Subjective Perception of Visual Distortions or Scotomas in Individuals with Retinitis Pigmentosa
Print edition page number(s) 50-55
Graduate fellowships to Walter Wittich were provided by the Canadian Institutes for Health Research, the McGill Tomlinson Foundation, and CNIB. This study was partially funded by the Institut Nazareth et Louis-Braille and the Fondation maladies de l'\kil, in partnership with the Vision Research Network of the Fondation de recherche en santé du Québec.
It is often assumed that persons who develop ocular disease have some form of visual experience that makes them aware of their deficits. Such experiences are frequently their reason for seeking the advice of an eye care professional. For example, those who develop cataracts often complain of blurry vision; those with macular holes or macular degeneration report distortions in their central vision, and those with diabetic retinopathy report that their vision has become "patchy" or that they see shadows. However, in the case of peripheral field loss or decreasing vision in dim lighting, as in retinitis pigmentosa, for example, symptoms are more obscure and may not be as easily identified by the persons who have them. This group of genetic diseases progressively reduces peripheral vision through the degeneration of photoreceptors.
Typically, these persons initially experience trouble with night vision under dim lighting conditions or may be labeled "clumsy" because they repeatedly bump into obstacles in their visual periphery.
In the case of retinitis pigmentosa, the large majority of research on visual function has focused on peripheral field loss, since its progressive decline is the hallmark symptom of this disease. From the subjective perspective of persons with retinitis pigmentosa, however, the remaining central vision becomes more important. Some aspects of central vision, such as contrast sensitivity (Alexander, Barnes, Fishman, Pokorny, & Smith, 2004; Alexander, Derlacki, & Fishman, 1995), color vision (Massof, Johnson, & Finkelstein, 1981; Sandberg & Berson, 1977), or spatial vision (Turano, 1991; Turano & Schuchard, 1991) have been shown to be affected early on. The latter studies have generally agreed that there is variability among persons with retinitis pigmentosa in that some display visual abilities comparable to typical observers but others deviate from normality, but, not always in predictable patterns. Little is known about whether these persons subjectively experience changes in spatial vision (that is, visual distortions).
Only one questionnaire that evaluates functionally relevant visual distortions has been published in a peer-reviewed scientific journal. Arimura and colleagues (2006) correlated perceptual distortions that are associated with the development of a macular hole with scores on their metamorphopsia questionnaire. The purpose of their study was to develop a subjective measure that provides information about the extent of visual distortion. Their results indicated that perceived distortions in everyday life situations that are due to a macular hole, as measured on their questionnaire, and quantified distortions on their M-Chart test (similar to a one-line version of the Amsler grid) correlated significantly.
Arimura et al.'s 10-item measure contains questions that evaluate far distance vision (such as the perception of telephone poles), intermediate distance vision (such as the outline of a television set), and near vision tasks (reading). It was originally developed for persons with macular disease; however, if those with retinitis pigmentosa experience visual distortions in their remaining central field, some of the items in this measure may also be appropriate for evaluating perceptions of spatial vision of individuals with that condition. The goal of administering this questionnaire to individuals with retinitis pigmentosa was to investigate whether these individuals are indeed experiencing visual distortions or scotomas or both, since no studies have yet addressed this question.
The protocol was approved by the Research Ethics Committee of the Center for Interdisciplinary Research in Rehabilitation (CRIR) of Greater Montreal, the supervising research ethics board for the Institut Nazareth et Louis-Braille (INLB), and the Institutional Review Board of the Jewish General Hospital in Montreal, in accordance with the Canadian Tri-Council Policy Statement of ethical conduct for research involving humans.
A review of the charts of patients at the INLB location in Longueil, Quebec, in September 2008 identified 614 individuals with a diagnosis of retinitis pigmentosa. Of these patients, 68 fulfilled the recruitment criteria of visual acuity better than 20/60 in at least one eye and a horizontal visual field diameter of larger than 6 degrees in at least one eye. These parameters were required for a larger research protocol (Wittich, Faubert, Watanabe, Kapusta, & Overbury, in press). A total of 28 participants were enrolled, 2 of whom were then excluded from the analysis because they had insufficient acuity to complete the protocol.
