Paraesthesiae from ectopic impulse generation in human sensory nerves.

Intraneural microelectrode recordings were obtained from sensory nerve fascicles in awake normal subjects experiencing post-ischaemic paraesthesiae. Spontaneous activity in afferent fibres started about half a minute after re-establishment of circulation; it peaked by the second minute and slowly waned thereafter. This parallelled the time course and intensity of the perceived paraesthesiae. During the most intense stage of paraesthesiae, single unit discharges consisted of prolonged bursts of high-frequency impulses (200-300 imp/s) lasting for 1 to 7 seconds. Repeated bursts became progressively shorter and often exhibited marked rhythmicity as paraesthesiae became weaker. Hyperventilation increased paroxysmal firing and paraesthesiae, whereas ischaemic periods repeated at short intervals exhausted the impulse generating mechanism. The outcome of local anaesthetic blocks indicated that the impulses were generated ectopically along the nerve fibres previously subjected to ischaemia and not primarily from the receptors in the skin. The high frequency of the discharge and the quality and distribution of various paraesthesiae suggest that sensory units with myelinated fibres participated in the paroxysmal discharge; normal activity, but no ectopic discharges were recorded in unmyelinated fibres. It is proposed that the perverted quality of paraesthesiae is due to an abnormal spatio-temporal pattern of ectopic impulses from different types of sensory units. creating a chaotic percept consisting of an assortment of sensations referred in irregular succession to multiple areas.