Visual function refers to how well the eye and overall visual system work to produce the ability to objectively observe the outside world. Visual impairment refers to any condition that decreases maximum visual function. It is sometimes considered as vision loss that cannot be corrected by standard means, such as spectacles. There are a whole host of causes of visual impairment (generally considered to be best corrected vision below 20/40 or sometimes 20/60). Some of the most common causes of vision loss include cataracts, glaucoma, diabetic retinopathy, macular degeneration, uveitis, amblyopia, retinal detachment, retinitis pigmentosa, and sometimes even stroke.
When considering visual function and impairment, one should consider that there are many ways of looking at anything depending on perspective. Artists all recognize this. Famous ones have unique perspective. Indeed, some even have altered color value in the cones of their retina, for example, yellow-blue blind.
One alters one’s perspective when placed in large or small lecture rooms. Magicians can create visual illusions and fool us into seeing things that are not there or making things disappear before our eyes; for example, Houdini made an elephant disappear on stage in front of a large audience and David Copperfield made the Statue of Liberty disappear in front of a large audience.
The eye care doctor’s goal is to achieve the best visual function for the individual and to determine if it is possible to minimize any impairments to vision that may be present.
Vision loss can be observed from many different points of view. The most commonly used set of aspects is the International Classification of Impairments, Disabilities, and Handicaps promoted by the World Health Organization (WHO), and the International Classification of Diseases. WHO produced on October 8, 2019 (before World Sight Day on October 10th) a publication entitled “World Report on Vision” ( www.who.int/news-room/detail/08-10-2019-who-launches-first-world-report-on-vision ). The report indicated that more than 2.2 billion have vision impairment or blindness and that “more than 1 billion people worldwide are living with vision impairment because they do not get the care they need for conditions like short and farsightedness, glaucoma, and cataract.”
The aspects and ranges of vision loss are also the subject of a recent standard of the International Council of Ophthalmology ( www.icoph.org ).
Types of vision
The basic structure of the eye is shown in Fig. 1.1, Fig. 1.2 . The retina contains receptor cells, rods, and cones, which, when stimulated by light, send signals to the brain (as described in another chapter). These signals are subsequently interpreted as vision.
Most of the receptors are rods, which are found predominantly in the periphery of the retina, whereas the cones are located mostly in the center and near periphery. It is estimated that there are 120 million rod versus 6 million cones in the human eye. There are three types of cone detectors in the human eye roughly corresponding to red, green, and blue detectors.
According to the duplicity theory of vision, the rods are responsible for vision under very dim levels of illumination (scotopic vision) and the cones function at higher illumination levels (photopic vision).
This dual-receptor system allows the human eye to maintain sensitivity over an impressively large range of ambient light levels. Between the limits of maximal photopic vision and minimal scotopic vision, the eye can adapt rather effectively to changes in brightness of as much as 1 billion times. The sensitivity of the eye automatically adjusts to changes in lumination.
Although the human eye can function over a vast range of brightness, the retina is sensitive to damage by light, such as from lasers, bright flashlights (>600 lumens), or unprotected sun gazing or bright computer screens. This potential for light injury exists because the optics of the eye can concentrate light energy on the retina by a factor of 100,000 times.
Both rods and cones function over a wide range of light intensity levels and at the intermediate levels of illumination, they function simultaneously. The transition zone between photopic and scotopic vision where the level of illumination is equivalent to twilight or dusk is called mesopic vision . Here is a summary of each type of vision depending upon illumination:
Cone photoreceptors/color vision
Resolves fine detail 20/20 or better: high acuity
Functions only in good illumination
Both rods and cones function over a wide range of light at simultaneous levels.
Intermediate levels of chromatic function simultaneously.
Twilight or dusk occurs in a mixed rod/cone mode.
Ambient illumination should be from dim to dark.
No surface (including reflective surfaces) within the subject fields should exact the eye chart luminance. (Standardizing the condition under which visual acuity is measured, that is, chart luminance, is important in determining whether the patient meets required occupational vision standards, as well as being an indicator of any pathologic conditions.)
