Repetitive nerve stimulation findings in iatrogenic botulism compared with myasthenia gravis

The widespread use of botulinum toxin has been accompanied by an increase in complications, including iatrogenic botulism (IB). This study investigated repetitive nerve stimulation (RNS) findings in IB and myasthenia gravis (MG) patients.

Clinical and neurophysiological data were retrospectively reviewed in IB patients negative for both acetylcholine receptor and muscle-specific kinase antibodies, and in MG patients who were positive for either. RNS was performed on the ulnar nerve at 2, 3, 5, and 50 Hz, and on the facial and spinal accessory nerves at 2, 3, and 5 Hz.

Fifteen IB and 23 MG patients were included. Compound muscle action potential (CMAP) amplitudes of the ulnar and facial nerves were lower in IB (p = 0.014, p = 0.006). A progressive decrement at low-frequency RNS occurred in 60% of IB and 17% of MG patients. At 5 Hz facial RNS, 1st-2nd (p = 0.044) and 1st-4th (p = 0.008) decrements were greater in MG, whereas the 4th-10th decrement was higher in IB (p = 0.031). Ulnar CMAP changes at 50 Hz were greater in IB (p < 0.001); nine patients showed increments > 40%, four > 60%, and one > 100%, while none of the MG patients exceeded 40%.

Reduced CMAP amplitude, an incremental response at high frequency, and a progressive decrement at low frequency were more commonly encountered in IB. In MG, a U-shaped pattern at low frequency and a more pronounced decrement between the 1st and 2nd responses were noted.