Muscle relaxants

Studies on the toxic effects of muscle relaxants are difficult to design because of the need for mechanical ventilation and, consequently, concomitant administration of anaesthetic drugs which may influence the results. The following overview shows that muscle relaxants are weak toxic agents with regard to their teratogenicity, carcinogenicity and cytotoxic effects (including tissue- and organ-damaging effects). Moreover, this chapter presents other side-effects of muscle relaxants under the broad heading of toxicity: the succinylcholine-triggered cytotoxic effects on skeletal muscle cells with different aetiology, for example, or persistent muscle weakness after long-term administration of non-depolarizing muscle relaxants. Receptor stimulation in the central nervous system may cause acute excitement and seizures. Muscle relaxants and their metabolites may interact with muscarinic and nicotinic receptors in other organs and the ganglionic system, for example in the cardiovascular system. Direct stimulation of mast cells, with consequent release of histamine, after administration of muscle relaxants may clinically impose as toxic reactions.

Succinylcholine-associated postoperative myalgia

The subject of postoperative myalgia associated with the use of succinylcholine is reviewed. We discuss the mechanisms of succinylcholine-induced myalgia and the techniques available to prevent and treat the myalgia. In situations where patients are at risk of developing myalgia and succinylcholine is the neuromuscular blocker of choice, the use of a combination of techniques may prove to be a useful strategy.

Phenytoin reduces suxamethonium-induced myalgia

A prospective, randomised trial was undertaken in 60 healthy adults to determine the efficacy of intravenously administered phenytoin in doses of 5 mg.kg-1 for the prevention of suxamethonium-induced fasciculations, a rise in serum K+ and myalgia. This was compared with tubocurarine pretreatment and no pretreatment (control group). Phenytoin pretreatment significantly reduced myalgia from 45% (nine patients) in the control group to 10% (two patients) (p less than 0.05). It also decreased the duration and mean intensity of fasciculations. Incidentally, phenytoin was also found to decrease significantly mean serum Na+ levels (p less than 0.001) both at 5 and 20 min after administration. Tubocurarine pretreatment (3 mg) resulted in a significant decrease in fasciculations, but myalgia, which occurred in five patients, remained the same. No significant correlation was found between muscle fasciculations, postoperative myalgia and K+ changes, but patients with myalgia had a significant decrease in mean serum Na+ levels at 5 and 20 min after suxamethonium (p less than 0.01).

Prevention of succinylcholine-induced myalgia with lidocaine pretreatment

We studied the effects of 3 mg.kg(-1) lidocaine iv on the succinylcholine (SCh)-induced myalgia in 94 unpremedicated ambulant patients undergoing dilatation and curettage of the uterus. The post-SCh myalgia was confirmed through interview by telephone. The data were correlated with the degree of fasciculation and changes in the serum electrolytes and creatine kinase (CK) levels following SCh administration. Pretreatment with lidocaine, 3 mg.kg(-1) iv, significantly reduced the incidence of myalgia from 40.4% of control group to 12.8% lidocaine-treated group, but not the CK levels. The severity of myalgia was not related to the intensity of fasciculation assessed by visual observation. The pretreatment with lidocaine had no untoward effect on the circulation, although the peak arterial and peak venous lidocaine levels achieved were 29.6 +/- 23 micro g.ml(-1) and 10.1 +/- 3.3 micro g.ml(-1) respectively. These finding indicated that the pretreatment with lidocaine, 3 mg.kg(-1) iv, was effective in prevention of SCh-induced myalgia.