Uses rod photoreceptors
Occurs in very low light levels
Exhibits poorer quality vision
Is limited by resolution (usually 20/200 or less) for acuity
Provides ability to discriminate only shades of black and white (confirmed by noting that at dusk different colors of the flowers in a garden become virtually indistinguishable)
Provides enhanced sensitivity and low detection threshold under marked reduced illumination
Color vision is absent
The luminance shows typical situations in which the eye would be in each operating mode. The ambient light level created by the sun level is almost independent of position until the sun falls to 5 to 10 degrees above the horizon. The human eye’s contrast sensitivity is roughly constant when the sun is much above the horizon. Once the sun is over the horizon, twilight begins and the change from photopic to mesopic and eventually to scotopic vision begins. Pure scotopic operation occurs only when there is no significant light source. Even good moonlight can prevent full scotopic operation.
Adaptation (the response to changing levels of stimulation, which for photoreceptors means light) also differs for rods and cones. Because the cones have three separate “channels,” the overall sensitivity is lower and the rods are much more sensitive than cones at lower levels of light. Because of the nature of human visual response, these levels are typically measured in terms of luminance (candela per unit area), but on a logarithmic scale. This means that differences of 1.33:1 or 1:0.75 are the smallest discernible steps, despite the seemingly large changes in the associated luminance values.
Luminance versus illumination
Illumination refers to the amount of light striking a surface. Luminance is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. According to the International System of Units (SI), luminance is candela per square meter (cd/m 2 ). One way to think about this relationship is by considering that illumination times reflectance equals luminance. Luminance may also be considered as the perception of brightness by the human eye, that is, how bright light appears to be that is reflected from a surface.
Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be perceived by an eye looking at the surface from a particular angle of view. Luminous energy in the field of photometry is the perceived energy of light, which is sometimes called “luminous flux,” that is, luminous energy per unit time. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye’s pupil. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and 300 cd/m 2 . The sun has a luminance of about 1.6 × 10 9 cd/m 2 at noon.
It is accepted that luminance is invariant in geometric optics and this means that for an ideal optical system, the luminance at the output is the same as the input luminance and that for real, passive, optical systems, the output luminance may be at most equal to the input. As an example, if you form a demagnified image with a lens, the luminous power is concentrated into a smaller area, meaning that the illuminance is higher at the image. The light at the image plane, however, fills a larger solid angle so the luminance comes out to be the same, assuming there is no loss at the lens. The image can never be “brighter” than the source. (Brightness is the term for the subjective sensation or impression of the objective, actual measured luminance.)
Measurement and assessment of visual loss
When considering visual functioning, we can perceive many different aspects of vision loss, depending on our point of view and the interventions, such as surgery and rehabilitation.
In considering how various causes may result in structural changes, such as scarring, atrophy, or loss, the focus is on the tissue. However, structural changes do not reveal how well the eye functions as a whole. For that we must widen our view from the tissue to the organ, and a clinician is needed to measure aspects of organ function, such as visual acuity, visual field, and contrast sensitivity.
Yet knowing how the eye functions does not disclose how a person functions. Our perspective thus has to expand even more to encompass the individual level and consider tasks, such as reading, mobility, and face recognition. For this perspective, various vision rehabilitation professionals are needed to work with a patient. Beyond this scope, the person has to be viewed in a societal context and assessed for how all of these changes have an effect on the person’s participation in society, such as causing job loss or reducing quality of life. The goal of all of these interventions is to ensure the patient is satisfied with the resulting condition.
Aspects of visual impairment
When organ functions are reduced, we speak of impairments. The most common ocular visual impairments are a result of ocular disorders. More recently, increased attention is being given to cerebral disorders, which may cause cerebral vision impairment . In infants and children, the cause may be perinatal cerebral ischemia, in adults, it may be traumatic brain injury, and in older adults, it may be a stroke. Cerebral visual impairments may cause abnormal visual functioning, which can be captured under the term visual dysfunction .
