In Glasgow in 1877 James Dewar first observed that if you flash a bright light into the eye, you can detect the reaction of the cells within the eye. This simple principle is being used in Glasgow today in a technique in which different areas of the retina are stimulated simultaneously and the resulting impulses are analysed by computer. These detected signals are then turned into useful maps. This is a technically demanding job, as the nerve signals are in the order of nano-volts (around 1/100,000 of the strength of an ECG -- Electrocardiogram -- or heart monitor) and sophisticated software is needed to process the data.
To undertake the test the patient sits looking at a screen on which a pattern of hexagonal shapes is presented. These shapes flicker at complex predetermined frequencies. While the patient sits there, a real time map of their retinal function is formed and can be used instantly for diagnosis. The use of standard electroretinograms - which simply flash light in and detect output - was pioneered in the 1940s and '50s. Over the past 10 years the technology has advanced to the point where spatial mapping is now possible and can be carried out sufficently quickly and accurately to be a valuable clinical tool.
This mapping technique - known as multi-focal electrophysiology - builds up a 3-dimensional picture of the retinal function, showing exactly the location and severity of any damage to the retina. The retina is a complex structure comprising layers of cells with different functional jobs. The top layer - with rods and cones - absorbs the light like film in a camera. The layers underneath - the mid retina - process this information and then feed it into the nervous system. The research can tell which layer has function impairment - something of great value for more effective diagnosis and treatment.
Drs Stuart Parks and David Keating have been working on this system since the early 1990s. Their research group is one of only three in the world to have developed this technology and is unique in being situated in a hospital eye department. Dr Parks says: 'We have been trying to develop a user-friendly tool for ophthalmologists to use to map the visual response. This technique has the advantage of being an objective test. All the others rely on what the patient says, and this information tends to be of poor quality and certainly doesn't tell you where the defect is in the visual pathway. It is a valuable tool in assessing retinal function and is good for detecting a range of eye disorders."
Current investigations include glaucoma, retinitis pigmentosa, retinal vein thrombosis and age-related macular degeneration. These conditions are either hard to diagnose or difficult to monitor with conventional tests. For example, with glaucoma if you wait until the patient can detect a difference, 40% of the retina will already be damaged.
The unique strength of the research team is that it is interdisciplinary, drawing on expertise from the University's Department of Clinical Physics. It also has the input of clinicians working in the hospital environment. The team includes medical physicists, research assistants, a mathematician, optometrist and ophthalmologist. The actual hardware used to test patients is fairly standard. The complex bit is the data processing side requiring powerful computing and software to give the information in real time, so clinicians can use it effectively in a clinical setting.
Dr David Keating says: "There is a huge amount of data gathered from each patient and we could spend our entire research time just looking at one patient. But to make it work in practice, the technique must be quick and easy. The patient is scanned for ten minutes and while they are there you can see the retinal map forming. The advantage of this technique is it allows the detection of retinal abnormalities before the patients are aware of any problems."
The team are currently working on building up databases of patient information, to act rather like a finger print library. The idea is that a particular set of findings from a patient could be matched with those in the library and act as a tool to aid diagnosis. Another useful aspect is that patients can be retested and their progressive results compared at the same time to see if the condition has worsened or improved, perhaps due to drug therapy.
But it is the clinicians who give this technique the most convincing thumbs up. Dr Parks says: "This is absolutely not abstract research. The clinicians like using this technique as it gives them a much stronger diagnostic tool than they have previously had available.It allows them to diagnose disorders that would have been difficult with conventional methods."
FURTHER INFORMATION:
Dr David Keating
Clinical Physics
tel: 0141 211 2758
fax: 0141 211 6746
e-mail:d.keating@clinmed.gla.ac.uk
John Hancox
University of Glasgow Press Office
0141 330 3683
j.hancox@admin.gla.ac.uk