Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 15

RPE, making the outer retina dysfunctional. In the long term, a scar may form
under the retina (known as disciform macular degeneration).
The range of central visual loss in AMD can vary from very mild to very
severe. Nevertheless, because the peripheral retina remains relatively normal,
affected individuals retain peripheral vision; i.e. they may not be able to see the
objects they are directly looking at, but they can see other objects around them.
Current treatments for AMD : Unfortunately there is no effective treatment
for dry AMD which comprises 90% of AMD patients. However, progression to
advanced disease may be reduced in high-risk patients by daily supplementation
of specific vitamins and zinc [16].
Unlike dry AMD, wet AMD often results in severe visual loss, and
therefore the majority of research has been concentrated in treating this type
of disease. Until the late 1990s, the only proven treatment was laser photo-
coagulation [17], but since then a growing number of treatments have entered
human clinical trials. The aim of treatment in wet AMD is to destroy the
CNV without damaging the retina or RPE. Other treatment modalities for
wet AMD include the following: surgical treatments [18, 19], photodynamic
therapy [20], transpupillary thermotherapy [21], intravitreal injection of antian-
giogenic drugs [22-24], and periocular injection of anecortave acetate [25].
Retinitis Pigmentosa
This term is used for a heterogeneous group of hereditary disorders charac-
terized by dysfunction and loss of photoreceptors and RPE, and has a worldwide
prevalence of 1:3000 to 1:5000 [26]. Rod photoreceptors are predominantly
affected; therefore, clinical symptoms begin with the loss of peripheral and night
vision. Depending on the specific gene mutation, clinical symptoms may start
in childhood or later in life. The disease is bilateral and progressive, and finally
results in loss of central vision in the majority of affected individuals. In extreme
cases even light perception may disappear.
The appearance of the fundus in patients with RP is characteristic and displays
a narrowing of the vessels, peculiar pigmentation of the peripheral retina, and
optic disc pallor (Figure 1.16). The ERG is usually abnormal, even in early stages
of the disease when patients still have relatively good vision (Figure 1.17).
Current treatments for retinitis pigmentosa : Like dry AMD, there is no proven
treatment for RP. However, in a randomized, controlled clinical trial on 601
patients with RP and with 4-6 years follow-up, it was demonstrated that vitamin
A supplementation could slow the rate of decline in the retinal function in
these patients. Potential treatments are being investigated in animals, and may
some day cure the disease, but are unlikely to be helpful to those who have
already lost their sight. These experimental treatments include the following:
gene therapy [27], stem cells [28], and RPE and retinal transplantation [29].
Viability of Inner Retinal Cells in Outer Retinal Disease
For a retinal prosthesis to be possible, there must be some part of the retina
remaining to which the prosthesis can communicate. Morphometric studies of