Ophthalmology has unique tools that can measure visual function with great precision. Those with the greatest effect on general functioning are: (1) visual acuity and (2) visual field , followed by (3) contrast sensitivity . Many other functions, such as color vision, stereopsis, light and dark adaptation, and psychophysical and electrophysiologic tests (e.g., electroretinography, visual-evoked potentials) can assess visual function, but they are poor predictors of functional consequence. Because loss of visual acuity has many different causes, it is a good screening test, but adds little to the differential diagnosis. Yet whatever its cause it can help in predicting the effect on activities of daily living (ADLs).
Measurement and assessment of functional aspects
The different functional aspects are measured and assessed in very different ways. Visual functions measure how the eye functions by varying one parameter at a time in a simplified, artificial environment. For example, the visibility of test objects depends on their size, contrast, and illumination. If we vary the size while keeping contrast and illumination constant, we create a contrast sensitivity test, like the CV-1200. If we vary the illumination while keeping size and contrast constant, we perform a dark adaptation test. Each test provides a threshold value for the measured stimulus parameter. The threshold criterion is generally defined as the response level that is 50% greater than guessing. Threshold measurements are chosen not because threshold performance is the most relevant performance level for ADLs, but because they enable more precise psychophysical calculations (psychophysics deals with the relationship between external physical stimuli and the human reaction/mental response to the stimuli).
For visual functions we measure the variable stimulus needed for a fixed response; for functional vision we measure the variable performance for a fixed task, either objectively (e.g., timing or error rate) or subjectively (questionnaires).
Finally, we must consider the societal context or quality of life. For subjective judgments, such as making and keeping friendships, social skills, and self-confidence, measurement is more difficult. The ultimate goal is satisfaction, which describes the subjective balance between individual achievements and individual expectations.
Parameters of ocular function
Provides binocular vision
Visual acuity better than 20/200
Central vision less than 10 degrees
Basic visual functions and essential ADL are:
Acuity testing for:
Monocular (each eye) and binocular (both eyes) vision
Both distance and near visual acuity
With and without correction (the person’s eyeglasses or contact lenses)
Stereopsis findings as a baseline; note subsequent changes, if any
Color perception, of the three visual hues
Visual fields to determine visual acuity from the fovea to the ciliary body
Muscle balance (distance and near), also called binocular balance . The examination should be for vertical and horizontal phorias
General limits of normal functional balance are set for far and near vision when performing the tests.
Ocular (visual screening)
Visual screening (oftentimes called an eye test and performed by ancillary personnel rather than an eye doctor) is an eye examination that quickly attempts to determine if there are any eye disorders or visual problems that needs to be examined further.
Vision screening is especially important in children, including babies, as good vision is critical to the child’s development and overall wellbeing. The American Academy of Ophthalmology states that “It is essential to check children’s vision when they are first born and again during infancy, preschool and school years” ( www.aao.org/eye-health/tips-prevention/children-eye-screening ). A paper entitled “Vision Screening: Program Models” by Mae Millicent W. Peterseim, MD and Robert W. Arnold, MD (Nov. 10, 2015) reviews in-depth information on the topic of vision screening for children ( www.aao.org/disease-review/vision-screening-program-models ).
Visual acuity worse than 20/200
Peripheral visual fields more than 10 degrees
Scotopic vision—night vision—exclusion area of macula/fovea control
Preventive medicine guidelines, CPT codebook’s evaluation/management guidelines, requirements
Ocular history, which includes a general overview of the individual’s visual history
Complete visual (ocular) screening examination
Visual acuity quantitative bilateral tests are measured for far, at infinity (at minimum and especially in pediatrics), for near, and for intermediate distances (based on job description); contrast sensitivity is done periodically; all examinations are performed with and without corrective devices (i.e., glasses, contact lenses).
Gross visual fields
Heterophoria/heterotropia (horizontal and vertical) and depth perception
Intraocular tension (e.g., puff tonometers) by separate glaucoma screening
Why perform visual screening
Visual function and visual tasks can analyze anatomic and structural changes caused by disease or injury. Also included here is a discussion of the relationship of these functional changes to the visual requirements of ADL or specific occupational requirements.
The precision and accuracy with which the eye can contrast is exceptional, exceeding many sophisticated cameras. It has unique focusing capabilities, and its ability to work with the brain allows people to undergo special training (e.g., finding specific details or characteristics in a product, such as a flaw in a factory-made part). The eye also can differentiate and distinguish between subtle shades of color and images under conditions of high and low contrast.
