Thermal sensibility changes during ischemic nerve block.

Changes in perception of pure thermal stimuli delivered to the hand at threshold intensity were observed during ischemic nerve block in 27 healthy subjects in order to study the significance of unmyelinated cutaneous cold receptors for thermal sensibility. Paresthesias and an increasing feeling of numbness were followed by a sudden change in cold sensation which developed a clearly dysesthetic quality. When complete motor block was reached and no myelinated axon functions were left, cold stimuli were still clearly but abnormally perceived. Warm sensation was little affected during the whole course of nerve block. The results indicate that the information from unmyelinated low threshold cold receptors alone leads to a dysesthetic cold sensation which normally is suppressed by the activity of myelinated cold afferents. Obviously these receptors are not needed for the discrimination of minor cold stimuli but they seem to be essential for the perception of the first burning cold pain.