RETINAL PROSTHESIS

Formal testing demonstrates significant improvements in variety of visual tasks

The Argus II retinal prosthesis (Second Sight Medical Products Inc, Sylmar, CA) provides long-term major improvements in the lives of patients with advanced retinitis pigmentosa, reported Mark S Humayun MD, PhD at the annual meeting of the Association for Research in Vision and Ophthalmology.

Dr Humayun presented results from 30 patients implanted for a mean of 2.4 years as part of a prospective, single-arm, international study currently ongoing at 10 centres. The patients have severe to profound outer retinal degeneration secondary to retinitis pigmentosa or related dystrophies with remaining visual acuity of bare light perception or worse.

He reviewed safety data and presented outcomes of structured performance testing with high-contrast environments that consistently showed use of the system was associated with significant improvements in localisation and direction of motion tasks, visual acuity, character recognition, orientation, and mobility. Some patients also recovered colour perception and reading ability.

'All implanted patients detect phosphenes and are using the system outside of the clinic where they report it improves their daily lives. With the Argus II, patients are able to locate and find objects indoors and outside, and they are able to localise their position and pass through doors and into elevators without hitting door frames. As a first achievement for any retinal prosthesis, some patients are even reading short sentences, and with continued training they have been able to quickly increase their reading speed. Now efforts to develop a rehabilitation programme have begun,' said Dr Humayun, professor of biomedical sciences, ophthalmology, biomedical engineering, cell and neurobiology, Doheny Eye Institute, University of Southern California, Los Angeles.

He told EuroTimes: 'The development of a retinal prosthesis has been extremely challenging but also very rewarding. After being involved for almost 25 years from the inception of the project, it is truly wonderful to see that the retinal implant has gotten CE mark approval and will be available in Europe for patients for whom otherwise there was no foreseeable cure.'

In a square localisation task, subjects identify a static square, 2.8 x 2.8 inches, displayed on a touch screen monitor. Results from testing performed with the retinal prosthesis system on versus off showed 86 per cent of the 30 patients performed significantly better using the system.

Motion detection was tested using a touch screen monitor displaying a moving 1.4-inch wide bar. For this task, 54 per cent of subjects performed significantly better with the system on versus off. 'Motion detection is a more difficult test because of the temporal aspect, and the results give us insight on where we need to be working to improve device functionality,' Dr Humayun said.

Visual acuity is tested using square wave gratings on a CRT. Using the Argus II, 27 per cent of patients had improved visual acuity, with the best achieved being 20/1260.

'These are encouraging results. With the first generation of our retinal prosthesis, which had 16 electrodes, the best visual acuity was about 20/4000. We would predict that a future generation device with 1000 electrodes might provide visual acuity of 20/80. Of course, there are many variables,' Dr Humayun commented.

For character recognition, patients were presented with a series of letters and numbers organised into four sets according to recognition difficulty. Across all four sets, a subset of patients were able to accurately identify only about 10 per cent of characters when the testing was performed with the system switched off. However, when using the retinal prosthesis, they accurately identified 80 per cent of characters in the most difficult set and up to 95 per cent in the easiest series.

Orientation and mobility testing was performed by having subjects perform walking tasks involving finding a door and following a line, and the testing conditions were varied to increase difficulty. Even in the more challenging tasks, performance was significantly better with the system on versus off.

Perception of eight different colours was achieved in a subset of patients who underwent testing of this function. Elicitation of colour perception is provided through frequency-based stimulation using specific electrical patterns for different colours. The information being gathered in this initial testing will be applied in the future design of the electrode array where various electrodes will represent different colours. 'If something is red, the visual processing unit will stimulate the electrode array with a corresponding pattern that allows the patient to perceive that colour,' Dr Humayun said.

Reliability and safety

As of May 2011, time post-implantation ranged up to four years, and during that time, the system has continued to perform dependably except for one device that was affected by a communication failure after 10 months. In the latter case, the telemetry coil was believed to have been damaged during implantation, but the system functions intermittently, and the epiretinal electrode array remains implanted in this patient.

Complications have included 10 per cent rates of both conjunctival erosion and endophthalmitis. Recurrent conjunctival erosion accounts for the single case of device explantation in the series. 'Problems with conjunctival erosion have been addressed with modifications to the device and its placement, and we've learned to better maintain sterile techniques intraoperatively so that the electrode array is protected from contamination. The risk of endophthalmitis is something that concerned us early on, but with institution of new practices, there have been no cases in the last 15 patients implanted,' Dr Humayun said.

The Argus II system received the CE mark in February 2011, and Second Sight plans to submit an application for FDA approval this year.