter 8).
Color is an attribute of human vision that adds aesthetics and diversity to the visual percept. It provides unique and characteristic information about the world and the objects in it, and it is often used as the primary descriptor for identifying specific items (e.g. “the blue one over there”).
Many people, however, have abnormal color vision and are unable to utilize color information with the same efficiency or surety as the rest of the population at large. These individuals, mostly males, are at a distinct disadvantage or are incapacitated when faced with performing activities based heavily on color information. The seriousness of the consequences of this handicap can range from merely inconvenient to life threatening.
Although patients are often labeled by the term color blind, this is a misnomer. These patients do perceive color, albeit differently than those with normal color vision. For the purposes of this chapter, the term color deficiency, a more apt description, is used in place of color blindness.
Most persons with color deficiencies are unaware, or at best minimally aware, of their problem, and it behooves the conscientious eye care practitioner to identify and educate these patients about the implications of their condition, both for their own personal safety and knowledge as well as for the safety of the general public.
Because color is such a vital part of the preschool and elementary school experience, it is especially important to identify children with color deficits at the earliest possible age. The rationale for early identification is twofold. First, short-term implications involve the need to isolate those children with the potential for academic difficulties because of their inability to respond to color-based curricular items and, if necessary, to plan appropriate interventions via altered curricula. Second, long-term implications are concerned with career counseling and the discussion of realistic options and goals for higher education and employment.
The main emphasis of this chapter is on the diagnosis and classification of hereditary red-green color vision defects, the overwhelming bulk of color anomalies. Hereditary blue-yellow defects are so rare that few color vision tests bother to check for them. Likewise, acquired color defects are not common in a normal pediatric population, especially without other more serious and obvious visual signs and symptoms.
Color Vision History
As with most aspects of health care, the first step in assessing color vision begins with a good case history. Hereditary red-green color vision defects, which affect approximately 8% of males and 0.5% of females, are transmitted in a predictable manner via the x-chromosome. A positive history of color vision problems on the maternal line (grandfather, sibling, or cousin) places a child, particularly males, in a high-risk category and warrants more aggressive screening protocol. A positive history from both parents should red flag even females for careful scrutiny.
It should be appreciated, however, that taking a family history for color vision anomalies has its limitations. Accurate information on lineage is often lacking because many affected family members may be unaware of a problem, or they may actively hide its presence. Therefore, although a positive history is helpful, a negative history is of uncertain value.
Another aspect of the case history for children is the parents’ report of color vision performance problems. When questioning parents about their child’s color ability, it is important to distinguish between color confusions and color naming skills. Color discrimination is present by 2 to 3 months of age (1), whereas accurate color naming may not develop until a child is 4 to 6 years of age (2). A child who is unable to discriminate between different confusion colors is likely to have a color deficiency, whereas a child who is unable to consistently name colors may be perfectly normal.
As noted earlier regarding lineage, parental observation of color vision performance can be misleading because of the child’s desire to conform and the natural coping strategies used by color-deficient children. For example, many of my adult color-deficient patients report that, as children, they would only color with whole crayons so that they could read the color name on the wrapper. Another patient, who was deuteranopic, recounted how his family car appeared brown to him, however, he called it green because everybody else did.
Although adult patients may deny or trivialize their color defect, on careful questioning, those with significant anomalies will usually begrudgingly admit to some difficulties and to an awareness of their variance from the color normal population. Thus, despite its shortcomings a good case history still remains one of the key factors in identifying potential color vision abnormality.
Significance of Test Design
The actual determination of color deficiency, however, is based on the results of clinically administered color vision tests. A wide assortment of color vision tests has evolved over the years. Many tests were never commercially available or are no longer available, and many tests have been shown to be unreliable. Our discussion will be limited to those tests with established reliability that are commonly available and used in the United States.
Doctors use many types of tests for assessing color vision. They are designed for a variety of purposes and measure different aspects of the color sense. Therefore, the proper administration of a color test requires a basic understanding of the test’s design and strategy to ensure that it is appropriate for the age of the patient as well as the color function it is presumed to be measuring.
No single color vision test can provide complete information about a patient’s color vision status. Instead, a battery of tests is selected to separate those with normal color vision from those who are color defective and then to further classify and grade the severity of any defects. Additional tests can be given to rate the patient’s color discrimination abilities to determine that patient’s aptitude for performing color-based activities.
The design of a color vision test will usually be influenced by its intended function. Screening tests are meant to be short, sensitive, and easily administered tests that differentiate patients with normal color vision from color-deficient patients, or those with generally good color skills from those with problematic color skills. They can also include provisions for classifying the type and severity of the defect.
Performance tests are designed to evaluate the level of some aspect of color ability. These tests are usually lengthier and broader in scope than screening tests. Tests that utilize color from
the entire color circle give a better indication of real world function than those that use only a narrow range of colors along the red-green confusion axes.
the entire color circle give a better indication of real world function than those that use only a narrow range of colors along the red-green confusion axes.
Performance tests, such as lantern tests, attempt to simulate or duplicate color conditions of a particular task to predict those patients who will successfully manage the activity and those who will not.
Differences in design between tests can lead to differences in results. This is not unexpected and gives some balance to the testing process. Keep in mind when evaluating the results of any color vision test or tests is that they present a narrow, unnatural, preselected set of parameters that capitalize on the known vulnerable portions of the color circle. Be cautious not to overstep the bounds of the test design and to make unwarranted generalizations about a patient’s color skills in the uncontrolled natural environment where a host of additional cues and adaptive behaviors may improve performance.
