Brief Alerts: Botulism: Making the (Electro)Diagnosis
Brief Alerts
Botulism: Making the (Electro)Diagnosis
Source: Gutmann L, Bodensteiner J. Electrodiagnosis of botulism—revisited. J Clin Neuromusc Dis. 2001;2:121-122.
Botulism may be correctly diagnosed at the bedside in a timely fashion using electrodiagnostic criteria, provided that mindfulness to detail avoids pitfalls. Compound muscle action potential (CMAP) amplitudes should be low but are generally not so in every nerve. Consequently, several nerves must be studied. Decrement of the CMAP amplitude on repetitive low-frequency (2 Hz) nerve stimulation is inconsistent and unreliable for botulism but should be pursued. Botulism may, after all, not be the correct diagnosis! CMAP increment on rapid (tetanic) repetitive high-frequency (20-50 Hz) nerve stimulation for 10 seconds is the electrical sine qua non for diagnosis, but again must be sought in several nerves if initially elusive. It is painful but unavoidable in infants. In adults, recording CMAP amplitude pre and post 10 seconds of maximal contraction can supplant tetanic stimulation. Increments less than 20% are physiologic and of no clinical significance. Increments of 23% to 313% may be seen in 92% of cases. Perhaps of greatest value is observing CMAP changes in the later post-tetanic (or post-exercise) period. Post-tetanic facilitation (increment) persists for several minutes in botulism, whereas it lasts less than a minute in other junctionopathies, where post exercise exhaustion may be seen. This is absent in botulism. —Michael Rubin
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