Variable ptosis, diplopia, ophthalmoplegia, and facial weakness are the presenting symptoms. Dysphagia, poor headcontrol, and extremity weakness may occur. Rapid fatigueof muscles distinguishes myasthenia from other neuromuscular disorders, with progressive worsening over theday or with repetitive activity. In some children, the disease never advances beyond ophthalmoplegia and ptosis (ocular myasthenia). Others have a progressive andpotentially life-threatening illness that involves all musculature, including that of respiration and swallowing.Treatment includes pyridostigmine, an inhibitor of acetylcholinesterase, and, depending on severity, various forms ofimmunosuppression.
A transient myasthenic syndrome develops in 10% to 20% of neonates born to mothers with myasthenia gravis, presenting in the first hours to days after birth. Almost all infants born to mothers with myasthenia have maternal anti-AChR antibodies. Signs include ptosis, ophthalmoplegia, weak facial movements, poor feeding, hypotonia, respiratory difficulty, and variable extremity weakness. Neonates with transient myasthenia gravis require cholinesterase inhibitors and supportive care for a few days to weeks until the weakness remits.
A variety of rare disorders of the neuromuscular junction have been reported that are not autoimmune mediated. The congenital myasthenic syndromes (CMS) are due to gene mutations in the components of the neuromuscular junction. They typically present in infancy with hypotonia, ophthalmoparesis, facial diplegia, and extremity weakness, although they can present throughout childhood. Respiratory function and feeding may be compromised. Children with CMS usually have lifelong disability. Some children will respond to pyridostigmine or other drugs that improve neuromuscular junction function.
Autoimmune myasthenia is most commonly diagnosed through the combination of clinical symptoms and antibody testing. The majority of individuals have antibodies to the AChR, although some have antibodies to other components of the neuromuscular junction. Neurophysiology may exhibit the classic feature of electrodecrement with 3-Hz repetitive stimulation during nerve conduction studies. Administration of a cholinesterase inhibitor (edrophonium chloride) can result in transient improvement in strength, particularly of ptosis, and thus can additionally be used for diagnostic verification.
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Infant botulism results from intestinal infection by Clostridium botulinum, which produces a neurotoxin that blocks presynaptic cholinergic transmission. Young age and the absence of competitive bowel flora predispose infants to this disease. Infants may ingest dust, soil, or food (honey or poorly canned foods) contaminated with spores. The progressive neuromuscular blockade ranges from mild to severe. Infants often present with constipation and poor feeding. Hypotonia and weakness develop, along with cranial nerve dysfunction manifested by decreased gag reflex, diminished eye movements, decreased pupillary contraction, and ptosis. Heart rate and blood pressure may fluctuate. Affected infants may develop respiratory failure. The diagnosis is made by the presence of C. botulinum spores and toxin in stool samples. Therapy with botulism IVIG should be administered as soon as the diagnosis is suspected.
With prompt treatment and respiratory and supportive care, the prognosis is good.
Muscular dystrophies are a group of genetic muscle diseases characterized by progressive myofiber degeneration and the gradual replacement of muscle by fibrotic tissue. Duchenne muscular dystrophy is the most common muscular dystrophy and one of the most common genetic disorders of childhood.