Seeing the Whole Picture - the Quality of Vision

Bruce Rosenthal, OD, FAOO (Chair of the AMD Alliance, and Chief, Low Vision Lighthouse International)
January 24, 2003 (Reviewed: January 21, 2005)

Introduction

Age-related macular degeneration (AMD) can cause a progressive loss of central vision amongst people affected by the disease. Without treatment, the more aggressive form, so-called wet AMD, usually results in a devastating loss of central vision. AMD is the leading cause of legal blindness in people over 50 in the western world, affecting about 500,000 patients worldwide each year.

Visual impairment can dramatically reduce the patient's quality of life.¹, ², ³, ⁴ People with impaired vision are less independent, e.g. if they cannot drive, they are dependent on others for travel and shopping.⁵ They can also have difficulty with performing many easy daily tasks, such as using kitchen utensils, walking around a strange area, and recognizing facial features from across a room.⁶ The impact of reduced quality of life is also demonstrated by the fact that visually impaired people need more help from their families, use more community social support services, and are more likely to be admitted to a nursing home than the general elderly population.⁷, ⁸

Studies have also shown that a high proportion of serious falls in the elderly, including hip fractures, are due to impaired vision, as are an increase in recurrent falls.⁹, ¹⁰

When considering emotional and personal lives, people with vision loss, such as that caused by AMD, are more likely to suffer mental health problems compared to individuals with normal sight. This results in increased occurrence of depression and, more worryingly, increased thoughts of suicide in this group.¹¹, ¹²

Interestingly, in one study the emotional distress associated with retinal disease was found to be greater in people who had the disease for shorter periods of time. This suggests that patients, if motivated and given time, can learn to cope with their disability, to some extent.¹³

These findings correspond with recent market research which found that AMD patients also frequently feel socially isolated - cut off from family, friends and their community - and a burden on their loved ones.⁴

Standard clinical practice is to evaluate visual function by measuring just the visual acuity (VA), either in terms of distance visual acuity (reading letters at a set distance, the "letter chart" familiar to all of us), or near visual acuity (reading letters at a set distance of 40 cm). VA measures our ability to detect spatial changes at high contrast.⁶ Historically, VA measurements have been equated to a person's ability to see during everyday situations and tasks.

However, as our understanding of sight and eye diseases has increased, it's become clear that, due to the highly complex nature of vision, other measures of visual function are needed to properly quantify a person's true quality of vision. For example, reading a text is dependent on several components of vision, such as the ability to focus on a fixed point, and not just VA. Indeed, in patients with eye disease visual components are affected to varying degrees, which means that visual impairment differs between individuals.⁶

How do we assess Quality of Vision?

What are the visual components that are key to quality of vision? Assessment of overall vision quality can take the following into account:

• Visual acuity
• Contrast sensitivity: the ability to distinguish between degrees of contrast e.g. seeing text on a page, the edge of a step
• Visual field measurement: presence of any "blind spots", known as scotoma
• Fixation behavior: the ability to focus on a fixed point, which is very important in tasks such as reading
• Metamorphopsia: distortion of the visual image, i.e. the image appears to bend or be at an angle

Together these visual components provide a more accurate measure of a person's ability to perform everyday tasks. By considering these critical components it's possible to understand the patient's overall quality of vision and how this affects their quality of life. It's therefore important to measure more than VA when evaluating an individual's visual function.

Contrast sensitivity is increasingly recognized as being essential to quality of vision. Contrast sensitivity measures the eye's ability to distinguish between areas of different contrasts (put simply, the better your contrast sensitivity the more able you are to distinguish between areas with little contrast), e.g. seeing lines, edges and fine details

VA only measures sight at one level of contrast - black letters on a white background - and so provides no information on contrast sensitivity. There are specific activities that are particularly contrast-dependent, e.g. watching TV, cutting food on a plate. Together with its role in general vision, contrast sensitivity is an important factor in visual function.⁶

New visual measurement tools that help to predict the ability to perform daily tasks are being used to assess patient visual functioning. These tools include:

• Daily living tasks dependent on vision (DLTV): a self-complete patient questionnaire on visual dependent task performance
• Reading index: this relates to both the size of the text and the person's reading speed
• Computer task accuracy: the ability to perform tasks on computers, which correlates closely with visual components such as VA, contrast sensitivity and color vision defects.¹³

The DLTV evaluates a patient's perception of their ability to perform vision- dependent tasks. It is a validated index developed to assess daily functioning in AMD patients. The DLTV questionnaire, which includes 22 questions on tasks, asks patients to give information on how their visual function affects task performance, e.g. no difficulty, vision prevents . . ., etc. The tasks (e.g. signing a document, identifying money) involve visual elements such as distance/near acuity, depth perception, contrast sensitivity, and visual fields.⁶ The DLTV found that AMD patients had particular difficulties distinguishing facial characteristics, reading text and pouring a drink.¹⁴

The reading index comprises reading speed (number of words per minute) related to print size. This index has been shown to correlate with visual components including contrast sensitivity, and also correlates with the majority of DLTV items. The reading index can therefore help clinicians to predict both the patient's ability to perform tasks and the benefit of optical and non-optical aids.⁶ Research has demonstrated that, for the better-seeing eye, the DLTV results correlate well with distance and near acuity, contrast sensitivity and, of course, the reading index.⁶

Another approach to developing instruments to measure quality of vision and daily task performance is Computer Task Accuracy. This is the ability to perform tasks on computers using icons and texts, which have been proven to correlate closely with visual components such as VA, contrast sensitivity and color vision defects.¹⁵

