The Argus II retinal prosthesis is a relatively novel medical device that can marginally restore some functional vision in patients suffering from retinal dystrophies. Numerous studies have demonstrated that a statistically significant improvement in visual function can occur in some patients, and it can no doubt impact their lives tremendously7,8,9,10. Our study’s analysis of SL results in patients with the device further demonstrates that a significant improvement in visual function is possible, with 1 patients performing better with the device ON. An exploration of the factors that influence these positive or negative outcomes is crucial to our understanding of how to further improve visual prosthetic technology. Ahuja et al3 determined that the electrode array-retina gap distance strongly affects electrical threshold in patients implanted with the Argus II retinal prosthesis, with complete apposition correlating with lower perceptual thresholds. Our study aimed to determine how the gap distance correlates with functional measures of visual acuity such as Square Localization and Direction of Motion testing.
As our results demonstrate, there does not seem to be a correlation between the electrode-array retina gap distance and visual outcome as determined by SL and DOM testing. The patient with the largest average gap distance (patient 04, Avg = 399.37 µm) had the greatest average improvement in SL testing (108.46 pixels). Conversely, the patient with complete apposition of the electrode (patient 02, Avg = 0 µm) at all visits demonstrated decreased visual functions in both SL and DOM testing. While there may not be a correlation between gap distance and visual function, this leads us to hypothesize that complete apposition of the electrode array on the retina may not be necessary for optimal visual outcomes in all patients. This becomes a crucial element in determining patient eligibility for patients with anatomical defects that could affect apposition. Rizzo et al2 and Delyfer et al11 have demonstrated that complete apposition of the electrode array on the retina may not be possible in patients with a posterior staphyloma. However, our results demonstrate that patients with staphylomas may not necessarily be precluded from implantation of the Argus II retinal prosthesis, as visual improvement may still occur in the presence of an electrode array-retina gap. An improvement in visual function in patients with a posterior pole staphyloma implanted with the Argus II retinal prosthesis has been demonstrated by Seider et al.12. It is possible that other staphyloma factors such as size, shape, and location may affect visual function outcomes rather than its presence alone.
One patient with almost complete apposition of the electrode array on the retina (average gap distance 0.38 µm, 0.27 µm, 8.70 µm at M3, M6, Y1, respectively) developed clinically significant retinoschisis. It is possible that complete apposition of the array may increase the risk of adverse postoperative outcomes due to traction, overstimulation of the retina, and inflammatory changes resulting in remodeling of the inner retinal layers.
SL and DOM testing are inherently arduous and time consuming, requiring patients to remain focused and motivated in order to perform optimally. Testing results may be influenced by frustration and avolition, although all attempts were made to ensure that patients completed testing in a comfortable environment. One patient (03) was on an SSRI antidepressant during all testing visits and seemed more motivated to perform well. SL and DOM testing although requires some degree of hand-eye coordination, a skill which may have slightly regressed in these patients due to a long-standing history of blindness. Testing methods that are less time consuming and easier for low-vision patients to perform would allow for a more accurate determination of post-operative visual function. Subjective measures that allow for a better assessment of what patient goals for the device may be and how they hope the device could be used to achieve them would also provide more crucial information for the device’s potential for patient benefit. Furthermore, optimal utilization of the Argus II retinal prosthesis requires visual rehabilitation and self-learning by the patient during their day-to-day life. Depending on the quality and quantity of visual rehabilitation, and the amount of time a patient spends with the device turned ON, their ability to interpret the generated phosphenes would be affected. It is also possible that overuse of the device could negatively impact visual function through constant electrical stimulation. These factors could ultimately influence performance on SL and DOM testing. Lastly, we noted that gap distances changed over time, with three patients having changes of over 50 µm between testing visits. This may be due to the positioning tack coming into contact with extraocular muscles at the scleral interface, or anatomical changes post-operatively. Our study is also limited by a small sample size (n = 5). Future studies with a larger data set may provide any opportunity to further explore the relationship between electrode-retina gap distance and visual outcomes.