Genetic aspects of succinylcholine sensitivity

Goodbye suxamethonium!

No drugs in anaesthesia are more problematic than suxamethonium. Yet, no drugs have survived as suxamethonium does in spite of crisis after crisis, and attempt after attempt at its replacement. For decades, suxamethonium has taught us neuromuscular pharmacology and provided us with an encyclopaedia of side effects, while benefiting millions and millions of our anaesthetised patients. With the arrival of sugammadex, it finally appears that suxamethonium can be retired. Suxamethonium has done its job and seen its days! The present review is intended to offer a eulogy for suxamethonium.

The genetic basis of variability in drug responses

It is almost axiomatic that patients vary widely in their beneficial responses to drug therapy, and serious and apparently unpredictable adverse drug reactions continue to be a major public health problem. Here, we discuss the concept that genetic variants might determine much of this variability in drug response, and propose an algorithm to enable further evaluation of the benefits and pitfalls of this enticing possibility.

Reversal of prolonged suxamethonium apnoea with fresh frozen plasma in a 6-week-old infant

A period of apnoea lasting 480 minutes following suxamethonium in a 6-week-old male infant is described. Neuromuscular function recovered following the administration of fresh frozen plasma. The infant was found to be homozygous for atypical cholinesterase (E1a E1a). This is believed to be the youngest reported case of suxamethonium apnoea.

Neuroendocrine carcinoma of lung associated with bradycardia and episodic cardiac asystole

A neuroendocrine carcinoma of the lung associated with bradycardia and episodic cardiac asystole is reported. Cardiac dysfunction may have been caused by a hormonal factor produced by a carcinoma developing in the pulmonary neuroepithelial bodies because the bradycardia and periods of asystole disappeared after pneumonectomy, only to return months later when pleural metastatic tumor developed. No neoplastic involvement of the heart was present. Implications of a cholinesterase isoenzyme involvement in the cardiac dysfunction are discussed.

Inhibition of the plasma cholinesterase variants by pancuronium bromide and some of its analogues

Pancuronium bromide, a 3,17-diacetoxy-5 alpha-androstane, and three of its analogues, the 17-desoxy, the 3,17-dibutyryloxy and the 16-N-monoquaternary ammonium derivatives have been used as inhibitors of the usual and atypical plasma cholinesterase variants. In all cases the usual enzyme is more sensitive to inhibition by the substituted steroids than the dibucaine resistant enzyme. The relative affinities of the bis-quaternary ammonium compounds for either enzyme is in the order dibutyryloxy derivative > 17-desoxy derivative greater than or equal to pancuronium bromide. The monoquaternary compound has the least affinity of all the inhibitors for the usual enzyme but the greater affinity for the atypical enzyme. These observations show that the bis-quaternary compounds are very powerful differentiators of the variants. The monoquaternary derivative shows less differential inhibition, but provides additional evidence that the usual and dibucaine resistant variants differ in structure at or near their esteratic active site.

Plasma cholinesterase variants and the anaesthetist

Biochemical properties of plasma cholinesterase of significance to the anaesthetist are reviewed. The role of the genetic variants of the enzyme in suxamethonium sensitivity and hyperthermia are discussed with emphasis on the pregnant patient. Altered gene frequencies of the enzyme variants in some mental disorders is commented upon.

Suxamethonium apnoea masked by tetrahydroaminacrine

A case is reported of a patient who was given THA with suxamethonium and who demonstrated a prolonged apnoea. This patient was later shown to have a variant cholinesterase enzyme. The different techniques of THA and suxamethonium administration are discussed and it is suggested that an initial dose of suxamethonium should precede a THA-suxamethonium combination in order to try and avoid a prolonged apnoea.

Plasma cholinesterase variants in patients having lithium therapy

The plasma cholinesterase variants of 190 mentally ill individuals having lithium prophylaxis have been examined. A significantly increased frequency of the E1f gene is reported. The effect of lithium nitrate and sodium nitrate on the plasma cholinesterase variants have been shown to be identical in the concentration range 25.0-50.0 mmol/l. The usual enzyme is slightly less sensitive to inhibition by either salt than the dibucaine resistant variant. The evidence suggest that the increased frequency of the E1f gene could be a genetic marker associated with some mental illness and not the result of lithium prophylaxis.

Atypical serum cholinesterase in a family with congenital distichiasis

This paper describes the coexistence of genetically determined reduced cholinesterase activity and congenital distichiasis in the same family. The pedigree suggests that these two autosomal dominant diseases are segregated independently and do not show evidence of linkage.

Suxamethonium apnoea terminated with commercial serumcholinesterase

A case of prolonged suxamethonium apnoea successfully terminated by the infusion of a commercial preparation of serumcholinesterase is reported. The patient appeared to be homozygous for the dibucaine resistant gene, having only 15% of normal activity in his serum. His dibucaine number was 21, and the Michaelis constant was 5.5 times that of normal sera. One and a half hours after receiving 110 mg suxamethonium for oesophagoscopy, the patient was still apnoeic with no response to ulnar nerve stimulation. Intravenous administration of 90 mg commercial serumcholinesterase, the equivalent to 1000 ml fresh human plasma, restored twitch and tetanic responses and the patient could lift his head 15 min after the beginning of the enzyme injection. The serumcholinesterase activity of the patient's serum increased by 55% (from 15% to 70%) following the injection. This rise was halved over the next 8 days.

Idiosyncratic reactions to the antiepileptic drugs

Idiosyncratic drug reactions can be defined as those adverse drug effects caused by genetically determined enzymatic abnormalities. For the clinician, however, this definition is too limited, and other rare and unusual adverse reactions to the antiepileptic drugs are discussed, including drug interactions, drug allergies, and organ toxicities, as well as true idiosyncrasies. Responsibility for initial recognition and later diagnosis of these reactions falls heavily upon the clinician. In addition to discussing the more common or serious rare reactions, the epidemiology and general mechanisms underlying the reactions are discussed. Treatment involves drug withdrawal, plus general supportive and specific therapy determined by the type and severity of the individual reaction. If primum non nocere should guide the treatment in general, then secundum non plus nocere should guide the treatment of adverse drug reactions.

An original approach to the diagnosis of scoline-induced apnoea

A new technique is described for measuring the rate of scoline hydrolysis by plasma cholinesterase, which may prove useful in the diagnosis of scoline sensitivity. The importance of extensive family screening is emphasized and three cases possessing a silent gene are described.