During the enrollment visit, the purpose and procedure of the experiment were again explained, and informed written consent was obtained. ETDRS acuity with habitual refraction was taken monocularly at a distance of 4 meters (about 13 feet). All the participants were screened for the integrity of their cognitive status using the Montreal Cognitive Assessment (Nasreddine et al., 2005). Relevant characteristics of the participants, such as age and the diameter of their visual field (Goldman III4e target), were extracted from the patients' files.
The authors of the questionnaire (Arimura et al., 2006) provided an English version along with permission to use it in our study. The translation was adjusted for cultural content (Item 4 regarding the Japanese meditation room was replaced with a question about the appearance of door frames) and then translated into French. The 10 questions all address visual distortion of the items in the examples with either one eye or both eyes open. The participants were instructed to answer the questions as though they were wearing their habitual correction in performing each of the response-item tasks. In each case, the participant had the option of responding on a 4-point Likert scale, ranging from 0 = not at all to 3 = a great deal, with 4 = none of the above, in case the item was not relevant to the person's situation. The questionnaire was administered orally.
Results and discussion
When we administered the questionnaire, it soon became apparent that the original scoring instructions (the sum of all items) would not result in data that would be analyzable in a meaningful way, since few persons indicated distortions or scotomas or both. In addition, persons with retinitis pigmentosa rarely consciously perform visual tasks monocularly, such as was described in the questionnaire. Therefore, the responses reported here are predominantly from a qualitative perspective.
Of the 26 participants who completed the questionnaire, 17 (65%) did not report any symptoms of metamorphopsia, while 9 (35%) responded to one or more items with the perception of at least a minimal distortion. The item that elicited the most frequent complaints among 7 of the 9 participants with symptoms was Question 8, which states, "Do the parts of people's faces appear missing or distorted to you?" With regard to the visual acuity and visual field characteristics of these 9 individuals, it is worth noting that the participants had widely ranging visual acuities and that all but one had visual field diameters of less than 18 degrees in each eye. Overall, visual acuity in the better eye ranged from 20/18 to 20/66, M = 20/34 (6/5.4 to 6/19.8, M = 6/10.7). Horizontal monocular field diameters in the better eye ranged from 6.0 to 65.4 degrees (M = 21.1, SD = 12.1). When we compared the diameters of the visual fields of the participants who perceived distortions or scotomas with those who did not, we found that the rank order of field diameters differed statistically significantly between the two groups, U = 108.5, p < .05, whereby the participants with symptoms had smaller visual field diameters. We chose the nonparametric Mann-Whitney U test, since the data were not normally distributed. Furthermore, the individual with the most complaints across the items was also the one with the most restricted visual field in this group. Visual acuity did not differ between the groups.
Even though a quantitative analysis of the results of the questionnaire was not suitable, given the infrequent occurrence of distortions or scotomas, the qualitative findings revealed interesting trends. Two-thirds of the participants did not report any symptoms of metamorphopsia or scotomas in their central visual fields. During the initial stages of retinitis pigmentosa, predominantly peripheral areas of the visual field are affected, slowly reducing the size of the usable field. In addition, the central retina and cone function are often affected only by functional loss in the later stages of retinitis pigmentosa. Therefore, individuals with larger fields may not yet display symptoms because their maculae are still healthy. The individual with the most restricted visual field in the group of participants who experienced visual symptoms was also the person who reported the most complaints about distortions or scotomas in everyday viewing experiences, while all except one individual in this group had total horizontal field diameters of less than 18 degrees. This finding further supports the previous point that field loss that is advancing to involve the macula results in more prominent symptoms of functional distortions or scotomas.
The item in which most participants reported symptoms of distortion was the one referring to viewing faces. This finding is intriguing because the ability of persons with retinitis pigmentosa to perceive and discriminate faces has largely been neglected in the research literature but has been addressed in other areas of low vision research. Most reports have focused on persons with central visual field loss caused by such diseases as macular degeneration (Boucart et al., 2008; Bullimore, Bailey, & Wacker, 1991; Tejeria, Harper, Artes, & Dickinson, 2002). Intuitively, it is clear that the ability to see or discriminate a face would become compromised when central vision is impaired, since this visual skill depends on both acuity and contrast sensitivity and is facilitated by the integrity of the central retinal region (Melmoth, Kukkonen, Makela, & Rovamo, 2000). Barnes and Schuchard (2009) presented data indicating that persons with retinitis pigmentosa have longer reaction times when discriminating faces, especially when their contrast sensitivity is more impaired. However, beyond that study little is known about the perception and discrimination of faces by persons with retinitis pigmentosa.
