20.4.4  Vision Tests for Very Low Vision

PC-based tests to assess basic aspects of visual function in prosthesis recipients have recently been developed.

The BaLM test (Basic Light, Localization and Motion; Zrenner and Wrobel, Retina Implant AG) [112], measures several visual functions, including light detection, light location, temporal resolution and motion direction discrimination. All tests involve forced-choice responses, provide optional auditory feedback and allow the number of trials to be varied. Output includes percent correct and response times.

The Berkeley Rudimentary Vision Test was created by Ian Bailey and co-workers to assess individuals with very low vision, within the range typically described as hand motion or count fingers (i.e., worse than 20/800 (LogMar 1.6)). It contains a light and basic form perception test (BFPT) and an acuity screening and measurement test, the Single Tumbling E’s Test (STET). Visual fields can also be measured.

FrACT, the Freiburg Acuity and Contrast Test [11] is available online at http://www.michaelbach.de/fract/index.html. It uses Landolt C’s to assess very coarse acuity in the range of hand motion to count fingers, and contrast sensitivity. It also assesses vernier acuity. By combining the results of FrACT, clinical acuity measures, and ETDRS acuity, Schulze-Bonsel et al. found that CF and HM acuity can be reproducibly assessed and correspond to acuities of 20/1,400 and 20/4,000 [84].

Dagnelie and co-workers have developed a number of PC-based tests appropriate in this context. These include a visual field measure [13, 28], yet to be validated, that can bridge the gap between crude localization and standard field measures.

20.5  Visual Performance Assessment

As has been noted [25], it is not what the prosthesis recipient can see, but what they can do that is critical. The aim of prostheses is to improve the ability of the recipient to perform activities of daily living (ADL), instrumental tasks of daily living (IADL) and what we may call activities of life satisfaction (ALS). Visual performance affects independence, quality of life and “visual disability” [67]. Assessing this aspect of vision remains a major hurdle in the path of ­prosthesis evaluation and progress. Assessment of visual performance (preand post-implantation) addresses questions such as: How has everyday task performance changed since implantation? Is the increment in performance sufficient to be of real value to the recipient? Does the prosthesis increase efficiency, reduce risk and increase independence?

This aspect of visual assessment has two branches: measured performance and self-reported performance. Direct measurement has been argued to be superior to self-report, at least in aging [43]. Theoretical advantages of measuring function

include better reliability and validity, greater sensitivity to change and less influence by confounding factors such as culture and language [45]. However, questionnaires enable us to assess important issues beyond the realm of performance, such as whether the implant was of any benefit (e.g., all of the tasks can now do) to the recipient, the difficulty an individual has performing particular IADLs, and the impact that difficulty or inability has on quality of life. Questionnaires provide valuable information that may guide the choice of performance measurement. Both approaches are important for demonstration to patients, care-givers, government, and funding organizations the effectiveness of the prosthesis.

20.5.1  Measured Performance

Observation of blind individuals who have undergone rehabilitation reminds us that there are fewer tasks than we realize that cannot be accomplished without vision. However, the return of the ability to perform tasks visually is desirable to those with little or no vision – particularly those who, like the subjects in the current implant studies, have lost their vision later in life.

Among tasks that cannot be performed at all without vision, one of the most important is reading printed text with all its images, information provided by formatting (e.g., headings, emphasis), potential for scanning to find information, etc. (Braille comes in only one “font” and letter size). Other desirable tasks that require vision are driving and the ability to identify and locate at a distance (Josh Miele, personal communication).

There is no generally accepted test battery of even rudimentary task performance measures for use in low vision patients (including prosthesis recipients). Many of the performance batteries that have been developed are not relevant to or have not been assessed in those with very low vision (e.g., [46, 54, 93, 102]). In choosing items of ADL to assess for their index, Haymes et al. (2001) considered how common the ADLs were on existing instruments (questionnaires) and whether they were consistent with daily living problems reported by a very large number of people with vision impairment [46]. Unfortunately, the tasks chosen which include reading print in various contexts (e.g., newspaper, medication label), using a telephone, recognizing faces, threading a needle, require vision superior to that currently afforded by prostheses.

Optimally, performance would be measured in situ. However, it is more practical to measure tasks under controlled (laboratory) conditions. In either case careful consideration of task relevance is crucial.

The most commonly used tasks are reading, face recognition, and mobility and orientation (wayfinding). As an alternative or in conjunction with measurements of ADLs and IADLs, which are complex, one may assess “component” tasks such as eye-hand coordination, visual search, figure ground discrimination, and finding embedded objects [2, 54, 58].

Wilke et al. (2007) developed two sets of test which we shall refer to as “at table” tasks and orientation and mobility tasks for use in visual prosthesis recipients [112].