Neuromuscular junction disorders

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Respiratory failure can herald disorders of the neuromuscular junction (Table 11.2), which can be distinguished from neuropathic causes by the absence of sensory deficit and preservation of tendon reflexes. In myasthenia gravis respiratory failure usually occurs in the setting of established disease that has failed to respond to conventional treatment. Even in an acute case the diagnosis is usually evident because of ptosis, facial weakness, and bulbar palsy with muscle fatigue. The diagnosis can be confirmed by showing a decrement in the compound muscle action potentials elicited by a train of stimuli, neurophysiological tests, or detecting variable conduction block (jitter) in terminal motor nerve fibres in

Table 11.2 Disorders of neuromuscular transmission that cause respiratory failure

Condition

Specific test

Specific treatment

Myasthenia gravis

Single fibre EMG

IVIg, PE, S

for jitter

Anti AChR antibody

Edrophonium test*

Anticholinesterase

Negative edrophonium

Drug withdrawal

overdose

test

Antibiotic induced

Drug withdrawal

paralysis107

Hypermagnesaemia28

Plasma magnesium

Intravenous

calcium

EMG increment on

50 Hz stimulation

Botulism27

Injection of serum

Antitoxin

into mice

Snake, scorpions,

Identifying the snake

Antivenin

and spider bite108

or its venomf

Fish, shellfish, crab

Identifying the fish

Varies

poisoning31,32,108

Tick paralysis33

Finding the tick

Removal/antitoxin

depending on tick

species

Eaton-Lambert

EMG increment on

PE, S

syndrome34

repetitive stimulation

Anti VGCC antibody

PE = plasma exchange; S = steroids; AChR = acetylcholine esterase receptor; VGCC = voltage gated calcium channel.

*This runs the risk of fatal bradycardia and should only be performed by an experienced clinician after giving atropine and in an intensive care setting.

fKits are now available to detect some snake venoms (especially in Australia) and allow the most appropriate antivenin to be chosen.

almost all cases. Acetylcholine receptor antibodies are present in 90% of patients and in about half of the remainder there are antibodies to the muscle specific kinase, which is closely apposed to the acetylcholine receptor.26 The rare occurrence of asystole following intravenous edrophonium has led some experts to stop using it. If it is used, atropine should be given first and resuscitation facilities should be available. In treated myasthenia, weakness can be caused by overdose of anticholinesterase drugs causing depolarisation of motor nerve terminal in a cholinergic crisis. This will be accompanied by diarrhoea, colic, excessive salivation, and small pupils, and will be worsened rather than improved by intravenous edrophonium.

Other causes of neuromuscular junction blockade are rare and the diagnosis is usually obvious from the clinical setting. Suspect botulism when autonomic features, dry mouth, constipation, poorly reactive pupils, ptosis, and bulbar palsy have heralded acute descending paralysis. In the early stages the symptoms and signs are entirely anticholinergic and the reflexes are normal. These symptoms have usually been immediately preceded by nausea, vomiting, abdominal pain, and diarrhoea from eating foul smelling food contaminated by Clostridium botulinum.27 Magnesium-containing antacids and aperients in patients with impaired renal function can produce severe hypermagnesaemia. The increased magnesium interferes with the release of acetylcholine so as to cause weakness, which may develop into respiratory failure.28 The aminoglycoside and polymyxin antibiotics and some other drugs also cause neuromuscular blockade by interfering with the release of acetylcholine.29 This is usually only significant when weaning infected patients off ventilation.30 Physicians practising in the tropics have to cope with a much wider range of toxic causes of neuromuscular conduction blockade whose diagnosis will be obvious from the history (Table 11.2).31 Fish or shellfish toxin poisoning (usually Caribbean or Pacific fish) causes a gastrointestinal upset before the development of weakness.32 In North America paralysis is sometimes caused by the bite of a female tick, Dermacentor spp., whose saliva contains an unidentified toxin which interferes with terminal motor nerve conduction perhaps by inhibiting sodium flux across the axolemma. The tick may be difficult to find but its removal is curative.33 In Australia tick paralysis caused by Ixodes holocyclus is due to a toxin which inhibits acetylcholine release and causes neuromuscular conduction block. Respiratory failure occurs in the Lambert-Eaton myasthenic syndrome, but only rarely and then usually in the setting of gradually progressive weakness.34 The diagnosis may be suggested clinically by autonomic symptoms, including a dry mouth, the finding of depressed reflexes that are enhanced after exercise, and confirmed by electrophysiological tests showing an increment in muscle action potential amplitude following repetitive nerve stimulation. It may be associated with a small cell lung carcinoma or autoimmune disease.

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