The low vision evaluation includes a history, measurement of visual function, and creation of a vision rehabilitation plan. In contrast to an ophthalmic examination, in which visual function and ocular status are evaluated with the intent to diagnose and treat, the evaluation of the patient seeking vision rehabilitation aims to assess and address reading, valued activities of daily living, patient safety, continued participation despite vision loss, and psychosocial well-being.
As with other clinical encounters, the history provides important information that directs the remainder of the examination. The low vision history focuses on the limitations the patient’s vision imposes on their function: what they are having difficulty doing.
Ocular history The disease, rate of progression, and previous ocular treatments will typically correlate with the patient’s functional complaints. For example, patients with macular degeneration would be anticipated to have different difficulties than patients with slowly progressive glaucoma.
General history The current living situation, supports, employment, hobbies, illnesses, and use of glasses, devices, cell phones, and computers are all relevant issues in the history. Systemic diseases can impact rehabilitation interventions, such as when arthritis or tremors impair a patient’s ability to hold a book or a handheld magnifier.
Patient’s subjective report of difficulties or goals The patient’s goals and values help direct and prioritize rehabilitation efforts. Tasks may still be difficult after rehabilitation, but patients highly value success in accomplishing tasks that are important to them. The examiner should ask the patient about difficulties with (1) reading tasks, such as reading newspapers, mail, and handwritten notes; (2) activities of daily living, such as shopping, cooking, using a cell phone or computer, shaving, and watching television; (3) safety issues, including falls, reading medications, and kitchen safety; (4) barriers to participation, including driving status, transportation alternatives, and isolation; and (5) psychosocial status, such as anxieties, including worry about visual hallucinations experienced and depressive symptoms, and concerns about responsibilities, such as financial or caregiving responsibilities.
Charles Bonnet syndrome Patients with Charles Bonnet syndrome see images of objects that are not real. The condition affects up to one-third of visually impaired persons. Patients are often relieved to discuss their hallucinations. They may see vivid, recurrent formed images of patterns, such as wallpaper or barbed wire, or even images such as people or landscapes. Many patients are puzzled by this symptom. Some are anxious, as they do not understand what they are experiencing, and a small proportion are very upset. Most patients will not report the hallucinations unless the clinician inquires, for fear of being labeled as mentally unwell.
A diagnosis of Charles Bonnet syndrome can be made if the patient has 4 clinical characteristics: (1) vivid recurrent visual hallucinations; (2) some degree of vision loss; (3) insight into the unreality of the images, when it is explained to them; and (4) no other neurologic or psychiatric diagnosis to explain the hallucinations. Charles Bonnet syndrome is a diagnosis of exclusion, and patients should be referred for neurologic or psychiatric evaluation if they have any other neurologic signs or symptoms (see BCSC Section 5, Neuro-Ophthalmology).
Assessment of Visual Function
As in ophthalmology in general, visual acuity is an important and common measure of visual function in vision rehabilitation, as the task performance of a person with 20/70 visual acuity will likely differ from that of someone with 20/400 visual acuity. Other measures of visual function, however, are also important, especially contrast sensitivity and central visual field. Two patients with 20/40 visual acuity, for example, may have different contrast perception and different central visual field, and have very different reading performance and require different devices and training. In some disease settings, such as glaucoma or after stroke, peripheral visual field will also be important.
Visual acuity Accurate visual acuity measurements can be made to very low levels. Charts can be brought to closer-than-standard viewing distances. An ETDRS chart is commonly used at 1 or 2 m (Fig 9-1). For patients with very poor vision, the Berkeley Rudimentary Vision Assessment, a set of 25-cm cards held at 25 cm, is available for quantifying visual acuity as low as 20/16,000. Care is taken to carefully measure acuity in patients with low vision by optimizing the refraction, allowing patients adequate time to respond, and often using different testing distances.
Fixation Patients with normal vision fixate a visual acuity chart with their fovea (Fig 9-2). Patients with macular disease may fixate with eccentric areas of the retina, or preferred retinal loci (PRLs; Fig 9-3). Visual acuity can vary when using areas of differing retinal sensitivity. Clues to understanding fixation behavior include head and eye movement, patient subjective reports, and measured fixation with macular microperimetry.
Figure 9-1 Measuring visual acuity with the ETDRS chart at 1 meter.
(Courtesy of Scott E. Brodie, MD, PhD.)
Figure 9-2 Macular microperimetry showing normal central field. A, All targets were responded to (green circles). B, The fixation is at the fovea (blue crosses).
(Courtesy of Mary Lou Jackson, MD.)
Figure 9-3 Macular photo showing anatomic fovea (lower arrow) and location of eccentric fixation with a preferred retinal locus (PRL) (upper arrow).
(Courtesy of Mary Lou Jackson, MD.)
