THE EYE AS A WINDOW ONTO THE BRAIN

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THE EYE AS A WINDOW ONTO THE BRAIN
17 August 2003 - Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.

It is an undisputed fact that chronic hypertension can lead to a stroke. To identify this risk factor as early as possible, upper-arm blood-pressure measurements are not always adequate, for the vascular system consists of many branches where narrowing of the arteries can lead to localized zones of high pressure.

But how can pressure be measured inside organs without surgical intervention? The head, at least, offers a convenient natural window: the eye. The retina contains numerous blood vessels which the doctor can examine in the search for pathogenic changes. Some ophthalmologists are already using a quick and elegant, non-invasive tool to measure systolic and diastolic pressure: the contact lens dynamometer. Probably the most advanced version of this combined lens and measuring instrument is marketed by the firm Meditron.

The physician places the lens on the patient's eye and, while observing the blood vessels in the retina, gradually raises the force acting on the lens. This force is transferred to the blood vessels, which - as in the standard measurement technique - eventually start to pulsate. The physician presses a key to store the measured value in a device connected by cable to the lens. This value plus the previously recorded intra-ocular pressure represents the diastolic blood pressure. Increasing the force acting on the lens causes the blood vessels to stop pulsating, giving the systolic blood pressure. "A spring-mounted ring is attached to the contact lens," explains physicist Margit Biehl, describing the integrated technique developed by the Fraunhofer Institute for Biomedical Engineering IBMT. "Calibrated sensor strips on the springs expand proportionally. By measuring their electrical resistance, the device calculates the pressure exerted on the eye by the lens."

What many doctors don't realize is that they could also determine the pressure of cerebrospinal fluid in the same way. As published back in 1925, the optical nerve is bathed in cerebral fluid and hence subject to the same pressure as the brain. The central retinal vein runs through a short section of the optical nerve. The pressure of the cerebrospinal fluid can be derived from the blood pressure measured in this vein. "This parameter, which can only be determined more precisely by a painful puncture procedure, can often provide the physician with other vital information," points out Dr. Bernhard Löw, chief executive of Meditron and inventor of the method. "This very simple diagnostic technique has helped many of my thankful patients to avoid the risk of premature death as the result of a stroke."

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