Color Vision Tests
Color vision tests are made using a variety of strategies and tasks for measuring color vision and can be categorized by their strategy design. Test design strategies include aspects of color mixing, color matching, color discrimination, color arrangement, color naming, and color confusion.
Tests within a particular category are generally very similar and, within reason, will yield similar outcomes. Tests in different categories, however, do not necessarily measure the same color attributes and can produce differing results. When constructing a color vision test battery it is best, therefore, to use test probes with a variety of designs to gain information on the different aspects of the patient’s color vision and to develop a fuller understanding of that patient’s color abilities and limitations.
Anomaloscopy
The anomaloscope presents the patient with the task of mixing a spectrally pure red light with a spectrally pure green light to match a spectrally pure yellow light. It is the gold standard of color vision testing, and it is the only test that will definitively identify the abnormal retinal pigment (protan vs. deutan) and determine the number of functioning pigments (dichromacy vs. anomalous trichromacy).
The anomaloscope, however, is not a clinical office test, and it is not a pediatric test. It is an expensive instrument requiring a sophisticated tester to administer it, and it is rarely found outside of a teaching or research institution. The testing protocol is relatively complex and is usually beyond the attentional and cognitive capabilities of most young children. If anomaloscopy is deemed necessary, it is best to wait until the child is older (at least 8 years old) and to refer to a clinic providing the test.
Pseudoisochromatic Plates
Pseudoisochromatic plates (PIP) are the major clinical test of color vision. They are relatively inexpensive, need short administration time and have low cognitive demands. On the other hand, PIP tests require good attentional and fixation skills, as well as good contrast and figure-ground capability.
The PIP are screening tests, in that they differentiate color normalcy from color variants. They do not determine the type of defect nor grade the severity of the defect. Only those tests incorporating additional plates especially designed for that purpose could be used to classify and grade the color vision defect. The common clinical practice of the using the number of screening plates correctly identified as a means of gauging severity of a color vision defect has no scientific basis.
The test plates consist of a mosaiclike array of colored dots which, when viewed under the proper light source, presents letters, lines, or geometric shapes constructed of confusion colors paired to the background. Those with normal color vision will see a particular target, whereas color-deficient patients will see a different target or nothing at all.
The two major plate types are the vanishing plate and the transformation plate. Vanishing plates are made so that people with normal color vision will see a target and those with color deficiency will not see a target. This design is very frustrating for the color-deficient patients
because they are aware they are doing poorly, and the frustration and disbelief builds with each additional missed plate.
because they are aware they are doing poorly, and the frustration and disbelief builds with each additional missed plate.
Transformation plates are constructed so that the person with normal color vision sees one target and the color-deficient patient sees a different one. This is a much less stressful condition than the vanishing plate because, by seeing a target, the patient is unaware of the poor showing.
Classification plates are a variation of the vanishing plates. They present two targets, one of which is more visible to patients with protan defects and the other of which is more visible to patients with deutan defects. The differential response determines the patient’s classification. An estimate of severity can be made as well by using plates with different color saturations.
Although classification plates are useful, keep in mind that their results are not conclusive. Results of classification plate tests are not always in agreement with anomaloscopic findings and frequently do not agree with the results of other plate tests or arrangement tests. The reason it is necessary to perform a battery with multiple color tests is to establish a consensus of the findings. For a pediatric population, the most useful PIP are the Ishihara plates, the Hardy-Rand-Rittler plates, and Color Vision Testing Made Easy plates.
Ishihara Plates
The Ishihara Pseudoisochromatic Plate Test is the most widely used and accepted clinical color vision test. It is an unrivaled screening test for red-green color deficiencies with basic classification capability. The most appropriate test versions for the pediatric practice are the concise edition and the Test for Unlettered Persons.
The concise edition is a quick and efficient test with a total of 14 plates. The first 11 plates are for screening (1 demonstration plate, 9 number plates, and 1 wiggly line plate), and the last 3 are for classification (2 with numbers and 1 with wiggly lines). The wiggly line plates are intended for use by children (or adults) who may have difficulty with numbers (Fig. 19.1).
The Ishihara Test for Unlettered Persons uses only a circle, a square, and wiggly lines as targets. It works well with younger patients who may not respond well to numbers. It consists of 8 plates, including three-demonstration plates (circle, square, and wiggly line), 4 screening plates (circle, square, and two wiggly lines), and 1 wiggly line classification plate (Fig. 19.2). It is easy to administer and I have gotten responses on children as young as 2 years of age, although it is better suited for those 3 to 5 years.
Hardy-Rand-Rittler Test
After the Ishihara plates, the Hardy-Rand-Rittler Test (HRR) is the second-most widely accepted plate test. Screening sensitivity is less effective than the Ishihara plates, and a small number of patients who pass the HRR will fail the Ishihara test. The HRR, however, does have a more detailed classification capability.
The HRR is also one of the few plate tests that tests for blue-yellow color vision defects. The original test was produced by the American Optical Company as the AO-HRR Test and has been unavailable for more than 30 years. The test was recently re-engineered and is distributed by Richmond Products as the fourth edition of the HRR. Preliminary results indicate that the new edition has improved performance relative to the original (3).
The test consists of 24 plates (4 demonstration plates, 2 blue-yellow screening plates, 4 red-green screening plates, 10 red-green classifications plates, and 4 blue-yellow classification plates) using geometric shapes (a circle, a triangle, and a letter x) as targets (Fig. 19.3).
Each plate contains either one or two shapes and a patient must correctly identify each shape and its location on the page to gain credit. The shapes are familiar to even very young children and, by modifying test protocol by permitting shape matching and shape tracing, the age range over which children respond to the test can be expanded.
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