5.1  Introduction

Retinal degenerations are characterized by a loss of vision. The loss of photoreceptors leads to the development of blind spots (scotomas) or reduction in the visual field area. The features of the scotomas (whether they are peripheral or central, the time course of development, etc) are characteristic of the particular retinal disease, but all have in common the loss of vision.

However, as a consequence of the retinal disease, whether at the level of “sick” retinal cells, changes in the optic nerve, or changes in the brain, there is the generation of new visual phenomena. Patients may report flashing lights (photopsias), positive scotomas (perceived as blurry or missing areas of vision), filling-in phenomena, and visual hallucinations. While the basis of these phenomena is not clearly understood, they are reported by a large number of patients, and must be taken into account both in the design of visual prostheses and when interpreting visual responses from patients implanted with such devices.

5.2  Vision Changes Experienced by RP Patients

5.2.1  Overview

The most prominent and earliest symptoms of RP are progressive night blindness and field loss, though central vision may also be reduced. The vision loss is bilateral and symmetrical. There are two patterns of night vision loss [35]. In type 1 rod-cone degeneration, there is reduced night vision from birth. In these patients, early rod dysfunction may be demonstrated by dark-adapted two-color static perimetry. In type 2 (sometimes called regional), night vision is normal until field loss begins. In type 2 patients, dark-adapted visual field perimetry shows rod photoreceptor function in non-scotomatous retinal areas. Usually the patients with type 2 degeneration once had the ability to see stars at night, while patients with type 1 were never able to see stars. In both forms, however, the initial symptoms typically include either mobility problems in dim or dark illumination or an inability to change quickly from one light level to another.

Most individuals are first symptomatic between the ages of 5–30 years, although some cases have been reported to emerge later in life [39]. The age of onset of RP varies for different genetic mutations, but across all patients, the average age at which RP was diagnosed by an ophthalmologist was reported as 35 years [56]. As a generalization, patients with X-linked RP begin having visual field loss the earliest (typically during the teenage years), patients with autosomal recessive RP are in the middle, and patients with autosomal dominant RP may not develop significant field loss until the 40s or later. The proportion of RP patients with autosomal recessive inheritance is approximately 30–40%, autosomal dominant is presumed to occur in about 50–60%, and x-linked inheritance is estimated in

5–15% [22]. For patients without a prior family history of RP, it is often diagnosed incidentally during a routine eye examination, and sometimes on the basis of subjectively reported reduced night vision.

5.2.2  Visual Field Loss in RP

Peripheral visual field loss is universal in RP. It typically starts in the midperipheral region of the retina and spreads both out toward the periphery and in toward the macula [39]. Patients may develop a full or partial mid-peripheral ring scotoma, which then expands outward and inward. Since the nasal retina extends more peripherally than the temporal retina, the far periphery of the temporal visual field may be spared when the scotoma reaches the edge of the nasal, superior and inferior fields. Some RP patients may retain far peripheral, temporal islands of vision later in the course of the degeneration, even when the central field is <20° or in some cases when there is no remaining central vision. If the peripheral spared areas are large enough, they can enable patients to detect moving objects from the side or give valuable information during mobility to avoid bumping into objects or people. Early in the course of the disease, individuals with RP may be labeled as being clumsy or careless in terms of mobility, bumping into people and obstacles hidden by their (as yet unknown) scotomas. As peripheral visual field loss progresses, they are increasingly prone to bumps, bruises and falls.

The rate of visual field progression in RP is typically slow, with estimates of about 5–14% lost per year [5, 20, 27, 34]. Figures 5.1 and 5.2 show examples of visual field progression in retinitis pigmentosa measured by Goldmann perimetry over 13 and 16 years, respectively [21]. For most individuals, the progression is steady, but some report that their rate of visual field loss is variable over time, with occasional lengthy periods of perceived stabilization. The slow rate of progression enables RP patients to adapt well to their vision loss, and they often do not seek mobility training or assistance until late in the disease when only a few degrees of central vision remain. A previous survey indicated that about 23% of RP patients were not aware that they had visual field loss, although they showed constriction of their field [24]. Often patients who have good central acuity but substantial field loss, who would be characterized as legally blind on the basis of a visual field diameter <20°, are surprised to learn the extent of the loss through visual field testing. This is because they have adjusted gradually to the progressive field loss and are still fully functional in their daily activities. Due to constrictions in the visual field, RP patients tend to use scanning to survey their environment for orientation and mobility. When walking in unfamiliar areas, instead of gazing straight ahead at a distant target, RP patients tend to direct their gaze at nearby objects on the walls, downward, or at the layout (i.e., edge-lines or boundaries between walls) [57]. The smaller the horizontal visual field extent, the more they tend to use downwarddirected fixations, which are important to detect changes in the walking surface and avoid low-lying obstacles.

Fig. 5.2  (a–e) Goldmann visual fields obtained on the right eye of a patient with retinitis pigmentosa, with targets as marked, showing a pattern IIB loss of visual fields over a period of 16 years. Reprinted from [21], with permission

5.2.3  Changes in Color Vision and Glare Sensitivity in RP

Early in RP, color vision is typically normal since the central visual field where the vast majority of cone photoreceptors are located is not affected by the initial rod photoreceptor degeneration. However, as the disease progresses, abnormalities in color vision are highly correlated with the extent of visual field loss. Among those with a visual acuity of 20/30 or better, autosomal dominant cases are less likely to show extensive color defects when compared to other genetic types of RP [15]. As central visual acuity is initially lost, the development of dyschromatopsia to pale,