Free-electron laser used in human eye surgery for first time

Photos by Dana Johnson

Dr. Karen Joos (second from left) uses a microscope and infrared laser probe to cut the sheath surrounding the optic nerve in the first operation of its kind performed on a human patient. She is assisted by Dr. Louise Mawn (left), who performed the conventional portions of the surgery.

by David F. Salisbury

A successful ophthalmic surgery performed with a special kind of laser is a major step toward giving doctors access to one of the last "invisible spaces" in the body: the area behind the eye.

In the operation performed at the W.M. Keck Foundation Free-Electron Laser Center at Vanderbilt Sept. 29, an infrared beam of laser light was used to cut a two-millimeter flap in the sheath surrounding the optic nerve without damaging the nerve itself. Because it is an experimental procedure, the operation was first performed on a patient with end-stage traumatic glaucoma who was having the eye removed.

Vanderbilt's free-electron laser (FEL) is a powerful type of laser that was adopted by the U.S. Department of Defense as part of the "Star Wars" missile defense program. An FEL works by passing a stream of electrons traveling at nearly the speed of light through a wiggler, a device that produces alternating magnetic fields. The fields cause the electrons to "vibrate" at a specific frequency that stimulates them to emit pulses of laser light. Such a design allows free-electron lasers to produce extremely powerful beams of coherent light. By varying the energy in the electron beam, an FEL can also be tuned to produce light in a wide range of frequencies. As a result, the laser beam can be set to frequencies that cut tissue and bone more smoothly and cleanly than conventional surgical devices.

The Sept. 29 ophthalmic surgery was only the third operation performed on a human patient using the Vanderbilt FEL, which is the only free-electron laser facility in the world licensed to perform human surgery. The first operation occurred last December, when the laser was used successfully to remove part of a benign tumor from the brain of Virginia Whitaker, 78, from Kansas City, Mo. A similar neurosurgery was performed on Paula Parrish, 41, from Springfield, Tenn., Sept. 15.

The ophthalmic operation consisted of two procedures. The first was performed using the well-established procedure of detaching a muscle on one side of the eye and carefully "rotating" it in the socket to expose the optic nerve. Currently, the only alternative is to cut an opening in the facial bones surrounding the eye, which causes permanent scarring or disfigurement. The second involved cutting the tiny flap in the optic nerve sheath. Dr. Louise Mawn and Dr. Karen Joos, both assistant professors of ophthalmology, performed the operation. Mawn performed the conventional portion while Joos handled the laser probe, which was designed by research assistant professor Jin H. Shen.

The procedure is typically used to treat a condition called pseudotumor cerebri, a relatively common neurological illness among young, obese women. A build-up of cerebral-spinal fluid in the optic nerve causes blurred vision, headaches and even loss of vision. Because pseudotumor cerebri occurs eight times more frequently in young women than in young men, scientists suspect that hormones play a role, but little is known about its cause. Surgery is called for when the condition does not respond to dietetic and medical treatment. Cutting a small opening in the sheath surrounding the optic nerve relieves the pressure buildup, preventing further vision loss and, in some cases, even restoring lost vision.

"I'm a traditional surgeon, so I was very skeptical when I was first confronted with the idea of using a laser for this kind of an operation," said Mawn. "After trying it out several times on animals, however, I became convinced that this is a better, safer and more efficient approach."

Her confidence was bolstered by the results of several years of basic science research that established the feasibility of using the FEL for ophthalmic surgery. As part of this effort, Joos worked closely with Vivien Casagrande, professor of cell biology, psychology and ophthalmology. They compared the cutting characteristics of the FEL beam with those of conventional surgical methods on a number of different animal species. The studies included a detailed analysis of the biological response of the nerve cells to these injuries and their effects on visual function.

In particular, Mawn was convinced by the fact that the laser beam can be focused to a much smaller size than a scalpel or scissors, allowing it to be handled more precisely. In addition, a microscopic layer of fluid between the nerve and nerve sheath provides an additional measure of safety. The liquid does nothing to stop a steel cutting edge, but it appears to act as a barrier against the infrared laser, and so adds an extra layer of protection for the delicate nerve fibers.

The actual importance of this line of research extends well beyond a better treatment for a single condition, emphasizes Dr. Denis O'Day, the George Weeks Hale Professor and chair of ophthalmology at Vanderbilt. He was involved in the early studies that showed a free-electron beam can cut tissue with a minimum amount of collateral damage when tuned to the proper frequency.

"One of the last invisible spaces in the body is behind the eye," O'Day said. "It is one place we haven't visited yet, and this approach has the potential to take us there."

The extreme delicacy of the eye makes accessing the region behind extremely difficult. As a result, it remains one of the least-studied parts of the body and one of the most difficult to treat, the ophthalmologists maintain. The process of rotating the eye in the socket, for example, is technically difficult: there are a number of things that can go wrong that damage the eye, Mawn said.

So the ultimate aim of the medical researchers is to avoid altogether the problems caused by moving the eye by combining the FEL beam with an endoscope, a slender optical instrument that allows the user to see parts of the body that are ordinarily hidden from view, Joos said.

They have developed such a combined probe, which is only 1.5 millimeters (about one-twentieth of an inch) thick, and have been using it in animals to evaluate its effectiveness for treating conditions like congenital glaucoma. In a paper published earlier in the year in the Journal of Glaucoma, for example, they report using this method to treat congenital glaucoma. The combination endoscope/laser allowed them to locate and cut open the eyes' natural drainage areas located around the outside rim of the iris even though they were hidden beneath opaque corneas.

Other uses of the Vanderbilt FEL may include welding tissue to assist in wound healing, repairing nerves, reattaching retinas or monitoring neurological activity -- applications where infrared light proves superior to other wavelengths.