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Up Front | Oct 2008

Evaluating Patients After Presbyopia Correction

The Salzburg Reading Desk is a precise method for testing visual function.

Presbyopia regularly occurs during an individual's most productive stage in life, and its effect on accommodative power makes near vision tasks, such as reading, more difficult. Losing near vision severely reduces one's independence, especially in our information-based age,1 and thus has a severe impact on perceived quality of life.2,3

According to current estimates, more than 1.3 billion people worldwide have presbyopia. The number of potential patients who want surgical correction for presbyopia is projected to increase in the coming decades; the direction of interest in refractive surgery is rapidly shifting toward presbyopia, considered by many to be its final frontier.4 Coupled with the sheer number of candidates for presbyopia correction, the size of the ophthalmic market for presbyopia-correcting devices and procedures is the major driving force for recent presbyopic developments.5 To be an attractive option for this large patient market, a fully satisfying surgical solution for presbyopia is needed. Although this has proved to be an elusive goal, several interesting options are currently under investigation.5

The means to the presbyopia correction endpoint are only as sucessful as the perceptions of the patient who has had surgery. It is critical to obtain meaningful evaluation of postoperative visual function. We will discuss the standard visual acuity testing, as well as an alternative testing method that we have found particularly valuable in evaluating visual function in patients who have undergone surgical correction of presbyopia.

Visual performance is the most important clinical examination when determining the most valid method of presbyopia correction for a given patient. Visual acuity should consider the testing distance and optotype size. In Europe, the official international standard is to evaluate visual acuity with the logarithmically graded Landolt optotypes.6 Print size used in test paragraphs is required to be logarithmically scaled, and the reading test items should be as comparable as possible1 to allow accurate and standardized measurements of reading acuity and/or reading speed at every viewing distance. Therefore, testing parameters for various procedures must be standardized for both distance visual acuity and reading performance.1,6

Refractive surgeons tend to overlook the optical component of the visual system. We must keep in mind that the important sensory component begins at the level of the retinal photoreceptor and ends at the optical cortex.5 Indeed, true reading is more than simply discriminating single optotypes in an almost unlimited time. Therefore, reading acuity—not near visual acuity— should be tested in all refractive patients.

Jaeger charts have been used to test reading acuity for decades, despite the variability between different designs.8 Colenbrander6 randomly collected 20 currently in-use Jaeger charts and found a variability of print sizes representing a difference spanning six lines.7 Therefore, Jaeger numbers are not standardized enough to be used in visual acuity studies, particularly clinical trials comparing methods of presbyopia correction.

Some of the previously outlined mandatory principles of standardization for near vision tests have been used to design new reading charts.1,8,9 Bailey and Lovie8 used unrelated words of similar legibility to simultaneously determine reading acuity and speed, a method that has also been applied to the MNRead acuity chart9 and Radner reading charts (RRC).1 Radner and coworkers were the first to investigate the test-retest and interchart reliability of their own reading charts under clinical conditions.10 Radner designed the RRC by implementing sentence optotypes to minimize variations between the test items and to keep the geometric proportions as constant as possible at all distances. For the precise documentation of reading acuity, Radner implemented the term logRAD (logarithm of reading acuity determination) which represents the reading equivalent of logMAR.1

In clinical trials with short sequences, reading tests should be administered in a randomized order, possible by using different RRCs. A choice of such test items that are highly comparable in lexical difficulty, reading length, and text construction allows optimized reliability and validity of reading performance analysis when short sentences are used. This is especially true when they are used for comparative analysis of reading acuity.1 Reading speed with RRC can be calculated on the basis of the number of words per sentence (eg, 14) and the time needed to read the sentence. We use the following formula: number of words x 60 sec ÷ reading time.1

Most modern reading charts are standardized according to the requirements for international standards of visual acuity tests;6,9 however, reading acuity is now either evaluated (1) under fixed distances, (2) with estimated distances, or (3) ignoring the reading distance. Because a patient chooses his optimal reading distance according to his body size and posture, reading preference, and print size, he should choose his own subjectively convenient reading distance during a reading test.

To the best of our knowledge, no published method exists to test reading acuity under standardized circumstances while simulating a natural reading process. Since 2004, the University Eye Clinic of Salzburg has been continuously working on the development of such a device.

The Salzburg Reading Desk (SRD; Figures 1 and 2) consists of a specially designed reading desk, a computer, a high-resolution display, and a printer. Data acquisition and processing are managed with a USB multifunction DAC module and the SRD software. The reading desk is mounted on a flat case that carries the electronic measurement equipment; a chain-impulse driver, which allows adjusting the inclination of the table from 0° to 40°; and a mounting device for the two video cameras that face the patient.

