Crises in myasthenia gravis

A 40-year-old woman known to have myasthenia gravis presents to the emergency

department with severe global weakness. She is pale, sweaty and cyanosed. Her

partner explains that she was diagnosed some time ago and she is, to the best of his

knowledge, compliant with her oral pyridostigmine therapy. She is a smoker and has

been coughing more than usual recently. He has been worried about her low mood in

past months. In order to distinguish between an excess or inadequacy of her myasthenia

treatment, which one of the following features is likely to be the MOST HELPFUL?

a) Rapid onset of ventilatory failure

b) Response to dose of cholinesterase inhibitor

c) Flaccid muscle paralysis

d) Presence of bronchospasm

e) Loss of deep tendon reflexes

Answer: b

Explanation:

Distinguishing myasthenic crisis from cholinergic crisis is difficult because the two

conditions share many clinical features. A myasthenic crisis may be precipitated by

non-compliance with medications, concurrent infection or any physiological insult, but

the condition naturally varies in severity and it is expected that cholinesterase inhibitor

therapy will have to be regularly adjusted. A cholinergic crisis is caused by relative

overdosage of cholinesterase inhibitor medication causing acetylcholine excess and

stimulation at nicotinic and muscarinic receptors in the locomotor and autonomic

nervous systems. This produces a syndrome similar to organophosphate poisoning.

Both may present with rapid onset flaccid paralysis. A curiosity of myasthenia gravis

and cholinergic crisis is that deep tendon reflexes are preserved. Bronchospasm is a

described feature of cholinergic crisis but may equally be present in myasthenic crisis,

as inability to cough causes retained secretions and airway irritation. Both may cause

sweating and cyanosis via ventilatory failure. Cholinergic crisis is associated with the

SLUDGE syndrome (salivation, lacrimation, urinary incontinence, diarrhoea, gastrointestinal

hypermotility and emesis) but is not consistently present and does not

distinguish from myasthenic crisis. Despite the fact that one of these emergencies is

caused by an excess of cholinesterase inhibitor, the conditions are differentiated by

administration of intravenous edrophonium at escalating doses (1 mg, 3 mg, 5 mg).

Myasthenic crisis will show transient improvement whereas in cholinergic crisis the

SLUDGE syndrome, bradycardia or very rarely asystole may be precipitated. This is

only acceptable as a diagnostic tool as the half-life of edrophonium is short. Atropine

should be immediately available to administer, if required. The arrangement of this test

should not distract from appropriate airway and ventilatory management with a small

dose of non-depolarising muscle relaxant, if necessary, to facilitate endotracheal

intubation.