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Applied Voltages and Electrical Stimulation
Dr. Richard Borgens at the Center for Paralysis Research has shown that spinal cord nerve fibers will grow within a matter of weeks in a steady and very weak electrical field. This discovery has led to the application of electric fields for approximately 14 weeks over the injury to the spinal cord using a new implantable medical device called an Extraspinal Oscillating Field Stimulator (OFS). This procedure is meant to promote better functional recovery in patients through regeneration of injured spinal cord nerve fibers. These OFS units have been surgically implanted in dozens of dogs and are currently being tested in human cases of spinal cord injury at Indiana University Medical Center.
All animals, including human beings, are truly walking batteries. That is to say, our microscopic anatomy and our functioning - even our thoughts, feelings, and dreams - are made entirely of electrical energy. We have shown that in early salamander embryonic development, naturally produced electrical voltages help lay out the geometry of the individual head/tail, up/down, and right/left. Research at Purdue has revealed this invisible "blueprint" for development of the animal. Natural voltages also play a role in the regeneration of tissues in animals, which in some respects are carried out in a manner similar to embryonic development. In particular, regenerating nerves are sensitive to cues provided by applied voltages. For nearly 50 years it has been established that nerve fibers will grow toward the negative pole and away from the positive pole of an electrical field.
Dr. Borgens was the first to convincingly show that artificially applied voltages could influence regeneration of spinal cord nerves in the living animal. This work was first carried out in 1979, and the work continued for the next 20 years in guinea pigs and rats. Eventually a clinical tool was produced - the OFS device, which was tested in naturally produced spinal cord injuries in dogs at Purdue's School of Veterinary Medicine (first published in 1993). Dr. Jim Toombs, a veterinary surgeon, led the team that showed severely injured dogs could benefit from this therapy. OFS treatment has been most successful when implanted within 18 days of initial SCI. This small device is implanted in paraplegic dogs that are brought to the Purdue Small Animal Hospital by their owners. The OFS unit is implanted by a surgeon near the spine with pairs of electrodes on either side of the spinal cord injury on the outside of the vertebrae to produce a weak electrical field across the injury site. An internal battery powers the OFS unit for approximately 14 weeks, and then it is surgically removed.
X-rays of implanted spinal cord stimulators. The left shows a laboratory
guinea pig with an implanted unit and to the right of it, an OFS unit within
a naturally injured paraplegic dog.
An Imposed Oscillating Electrical Field Improves the Recovery of
Neurologically Complete Paraplegic Dogs
R. Borgens, J. Toombs, G. Breur, W. Widmer, D. Waters, A. Harbath, P. March, and L. Adams, Center for Paralysis Research, School of Veterinary Medicine, Purdue University
We show that an applied electric field in which the polarity is reversed every 15 minutes can improve the outcome from severe, acute, spinal cord injury in dogs. This study utilized naturally injured, neurologically complete, paraplegic dogs as a model for human spinal cord injury. The recovery of paraplegic dogs treated with oscillating electric field stimulation (OFS) (approximately 500-600 ÁV/mm; n = 20) was compared to sham-treated animals (n = 14). Active and sham stimulators were fabricated in West Lafayette, Indiana. They were coded, randomized, sterilized, and packaged in Warsaw, Indiana, and returned to Purdue University for blinded surgical implantation. The OFS stimulators were of a previously unpublished design, and meet the requirements for phase I human clinical testing. All dogs were treated within 18 days of the onset of paraplegia. During the experimental applications, all received the highest standard of conventional management, including surgical decompression, spinal stabilization (if required), and acute administration of methylprednisolone sodium succinate. A radiologic and neurologic examination was performed on every dog entering the study, the latter consisting of standard reflex testing, urologic tests, urodynamic testing, tests for deep and superficial pain appreciation, proprioceptive placing of the hind limbs, ambulation, and evoked potential testing. Dogs were evaluated before and after surgery and at 6 weeks and 6 months after surgery. A greater proportion of experimentally treated dogs than of sham-treated animals showed improvement in every category of functional evaluation at both the 6-week and 6-month recheck, with no reverse trend. Statistical significance was not reached in comparisons of some individual categories of functional evaluation between sham-treated and OFS-treated dogs (ambulation, proprioceptive placing); an early trend towards significance was shown in others (deep pain), and significance was reached in evaluations of superficial pain appreciation. An average of all individual scores for all categories of blinded behavioral evaluation (combined neurological score) was used to compare group outcomes. At the 6-month recheck period, the combined neurologic score of OFS-treated dogs was significantly better than that of control dogs (p = 0.047; Mann-Whitney, two-tailed). J Neurotrauma 16(7), 639-657 (1999)
View the Institute's Fall 2000 newsletter for more information on this treatment.