The SRD equipment allows accurate measurement of reading acuity, taking into account the individual's freely chosen reading distance and other parameters, such as reading time, illumination, and inclination of the reading table. The available RRCs are simultaneously used to prevent possible recognition of a previously read sentence.1,10,11

Two fluorescent tubes, emiting light similar to daylight (5,400 K; 40 kHz), uniformly illuminate the surface of the desk. A microphone, fixed on the plane of the adjustable table, signals the computer when the patient is reading the sentences aloud. The patient's reading process is visualized on the user interface (Figure 3). The examiner defines the beginning and the end of each reading process (ie, a complete sentence) by positioning two vertical lines (green line = beginning, red line = end) on the user interface (Figure 4). The software automatically calculates and displays the following parameters: (1) reading time (seconds), (2) the perpendicular distance between the text line of the RRC and the green coding dot, placed on the root of the nose of the patient to display the mean reading distance (measured in cm), (3) illumination of the RRC (preset to 500 lux), and (4) inclination (reading angle, 0°–40°) of the SRD. From these values, the computer calculates and displays distance-corrected reading acuity in logRAD and reading speed in words/minute. A sentence can be statistically analyzed as soon as a whole RRC-sentence has been read by the patient aloud, with a minimum reading speed of 80 words per minute, which represents the lower limit for recreational sense-capturing reading.12

Distance measurement is the most important parameter when testing reading performance with the SRD. To check the testing spectrum, we performed validity and reliability checks with 2,184 single measurements in distances between 15 and 63 cm and inclinations of the reading desk between 0° and 40° (in 5° steps), correlating the calculated distances from the SRD to a fixed reference value. As a limit of agreement for validity, a possible deviation (95% confidence interval) to the reference value of ±0.5 cm was subset.

As a future improvement, the SRD-advanced will include a high-resolution computer display, which will allow the use of different luminance and contrast levels. Currently, reading charts typically use high contrast levels (approximately 85–95%). By adding a feature to test with different—especially reduced—contrast levels, even smaller differences in reading ability will be realized. This holds great promise in the comparison of different surgical procedures for the improvement of reading acuity.

As the investigating surgeons' intention should be to test under everyday conditions, this will be another step to achieve more accurate, repeatable, and comparable results. By using the above-mentioned display, patients can either be presented with different charts for testing reading acuity or with a setup for assessing simple near visual acuity. Additionally, other testing patterns, such as road maps, music sheets, or similar graphics, can be used.

The experimental design of the SRD-advanced seems to be more up-to-date for today's patients who receive most of their information on the Internet. In the future, handheld reading charts may be replaced with computerized reading acuity testing as the standard of care in all clinical settings.

The SRD precisely and objectively tests distance-corrected reading acuity for the first time and should be considered for assessment of patientsÔ everyday reading ability, as each individual can now use his own subjectively convenient reading distance. Use of SRD to test reading ability after surgical procedures, including multifocal IOL implantation, new laser ablation profiles, or corneal implants, is currently under way in Salzburg and other European centers to establish the validity and reliability of this highly refined method for the evaluation of reading acuity.

Alois K. Dexl MD, MSc, practices at the University Eye Clinic, Paracelsus Medical University of Salzburg, Austria. Dr. Dexl states that he has a proprietary interest as a patent assignee of the SRD. He may be reached at tel: +43 662 4482 57288; fax: +43 662 4482 3703; e-mail: alois@dexl.at.

Günther Grabner, MD, is a Professor of Ophthalmology and Chairman at the University Eye Clinic, Paracelsus Medical University of Salzburg, Austria. Professor Grabner states that he has a proprietary interest as a patent assignee of the SRD. He may be reached at tel: +43 662 4482 3701; fax: +43 662 4482 3724; e-mail: g.grabner@salk.at.

Horst Schlögel, PhD, practices at the University Eye Clinic, Paracelsus Medical University of Salzburg, Austria. Dr. Schlögel states that he has a proprietary interest as a patent assignee of the SRD. He may be reached at tel: +43 662 44823747; fax: +43 662 4482 3703; e-mail: h.schloegel@utanet.at.

Michael Wolfbauer practices at the University Eye Clinic, Paracelsus Medical University of Salzburg, Austria. Mr. Wolfbauer states that he has a proprietary interest as a patent assignee of the SRD. He may be reached at tel:+43 662 4482 3747; fax: +43 662 4482 3703; e-mail: michael@wolfbauer.at.