Measuring visual components

By accurately measuring and monitoring overall visual function and vision loss, physicians, ophthalmologists and eye care professionals will be able to correctly:

• understand patient disability so as to facilitate tailored support: counseling, low vision devices and vision rehabilitation training
• identify patients for timely and appropriate treatm
• monitor outcomes of treatment

The key methods of measuring critical visual components are summarized in the following table:

VISUAL COMPONENT	MEASUREMENTS AND COMMENTS
Visual acuity (distance & near)	Measurement: by reading letters on a letter chart from a set distance Accepted, widely used method
Contrast sensitivity: the ability to distinguish between degrees of contrast	Measurement: using a test which asks the patient to distinguish between patterns (lines of both differing thickness and distance apart) of varying contrast. A number of different test methods are available. Contrast sensitivity is increasingly been measured by clinicians
Visual field measurement: presence of any "blind spots", known as scotoma	Measurement: uses computer-aided technology. The patient places their face in instrument that provides a 180o degree visual field, using fixation to prevent head movement. Lights in different areas of the field are lit and the patient presses a button when she/he sees a light. In this way, the extent of the patient's visual field is determined. Visual field measurement is a standard test for glaucoma patients
Fixation behavior: Relates to preferred area within the macula used for reading	Measurement: uses computer-aided technology. The patient faces of an instrument that provides fundus-controlled small stimuli within the macula. The patient is asked to press a button when they can see the stimuli. In this way, the area of fixation and its stability are determined.
Metamorphopsia: distortion of the visual image i.e. the image appears to bend or be at an angle	Measurement: using an Amsler Grid, a chart composed of black grid lines on a white background. The Grid is held 30 cm from the patient's head and the patient is asked to describe any distortion of the image, e.g. bending lines or lines at unexpected angles.
DLTV	Measurement: self-complete patient questionnaire of performance of 22 vision-dependent tasks. Validated AMD instrument that provides the patient perspective on the ability to perform tasks.
Reading Index: relates to both the size of the text and the person's reading speed	Measurement: firstly a patient's reading speed (number of words per minute) is evaluated, using standard charts of different sized fonts. This reading speed is then related to print size, to give the Reading Index.
Computer task accuracy: the ability to perform tasks on computers	The CTA correlates closely with the VA, contrast sensitivity and color vision defects.

The way forward

In providing healthcare, there is considerable pressure on clinicians to balance time and cost with clinical outcome in patient management protocols. In the evaluation of quality of vision, it has become clear that a better understanding and monitoring of sight is essential in order to ensure correct management of both disease and patient quality of life. In order to improve care, clinicians need to move away from only measuring VA, combining it with ad hoc history taking, to a more systematic method of monitoring a wider range of visual components that truly capture quality of vision in patients.

Footnotes
1. Utility values associated with blindness in an adult population. MM. Brown,  et al., Br J Ophthalmol, 2001, vol. 85, pp. 327--331
2. Utility values and age-related macular degeneration. GC. Brown,  et al., Arch Ophthalmol, 2000, vol. 118, pp. 47--51
3. Vision and low self-rated health: The Blue Mountains Eye Study. JJ. Wang,  et al., Invest Ophthalmol Vis Sci, 2000, vol. 41, pp. 49--54
4. ZMET Patient research. Novartis Ophthalmics, data on file 2002.
5. Impaired vision and other factors associated with driving cessation in the elderly: The Blue Mountains Eye Study. JS. Gilhotra,  et al., Clin and Exper Ophthal, 2001, vol. 29, pp. 104--107
6. Macular degeneration: do conventional measurements of impaired visual function equate with visual disability?  ME McClure,  et al., Br J Ophthalmol, 2000, vol. 84, pp. 244--250
7. Changes in visual acuity in a population over a 10-year period. The Beaver Dam Eye Study.  Klein,  et al., Ophthalmology, 2001, vol. 108, pp. 1757--1766
8. Impact of visual impairment on use of community support services by elderly persons: The Blue Mountains Eye Study.  Wang,  et al., Invest Ophthalmol Vis Sci, 1999, vol. 40, pp. 12--19
9. 9. Ivers R et al. Poor vision and risk of falls and fractures in older Australians: The Blue Mountains Eye Study. NSW Health Bulletin Vol13: No 1-2, 8-10
10. Visual impairment and falls in older adults: The Blue Mountains Eye Study. RQ. Ivers,  et al., J AM Geriatr Soc, 1998, vol. 46, pp. 58--64
11. Depression, visual acuity, comorbidity, and disability associated with age-related macular degeneration. BL. Brody,  et al., Ophthalmology, 2001, vol. 108, pp. 1893--1901
12. Burden of illness and suicide in elderly people: case-control study. M. Waern,  et al., BMJ, 2002, vol. 324, pp. 1355--1358
13. Emotional distress in patients with retinal disease. IU. Scott,  et al., Am J Ophthalmol, 2001, vol. 131, pp. 584--589
14. A vision specific functional index for use in patients with age related macular degeneration. PM. Hart,  et al., Br J Ophthalmol, 1999, vol. 83, pp. 1115--1120
15. Impact of visual function on computer task accuracy and reaction time in a cohort of patients with age-related macular degeneration. IU. Scott,  et al., Am J Ophthalmol, 2002, vol. 133, pp. 350--357

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Age-Related Macular Degeneration (AMD) - An Overview
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