Abnormally large evoked potentials arising from dorsal column fibers in the region of chronically compressed spinal cord

Objective: To understand the origin and mechanism of the large evoked poten tials known to occur in chronically compressed spinal cord. Methods: Electrophysiological analysis was made of evoked potentials produc ed in the cervical cord compressed for more than 1 year by rotating a screw in the vertebral body, in 4 adult cats. Results: In all cats, manifesting no motor disturbance, large positive or n egative evoked potentials were recorded from the cord dorsum at the compres sed region following lumbar cord stimulation. These potentials followed hig h-frequency stimulation and were judged as conducting volleys of directly a ctivated fibers. Intraspinally, single fiber spikes, normally barely record able with the glass microelectrodes, were recorded extracellularly from loc alized areas within the dorsal column restricted to the compressed region. Such single fibers showed slowed conduction at the compressed region withou t a conduction block. Histologically, demyelinated fibers were observed in localized areas in the dorsal column of the compressed region. Conclusion: We concluded that the large abnormal surface potentials origina ted from the dorsal column and were attributed to an increase in action cur rent in affected fibers, which presumably occurred associated with demyelin ation. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.