NEUROMUSCULAR JUNCTION DISORDERS
Infantile botulism primarily occurs in infants 2 to 6 months of age. Clinical findings include diffuse weakness, hypotonia, weak cry, poor feeding, constipation, and occasionally respiratory distress. The onset is fairly rapid. Electrophysiologic studies may show a reduced CMAP amplitude, preserved MCVs and snaps, and abnormal repetitive nerve stimulation findings at high rates of stimulation (see Figure 6.11). One study demonstrated an incremental response to repetitive nerve stimulation at rates of 20 to 50 Hz in 92% of infants with infantile botulism (25). The mean increment was 73% with a range of 23% to 313%. For lower frequency stimulation (2-5 Hz), variable changes occurred, but the majority of infants showed décrémentai responses. A recent study demonstrated that the isolation of Clostridium botulinum from stool obtained by enema effluent
FIGURE 6.11 High frequency repetitive nerve stimulation in a 7-week-old infant with marked progressive weakness, respiratory failure, and botulism. (A) Several days into the course, the repetitive stimulation study of the ulnar nerve at 50 Hz is normal; however, the compound muscle action potential amplitude is severely reduced (1.63 mV). (B) Twelve days later, the infant is slightly improved clinically. A repeat study of the ulnar nerve at 50 Hz is diagnostic of infantile botulism with a 33% increment obtained between first and tenth stimuli. Clostridium botulinum was isolated from the stool.
was actually more sensitive for the diagnosis of infant botulism than EDSs (131).
EMG in infants with botulism demonstrates abnormal spontaneous rest activity with fibrillation potentials and positive sharp waves and short-duration low-amplitude MUAPs (25).
Transient Neonatal Autoimmune Myasthenia Gravis
This disorder is caused by the passage of antibodies from myasthenic mothers to their fetuses. Infants often present with hypotonia and respiratory distress. The diagnosis may be made by repetitive nerve stimulation studies. Given that normal infants exhibit less neuromuscular reserve than older children or adults, repetitive stimulation studies in this clinical setting utilize rates of 2 to 5 Hz almost exclusively. A decrement of greater than 8% to 10% between the first and fifth CMAP at low rates of stimulation is considered positive for myasthenia. The combination of repetitive motor nerve stimulation and edrophonium or neostigmine testing may improve the accuracy of the diagnosis (132). If a décrémentai response is obtained, the repetitive nerve stimulation may be repeated at 30 to 120 seconds after administration of edrophonium utilizing a stimulation rate of 2 to 5 Hz. Near complete repair of the décrémentai response may be evident in the myasthenic infant (see Figure 6.12). Serologic antibody testing may be helpful if the mother has documented antibodies. Transient neonatal myasthenia gravis is self-limited with a reported duration of 5 to 47 days with a mean duration of 18 days (133).
Toxic Neuromuscular Junction Disorders
Medications can interfere with neuromuscular transmission by inhibiting the release of acetylcholine, impairing the function of AChE, or binding directly to the acetylcholine receptor. Two drugs that may produce clinically
FIGURE 6.12 Low-frequency repetitive nerve stimulation study of the ulnar nerve in a 2-week-old infant with respiratory failure secondary to congenital myasthenia. (A) At baseline, a 68% decrement in amplitude and a 59% decrement in area is present between first and fifth stimuli with a stimulation frequency of 2 Hz. (B) Twenty minutes after intravenous neostigmine is given, the initial compound muscle action potential has improved from 2.32 to 2.64 mV and the decrement has improved to 14%. The infant was treated with Mestinon and later extubated.
significant weakness in normal children are magnesium and organophosphates (134,135).