Aspects of vision loss and function
Vision loss can be observed from many different perspectives in addition to those of the patient, whether it is the treating physician, a family member, or a caregiver. Each aspect is different, but they all revolve around the same clinical case and reveal something about the patient.
Older adults in North America need to be assessed for the most common sight-threatening conditions they may face, that is, cataract, age-related maculopathy, glaucoma, and diabetic retinopathy. These conditions cause visual sensory impairments, even in early and moderate stages and in later stages, reductions in health-related quality of life, including difficulties in daily tasks and psychosocial problems. Other older adults that may be free from these common conditions, may still experience visual perceptual problems as a result of simple aging relating from changes in the optics of the eye and/or degeneration of visual neural pathways.
Functions describe how the eyes and the visual system function. Functional vision describes how the person functions. When organ functions are reduced, they are referred to as impairments. The most common ocular visual impairments are caused by ocular disorders. More recently, increased attention is being given to cerebral disorders, which may cause cerebral visual impairment . Cerebral vision impairments may cause abnormal visual functioning, which can be considered under the term visual dysfunction .
Another scenario involves screening patients with an identifiable defect during an evaluation required by federal law for entrance into a work position, such as the evaluation that a pilot might undergo. In particular, the Americans with Disabilities Act (ADA) of 1990 requires that an individual be evaluated to determine whether that person is qualified to fulfill the essential tasks of the position, with or without accommodation, without significant increase in risk to self or others.
Binocular vision (visual perception using two eyes) is normal and confers three benefits: it makes hard-to-see objects easier to detect, it enlarges the total field of view, and it improves a person’s capacity to distinguish small differences in depth (allows for more effective depth perception).
The most distinctive benefit of using two eyes derives from the fact that because they are horizontally separated, they do not have exactly the same view of the visual world. The small differences between the images in the two eyes are systematically related to the arrangement of objects in depth, providing information from which the visual system is able to distinguish small differences in the distances at which objects lie. This capability, known as stereopsis , is most beneficial for making fine depth judgments, especially when objects are nearby (i.e., within arm’s reach).
For all three of these capabilities (enhanced acuity, field of view, and stereopsis), the brain must properly combine information from the two eyes. If the vision in the two eyes differs substantially, the brain may not be able to combine the information in a unified view (binocular single vision) or may be unable to use the differences between the images to distinguish small differences in the depth. Binocular vision can be disturbed even though each eye alone is functioning normally. Abnormalities in the brain, or improperly coordinated movements of the eyes, or misalignment of them, can disrupt normal binocular vision. When the brain is unable to combine information of the two eyes, a person may experience double vision (diplopia) or binocular rivalry , a sometimes haphazard switching of the eyes from one eye to the other. Failure to combine information from the two eyes can lead to a reduced ability to use small differences in depth. Moreover, under some circumstances, vision of the two eyes might conflict, making vision poorer than if one eye alone were used.
Monocular vision is one-eyed vision. A person with monocular vision usually has one of the following:
Visual acuity quantitative bilateral tests are measured for far (or at infinity), for near, and for intermediate distances (based on job description); all examinations are performed with and without corrective devices (i.e., glasses, contact lenses).
A visual acuity test may be an evaluation for those especially who have had cataract implant operations or refractive surgery or are older.
Contrast sensitivity (CS) determines the lowest contrast level that can be detected by a person for a given size target. Normally a range of target sizes is used. Unlike visual acuity, CS measures two variables, size and contrast; acuity measures only size.
Fig. 9.1A demonstrates normal contrast, whereas Fig. 9.1B demonstrates a loss of contrast in which the image is hardly visible. CS measures the degree to which the visual system can discriminate between adjacent areas of light and dark. Unlike measures of static focal acuity, such as the Snellen test, measurement of CS does not yield a single figure, describing performance. It does, however, detect functional vision loss often caused by early eye disease. Moreover, it can produce a more sensitive and comprehensive measurement of visual capability and performance than is provided by Snellen visual acuity alone.