Hydrophilic Polymer Repair of Nerve
Polyethylene glycol (PEG) is a hydrophilic polymer proven to rapidly (minutes to hours) restore nerve impulse conduction and recovery of sensory and motor functions dependent on those nerve impulses in severely spinal cord injured dogs. This recovery is produced by forming a sealing film - like a detergent soap bubble - across the breaks in nerve fiber membranes caused by mechanical damage. After a traumatic injury, many nerve fibers die instantly but many more are injured with holes in their cell membranes. If these "breaches" get sealed quickly then the nerve cell will survive. If these "breaches" stay open then extracellular ions will flow into these holes and eventually kill the cell. As more cells die, toxic by-products from dying cells and free radicals are released in the spinal cord environment, killing healthy bystander nerve cells. This cascade of events is called "secondary damage" and occurs hours to weeks after the initial "primary" insult. PEG has been shown to seal and repair the cell membranes of injured nerve fibers - not only, sparing these cells but also decreasing the secondary cell death cascade and consequently, protecting a large number of bystander nerve cells.
PEG is thought to act by extracting the water content from between the damaged membranes, allowing the fatty, oily components in the center of cell membranes to flow together, providing a permanent seal - even after PEG is removed. Consequently, PEG rescues nerve processes (called axons) from further degeneration and ultimately death, resulting in very rapid recoveries in function after a spinal cord injury.
For PEG treating paralyzed dogs, usually caused by disk herniation or "slipped disk", PEG is injected directly to the damage site of the spinal cord during hemilaminectomy surgery to alleviate the pressure on the spinal cord. PEG must be administered "acutely" within 48 hrs. post-injury to be most effective. PEG appears to be similarly effective if administered intravenously.
Cell Fusion: A Common Ingredient of Detergents May Soon Revolutionize Treatment of Acute Spinal Injuries
Similar to polyethylene glycol, Poloxamer 188 (P188) is a new type of polymer that may do an even better job of repairing holes and breaks in nerve membranes. P188 is believed to physically plug holes in the cell membrane. Once the nerve cell membrane forms a seal, the P188 molecules get squeezed out the cell membrane and eliminated. Testing of this chemical has been completed in guinea pig spinal cord injuries and is in clinical trials involving dog patients. If needed, either PEG or P188 may be used in conjunction with other therapies, like OF stimulation and 4-AP.
Polyethylene Glycol Repairs Mammalian Spinal Cord Axons After Mechanical
Riyi Shi and Richard B. Borgens, Center for Paralysis Research, Department of Basic Medical Sciences, Purdue Univ., W. Lafayette, IN 47907
The most significant structural damage following mechanical injury to the spinal cord is caused by the disruption of large numbers of nerve fibers which interrupts sensory and motor function. At the cellular level, the loss of spinal cord function is mainly due to damage to axonal membranes which can partially or completely sever the axon. Such insults to the axon cause the dissolution of the distal segment and sometimes the atrophy of the target tissue. In order to survive and functionally recover from a mechanical injury, axons must first seal the breach in the membrane to avoid secondary deterioration leading to axotomy and cell death. Here we describe an in vitro technique using polyethylene glycol (PEG) which can functionally reconnect the two segments of transected adult guinea pig spinal axons within minutes of the injury. Strips of isolated spinal cord white matter and a double sucrose gap chamber were used for electric recording. The cord strips were cut, using a microknife, in the chamber after Compound Action Potential (CAPs) recording was stabilized. PEG, in a solution of 50% (w/w in water), was applied directly onto the lesion site for 2 minutes through a glass pipette immediately after transection. The initial recovery of CAPs was evident in 5 to 10 min. following the application of PEG. Successful fusion was documented by the restored conduction of CAPs and the diffusion of two intracellular fluorescent markers through fused axons.
