Stimulation technique holds new promise for spinal cord injuries

June 18, 1999

An innovative method to stimulate the spinal cord directly could offer new hope to control bladder, bowel, sexual function, and leg movement in people with spinal cord injuries.

Warren Grill, a senior research associate in the Department of Biomedical Engineering at Case Western Reserve University, recently received almost $1 million from the National Institutes of Health's National Institute on Neurological Disorders and Stroke.

He will work with Musa Haxhiu, CWRU professor of medicine, and other researchers from the Cleveland FES (Functional Electrical Stimulation) Center to assess the feasibility of neural prostheses based on microstimulation inside of the spinal cord.

The Cleveland FES Center is a consortium of CWRU, the Cleveland VA Medical Center, and the MetroHealth Medical Center. Neural prostheses technology uses electrical activation of the nervous system to restore lost function in persons with neurological impairment.

Usually with FES technology, minute levels of electrical current are delivered to nerves in paralyzed muscles through electrodes placed on or near them.

Grill is investigating intraspinal microstimulation, in which electrodes are placed inside the spinal cord, which has matter that is similar to the brain in its structure and consistency. He believes this kind of neural prosthesis could improve the health and independence of people with quadriplegia (paralysis of the arms and legs) and paraplegia (paralysis of the legs).

Most current neural prosthetic systems either use muscle-based electrodes that are placed on or in the skeletal muscle and activate the nerve as it enters the muscle, or they employ nerve-based electrodes that are placed on or around the peripheral nerve, Grill explained.

Both of these approaches drive the last-order neuron, a connection between the cells and the muscle that when stimulated causes a certain action. The last-order neuron is a one-synapse link, Grill said. It is the last neuron in a long chain that starts at the cerebral cortex and works down through the spinal circuitry.

"What we are trying to do is to access higher-levels in the nervous system and use the biological control circuits which are present in the spinal cord," Grill said.

Although the connections between the brain and the higher-order neurons are severed in spinal cord injuries, creating a region of dead cells immediately around the injury, the neurons below the injury level are alive. That is where the CWRU researchers plan to put the neural prosthetic device.

"The idea is to put electrodes below the spinal injury to artificially re-establish the communications link from the brain," Grill said.

Injuries that are farther up the spinal cord cause greater loss of function. People with these kinds of injuries could benefit more from this new technology, according to Grill.

"The greater the level of disability, the more impact even a modest intervention could have," Grill said. "If a person can't use his hand, being able to move the hand even crudely is a huge step forward."

The neural prosthesis would be surgically implanted and powered most likely by a radio frequency link with a small box worn outside the body. The electrodes, which are half as thick as a human hair, are made of a metal called iridium. Grill expects that the new technology will require fewer electrodes. By stimulating higher-order neurons in the nervous system, Grill expects to coordinate activation of the nerves using only a few stimuli.

"One of the challenges we face is that the mechanical properties of the electrode are very different than the mechanical properties of the underlying tissue," Grill said. "The tissue is like cooked pasta and the electrode is like a sewing needle. I think the solution is likely to be a conductive polymer."

The CWRU researcher believes that the first human applications for the technology will be in about 10 years. He contends that the risk for implanting the neural prosthesis would be comparable to an artificial hip replacement.

Case Western Reserve University

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