Extra Topic 5.5 -- Myasthenia Gravis Flashcards
What do you think may be the cause of her prolonged neuromuscular blockade?
(A 13-year-old girl, was recently diagnosed with thyroiditis, has been complaining of difficulty when swallowing. She was subsequently scheduled for tonsillectomy at your facility and your partner supervised the case. Your partner calls you to the room 45 minutes after the procedure was completed to evaluate the patient, who remains intubated and exhibited no twitches by peripheral nerve stimulator. You subsequently learn that she was given a defasciculating dose of rocuronium and an intubating dose of succinylcholine during induction to provide adequate muscle relaxation.)
Her prolonged neuromuscular blockade could be the result of –
- pseudocholinesterase deficiency,
- drug error,
- a defective nerve stimulator,
- an underlying neuromuscular disease:
- myasthenia gravis,
- Eaton-Lambert syndrome,
- botulism toxin,
- amyotrophic lateral sclerosis,
- multiple sclerosis, or
- muscular dystrophy,
- electrolyte abnormalities,
- hypothermia,
- acidosis,
- hypercarbia, or
- aminoglycoside administration.
Assuming the equipment was properly functioning and the proper dosage of neuromuscular blocker was administered, and considering her age, gender, diagnosis of thyroiditis, and difficulty when swallowing, I would be concerned that this clinical picture represented myasthenia gravis.
Thyroiditis is often associated with myasthenia gravis (autoimmune disease) and her difficulty swallowing may represent disease involvement of the bulbar musculature, a finding less likely to be found with Eaton-Lambert syndrome.
Her age and gender make other neuromuscular diseases such as ALS, multiple sclerosis, and muscular dystrophy less likely.
However, her prolonged neuromuscular blockade may be the result of pseudocholinesterase deficiency, with her difficulty swallowing being simply due to enlarged tonsils.
What is the pathophysiology of myasthenia gravis?
(A 13-year-old girl, was recently diagnosed with thyroiditis, has been complaining of difficulty when swallowing. She was subsequently scheduled for tonsillectomy at your facility and your partner supervised the case. Your partner calls you to the room 45 minutes after the procedure was completed to evaluate the patient, who remains intubated and exhibited no twitches by peripheral nerve stimulator. You subsequently learn that she was given a defasciculating dose of rocuronium and an intubating dose of succinylcholine during induction to provide adequate muscle relaxation.)
Myasthenia gravis is an autoimmune disorder of the neuromuscular junction, in which postsynaptic acetylcholine receptors at the endplates of affected muscles are destroyed or inactivated, resulting in weakness and easy fatigability that improves with rest.
The disorder is usually characterized by a slow onset and is associated with relapses and remissions.
While any muscle group may be affected, the ocular muscles are most commonly involved, leading to ptosis and diplopia.
Clinical findings such as dysarthria, difficulty chewing and swallowing, an inability to effectively clear secretions, difficulty breathing, and pulmonary aspiration, indicate – involvement of bulbar musculature and suggest more severe disease.
This condition is often treated with –
- anticholinesterase drugs (i.e. pyridostigmine),
- immunosuppressive drugs (i.e. steroids, azathioprine, cyclophosphamide, and cyclosporine) and
- thymectomy (the treatment of choice in most patients).
Should succinylcholine have been avoided for this case?
(A 13-year-old girl, was recently diagnosed with thyroiditis, has been complaining of difficulty when swallowing. She was subsequently scheduled for tonsillectomy at your facility and your partner supervised the case. Your partner calls you to the room 45 minutes after the procedure was completed to evaluate the patient, who remains intubated and exhibited no twitches by peripheral nerve stimulator. You subsequently learn that she was given a defasciculating dose of rocuronium and an intubating dose of succinylcholine during induction to provide adequate muscle relaxation.)
Assuming the cause of this patient’s prolonged neuromuscular blockade is due to myasthenia gravis rather than pseudocholinesterase deficiency, amyotrophic lateral sclerosis, multiple sclerosis, or muscular dystrophy,
the use of succinylcholine is acceptable when indicated.
