| Myasthenia gravis (MG) is an immune mediated disorder that causes muscle weakness and is caused by antibodies that bind to acetylcholine (ACh) receptors. These receptors normally bind acetylcholine and help with the conduction of nerve impulses. In patients with myasthenia gravis, the antibodies bind to the receptors and block acetylcholine from also binding, so that nerve impulses do not get through to the muscle.
The main symptoms of myasthenia gravis include muscle weakness, ptosis (drooping of the eyelids), diplopia (double vision), dysphagia (difficulty swallowing), and trouble feeding and poor head control in infants. Symptoms usually appear after the age of 10 years in 75% of children with this disorder.
Patients with myasthenia gravis also have rapid fatigue of their muscles, so that symptoms can be worse toward the end of the day. Also, symptoms of muscle weakness usually also get worse with repetitive activities, such as blinking or opening and closing a fist repeatedly. The muscle weakness can also make it difficult to hold their head or arm up for more than a few minutes. Without treatment, the muscle weakness can lead to trouble breathing.
Testing can include an electromyogram (EMG), which will show a characteristic decreased response to repeated nerve stimulation. Other testing can include checking the blood for anti-acetylcholine antibodies, , and sometimes a muscle biopsy.
The main clinical testing that can be done is the edrophonium chloride test, which is a short acting cholinesterase inhibitor. Cholinesterase is the enzyme that normally breaks down acetylcholine in the body, and by inhibiting it, you can increase the amount of acetylcholine available and reverse the symptoms. Children with myasthenia gravis who are given edrophonium chloride have improvement of the ptosis and muscle weakness within a few seconds. Another test involves using an intramuscular (IM) dose of neostigmine, which has similar results in 20-40 minutes after it is given.
Treatments include the use of cholinesterase inhibiting drugs, such as neostigmine given IM or orally, or pyridostigmine for children with moderate to severe symptoms. Children with mild symptoms may not need treatment and may undergo spontaneous remission. Other treatments include the use of prednisone (a steroid), plasmapheresis or intravenous immunoglobulin (IVIG), especially during a myasthenic crisis that can lead to difficulty breathing, or thymectomy (surgical removal of the thymus gland).
While some patients do undergo spontaneous remission in months or years, other children continue to have problems as adults.
Infants that are born to a mother with MG can also have symptoms because the acetylcholine antibodies can cross the placenta. These babies may have difficulty breathing or eating and generalized muscle weakness. Fortunately, they return to normal once all of the antibodies have disappeared and they are not at an increased risk of developing MG later in life. This condition is called transient neonatal myasthenia gravis.
Another form of MG that can affect newborns is congenital myasthenia gravis, which is a rare disorder caused by an abnormality in the ACh receptor.
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