Summary

The large majority of low vision patients suffer from loss of central vision due to macular pathology. A variety of optical low vision aids (LVAs) may be prescribed to optimize the residual visual function, ranging from high power reading glasses and more complex magnifying aids to a closed circuit television. The indications, the advantages and the disadvantages of the various aids for low vision patients are thoroughly discussed in the first part of this thesis.

Most patients with central visual field loss develop a form of eccentric fixation, using an alternative retinal locus as alternative for the damaged macula. Eccentric viewing spectacles (EVS) constitute an alternative approach in visual rehabilitation by facilitating eccentric fixation. This thesis focuses on the various aspects of the EVS.

In chapter 1, general aspects of low vision and low vision rehabilitation are discussed. Low vision may be the result of central visual field loss, most commonly end stage age-related macular degeneration (AMD), but may also be associated with peripheral visual field loss with sparing of the posterior pole, as in retinitis pigmentosa and glaucoma. Satisfactory and effective use of LVAs depends on the possibilities and desires of the patient, the characteristics of the type of low vision aid, and on training in the use of the specific low vision aid. Visual rehabilitation with non-magnifying devices aims to improve the environment of the low vision patient and includes adequate illumination and reduction of glare. Various magnifying devices as well as telescopic devices may be employed in more severely affected patients. Finally, closed-circuit television systems can offer high levels of magnification for patients with very low residual visual acuity.

Chapter 2 contains a thorough discussion of the principles and practices of EVS. This part of the thesis includes an explanation of the basic optics of prisms and reviews the most common medical applications of prisms. The principle of normal fixation is compared to eccentric fixation. The principle that prisms have the property to refract light is used to shift the focal point

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from the fovea to an alternative point in the retina where a patient with central dense scotoma has developed a PRL. The patient no longer needs to adjust the position of his eye and/or head to make effective use of the PRL. A review the literature on the use of EVS is provided.

In chapter 3, our first experiences with EVS are described in patients with end-stage AMD. The patients in this study depended on their eccentric viewing for distance and near vision. The PRL in these patients was determined and high power prism glasses were fitted. After a mean follow-up period of 13 months 61% of the patients considered the EVS an improvement in everyday life. Dizziness was the most frequent reason to stop wearing the prismatic spectacles.

In chapter 4, the possible benefits of the EVS on the visual search time are quantified. We conducted a study to compare the visual search time with and without the EVS. Previous studies demonstrated that the visual search time is longer in patients with a central scotoma. When the visual search time with and without the EVS was compared in full-time EVS wearers, we observed a 34% decrease in visual search time with EVS.

In chapter 5, the long-term success and patient satisfaction of the EVS are evaluated. By means of a questionnaire, 191 patients were questioned about wearing characteristics, advantages and disadvantages of the EVS. The average follow-up time was 4.5 years. Regular users were compared with persons who no longer used their EVS. Forty percent of all patients still used the EVS. The EVA appeared to be associated with a number of disadvantages and side effects, such as the heavy weight of the spectacles, perception of curved lines, dizziness during walking and poor cosmetic appearance of the massive frame and thick glasses. Even so, many successful long-term users considered the EVS helpful for home-activities and walking in the street. They reported that this device reduced head and eye adjustments, improved vision in the centre of their visual field and helped with the recognition of objects and faces. New technical developments can probably reduce some of these disadvantages. As always, patients need to be realistically informed about the advantages and disadvantages of low vision aids such as the EVS.

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Chapter 6 focuses on the spontaneously developed EVGD in patients with bilateral central scotomas. The author employed the cornea reflex method to determine the binocular gaze directions in 434 patients. AMD accounted for 77% of the underlying ocular pathology in this group of patients. The majority demonstrated a binocular EVGD to the right (50%). In 25% of the patients a superior binocular EVGD was found. Less often, a binocular EVGD to the left (14%) and an inferior binocular EVGD (11%) were encountered. The binocular EVGD appeared to be determined by the eye with the best visual acuity. In patients with eyes with equal vision, the monocular EVGD is similar in both eyes, presumably in an attempt of the visual system to make optimal use of both eyes. A review of the literature on this subject suggests that the method of investigation is of influence on the observed EVGD. Most other studies determined the monocular EVGD and observed a superior EVGD (mean 62%) in the majority of eyes. The monocular EVGD may be an inadequate reflection of the binocular reality in most patients, in our opinion, a binocular measurement of the EVGD should therefore be considered.

In chapter 7, we discuss the most important issues regarding the EVS and eccentric viewing. We speculate on the functional aspects of EVGD and PRL location, as well as the many uncertainties in our knowledge of the development of the naturally developed PRL. In view of this lack of understanding, we would not encourage the development of a trained retinal locus (TRL) to replace the naturally developed PRL.

A common misunderstanding concerning bilateral prisms in patients with bilateral central scotomas is the often described positive effect on the visual acuity. In this regard, the off-centre visual acuity becomes a key factor. The introduction of a prism may seem to improve visual acuity, but the off-centre visual acuity will always remain the same. The issue of high and low power prisms is also addressed in chapter 7. The current thesis focuses on high power EVS (≥ 15∆) for daily, continuous use. High power prisms are used in patients with relative large scotomas and therefore a more profoundly reduced visual acuity.

We discuss the advantages and the disadvantages of the EVS in supporting eccentric viewing. Obviously, reducing torticollis and the associated neck

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and shoulder strain benefits the patients, but social factors are also of importance, such as restoring the eye-to-eye contact. As with other low vision aids, there is some concern regarding the compliance of patients using EVS. After a mean of 54 months, almost 60% of the patients discontinued the use of their EVS. Although this is certainly not higher than with other low vision aids, better training in the patient’s own surroundings may help in lowering this number.

In this general discussion, a flow chart is provided, with a two step technique for prescription and correct fitting of the EVS.

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