Future studies in this area should include a measure of contrast sensitivity, since the ability to discriminate faces is related to the level of contrast sensitivity across several spatial frequencies (Melmoth et al., 2000). The findings regarding visual distortions and scotomas in persons with retinitis pigmentosa and these individuals' resultant ability to discriminate faces point to a potentially interesting new direction for research. Although this study did not have a sufficient number of participants or types of data to draw conclusions and was limited by methodological concerns, we believe that the topic is interesting and suggest that a larger and more comprehensive data set may produce significant results. Given that the data were collected in a rehabilitation center, no assessment of clinical variables was available, such as OCT (optical coherence tomography) scans of the macular area or a detailed ophthalmic examination to establish the presence of cystoid macular edema or early cataracts. Future investigations in this area should also consider microperimetry of the macular region because it is possible that some persons may have paracentral or midperipheral scotomas that interfere with spatial vision but do not affect central fixation.
In this context, some of the limitations of the questionnaire become apparent. The questions generally asked whether an object appeared "missing or distorted." This wording poses a problem, because if the participant confirmed the presence of such a symptom, it was not actually specified whether part of the visual information was missing or distorted rather than both missing and distorted. Although missing an object may indicate larger central or paracentral scotomas, an object that appears distorted may indicate macular edema or some perceptual distortions in or around a scotoma that may be due to perceptual "filling in" (Wittich, Overbury, Kapusta, Watanabe, & Faubert, 2006). Also, the questionnaire did not ask whether the presence of missing or distorted information interferes with the discrimination of faces, which is a vital aspect of social interaction. In addition, the scoring system of the measure will need to be adjusted because, in its original version, each question is considered for the right eye and the left eye separately, as well as for binocular vision. In the case of unilateral diseases, such as macular holes, questions about monocular vision are more intuitive to individuals who frequently check their own vision by closing the healthy eye; however, for bilateral conditions, such as retinitis pigmentosa, individuals have difficulty answering questions about monocular functioning.
Given that only a small number of questions were sensitive enough to detect functional effects in central vision among this group of persons with retinitis pigmentosa who had relatively good acuities (better than 20/60 or 6/18), some of the content will need to be revised to make this measure more informative for both clinicians and rehabilitation specialists. For example, clinicians may specifically investigate potential causes of distortions or scotomas if patients report such symptoms, while orientation and mobility instructors may be interested in perceived distortions of space when they teach skills related to independent travel. In addition, revised questions could include topics on color vision, depth perception, or visual skills under various luminance conditions. Such questions should also include more precise information about the viewing distance at which the visual tasks are being accomplished. All this information could identify areas of need for medical and rehabilitation services and ultimately improve the quality of life of persons with retinitis pigmentosa.
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Walter Wittich, Ph.D., postdoctoral fellow, Centre de recherche institut universitaire de gériatrie de Montréal, University of Montreal, and research coordinator, MAB-Mackay Rehabilitation Center, 7000 Sherbrooke Street West, Montréal, Québec, H4B 1R3, Canada; e-mail: <firstname.lastname@example.org>. Donald H. Watanabe, M.Sc., research assistant, MAB-Mackay Rehabilitation Center; e-mail: <email@example.com>. Michael A. Kapusta, M.D., FRCSC, ophthalmologist-in-chief, Department of Ophthalmology, McGill University, Jewish General Hospital, 3755 Cote-Sainte-Catherine Road, E-008, Montréal, Québec, H3T 1E2, Canada; e-mail: <firstname.lastname@example.org>. Olga Overbury, Ph.D., associate professor, School of Optometry, University of Montreal, and Department of Ophthalmology, McGill University, Case postale 6128, succursale Centre-Ville, Montréal, Québec, H3C 3J7, Canada; e-mail: <email@example.com>.
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