While patients with myasthenia gravis tend to be resistant to depolarizing neuromuscular blockade, a normal dose of succinylcholine results in adequate relaxation for intubation and a normal recovery time.
However, if the patient had been previously diagnosed with myasthenia gravis and was receiving anticholinesterase therapy, the duration of neuromuscular blockade following succinylcholine administration may be prolonged due to reduced metabolism (anticholinesterase therapy may result in decreased plasma cholinesterase activity).
This undiagnosed patient who was not receiving anticholinesterase therapy is most likely experiencing prolonged neuromuscular blockade secondary to the profound sensitivity to nondepolarizing drugs associated with myasthenia gravis.
Assume that this patient does have previously undiagnosed myasthenia gravis.
What are you going to do?
(A 13-year-old girl, was recently diagnosed with thyroiditis, has been complaining of difficulty when swallowing. She was subsequently scheduled for tonsillectomy at your facility and your partner supervised the case. Your partner calls you to the room 45 minutes after the procedure was completed to evaluate the patient, who remains intubated and exhibited no twitches by peripheral nerve stimulator. You subsequently learn that she was given a defasciculating dose of rocuronium and an intubating dose of succinylcholine during induction to provide adequate muscle relaxation.)
Assuming this was myasthenia gravis, I would –
- provide supportive care (including post-operative ventilation),
- consult a neurologist, and
- consider administering an anticholinesterase to prolong the action of acetylcholine at the postsynaptic membrane (these drugs may, themselves, have some limited agonist effect at the acetylcholine receptors),
- keeping in mind that overaggressive treatment with an anticholinesterase has the potential to induce a cholinergic crisis (excessive muscarinic effects of acetylcholine), which may also result in weakness.
If the patient then demonstrated adequate respiratory effect, I would –
- extubate her and
- provide close monitoring for a duration of time that exceeded the duration of action of the specific anticholinesterase I had administered (to avoid a potential relapse of respiratory insufficiency with the cessation of anticholinesterase activity).
After receiving neostigmine, the patient’s respiratory effort improves and she is extubated.
Forty-five minutes later, she complains of weakness and difficulty breathing.
What do you think is the cause?
(A 13-year-old girl, was recently diagnosed with thyroiditis, has been complaining of difficulty when swallowing. She was subsequently scheduled for tonsillectomy at your facility and your partner supervised the case. Your partner calls you to the room 45 minutes after the procedure was completed to evaluate the patient, who remains intubated and exhibited no twitches by peripheral nerve stimulator. You subsequently learn that she was given a defasciculating dose of rocuronium and an intubating dose of succinylcholine during induction to provide adequate muscle relaxation.)
Given her disease process and recent treatment with an anticholinesterase, her weakness could be the result of either a myasthenic or a cholinergic crisis.
Both conditions result in muscle weakness, salivation, and sweating.
The two conditions may be differentiated by administering 10 mg of edrophonium (Tensilon test), with improved strength implying myasthenic crisis and increased weakness suggesting a cholinergic crisis.
Additionally, an examination of the patient for other signs of cholinergic crisis, such as –
- constricted pupil size (pupils are dilated in a myasthenic crisis, secondary to sympathetic activation),
- weakness and muscle fasciculations (depolarizing phenomenon occurs at the neuromuscular junction),
- bradycardia,
- bronchorrhea (an excessive discharge of watery mucous from the lungs),
- salivation,
- nausea,
- vomiting,
- abdominal cramps,
- diarrhea,
- urinary frequency and urgency,
- pallor, and
- diaphoresis, would help in making the diagnosis.
Common mnemonics for organophosphate poisoning or cholinergic crisis include the “killer B’s” of bradycardia, bronchorrhea and bronchospasm because they are the leading cause of death,[4] and SLUDGE - Salivation, Lacrimation, Urination, Diarrhea, Gastrointestinal distress, and Emesis.
An alternative mnemonic is DUMBBELLS - Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm, Emesis, Lacrimation, Lethargy and Salivation and seizures.