Decreasing post-succinylcholine myalgia in outpatients

The effectiveness of four pretreatment regimens in decreasing succinylcholine-induced myalgias was studied in healthy outpatients undergoing general anaesthesia for ambulatory surgery. Four hundred and forty adult females were randomly assigned to one of four pretreatment groups. Three hundred and ninety-five patients completed the study. One of the following pretreatments was given prior to injection of 1.5 mg X kg-1 of succinylcholine: normal saline IV three minutes and again immediately prior to succinylcholine; 0.06 mg X kg-1 d-tubo-curarine (dTc) IV three minutes prior and normal saline IV immediately prior; normal saline IV three minutes prior and 1.5 mg X kg-1 lidocaine IV immediately prior; 0.06 mg X kg-1 dTc IV three minutes prior and 1.5 mg X kg-1 lidocaine IV immediately prior. Fasciculations after injection of succinylcholine were observed and recorded. Patients were contacted by telephone 40-48 hours postoperatively and questioned about the presence of muscle pains. These pains, if present, were graded either mild or moderate to severe. The patients in the two dTc-containing groups exhibited less fasciculations than patients in the other two experimental groups. The dTc-lidocaine group had a lower incidence of moderate to severe fasciculations than in any of the other three groups. Patients in the dTc, lidocaine, and dTc-lidocaine experimental groups reported a higher incidence of absence of muscle pain and a lower incidence of moderate-severe pain than did patients in the saline group. The dTc-lidocaine group had more patients without myalgia and less patients with moderate to severe myalgias than any of the other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Oral diazepam premedication reduces the incidence of post-succinylcholine muscle pains

In a prospective, double-blind placebo controlled trial, the effect of oral diazepam premedication on post-succinylcholine fasciculations and myalgia was studied. Forty patients undergoing septoplasty procedures received orally identical capsules containing either 10 mg of diazepam or a placebo 90 minutes preoperatively. A standardized anaesthetic regimen included induction with 5 mg X kg-1 of thiopental and 1 mg X kg-1 of succinylcholine. The diazepam and control groups did not differ significantly in extent of fasciculations or ease of intubation. However, only 15 per cent of the diazepam premedicated patients had myalgias postoperatively compared to 50 per cent of the control patients, a statistically significant difference (p = 0.04).

Diazepam and suxamethonium muscle pain (a dose-response study)

Postoperative muscle pain occurred in 17.2% of patients given diazepam intravenously 5 minutes prior to suxamethonium. The incidence of muscle pain was 48% in those patients who were not given diazepam before suxamethonium. The difference is statistically significant (p less than 0.01). Five mg diazepam reduced the incidence of pain but not significantly (p = 0.30); 10 mg diazepam significantly reduced the incidence (p less than 0.05); while 15 and 20 mg diazepam showed better effects (p less than 0.01). Diazepam pretreatment also reduced the severity, frequency and duration of muscle pain. The incidence of fasciculations was significantly reduced but there was no correlation between the incidence of fasciculations and the occurrence of muscle pains.

Suxamethonium fasciculations: a topographical study

It has been found in a group of 455 gynaecological patients that not all the voluntary muscles of the body respond to suxamethonium in the same way. One group of muscles (e.g. biceps) which is sensitive to suxamethonium, responds by fasciculating in almost every individual. A second, resistant, group of muscles (e.g. pectoralis major) is almost totally unresponsive to suxamethonium. A third muscle group (e.g. triceps) shows fasciculations in about 60% of cases and appears intimately associated with the development of suxamethonium pains, for if fasciculations do occur then suxamethonium pains are likely to ensure, but not necessarily involving all the fasciculating muscles. A theory to account for these different groups has been proposed involving the known variation in distribution of red and white muscle fibres throughout the body.

Dantrolene and suxamethonium. The effect of pre-operative dantrolene on the action of suxamethonium

A single oral dose of dantrolene (100--150 mg) given at least 2 hr pre-operatively has been found to reduce significantly the strength of muscular fasciculations, the hyperkalaemia and the incidence of muscle pains following suxamethonium (from 56 to 4%) in a series of forty-eight patients compared with controls. The biceps EMG of the fasciculations was unchanged by dantrolene; the incidence of troublesome side effects was low (9%) and there did not appear to be any alteration in the duration of action of suxamethonium.

Suxamethonium pains and early electrolyte changes

Significant changes in the plasma electrolytes were found in a group of ninety-two female patients in the first 5 min following suxamethonium administration. The plasma potassium rose to a higher level in those patients who developed suxamethonium pains than in those who did not; the plasma calcium decreased in the former group, but increased in the latter. The plasma sodium decreased early in most patients, with the greatest reduction in those patients who did not develop symptoms. A decrease in the level of plasma calcium at 1 min correlated well with the incidence of 'the pains'. There were significant differences in the electrolyte changes, depending on whether thiopentone or Althesin was used for induction, but there was no difference in the incidence of suxamethonium pains. Suxamethonium pains are attributed to damaged muscle-spindles with enhanced calcium release playing an important part.