Medical Information on the Electronic Eye
by Dr Andrew Blaikie for VI Scotland
This document is written with the minimum use of medical terms and jargon. It is impossible to avoid all medical terms but where we have used them we have attempted to explain them as clearly as we can. Although the information is intended to describe most aspects of the condition each child is different and there will always be exceptions to the rule. As far as we can determine these pages are true and accurate and have been written in good faith.
What this information is not for
This document is not a substitute for a consultation with a Health Professional and should not be used as a means of diagnosing a condition.
We hope the information will help you to:
- Have a better understanding of the condition
- Know what tests and treatments are normally available
- Know when to seek professional advice
- Be able to discuss the condition in a more informed way
- Make the most of consultations with carers, teachers and health professionals
- Be reassured and more able to cope
Due to staffing limitations we are not able to offer telephone or email advice to parents of children.
Several different scientific groups around the world have focused their efforts on developing an ‘electronic eye’ that might be able to restore vision by replacing non-functioning parts of the human eye with clever electrical devices. In blinding conditions affecting the light-receptor cells (photoreceptors), such as retinitis pigmentosa, the ‘electronic eye’ has been shown to have some effect.
How the photoreceptors normally work?
The retina is a thin nerve tissue in the back of the eye consisting of many layers. The photoreceptors form the first of these layers and are responsible for converting light into electrical current. This electrical information will pass through the inner layers of retina onto the last layer of long, thin nerve cells called ganglion cells. These cells will then send the signal, via the optic nerve, onto the brain where the image is created in consciousness.
Diseases affecting the photoreceptor cells such as Retinitis pigmentosa (RP) ‘block’ the very first step of the visual pathway. RP is an inherited disease characterised by gradual degeneration of the photoreceptors leading to progressive sight loss. The onset of symptoms varies form infancy until later in life and the patients experience difficulty in adaptation from light to dark or dark to light, night blindness, loss of peripheral vision or less frequently loss of central vision.
In the past few years, gene therapy (identification and replacement of the ‘altered’ information carried by the photoreceptor cells) offered promising results. However, the significant variety of possible ‘gene alterations’ leading to RP means that a successful treatment for one case would not necessarily treat other cases. This restricts the development of widely effective therapies. Also, the concept of stem cell transplantation (replacement of photoreceptors with new healthy ones) is only in relatively early experimental stages.
In this context, an ‘electronic eye’ that would restore vision has been a popular subject amongst scientists and various electric devices have been designed.
The idea of the Electronic Eye
The idea behind the electronic devices is to ‘by-pass’ the non-functioning layer of photoreceptor cells by targeting the information directly onto the healthy inner layers of the retina (sub-retinal devices) or ganglion cells (epi-retinal approach).
How does the electronic eye work?
To replace the photoreceptors a device needs to translate the visual information into electrical signals. There are 2 different ways to achieve that:
1) Devices almost entirely inside the eye.
A light-sensitive device, described as microphotodiode array, is implanted at the natural position of photoreceptors (sub-retinal) and converts the light to electrical signals. These signals will activate the inner layers of retina through attached microelectrodes (small metal electrodes). The benefit of these implants is that they do not only respond to light but also act as a small solar panel to generate power to drive the device (by using infra-red light). This means there is no need for an external power supply which simplifies their design.
2) Devices with external capturing, powering and processing systems.
A camera attached to a pair of spectacles captures the image. The signal is then wirelessly sent to a mini processor hidden under the skin over the temple. Small wires leaving the processor will pass through the wall of the eye and transfer the electrical signals to an array of microelectrodes implanted on the surface of the retina (epi-retinal approach). The microelectrodes will then stimulate the ganglion cells. This device requires a video processing unit and battery attached to a belt around the patient.
Both these types have their advantages and drawbacks. Completely internal microphotodiodes act like ‘true’ photoreceptors producing current in response to the light they detect. If the device is implanted in the layer normally occupied by healthy photoreceptors, the information that is ultimately passed to the brain is more likely to be correctly interpreted by visual centres in the brain - potentially creating more realistic vision. On the other hand, the external processing devices provide the option of modifying the visual information that triggers the visual pathway addressing the needs of each patient individually. However, these devices in theory do carry a higher risk of infection, due to the wires that traverse the wall of the eye.
Is it effective?
Studies measuring the results of these electronic devices implanted in eyes of blind patients show that the produced electrical signals enable them to see light. They also report that some patients can identify objects, distinguish patterns or even read large letters.
Why the outcome is not the same in all patients remains to be answered. Factors that may interfere with the results are the design of the device, the exact location of the implant inside the eye, particularly the level of the retina that it is implanted, the stage of the deterioration of the disease, the functional capacity of the remaining retinal cells as well as the health of the visual centres in the brain and their ability to ‘recreate’ vision.
The future is likely to belong to devices that do not depend on an external powering and processing system. Early results do show that the ‘electronic eye’ can be a useful approach for restoring poor vision when it is due to acquired disease of the retina. Before expanding the idea to other retinal diseases it will need to be established how safe these devices are in the long term for the remaining parts of the eye. Nevertheless, the advancements in the field of the ‘electronic eye’ so far have been rapid and the future looks very promising.
Who wrote these documents?
These pages are the consensus of opinion of many different people They include parents of visually impaired children, visually impaired children themselves, Community Paediatricians, Ophthalmologists, Educationalists and Psychologists.
The main author and person responsible for their content is Dr Andrew Blaikie who was an Ophthalmology Research Fellow with Visual Impairment Scotland and is a member of the Royal College of Ophthalmologists.