Second sight for blindness

Utilising the same principle that lets a TV camera transform external images into electric signals, IST project OPTIVIP has tested an implantable visual prosthesis to stimulate the optic nerve and allow limited sight for certain sufferers of blindness.

The method used by the four-year project is based on the stimulation of the optic nerve by a cuff electrode. The prosthesis is operational only if the optic nerve is still healthy in spite of the complete blindness. For this reason, OPTIVIP has targeted retinitis pigmentosa, a disease that leads to the premature ageing and a disappearance of the retina receiving cells that leads to blindness over time. Hereditary in 60 per cent of the cases, it occurs to roughly 1 out of every 5,000 women and 1 out of every 4,000 men.

The origins of the project can be traced to February 1998, when within the framework of the ESPRIT project MiViP, an electrode designed to stimulate the optic nerve was implanted in a volunteer who was completely blind due to retinitis pigmentosa. The use of this electrode made possible the electrical stimulation of the optic nerve by means of external equipment. This revealed numerous successful visual perceptions called phosphenes, which are apparitions of luminous sensations, and it was decided to implant a telemetrically controlled neurostimulator in the same patient.

Following up these discoveries, the OPTIVIP project developed a system that works in the following ways. A small camera fastened on glasses picks up an image of what appears in the direction of the glance. The components of the prosthesis are connected between the camera and the stimulation electrode, which contains eight contacts that sends signals to some packets of optic nerve fibres, provoking the phosphenes.

The implanted stimulator produces the electric currents that are applied to the contacts of the electrode according to orders transmitted by the antenna. The heart of the implanted stimulator is a specific integrated circuit that takes care of the demodulation functions of the signal received from the antenna, formats the stimulation currents, and takes measurements. Its electric power is provided through a transcutaneous link (ie a small transformer with coils that act as antennas) implanted behind the ear and the outside.

After learning how to control the visual sensations generated by the optic nerve stimulation, OPTIVIP developed some practical uses of the system, which it documented in a report published in October 2003. Shapes or characters projected on a screen can now be recognised by the patient thanks to the prosthesis. During a recognition test of one out of 45 patterns, 87 per cent of the answers were correct. The report concluded that the implantation brought “good results” and the consortium has “made a step forward” from the original tests.

“Thanks to this system, our volunteer has leaned how to recognise signs looking like block capitals. In a simplified environment, she can also identify objects laying on a table and grasp them thanks to the prosthesis,” says Claude Veraart, head of biomedical aspects within the project. While these results may seem modest, he adds that they are important insofar as numerous possibilities of improvement have been identified. Also, important is the fact that the system is perfectly tolerated by the wearer. No deterioration could be noticed during the five years of evaluation, due to the use of biocompatible materials that resist their environment without aggressing it.

With such a promising future, “it was decided to continue the work while widely improving the system technically and carrying out an implantation in several new volunteers,” says Charles Trullemans, head of system engineering for the project. “From the technical point of view, the prototypes are close to being delivered for implantation. From the biomedical point of view, the implantation is going to occur within a few months.” However, he notes the project is a pre-clinical test, and that actual commercialisation is not foreseen for several years.

Beyond the implanted prosthesis, the consortium sees additional potential for the technology that is being developed. According to them, the results of the project will in a short time enable the availability of several products in a wider application field, such as electrodes for neural stimulation, neuronal stimulators with high-speed, bi-directional transcutaneous links for general use, and several models of external processors with high computing power.

Contact:
Charles Trullemans
Université Catholique de Louvain
DICE – Unite des Dispositifs et Circuits Electroniques
1 Place de l’Université
B-1348 Louvain-la-Neuve
Belgium
Tel: +32-10-472567
Fax: +32-10-472598
Email: Ctrullemans@dice.ucl.ac.be

Source: Based on information from OPTIVIP