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 potentials known to occur in chronically compressed spinal cord. Methods: Electrophysiological analysis was made of evoked potentials produced 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 negative evoked potentials were recorded from the cord dorsum at the compressed region following lumbar cord stimulation. These potentials followed high-frequency stimulation and were judged as conducting volleys of directly activated fibers. Intraspinally, single fiber spikes, normally barely recordable with the glass microelectrodes, were recorded extracellularly from localized areas within the dorsal column restricted to the compressed region. Such single fibers showed slowed conduction at the compressed region without 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 originated from the dorsal column and were attributed to an increase in action current in affected fibers, which presumably occurred associated with demyelination