Zones of differential sensory block during extradural anaesthesia.

We have measured spinal segmental levels of anaesthesia to light touch (LT), pinprick (PP) and cold temperature discrimination (TE) during 2% lignocaine extradural anaesthesia in 22 patients, to determine if zones of differential sensory block develop during extradural anaesthesia and, if so, the extent to which TE extends beyond PP or LT levels and how age affects differential block. The median thoracic dermatomal levels were 4.5 for LT, 2.0 for PP and 2.0 for TE. Zones of differential sensory block developed within 5 min of extradural injection of local anaesthetic, and persisted for the next 55 min. In all instances, PP extended more cephalad than LT, and TE extended above PP levels. There were no differences in the extent of zones between the two groups of patients with mean ages of 28 and 48 yr. Thus, during extradural anaesthesia, sympathetic denervation extended one to two spinal segments above the sensory levels of LT and PP anaesthesia, age (28 vs 48 yr) affected neither the cephalad extent nor the width of zones of differential block, and PP levels of anaesthesia were closer to presumed levels of sympathetic block than were LT levels.