June 5, 2000 COLUMBUS, Ohio - Ultrasound technology may soon play an important role in the treatment of glaucoma, an eye disease that can lead to blindness.
New research suggests that examining an eye with ultrasound while exciting it with audible sound waves will give an accurate reading of the fluid pressure behind the cornea. People who suffer from glaucoma - a disorder characterized by too much fluid build-up inside the eye - may one day be able to use the technology to take pressure readings on their own.
"In glaucoma, the pressure inside the eye fluctuates greatly throughout the day," said Mardi Hastings, an associate professor of biomedical and mechanical engineering at Ohio State University.
"An ophthalmologist prescribes drugs based on a one-time pressure reading, so it's difficult to get the right dose of medicine to treat the glaucoma properly. If a patient had a way to monitor changes in pressure inside the eye, she could determine her normal eye pressure, know when the pressure deviates from that norm, and take medication accordingly."
Hastings will present her findings on May 31 at the Acoustical Society of America meeting in Atlanta.
The most common way to currently measure pressure inside the eye is tonometry. In air tonometry, a short burst of air hits the cornea. In applanation tonometry, a doctor anesthetizes the eye, then presses against it with a tiny instrument and measures the depth of the indentation.
"Ultrasound technology may someday make these often-uncomfortable tests unnecessary," Hastings said.
In this study, Hastings used a small loudspeaker and two ultrasonic transducers - one to transmit an ultrasonic wave to the eye and the other to receive the reflected ultrasonic wave from its surface- to obtain pressure readings from each eye. A continuous tone from the loudspeaker would cause the eye to vibrate. The motion caused by the audible sound wave altered the ultrasound wave reflected from the eye.
"These minute displacements of the surface of the eye tell how much pressure is behind the cornea," Hastings said. "The ultrasonic wave reflected by the eye is going to be slightly different than the wave that went in. This change is where the information is."
An increase in fluid pressure in the eye makes the cornea stiffer. When the cornea increases in stiffness, the eye's response to sound waves change.
Hastings used disease-free eyes that had been either harvested from animals (cows, pigs, dogs, chickens and rabbits) or human eyes donated by the Lions Eye Bank. She took pressure readings in water by suspending the eye in a gauze sling, or in air, with each eye tethered to a surgical thread. Both methods allowed the eye to freely react to the audible sound wave.
"While both methods gave accurate readings, using the technology in the air would likely be more comfortable and less cumbersome to the patient," Hastings said. The eventual goal is to develop a hand-held device that contains a small loudspeaker, ultrasonic transducers, and the electronics needed to measure the eye motion from the reflected beam.
After taking initial pressure readings, she injected saline into each eye to mimic pressures similar to those found in the eyes of glaucoma patients. She repeated the measurements at three different pressure settings.
She is currently trying to determine the best frequency of excitation to use for measuring fluid pressure inside the eye.
"There are frequencies the cornea likes that cause it to vibrate a little more," she said. "Knowing these frequencies would allow a patient to calibrate her pressure measuring device in order to get the most accurate reading."
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