Page 49 - T-I JOURNAL19-3

ARGUS SYSTEM 609

a subretinal implant with 1,500 photodiodes in an REFERENCES
array (28,29), received CE mark approval in 2013.
Implants with photodiodes serving as the visible light 1. Grusser OJ, Hagner M. On the history of defor-
mation phosphenes and the idea of internal light
sensor often suffer from low photocurrent output. generated in the eye for the purpose of vision.
Therefore, Alpha-IMS relies on external power supply Doc Ophthalmol. 1990;74: 57-85.
to amplify the photocurrents such that the current 2. Brindley GS, Lewin WS. The sensations pro-
pulses delivered by the electrodes are sufficient to duced by electrical stimulation of the visual
drive neuronal activation (30). However, the addi- cortex. J Physiol. 1968;196:479-493.
tional power amplification circuitry increases the 3. Hossein Nazari PF, Lan Yue, James Weiland,
system complexity and surgical difficulty. An alter- Mark S. Humayun. Retinal prostheses: a clinical
native approach developed by Palanker et al. adopts perspective. J Vitreoretin Dis. 2017;1:204-213.
optical amplification by converting ambient visual 4. Tassicker GE. Preliminary report on a retinal
inputs into high intensity near infrared laser pulsing stimulator. Br J Physiol Opt. 1956;13:102-105.
that is projected onto the subretinal photodiode array. 5. Humayun MS, de Juan E, Jr., Weiland JD, Dagne-
Each pixel consists of a few photodiodes connected in lie G, Katona S, Greenberg R, Suzuki S. Pattern
series to further increase the output current (31,32). electrical stimulation of the human retina. Vision
This approach avoids complex electrical circuitry, Res. 1999;39:2569-2576.
but its safety and efficiency needs to be evaluated 6. Rizzo JF, 3rd, Wyatt J, Loewenstein J, Kelly S,
in humans. The clinical evaluation of the Alpha- Shire D. Perceptual efficacy of electrical stim-
IMS implant showed that the highest visual acuity ulation of human retina with a microelectrode
achieved was 20/546 measured with the Landolt C array during short-term surgical trials. Invest
chart, (30) lower than the acuity of 20/200 that would Ophthalmol Vis Sci. 2003;44:5362-5369.
be theoretically offered by 1,500 pixels in the array. 7. Yue L, Weiland JD, Roska B, Humayun MS.
This observation, along with findings from the animal Retinal stimulation strategies to restore vision:
models, suggests that increased electrode density fundamentals and systems. Prog Retin Eye Res.
only improves the visual acuity to an extent beyond 2016;53:21-47.
which other factors such as electrical properties of the 8. Busskamp V, Duebel J, Balya D, Fradot M, Viney
retina will weigh in. A major benefit of Alpha-IMS TJ, Siegert S, Groner AC, Cabuy E, Forster V,
and other systems that similarly implant high-density Seeliger M, Biel M, Humphries P, Pagues M,
light sensing units intraocularly is that the patients Mohand-Said S, Trono D, Deisseroth K, Sahel
can use eye movement, instead of head movement, to JA, Picaud S, Roska B. Genetic reactivation of
scan the visual field, which better mimics the natural cone photoreceptors restores visual responses in
vision. These advances and the continued excellent retinitis pigmentosa. Science. 2010;329:413-417.
work by different groups around the world in visual 9. Curcio CA, Owsley C, Jackson GR. Spare the
prostheses will further our understanding of the field rods, save the cones in aging and age-related
of artificial sight, leading to improved restored vision maculopathy. Invest Ophthalmol Vis Sci.
to otherwise blind patients for whom there is no 2000;41:2015-2018.
foreseeable cure. 10. Bourne RR, Stevens GA, White RA, Smith JL,
Flaxman SR, Proce H, Jonas JB, Keeffe J, Leasher
ACKNOWLEDGMENTS J, Naidoo K, Pesudovs K, Resnikoff S, Taylor HR,
Mark S. Humayun has commercial interest and Visions Loss Expert Group. Causes of vision loss
holds patents in the development of the epiretinal worldwide, 1990-2010: a systematic analysis.
prosthesis with Second Sight Medical Products, Inc., Lancet Glob Health. 2013;1:e339-349.
Sylmar, California. 11. Bressler NM. Age-related macular degenera-
tion is the leading cause of blindness. JAMA.
2004;291:1900-1901.
12. Friedman DS, O’Colmain BJ, Munoz B, Tomany