Sustained focal effects of low-dose intramuscular succinylcholine

We studied low-dose intramuscular succinylcholine in 9 subjects as part as an ongoing investigation of its potential to predict responses to botulinum toxin. We measured compound muscle action potentials (CMAPs) from the extensor digitorum brevis (EDB) muscles in each foot before and after intramuscular injections of 2.5 mg of succinylcholine into the EDB. Succinylcholine reduced mean CMAP amplitudes to 42% of baseline; the maximal reduction occurred at 19 +/- 6 (mean +/- standard deviation) minutes. Recovery to 73% of the baseline CMAP amplitude (approximately 50% recovery from block) occurred at 105 +/- 49 minutes after injection. Repetitive (train-of-four) stimulation at 2 Hz produced mild CMAP decrements (5-25%), but only during the recovery phase. Varying the succinylcholine concentrations (10, 20, or 50 mg/mL) while holding the total drug dose constant did not change the rate of onset or the extent of block. No systemic complications occurred. We conclude that: (1) 2.5 mg intramuscular succinylcholine can safely induce selective muscle weakness with a time course that differs from intravenously administered succinylcholine; and (2) further clinical studies comparing intramuscular succinylcholine and botulinum toxin are warranted.