Monitoring tiny eye movements
In many medical situations, such as coma or stroke, it is vital to assess a patient’s level of consciousness. This can be done by measuring tiny eye movements, called ocular micro-tremor, or OMT. There is currently no portable, patient friendly machine available to doctors to measure OMT, but Dr Gerard Boyle, a medical physicist and engineer, based at St James’s Hospital, Dublin, has Science Foundation Ireland (SFI) funding to develop just such a device.
Many of us would have heard of REM, or Rapid Eye Movements, which are known to occur during dreaming. However, there are also other eye movements going on, and unlike REM these are happening constantly, not just during dreaming. These movements, called OMTs, are so tiny that we are not aware that they are happening.
One reason that OMTs are very important medically is because of the link – which was found by Prof Davis Coakley at TCD back in the 1980s – between OMT and a person’s level of consciousness. The more rapid the person’s OMT, the more alert they are. The less rapid the movements, the less conscious a person is, and when OMT stops altogether, that is a diagnostic tool to indicate a person is ‘brain dead’.
Thus, the measurement of OMT is highly relevant to the work of an anaesthesiologist, as it slows down as a person goes deeper into anaesthesia, and speeds up if a person is becoming alert. As well as stroke and coma, measurements of OMT have recently been linked to neurological conditions such as Multiple Sclerosis or Parkinson’s Disease, as the OMT levels have been found to change in people with these diseases.
Building on the work of Prof Coakley, Dr Gerard Boyle became interested in developing a device that can measure OMT. Dr Boyle was involved in the building of a device at St James’s based on what are called ‘piezo-electric probes’ – probes that can measure OMT. The probes are dropped down onto the patient’s eye. Eye movement induces a current in the probes and this can be measured. This is not a comfortable process for the patient involved, however.
Device
“The piezo-electric probes work well from a scientific point of view,” said Dr Boyle, “but they are not ideal from a patient’s point of view.” The person’s eye must be anaesthetised, and the eyelids taped open before the probes are dropped onto the eye. From the clinicians point of view these probe machines were too large, and not easily transported to where they are needed in the hospital. Furthermore, as the procedure involves contact with the eye, and is, thus, determined to be ‘invasive’ there are ethical issues to address and permissions to be sought before the device can be used. Dr Boyle has experience working on a laser-based system, called ‘speckle-interferometry’, which can measure tiny eye movements in a non-contact manner. The system works by shining a laser at the white of the patient’s eye, picking up the light that is scattered from the eye, and extracting the measurements of movement.
“We have a demonstration system of this,” said Dr Boyle, “but what SFI funded us for is to look in a little more detail at the scattering process that happens when the laser light strikes the white of the eye. The laser that you use – the power levels are really low. It would be even much less than the laser pointers that are used in lectures, way down below that level, so it’s ‘eye safe’.”
Benefits
The goal for Dr Boyle is to develop a new diagnostic tool for clinicians to measure OMT that would be available at a fairly low cost, compared to say, an MRI machine where €2 million would be invested. The device will be portable and easily transported to theatre to use on people in a coma, for example, or be transported to ICU if an anaesthesiologist wants to determine a person’s depth of anaesthesia. “While a portable device to measure OMT and the use of piezoelectric probes are already quite novel developments,” said Dr Boyle, “the laser-based speckle-inferometry system is unique. No one anywhere is doing that.
“However,” Dr Boyle said “there is nothing about the demonstration model that has been already developed at St James’s that can’t be miniaturised. The technology the device will be based on is lasers, and some laser diodes now are the size of a one cent coin.”
Dr Boyle received funding in August 2008 under the SFI Research Frontiers Programme. The research goal is to have a portable device for measuring OMT that could be used by hospital clinicians – as part of their daily work – all around the world.
Dr Gerald Boyle
Department of Medical Physics & Bioengineering, St. James’s Hospital, Dublin 8.
