Myasthenia Gravis in Children
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Direct answer
Myasthenia gravis in children is a disorder of the neuromuscular junction that causes fatigable weakness, most often from acetylcholine receptor antibodies. Neonatal transient myasthenia results from transplacental maternal antibodies and settles within weeks to months, whereas congenital myasthenia is non-immune, present from birth and persistent. Juvenile myasthenia begins in childhood or adolescence with ptosis and diplopia, and is treated with pyridostigmine and immunosuppression.
What you must remember
- Neonatal transient: occurs in some infants born to mothers with myasthenia, with a weak cry, poor suck and hypotonia in the first days.
- Congenital myasthenia: non-immune, caused by inherited neuromuscular junction defects such as CHRNE, with receptor antibodies negative and no response to immunosuppression.
- Juvenile myasthenia: autoimmune, with acetylcholine receptor antibodies in most cases, ocular onset with ptosis and diplopia, and a female predominance after puberty.
- Edrophonium test: a short-acting cholinesterase inhibitor given intravenously, where improvement in ptosis or limb power supports the diagnosis, though it is used less now.
- Confirmation: acetylcholine receptor antibodies, a decrement on repetitive nerve stimulation and single fibre EMG, the most sensitive test.
- Pyridostigmine: the first-line symptomatic drug, titrated upward gradually because excess causes muscarinic effects such as salivation, colic and bradycardia.
- Immunotherapy: prednisolone is started cautiously because it can transiently worsen weakness, with azathioprine, mycophenolate or IVIG for ongoing disease.
- Drugs to avoid: aminoglycosides, fluoroquinolones, magnesium and beta blockers can precipitate weakness or crisis.
A pathway from ptosis to a treatment plan
A 7-year-old has ptosis and diplopia that worsen by evening, and the steps below guide confirmation and treatment.
- Confirm fatigability: look for ptosis and diplopia that worsen with sustained upgaze or by evening, and recheck limb and bulbar strength after repeated effort.
- Ask about the mother: in a neonate, a maternal history of myasthenia points to transient disease from transplacental antibodies.
- Send the confirmatory tests: acetylcholine receptor antibodies, repetitive nerve stimulation and single fibre EMG, with MuSK or LRP4 antibodies tested when receptor antibodies are negative.
- Image the mediastinum: chest imaging looks for thymic hyperplasia or thymoma, although thymoma is uncommon in children.
- Start pyridostigmine: begin at a low dose, titrate to effect and watch for cholinergic side effects.
- Add immunotherapy: steroids, azathioprine or mycophenolate treat juvenile disease, whereas congenital disease does not respond to immunotherapy.
- Plan for crisis: respiratory failure needs ventilatory support, with IVIG or plasma exchange and avoidance of precipitating drugs.
Where the exam tries to catch you
Myasthenia questions usually hinge on the age of onset and the presence of maternal antibodies.
- Trap: calling neonatal myasthenia a permanent disease. Correct: transient neonatal disease resolves as maternal antibodies clear, whereas congenital myasthenia persists.
- Trap: treating congenital myasthenia with immunosuppression. Correct: congenital disease is non-immune, so immunosuppression does not help and treatment targets the defect.
- Trap: giving magnesium or aminoglycosides freely to a child with myasthenic weakness. Correct: these drugs can precipitate a crisis, so they are avoided or closely monitored.
Frequently asked questions
Which test helps confirm myasthenia gravis in a child?
Acetylcholine receptor antibodies and a decrement on repetitive nerve stimulation are the main tests confirming myasthenia gravis in children. Single fibre EMG is the most sensitive electrophysiological test, but it is less widely available.
What is transient neonatal myasthenia gravis?
Transient neonatal myasthenia gravis results from transplacental transfer of maternal acetylcholine receptor antibodies to the infant. Weakness, poor feeding and a weak cry resolve within weeks to a few months as the antibodies clear.
How does congenital myasthenia differ from juvenile myasthenia?
Congenital myasthenia is non-immune, caused by inherited defects at the neuromuscular junction, and is present from birth. Juvenile myasthenia is autoimmune, usually with acetylcholine receptor antibodies, begins in childhood or adolescence, and responds to immunotherapy.
What is the role of pyridostigmine in myasthenia gravis?
Pyridostigmine is the first-line symptomatic therapy in myasthenia gravis, inhibiting cholinesterase to increase acetylcholine at the junction. Its dose is titrated to effect, because excess causes muscarinic effects such as salivation, colic and bradycardia.