Crossland MD, Culham LE, Kabanarou SA, Rubin GS. Preferred retinal locus development in patients with macular disease. Ophthalmology. 2005;112(9):1579–1585.
Refraction The goal of refraction for patients with low vision is to check for significant uncorrected refractive errors; however, only about 10% of low vision patients will benefit from alternate refractive correction, as the source of their poor vision is typically ocular disease, not refractive error. The vision rehabilitation clinician must temper unreasonable expectations, such as the expectation that new glasses can solve vision problems associated with the eye disease, and ensure that patients do not deplete their financial resources on spectacles that offer little benefit, especially when they could put that money toward other devices that significantly improve function. Purchase of new glasses is often best delayed until the patient can compare the benefit of other rehabilitation options and devices to the benefit of spectacles.
Specific strategies can assist the low vision refraction including using a trial frame, retinoscopy at a shorter distance with greater working-distance lens power, using a +1.00/–1.00 cross cylinder for patients with poorer acuity to allow them to appreciate the differences between choices, or using an automated refractor. The vision rehabilitation clinician watches for fluctuating acuity in diabetic patients and balance corrections in an eye that may now be the better-seeing eye. Full corrections, rather than balance corrections, are encouraged. Polycarbonate lenses can be considered for ocular safety. It is not uncommon that patients with macular disease have very small areas of foveal retina surrounded by dense scotoma: foveal-sparing scotomas (Fig 9-4A). This is seen in both dry and treated wet macular degeneration, Stargardt disease, and other macular diseases. Such patients may be unable to read the larger letters on a visual acuity chart, causing the examiner to abandon the testing and record very low acuity. More careful testing, or testing at a closer distance, however, may reveal that the patient can in fact discern smaller letters when he or she is able to align the limited central field with the targets on the eye chart (Fig 9-4B).
Contrast sensitivity The ability to discern contrast is a separate visual function from visual acuity, and the functions are not directly correlated. Patients with poor contrast sensitivity have difficulty seeing the edges of steps, reading light-colored print, driving in foggy or snowy conditions, and recognizing faces. Contrast sensitivity varies with target size (spatial frequency), and the relationship between contrast threshold and spatial frequency may be displayed as a contrast sensitivity curve (see Chapter 3 in this volume). Formal tests of contrast sensitivity include paper charts and computer tests, the latter allowing greater testing range. Paper charts may test a range of spatial frequencies (Fig 9-5A), or a single spatial frequency (Fig 9-5B).
Figure 9-4 Foveal-sparing scotoma. A, Macular microperimetry of paracentral scotoma: green dots indicate targets that are seen by patient; red dots indicate missed targets. B, Effect of foveal-sparing scotoma on visualization of large print.
(Courtesy of Mary Lou Jackson, MD, and American Academy of Ophthalmology/Vision Rehabilitation Committee.)
Patients whose visual impairment includes loss of contrast sensitivity may benefit from illuminated magnifiers or electronic magnification, and nonoptical strategies, such as task lighting, or modification of contrast in tasks, such as using a black felt-tip marker.
Central visual field The largest group of patients referred for vision rehabilitation are patients with central field loss due to age-related macular degeneration (AMD). Traditional field testing with Goldmann or Humphrey perimeters maps the visual field relative to a central fixation point. This is accurate in patients with stable central fixation, but results can be misleading in patients with unstable or eccentric fixation. Defects can be under-or overrepresented or displaced. Other nonautomated testing methods, such as Amsler grids, cannot assess fixation and will not detect approximately half of central or paracentral scotomas due to perceptual completion, or “filling in.” The macular microperimeter monitors fundus location and then determines the patient’s direction of gaze before each target is presented. Macular microperimetry (also called fundus-related perimetry), documents the patient’s retinal point of fixation, scotomas, and the relationship of the fixation point to the scotomas. Most patients with central scotomas spontaneously develop eccentric fixation but may have poor oculomotor control at the eccentric area or preferred retinal loci (see Fig 9-3). They may use multiple PRLs, change fixation depending on target size or illumination, or develop a sense of “straight ahead” related to their PRL, rather than their fovea.
Figure 9-5 Eye charts for measuring contrast sensitivity. A, Vistech chart. Spatial frequency increases from top to bottom; contrast decreases in each row from left to right. Patient must detect whether grating pattern is tilted to left, vertical, or tilted to right; see samples in bottom row. B, Pelli-Robson chart. Contrast of large Sloan letters decreases in groups of 3 from top to bottom and left to right within each line.
(Part A courtesy of Scott E. Brodie, MD, PhD; part B reprinted with permission from Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci. 1988;2(3):187–199. Copyright © 2002, Pelli DG, Robson JG. Distributed by Haag-Streit.)
