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New performance assessments suggest retinal prothesis augments native vision
"Even subjects who were able to perform the task with some success with their native residual vision showed a significant improvement using the [retinal prosthesis]," continued Dr Ahuja, research scientist consultant, Second Sight Medical Products, Sylmar, California, USA. "This provides evidence from the largest cohort...to date [of subjects with chronic implants] that artificial vision augments information from existing vision in a spatial-motor task."
The prosthesis is the second generation of retinal implant technology from the manufacturer and features an implanted epiretinal multielectrode array. The trial is a 3-year study in which the epiretinal multielectrode array was implanted in 30 subjects with bare light perception vision, in the eye with the poorer vision.
Spatial-motor coordination was investigated using a square localization task in which high-contrast 2.3- × 2.3-inch square stimuli were presented at random locations on a touch screen monitor. Subjects, using both eyes, were asked to locate and touch the square centre. The test was repeated 40 times with the visual prosthesis turned on and 40 times with the prosthesis turned off. For each trial, the coordinates of the square centre and location touched were recorded.
Patient performance was assessed based on differences in accuracy and repeatability during testing in the prosthesis on/off conditions. Accuracy was defined as the mean distance between the individual's response and the target square centre. Repeatability was defined as the mean of the distance of the individual responses to the mean of all responses.
Across the subjects studied, there was significant variability in the improvement achieved in the on-versus-off condition, in part due to the ability of some subjects to perform above chance with their native vision. Nevertheless, 25 (93%) of the 27 subjects showed statistically significant improvement in repeatability and 26 (96%) subjects showed statistically significant improvement in accuracy when performing the task with the prosthesis turned on relative to when they were using native residual vision.
"Interestingly, looking at the data for testing when the prosthesis was off, there is a cluster of five subjects [that showed] relatively better repeatability and accuracy than the rest of the group," Dr Ahuja revealed.
Measurement of monocular dark-adapted light threshold to a full-field white light stimulus [Full Field Stimulus Threshold Test (FST), Diagnosys] showed that these five subjects had a significantly lower light threshold than the rest of the cohort, confirming better bare light perception. Although there was a significant correlation between light threshold and performance with the system off (i.e., just subjects' native vision), there was no relationship between either of these parameters with either patient age or self-reported number of years blind.
"The fact that candidates are unlikely to have measurable function using standard clinical electrophysiological [e.g., mfERG, ERG] and psychophysical [e.g., kinetic or static perimetry] techniques underlies the importance of the test," Dr Ahuja added.
He also mentioned other ARVO presentations on the study made by multiple collaborators from various participating clinical sites, including a presentation by Lyndon da Cruz, PhD, FRCOphth, that showed subjects reading letters, numbers and words using the retinal prosthesis.
"These results, taken together with a presentation made by Dr Gislin Dagnelie demonstrating stability of light sensitivity in the [eye with the implant] during follow-up to 2.5 years, support intervention earlier in the time-course of the disease from both safety and efficacy perspectives," Dr Ahuja said.
Dr Brian Mech, PhD, MBA, vice president of business development, Second Sight Medical Products, told Ophthalmology Times, "As the largest clinical study for a device of this kind, the clinical trial has been very informative and demonstrated several meaningful improvements in visual function."
The company is currently seeking approval to market the device in Europe beginning later in 2010. More clinical trial results will be reported in the British Journal of Ophthalmology, as well as at several meetings.