Decreased sensitivity to metocurine in patients with upper motoneuron disease

Responses to the nondepolarizing muscle relaxant, metocurine, were studied in eight hemiplegic and eight unmatched patients with normal motor strength during the general anesthetic given for various neurosurgical operations. Metocurine, 0.3 mg/kg, was administered intravenously, and indirectly evoked thumb twitch tensions were measured on both sides in the hemiplegic patients, and on one side in the normal patients. Arterial blood samples were obtained as twitch tension was recovering, and serum metocurine concentrations were determined using a specific radioimmunoassay. Percentage of paralysis was plotted as a function of log [metocurine] and the data were compared by analysis of covariance. For the normal motor strength patients, r = 0.84; for the unaffected arm of the hemiplegic patients, r = 0.69; and for the affected arm of the hemiplegic patients, r = 0.86, all significant at P < 0.001. The mean plasma metocurine concentrations at 20, 25, 50, 75, and 80% paralysis were significantly different for all groups (P < 0.001). The regression lines, in turn, did not overlap and were significantly different, each from the other (P < 0.005). We were, however, unable to detect any significant deviation from parallelism among the three regression lines. We also measured time to 50% return of single twitch height for each data group as follows (mean ± SEM: for NMS patients, 242 ± 73 min; for the unaffected arm of hemiplegic patients, 116 ± 60 min; and for the affected arm of hemiplegic patients, 59 ± 36 min. By ANOVA and the Bonferroni test, each value was different from the other at P ≤ 0.01. We conclude that both the affected arm and the unaffected arm of hemiplegic patients exhibit decreased sensitivity to metocurine, and that the effect of stroke is, even in the long term, global. Thus thumb twitch monitoring in patients with hemiplegia may underestimate the degree of blockade of the muscles of respiration, no matter which side is monitored.