The vision rehabilitation clinician needs to appreciate the nature of the patient’s fixation (foveal or eccentric), the presence and nature of scotomas (central or paracentral), and the relationship of fixation and scotoma. For example a scotoma may surround fixation, as in foveal-sparing scotomas (see Fig 9-4A), be right of fixation (Fig 9-6A) and obscure next words (Fig 9-6B), or be left of fixation (Fig 9-7) and make it difficult to carry out an accurate saccade to the beginning of the next line of print. Although some believe that a PRL looking up (Fig 9-8) is optimal, as it would allow a horizontal span for left-to-right readers, no difference in reading speed with PRL location has been determined. Scotomas that surround seeing retina may interfere with the recognition of large objects, fluent reading, or using magnification, depending on the size of the central seeing field (see Fig 9-4B).
Scotomas can vary widely in size, shape, number, and density, and they may not correspond to the fundus appearance of atrophy, scarring, or pigment alteration. This lack of correspondence is particularly important to consider in patients with wet AMD who receive anti–vascular endothelial growth factor (anti-VEGF) injections. Such patients may not exhibit obvious scars yet still have significant scotomas in their central field.
Figure 9-6 Macular microperimetry. A, A scotoma (red targets) right of fixation (yellow crosses). B, This pattern of scotoma would decrease the horizontal perceptual span when reading, obscure the next word, and be anticipated to interrupt normal reading saccades.
(Courtesy of Mary Lou Jackson, MD.)
Figure 9-7 The scotoma is left of fixation and may cause difficulty finding the beginning of the next line of text.
(Courtesy of Mary Lou Jackson, MD.)
Figure 9-8 The patient has moved the eye up to use a preferred retinal loci looking up.
(Courtesy of Mary Lou Jackson, MD.)
Peripheral visual field Peripheral visual field defects cause patients to bump into objects or people, trip over objects and curbs, and lose their orientation particularly in unfamiliar areas. Goldmann fields, automated peripheral fields, or carefully conducted confrontational fields can be informative in the setting of patients with glaucoma, peripheral retinal disease, or optic nerve or neurologic disease affecting visual pathways.
Assessments of other visual functions Glare, color vision (discussed in BCSC Section 6, Neuro-Ophthalmology and in BCSC Section 12, Retina and Vitreous), binocularity (see BCSC Section 6, Pediatric Ophthalmology and Strabismus), eye movements and accommodation (see BCSC Section 6, Pediatric Ophthalmology and Strabismus) may be considered in some situations. Glare is the discomfort or impairment of vision caused by scattered light (mainly Mie-type scattering in the forward direction; see Chapter 2). It occurs commonly in ocular conditions such as corneal disease, cataracts, macular dystrophy, and albinism, and somewhat less commonly in AMD or glaucoma. Patients with reduced contrast sensitivity often require increased illumination, which may, in turn, exacerbate glare. Therefore, determining sensitivity to glare is important so that you can advise patients about optimal lighting.
The simplest way to assess glare is through the history; however, formal testing can be done with a brightness acuity tester, a handheld device that allows the patient to view a distant target through a small dome that floods the eye with off-axis illumination.
Performance of visual tasks
To assess patients’ current success with visual tasks, they can be observed doing tasks such as reading, using their cell phone or computer, writing, or ambulating. Reading tests vary and include tests to assess reading single numbers or words (spot reading), paragraphs (continuous print reading; eg, the International Reading Speed Texts [iReST]) or sentences with decreasing size of text (eg, the Minnesota Low-Vision Reading Test (MNREAD, Fig 9-9). Useful variables include the minimum size of print that can be read with current glasses or devices (reading acuity), reading errors, and the optimal size text for reading fluency (critical print size). The latter can inform magnification goals. It is important to note that different patients with the same distance visual acuity will read different sizes of near print if they are using different powers of reading add. The reading add power used and the distance from the eye to the text should always be documented when measuring near vision (eg, 0.4 M at 40 cm, with +2.50 add). For purposes of vision rehabilitation, it is often convenient to describe the print size of reading materials using M notation. Note: the M size of an optotype is the distance (in meters) at which the sample can be read by a person with normal acuity—thus, “1 M” print is normally legible at 1.0 m.
Figure 9-9 The MNREAD reading test has a range of text sizes. Patients may have difficulty reading the larger text yet find it easier to read smaller text when they have a limited area of central seeing retina.
(Courtesy of Mary Lou Jackson, MD.)
Observing a patient reading the actual material that he or she normally reads—such as the newspaper or medication labels, or doing a task that they value such as using a cell phone, writing, reading prices at arm’s length or walking independently—can reveal not only difficulties or successes, but also current adaptive strategies, such as head posture, manipulation of the material, or attempts to use current devices. Computer and cell phone difficulties may include losing the cursor into surrounding areas of field loss, misdialing phone numbers, or having difficulty with keyboards.
Observing a patient ambulate provides valuable insight into the need for orientation and mobility training, white-cane or support-cane use, or scanning training.
Many different questionnaires have been used to elicit patient reports of difficulties with visual tasks.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.