Suxamethonium Apnoea

What is it?

Suxamethonium (succinylcholine) apnoea is rare and occurs when a patient has been given this muscle relaxant prior to surgery but is incapable of metabolising the drug sufficiently rapidly. They remain paralysed and unable to breathe after the surgery is complete because they cannot regain their muscle function quickly enough.

Uses
Suxamethonium is a fast acting muscle relaxant and its main use in anaesthesia is to allow the rapid intubation of the trachea. It is also used to modify fits after ECT (electroconvulsive therapy).
Suxamethonium is useful in these situations because it acts and subsides quickly for the vast majority of patients lasting just long enough to meet the needs of the surgical procedure.

How it works
Suxamethonium depolarises skeletal muscle and is the fastest acting of all the available muscle relaxants and lasts for between 2 and 10 minutes. Further doses can be given as required. Part of the structure is similar to the neurotransmitter acetylcholine and as such it is able to prevent acetylcholine from binding at the neuromuscular junction. It binds non-competitively at the synaptic junction, which means that it cannot be reversed by other drugs, and as a result it wears off very slowly until the drug is eliminated by other routes.

Causes of Suxamethonium Apnoea
The majority of suxamethonium is metabolised by an enzyme called cholinesterase and about 10% of the drug is excreted by the kidneys. Suxamethonium apnoea is rare and can be either inherited or acquired. In the case of inherited suxamethonium apnoea the degree of sensitivity to this drug varies with the phenotype of the patient (see section on cholinesterase). The phenotypes (which express a particular form of the enzyme cholinesterase) that are most susceptible to apnoea are the atypical, silent fluoride resistant phenotypes and to a variable degree the UA phenotype. The action of suxamethonium can be increased from 30 minutes to over two hours in these cases.
In the acquired form situations such as pregnancy, liver disease, kidney disease, hypothyroidism and if taking drugs such as methotrexate or monoamine oxidase inhibitors, the action of suxamethonium can be increased by minutes as opposed to hours.

About Suxamethonium Apnoea

Tests
The total activity for the cholinesterase enzyme in blood is measured, also known as butyrylcholinesterase.

A cholinesterase phenotype for the patient is also reported as this will enable the doctor to predict the likelihood of suxamethonium apnoea. The phenotype is a study of the characteristics of the enzyme activity and is determined by the effect of a range of compounds that inhibit the cholinesterase enzyme

If a phenotype is difficult to confirm or there is an atypical type in the immediate family, genotype studies which identify the inherited genes should be considered.
Treatment

Suxamethonium apnoea is often only apparent at the end of an operation. These patients may remain paralysed for up to several hours after a standard dose of suxamethonium. They are kept anesthetised and ventilated in a high dependency unit until this muscle relaxant is cleared by other slower means and the patient begins breathing spontaneously. The length of support will depend on the defect in the cholinesterase phenotype/genotype.
FAQs

1. Is there another muscle relaxant which is metabolised by cholinesterase?
Yes and it is called mivacurium. This drug is also metabolised by cholinesterase but unlike suxamethonium it is a competitive inhibitor for the neurotransmitter acetylcholine. Although the effect of this drug can be more easily reversed, mivacurium should also be avoided in individuals with low enzyme levels and/or atypical cholinesterase phenotypes.

2. Are there alternative muscle relaxants that can be used?
Yes, there is a range of other muscle relaxants that are available for use in surgical operations.

3. What happens if I have experienced Suxamethonium Apnoea?
A blood sample will be taken to measure the total cholinesterase activity and frequently the enzyme phenotype. If the results show that the enzyme level is low or low normal and an abnormal or silent phenotype is present this should be recorded in your notes. In some cases this may include a genotype study. In addition you should be given a Medical Warning card giving your cholinesterase details so that it may be shown to a surgeon and anaesthetist before any future surgical operations.

4. What happens if a close relative experiences Suxamethonium Apnoea?
If a close relative such as a parent, brother, sister or child is affected in this way after some surgery and found to have an abnormal or silent cholinesterase phenotype, then family members should be offered blood tests to see if they are carrying the same phenotype or genotype. If this is the case they will also be encouraged to carry similar medical warning cards.