A successful method for healing spinal injuries in dogs has been developed by Purdue University researchers, offering hope for preventing human paralysis.
Lab tests have shown that an injection of a liquid polymer known as polyethylene glycol (PEG), if administered within 72 hours of serious spinal injury, can prevent most dogs from suffering permanent spinal damage. Even when the spine is initially damaged to the point of paralysis, the PEG solution prevents the nerve cells from rupturing irrevocably, enabling them to heal themselves.
"Nearly 75 percent of the dogs we treated with PEG were able to resume a normal life," said Richard Borgens, Mari Hulman George Professor of Applied Neuroscience and director of the Center for Paralysis Research in Purdue's School of Veterinary Medicine. "Some healed so well that they could go on as though nothing had happened."
The research, performed at Purdue, Indiana University—Purdue University Indianapolis, and Texas A&M University, appears in the December issue of the Journal of Neurotrauma.
In the study, 19 paraplegic dogs between 2 and 8 years of age were treated with a PEG injection within 72 hours of their injury as an addition to the standard veterinary therapy for spinal injury. This standard treatment includes injection of steroids, physical rehabilitation with swimming, and surgical removal of any offending chips of bone remaining in the spinal area after injury. This group of 19 was compared with a second group of 24 dogs that received only the standard treatment.
"The control group was taken from historical cases of dog injury that were similar to those in the 19 dogs we treated," Borgens said. "We didn't want to tell any owners who walked in with injured dogs that their pets were not going to receive something that might help. So we looked at the results that the standard treatment had on dogs that had suffered similar injuries in the past."
After treatment, the dogs' improvement was measured based on criteria including desire to move, deep and superficial pain perception, and transmission of electrical impulses through the nerve tissue.
"More than half of the dogs in this study were standing or walking within two weeks of treatment," Borgens said. "In most cases, you could usually notice positive signs within three to five days."
Another 16 dogs were injected with a different substance called P-188, a mixture of 80 percent PEG along with other chemicals, which also was thought to have potential as a treatment.
"However, dogs treated with the P-188 mixture did not perform as well as those treated with PEG," Borgens said.
Trauma to nerve cells causes their membranes to weaken and even rupture. Though the cells may survive, this membrane damage causes them to lose the ability to produce and carry nerve impulses along their membranes from one cell to the next.
"Worse yet, chemicals seeping out of the dying spinal cord cells send a 'suicide signal' to other nearby cells, causing a chain reaction that kills off more cells than the initial injury did," Borgens said. "Until now, the end result has been irreparable damage to the spinal cord, causing partial or complete paralysis to the victim."
PEG is able to intervene in this process by repairing the initial membrane damage. It has been known for decades that two cells that touch each other can become one big cell if PEG is added to the fluid they live in. Because of this surprising ability, PEG is sometimes called a "fusogen."
About five years ago, Borgens and his partner, Riyi Shi, found that they could actually fuse hundreds to thousands of severed nerve fibers of the guinea pig spinal cord with only a two-minute PEG treatment. This observation led to developing the polymer as a repair agent that would mend the broken membranes of nerve cells after traumatic injury.
Though PEG's action as a fusogen has been known prior to their work, the exact mechanism that occurs at the membrane to fuse or mend it is still poorly understood. Borgens said that many membrane specialists believe it has much to do with the ability of PEG to quickly and dramatically remove water from the cell membrane that floods into the cell after suffering damage. This makes it difficult for the cellular membrane to heal on its own.
"Imagine children blowing bubbles with wands, the kind with a small round hole at the end," Borgens said. "The polymer acts like a soap film that covers the hole and draws the water away. In the PEG-sealed membrane, the fatty oils that form the center of the membrane can mix again, free of the water that had likely repelled them. Once PEG dissolves away from the area, water molecules once again help to induce and preserve the restructured membrane."