Muscle Weakness after Muscle Relaxants: An Audit of Clinical Practice

Carisoprodol Single and Multiple Dose PK-PD. Part II: Pharmacodynamics Evaluation Method for Central Muscle Relaxants. Double-Blind Placebo-Controlled Clinical Trial in Healthy Volunteers

Centrally acting skeletal muscle relaxants (CMR) such as carisoprodol are used to treat acute, painful musculoskeletal conditions, though its precise mode of action has not been characterized. A double-blinded, placebo-controlled, randomized clinical trial was designed to evaluate the pharmacokinetics–pharmacodynamics (PKPD) of CMR after single (350 mg), double (700 mg), and multiple doses (up to 350 mg/8 h, 14 days) of carisoprodol. Muscular (Electromyogram–EMG, muscular strength dynamometry), central (sedation), and tolerability (psychomotor activity test, adverse events) parameters, as well as withdrawal symptoms, were evaluated. Thirteen healthy volunteers were enrolled. No evidence of direct muscle relaxation was evidenced, but some differences on sedation were evidenced throughout the study, suggesting that CMRs act, at least partly, through sedation. Most significant differences were detected at 1.5 h after dosing. The effect on psychomotor impairment was variable, most prominently after 1.5 h, too, suggesting that it is produced by carisoprodol rather than by meprobamate. No withdrawal symptoms were detected, so the risk of dependence following maximum doses and duration of treatment recommended, and under medical supervision, should be low.

Effects of shakuyakukanzoto and its absorbed components on twitch contractions induced by physiological Ca2+ release in rat skeletal muscle

Shakuyakukanzoto (SKT) is a kampo medicine composed of equal proportions of Glycyrrhizae radix (G. radix) and Paeoniae radix (P. radix). A double-blind study reported that SKT significantly ameliorated painful muscle cramp in cirrhosis patients without the typical severe side effects of muscle weakness and central nervous system (CNS) depression. Previous basic studies reported that SKT and its active components induced relaxation by a direct action on skeletal muscle and that SKT did not depress CNS functions; however, why SKT has a lower incidence of muscle weakness remains unknown. In the present study, we investigated which components are absorbed into the blood of rats after a single oral administration of SKT to identify the active components of SKT. We also investigated the effects of SKT and its components on the twitch contraction induced by physiological Ca²⁺ release. Our study demonstrated that SKT and five G. radix isolates, which are responsible for the antispasmodic effect of SKT, did not inhibit the twitch contraction in contrast to dantrolene sodium, a direct-acting peripheral muscle relaxant, indicating that the mechanisms of muscle contraction of SKT and dantrolene in skeletal muscle differ. These findings suggest that SKT does not reduce the contractile force in skeletal muscle under physiological conditions, i.e., SKT may have a low risk of causing muscle weakness in clinical use. Considering that most muscle relaxants and anticonvulsants cause various harmful side effects such as weakness and CNS depression, SKT appears to have a benign safety profile.

Influence of gender on the course of neuromuscular block following a single bolus dose of cisatracurium or rocuronium

BACKGROUND AND OBJECTIVE

There is increasing evidence of gender differences in the pharmacokinetics and pharmacodynamics of aminosteroid neuromuscular blocking agents. Compared to males, females are more susceptible, requiring approximately 30% less rocuronium to achieve the same degree of neuromuscular block. However, little information is available whether this difference is applicable to modern benzylisoquinolines (cisatracurium).

METHODS

In all, 848 patients (423 males, 425 females) undergoing general surgery under total intravenous anaesthesia with muscle relaxation, tracheal intubation and mechanical ventilation were studied. Patients were randomized to receive a single bolus dose of cisatracurium (0.1 mg kg-1, 221 males and 199 females) or rocuronium (0.6 mg kg-1, 202 males and 226 females). The onset time for 95% depression of T1, clinical duration until 25% recovery and recovery index (T1 from 25% to 75%) were determined with an NMT electromyographic module of the Datex-Ohmeda S/5 Anaesthesia Monitor. The data for male and female groups were compared with appropriate statistical tests (unpaired t-test, Mann-Whitney rank sum test and Fisher's exact test).

RESULTS

In both groups (cisatracurium and rocuronium), males were significantly taller (P < 0.001) and heavier (P < 0.001) than females, but the body mass index was comparable. For rocuronium, the onset time was shorter 91.7 +/- 14.3 s vs. 108.0 +/- 14.6 s (P < 0.001) and the clinical duration was increased in females 43.3 +/- 7.8 min vs. 31.3 +/- 5.5 min (P < 0.001). In the cisatracurium group, both onset times (248.9 +/- 60.7 s for males vs. 253.4 +/- 70.9 s for females) and clinical duration (42.6 +/- 6.9 min for males vs. 43.1 +/- 6.9 min for females) were similar. The recovery index was identical for males and females in both groups.

CONCLUSIONS

Females were more sensitive than males to a single bolus dose of rocuronium. Under the study conditions described, the onset time was shorter and the clinical duration was increased in female patients. This suggests that the routine dose of rocuronium should be reduced in females compared to males. On the contrary, we could demonstrate no gender differences in the onset time or clinical duration of cisatracurium.