The majority of RP patients indicated that photopsias interfere with their vision, and interference was more likely when photopsias occurred daily, increased in frequency over time, or were located across a larger area over time. About half of RP patients who report photopsias experience them daily. The increased frequency of photopsias in RP appears to be related to increased perceived stress and decreased positive mood. About a quarter experienced photopsias constantly, and over half experienced photopsias for only a few seconds at a time. The location or frequency of photopsias in later RP stages may obstruct vision at times, and is a potential issue for patients’ function or when obtaining vision measures.

5.8.2  Photopsias in AMD and Other Ocular Diseases

One report indicated that photopsias are common in patients with macular choroidal neovascularization [8], occurring in 59%, and of those, 59% experienced white colored photopsias and described them as typically lasting several seconds. Subretinal fluid, cicatrix formation and larger disciform scars were more common among individuals who noted photopsias than in those who did not. The occurrence of photopsias among those with macular choroidal neovascularization may potentially be due to sensory deprivation; when normal input to the visual system is repressed and the activity of other neural tissue may become more apparent.

Rare retinal diseases with scotomata and the possible presence of photopsias are acute zonal occult outer retinopathy (AZOOR) [61], multiple evanescent white dot syndrome (MEWDS) [29], acute idiopathic blind spot enlargement (AIBSE) affecting the retina around the optic nerve without optic nerve head swelling or choroiditis [16], autoimmune retinopathy (including cancerand melanomaassociated retinopathies) [23], photoreceptor dysfunction due to digitalis toxicity [38], and punctate inner choroidopathy (PIC) [19]. Some patients with optic neuritis secondary to multiple sclerosis [11, 33] and restrictive thyroid ophthalmopathy with tight inferior recti eye muscles [10] have also reported photopsias associated with eye movements, likely related to compression or traction.

5.9  Concluding Remarks

In addition to the anatomical and functional changes that occur in the retinal and visual cortex, it is important to consider and address the various types of changes in vision experienced by patients with RP and AMD that will impact both the objective and subjective outcomes with prostheses. In particular, emphasis should be placed on the patients’ perspectives of functioning with a retinal degenerative disease, considering the disability and uncertainty associated with the performance of activities of daily living and day-to-day visual fluctuations. In the assessment and rehabilitation of prosthetic vision, several types of visual phenomena that may ordinarily occur in those with advanced AMD or end-stage RP will potentially

interfere with the visual percepts produced by prosthetic devices. These aspects need to be better understood and managed by researchers and clinicians working in the area of prosthetic vision.

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Part II

Neural Stimulation of the Visual System

Chapter 6

Structures, Materials, and Processes

at the Electrode-to-Tissue Interface

Aditi Ray and James D. Weiland

Abstract  This chapter reviews the basic concepts of neural stimulation along with safety considerations for both the electrode and tissue. The section on electrode– electrolyte interface describes the basic mechanism of charge injection at the interface introducing the reader to the electrode double layer. The use of circuit models to represent the physical processes at the interface and in the bulk tissue is discussed. The next section provides a detailed description of the biopotential electrode along with measurement techniques used in electrode characterization. Following this, an overview of popular electrode materials for neural stimulation is provided for the reader. These include conventional materials such as platinum and iridium oxide, as well as newer materials like conducting polymers and carbon nanotubes. The next section reviews the concept of extracellular stimulation introducing the reader to Goldman Equation used to describe the membrane potential. Finally the section dedicated to safe stimulation of tissue describes the mechanisms of neural injury and parameters considered to ensure safe neural stimulation. Special emphasis is placed on safety studies of retinal stimulation.

Abbreviations

A. Ray (*)

Department of Biomedical Engineering, 1042 Downey Way, Denney Research Building (DRB) 140, Los Angeles, CA 90089, USA

e-mail: Aditi.Ray@AlconLabs.com