In addition, PEG can also significantly improve the physiological recovery of spinal cord axons following severe compression injury. A standardized compression was carried out in the recording chamber with the use of a rod attached to a motorized micromanipulator. The compression rod was advanced at a speed of 24 llm/s. CAPs were monitored during the compression which was stopped when CAPs disappeared. A recovery of conduction (reappearance of CAPs) was evident within 60 minutes of injury. The average amplitude of recovering CAPs was significantly different between the control and PEG-treated group (4% vs. 19%,10 in each group). Examination of the relationship between stimulus and response amplitude in control and PEG-treated groups indicated that PEG equally repairs axons of different stimulus thresholds. Furthermore, 4aminopyridine, a potassium channel blocker, when used in combination with PEG, can produce an additional 70% percent increase in CAP amplitude following injury. These techniques may lead to a novel acute treatment for central nervous system trauma.
A brief application of the hydrophilic polymer polyethylene glycol (PEG) swiftly
repairs nerve membrane damage associated with severe spinal cord injury in adult
guinea pigs. A two-minute application of PEG to a standardized compression injury to
the cord immediately reversed the loss of nerve impulse conduction through the
injury in all treated animals while nerve impulse conduction remained absent in all
sham-treated guinea pigs. Physiological recovery was associated with a significant
recovery of a quantifiable spinal cord-dependent behavior in only PEG-treated
animals. The application of PEG could be delayed for up to eight hours without
adversely affecting physiological and behavioral recovery, which continued to
improve for up to one month after PEG treatment.
FASEB J. 14, 27-35 (2000)
View the Institute's Spring 2000 newsletter
Drug Development/Administration: 4-Aminopyridine
4-aminopyridine (4-AP) helps increase electrical conduction along intact nerve fibers years after a SCI. Not unlike taping a bare spot on the rubber insulation of an electrical cord, 4-AP acts by increasing nerve impulse conduction through spinal nerve fibers that have lost their insulation, called myelin, at the injury site. 4-AP helps conduct impulses to and from the brain across these intact but nonfunctional nerve fibers. 4-AP accomplishes this by blocking potassium ions from leaking out of neurons so that these nerve cells become depolarized for a longer time, therefore more excitable.
In some cases, 4-AP can return both motor and sensory functions, reduce pain, decrease spasticity, and improve continence. When the drug is administered by injection, behavioral improvements have been observed within 15 minutes. Since the amount and position of spared but nonfunctional nerve fibers in any spinal injury is variable, behavioral responses to 4-AP are variable as well. 4-AP, though not a "cure", is an important first step towards a treatment for chronic (long-term) neurological injuries.
4-AP was first developed as a treatment for chronic SCI in dogs by Drs. Andrew Blight, Jim Toombs and R. Borgens in the early 1990's at Purdue's School of Veterinary Medicine. Both motor and sensory functions have been restored soon after injection or ingestion of the drug by paraplegic dogs in a published Purdue study. Similar results were found in human spinal cord injured volunteers in subsequent clinical testing. This is the first method developed at the Purdue Center to be moved to human clinical trials. Dr. Blight and Acorda Therapeutics, Inc. have conducted human clinical trials of 4-AP, called Fampridine-SR, for spinal cord injury.
Another way to deliver 4-AP (in high concentration)
directly to the damaged spinal cord is to use a fully implantable pump
and catheter (see x-ray).
The adjacent radiograph shows the round pump (approximately 2 1/2" in diameter) implanted within a paraplegic dog. Notice the drug delivery tubes which deliver the drug (4-AP) directly to the damaged spinal cord. The small arrows show the location of injury to the cord inside the vertebral column.