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What is it?

Malignant Hyperthermia [hyperpyrexia] is an inherited (pharmacogenetic) disorder of skeletal muscle, characterised by a hypermetabolic state, triggered by all volatile anaesthetics and suxamethonium.

As you know calcium is released into the cell as a key component in muscle contraction. In MH there is a problem with calcium reuptake. Intracellular calcium increases up to 500 fold leading to sustained muscle contraction. The cell incurs a severe oxygen debt, the constant demand for ATP leads to glycolysis and subsequent to lactic acidosis. Eventually this leads to membrane instability, cell rupture and rhabdomyolysis.

The exact cause is unknown but the primary defect may be related to a calcium release channel receptor known as ryanodyne. In 50% of families there is a genetic mutation on the short arm of chromosome 19, but multiple other chromosomal abnormalities have been detected, which is not suprising when one considers the complex control of intracellular calcium. The disease is of hetrogenous aetiology. Inheritance is autosomal dominant.

There is increased predominence in young males, children and patients with musculoskeletal disorders. Overall the incidence is 1:50,000.

Why is it important?

MH is important because it is a potentially fatal disorder.

Presenting features:

There are three main underlyingphenomena:

1. Heat production
2. Glycogenolysis -> lactic acidosis
3. Muscular rigidity

This is manifested as

• Tachycardia
• Increased end tidal CO2 (etCO2)
• Decreased oxygen saturation (SpO2)
• Increase in body temperature

Main Clinical Features

Treatment of established MH

Treatment involves, on the one hand, reversal of the primary disorder with dantrolene, and on the other, general resuscitative measures.

1. Call for help
2. The volatile agent is stopped
3. 100% oxygen is given,.
4. The patient is manually hyperventilated.
5. A clean breathing system is used.
6. Surgery should be aborted.
7. Unconsciousness is maintained with an appropriate sedative-hypnotic
8. Dantrolene 1mg/kg is given intravenously, and then 1 – 2.5mg/kg every 10 minutes until MH is under control (to a maximum dose of 10mg/kg).
9. Measure core temperature.
10. Intravenous fluids are given to maintain a steady urinary output, insulin and dextrose may be given to reverse hyperkalaemia. Haemodiafiltration may be necessary in resistant cases.
11. Cardiac arrhythmias are treated as appropriate.
12. The patient is cooled by tepid sponging, cooled fluids, cooling blanket, cool nasogastric lavage etc. as necessary.
13. The patient should be admitted into a High Dependency Unit for postoperative care.

Dantrolene acts by impairing calcium dependent muscle activity. The solution is prepared as 20mg dantrolene, 3g mannitol in 60ml water.

How to avoid MH

By taking a good preoperative history one should be able to establish a family history of MH – look for evidence of problems or even death among family (extended) during anaesthesia. The best way of determining true susceptiblilty is to perform a muscle biopsy (quadriceps under local / regional blockade). The muscle is exposed to halothane and caffeine. Susceptible (MHS) patients respond positively to both; equivical (MHE) patients respond to one or the other.

How to anaesthetise patients with known or suspected MH.

The options are local, regional or general anaesthesia. Local anaesthetics do not trigger MH. If general anaesthesia is required, then TIVA (total intravenous anaesthesia with propofol) with oxygen – air inhaled through a vapour free machine can be used.